Tv Overview

7/24/2019 Tv Overview

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Refresher Topics Television Technologyby Rudolf Musl


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Rudolf Musl, Professor at the University of applied Sciences Munich, gave a detailed overview of state-of-the-art television technol-ogy to the readers of "News from Rohde & Schwarz" in a refresher serial.

The first seven parts of the serial were published between 1977 and 1979 and dealt with fundamentals of image conversion, transmis-sion and reproduction, including a detailed description of the PAL method for colour TV signals. Further chapters on HDTV, MAC and

HD-MAC methods, satellite TV signal distribution and PALplus were added in two reprints.

In 1998, these topics were no longer of interest or in a state of change to digital signal transmission. This background has been fullytaken into account in the current edition of this brochure which also presents a detailed description of digital video signal processing inthe studio, data compression methods, MPEG2 standard and methods for carrier-frequency transmission of MPEG2 multiplex signals toDVB standard.

An even more detailed discussion of the subject matter as well as of state-of-the-art technology and systems is given in the secondedition of the book by Rudolf Musl "Fernsehtechnik - bertragungsverfahren fr Bild, Ton und Daten" published by Hthig Buch Ver-lag, Heidelberg 1995 (only in German).

Introduction


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1 Transmission method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1 Scanning1.2 Number of lines1.3 Picture repetition frequency1.4 Bandwidth of picture signal

2 Composite video signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1 Blanking signal2.2 Sync signal

3 RF transmission of vision and sound signals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1 Vestigial sideband amplitude modulation3.2 Sound signal transmission3.3 TV transmitter and monochrome receiver3.4 TV standards

4 Adding the colour information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.1 Problem4.2 Chromatics and colorimetry4.3 Luminance and chrominance signals, colour difference signals

5 Transmission of chrominance signal with colour subcarrier. . . . . . . . . . . . . . . . . . . . . . . . . 18

5.1 Determining the colour subcarrier frequency5.2 Modulation of colour subcarrier5.3 Composite colour video signal5.4 NTSC method5.5 PAL method5.6 SECAM method

6 Colour picture pickup and reproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

6.1 Principle of colour picture pickup6.2 Colour picture reproduction using hole or slot - mask tube

7 Block diagram of PAL colour TV receiver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

8 PALplus system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

8.1 Spectrum of PAL CCVS signal8.2 Colour plus method

8.3 Compatible transmission with 16: 9 aspect ratio9 Digital video studio signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

10 Data compression techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

10.1 Redundancy reduction for the video signal10.2 Irrelevancy reduction for the video signal10.3 MUSICAM for the audio signal

11 Video and audio coding to MPEG2 standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

11.1 Definition of profiles and levels in MPEG2 video11.2 Layers of video data stream11.3 Layers of audio data stream11.4 Packetized program and transport stream

12 Transmission of DVB signal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

12.1 Error correction12.2 Satellite channel12.3 Cable channel12.4 Terrestrial transmission channel

13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Contents


4/51

Refresher Topics - Television Technology 5

is

pattern

scanning direction

line123456

y

x

1 2 3 4 5 6

signal current

1 2 3 4 5 6line

brightness distributionin lines

white

black

t

t

1 Transmission method

The principle of TV transmission with aview to reproducing black-and-white pic-tures can be summarized as follows: theoptical image of the scene to be transmit-ted is divided into small picture elements

(pixels).

Fig 1

Principle of TV transmission.

An opto-electrical converter, usually acamera tube, consecutively translates theindividual elements into electrical infor-mation depending on their brightness.This signal is then transmitted at itsactual frequency or after modulation ontoan RF carrier. After appropriate process-ing at the receiving end, the informationis applied to an electro-optical converterand reproduced in accordance with the

brightness distribution of the pattern.Continuous transmission is ensured byproducing a defined number of frames asin cinema films.

1.1 Scanning

The pattern is divided into a number oflines which are scanned from left to rightand from top to bottom (Fig 1). The scan-ning beam is deflected horizontally andvertically, writing a line raster. Synchro-nizing pulses are transmitted to ensure

that the reading and the writing beamsstay in step, covering the correct, corre-sponding picture elements.

Scanning converts the individual pictureelements from the geometrical into thetime domain. Fig 2 gives a simplified rep-resentation assuming that the scanningbeam returns to the lefthand picture mar-gin within a negligible period of time. Ingeneral, the signal current obtained is atrain of multishape pulses of varyingmean value, corresponding to the mean

opt.

electr.

readingbeam

converter

opt.

electr.

writingbeam

converter

electrical signal

brightness of the pattern. This signal cur-rent, which may contain components ofvery high frequency due to fine picturedetails, must be applied to the receiverwithout distortion. This requirement

determines the essential characteristicsof the transmission system.

1.2 Number of lines

The quality of the reproduced picture isdetermined by the resolution, which isthe better the higher the number of lines,a minimum number being required toensure that the raster is not disturbing tothe viewer. In this context, the distance ofthe viewer from the screen and the acuityof the human eye have to be considered.

Fig 3

Angle of sight when viewing TV picture.

The optimum viewing distance is found tobe about five times the picture height, i.e.D/H = 5 (Fig 3). At this distance, the linestructure should just be no longer visible,i.e. the limit of the resolving power of theeye should be reached.

Under normal conditions the limit angle is about o= 1.5. From the equation:

(1)

H=L lines

D

H LD

--------------=tan )

where = o= 1.5 and tan o= 4 x10-4

the following approximation formula forcalculating the minimum line number isobtained:

(2)

For D/H = 5, this means a number ofL = 500 visible lines [1]. In accordancewith CCIR, the complete raster area hasbeen divided into 625 lines, 575 of whichare in the visible picture area due to thevertical flyback of the beam (525 lines inNorth America and Japan with about475 active picture lines).

1.3 Picture repetition frequency

When determining the picture repetitionfrequency the physiological characteris-tics of the eye have to be considered. Toreproduce a continuous rapid motion, acertain minimum frame frequency isrequired so that no annoying discontinui-ties occur. 16 to 18 frames per second, asare used for instance in amateur films,are the lower limit for this frequency. 24frames per second are used for the cin-ema. This number could also be adoptedfor television; however, considering thelinkage to the AC supply frequency, a pic-

ture repetition frequency (fr) of 25 Hz foran AC supply of 50 Hz has been selected(30 Hz for a 60 Hz AC supply in NorthAmerica and Japan).

However, the picture repetition fre-quency of 25 Hz is not sufficient for flick-erfree reproduction of the picture. Thesame problem had to be solved for thecinema where the projection of each indi-vidual picture is interrupted once by aflicker shutter, thus producing the

impression that the repetition frequencyhad been doubled.

L2500

D H-----------------=

Fig 2

Waveform of signal current in

case of line-by-line scanning of

pattern.


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6 Refresher Topics - Television Technology

This method cannot be used for televi-sion; here the solution found has beeninterlaced scanning. The lines of the com-plete raster are divided into two fields,which are interlaced and transmittedconsecutively. Each field contains L/2lines and is swept within the intervalTv/2. This means that lines 1, 3, 5 etc arein the first field and lines 2, 4, 6 etc in the

second field (geometrical line counting)(Fig 4).

Fig 4

Division of complete raster for interlaced scanning.

When reproducing the two fields it isessential that they be accurately inter-laced since otherwise pairing of l ines maycause the field raster to appear in a veryannoying way. In a system using an oddline number, for instance 625, the transi-tion from the first to the second field

takes place after the first half of the lastline in the first field. Thus no special auxil-iary signal is required to ensure periodicoffset of the two fields. This detail will bediscussed in section 2.2.

Fig 5

Coupling of horizontal and vertical deflection

frequencies in case of interlaced scanning

according to CCIR.

Thus 50 fields of 312 lines each aretransmitted instead of 25 pictures of 625lines, the field repetition or vertical fre-quency being f

v= 50 Hz.

line12345....

complete picture

line1

3

5

.

.line

24

.

.

2nd field

1st field

2 fh

fh

2

1

fv

625

1

The resulting line or horizontal frequencyis

fh= 25 x 625 = 50 x 312 = 15 625 Hz.

The period of the horizontal deflection isTh= 64 s, that of the vertical deflectionTv = 20 ms. The horizontal and verticalfrequencies must be synchronous and

phase-locked. This is ensured by derivingthe two frequencies from double the linefrequency (Fig 5).

1.4 Bandwidth of picture signal

The resolution of the picture to be trans-mitted is determined by the number oflines. With the same resolution in the hor-izontal and vertical directions, the widthof the picture element b is equal to theline spacing a (Fig 6).

Fig 6

Resolution of pattern by line raster.

At the end of a line, the scanning beam isreturned to the left. After sweeping a

field, it is returned to the top of the raster.

Fig 7

Periods of horizontal and vertical deflection

with flyback intervals.

During flyback both the reading and thewriting beams are blanked. The requiredflyback intervals, referred to the period Thof the horizontal deflection and Tvof thevertical deflection, are given in Fig 7.

a b

L lines

Ih

Th

tfh

t

Iv

Tv

tfv

t

In accordance with CCIR, the flybackintervals are defined as follows:

Tfh= 0.18 x Th= 11.52 s

tf v= 0.08 x Tv = 1.6 ms

Thus, for transmitting the picture infor-mation, only the line interval Thx(10.18)

= 52.48 s of the total line period Th andthe portion Lx (1 0.08) = 575 lines of theL-line raster (= 2 Tv) can be used, theraster area available for the visible pic-ture being reduced (Fig 8).

Fig 8

Raster area reduced due to flyback intervals.

For optical and aesthetic reasons a rec-tangular format with an aspect ratio of4:3 is chosen for the visible picture.

With the same horizontal and vertical res-

olution, the number of picture elementsper line is:

x 625(10.08) = 767

and the total number of picture elementsin the complete picture:

x 625 x (10.08) x 625 x (10.08)

= 440 833

This number of picture elements is trans-mitted during the time interval:

64 s x (10.18) x 625 x (10.08)=30.176 ms

Thus the time TPEavailable for scanningone element is:

=2Tv

=tfh

=Th^

=2tfv

visible picture

4

3---

4

3---

TPE30.176 ms

440833--------------------------- = 0.0684 s=


6/51


The highest picture signal frequency isobtained if black and white picture ele-ments alternate (Fig 9). In this case, theperiod of the picture signal is:

TP= 2 x TPE= 0.137 s

Due to the finite diameter of the scanningbeam the white-to-black transition is

rounded so that it is sufficient to transmitthe fundamental of the squarewave sig-nal. This yields a maximum picture signalfrequency of:

fPmax1TP----- = 7.3 MHz=

Fig 9

Rounding of picture signal due to fin ite

beam diameter.

pattern

brightness

picture signal

scanning direction

TPE

t2TPE

Considering the finite beam diameter, thevertical resolution is reduced comparedwith the above calculation. This isexpressed by the Kell factor K. With avalue of K = 2/3, the bandwidth of thepicture or video signal, and the value laiddown in the CCIR standards, results as:

BW = 5 MHz.


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B

S

end of2nd field

1st field

622 623 624 625 1 2 3 4 5 6 7 23 24

H2

2.5 H

3 H

2.5 H 2.5 H

field-blanking interval (25 H + 12 s)

V pulse

B

S

end of1st field

2nd field

309 310

12

2.5 H

(3+ )H

2.5 H 2.5 H

field-blanking interval (25 H + 12 s)

V pulse

336320319318317316315314313312311

2 Composite video signal

The composite video signal (CVS) is thecomplete television signal consisting ofthe scanned image (SI), blanking (B) andsync (S) components. The scanned imagesignal was dealt with in section 1.

2.1 Blanking signal

During the horizontal and vertical beamreturn, the scanned image signal is inter-rupted, i.e. blanked. The signal is main-tained at a defined blanking level whichis equal to the black level of the video sig-nal or differs only slightly from it. In mostcases the setup interval formerly used todistinguish between blanking level andblack level is nowadays omitted for thebenefit of making better use of the wholelevel range. The signal used for blankingconsists of horizontal blanking pulseswith the width:

tbh= 0.18 x Th

and vertical blanking pulses with thewidth:

tbv= 0.08 x Tv

Thus the signal coming from the videosource is completed to form the picture

signal (Fig 10).

Fig 10

Horizontal blanking signal and generation

of picture signal.

SI

B

P

t

white

black

white

black

setup blanking

level

tbh

2.2 Sync signal

Synchronization pulses are required sothat line and field of the picture at thereceiver stay in step with the scanning atthe transmitter. These sync signals drive

the deflection systems at the transmitterand receiver ends. The sync pulse level islower than the blanking level, thus corre-sponding to a "blacker-than-black" region(Fig 11).

Fig 11

Level range of composite video signal.

In this level range, the horizontal and ver-tical sync signals must be transmitted in adistinctive way. This is why differingpulse widths are used.

This and the different repetition fre-quency permit easy separation into hori-zontal or vertical sync pulses at the

receiver end.

white level

sync level

100%

0

-40%

picture signal

sync signal

P

S

black levelblanking level

The horizontal sync pulse is separatedfrom the sync signal mixture via a differ-entiating network.

Thus the leading edge of the pulse,

whose duration is 4.5 s to 5 s, deter-mines the beginning of synchronization,i.e. at the beam return. The front porchensures that the beam returns to thelefthand picture margin within the blank-ing interval tbh(Fig 12). The back porch isthe reference level. But it is also used fortransmitting additional signals, such asthe colour synchronization signal.

Fig 12

Horizontal sync signal.

The vertical sync pulse is transmitted dur-ing the field blanking interval. Its duration

of 2.5 H periods (2.5 64 s) is consider-ably longer than that of the horizontalsync pulse (about 0.07 H periods). Toobtain regular repetition of the horizontal

setup

P

S

H=Th=64s

a b c

^tbh

Fig 13

Vertical sync signal with pre- and postequalizing pulses.


8/51


sync pulse, the vertical sync pulse isbriefly interrupted at intervals of H/2. Atthe points marked in Fig 13, the pulseedges required for horizontal synchroni-zation are produced. Due to the half-lineoffset of the two rasters, the interruptiontakes place at intervals of H/2. Interlacedscanning also causes the vertical syncpulse to be shifted by H/2 relative to the

horizontal sync pulse from one field to thenext.

Since the vertical sync pulse is obtainedby integration from the sync signal mix-ture, different conditions for starting theintegration (Fig 14, left) would result for

the two fields due to the half-line offset.This in turn might cause pairing of theraster lines. Therefore five narrow equal-izing pulses (preequalizing pulses) areadded to advance, at H/2, the actual ver-tical sync pulse so that the same initialconditions exist in each field (Fig 14,right). In a similar way, five postequaliz-ing pulses ensure a uniform trailing edge

of the integrated vertical part pulses.

The following explanation of the linenumbering of Fig 13 is necessary. In tele-vision engineering, the sequentiallytransmitted lines are numbered consecu-tively. The first field starts with the lead-ing edge of the vertical sync pulse andcontains 312 lines. The first 22 linesare included in the field blanking interval.After 312 lines the second field beginsin the middle of line 313, also with theleading edge of the vertical sync pulse,and it ends with line 625.

After the complete sync signal with thecorrect level has been added to the pic-ture signal in a signal mixer, the compos-ite video signal (CVS) is obtained.

without preequalizing pulsesstart of 1st field

with preequalizing pulses

Vint

Vswitch

o

2H + o

start of 2nd field

Vint

Vswitch

UCo


9/51


3 RF transmission of vision and sound signals

For radio transmission of the televisionsignal and for some special applications,an RF carrier is modulated with the com-posite video signal. For TV broadcastingand systems including conventional TV

receivers, amplitude modulation is used,whereas frequency modulation isemployed for TV transmission via micro-wave links because of the higher trans-mission quality.

Fig 15

RF transmission of CVS by modulation of vision carrier:

top: amplitude modulation, carrier with two side-

bands;

center: single sideband amplitude modulation;

bottom: vestigial sideband amplitude modulation.

3.1 Vestigial sideband amplitude

modulation

The advantage of amplitude modulationis the narrower bandwidth of the modula-tion product. With conventional AM themodulating CVS of BW = 5 MHz requiresan RF transmission bandwidth of BWRF=10 MHz (Fig 15, top). In principle, onesideband could be suppressed since thetwo sidebands have the same signal con-tent. This would lead to single sideband

amplitude modulation (SSB AM) (Fig 15,center).

Due to the fact that the modulation sig-nals reach very low frequencies, sharpcutoff filters are required; however, thegroup-delay distortion introduced bythese filters at the limits of the passbandcauses certain difficulties.

AM

SSB AM

VSB AM

LSB USB

USB

VSB USB

fvision f

The problem is eluded by using vestigialsideband amplitude modulation (VSBAM) instead of SSB AM. In this case, onecomplete sideband and part of the otherare transmitted (Fig 15, bottom).

Fig 16

Correction of frequency response in vestigial

sideband transmission by Nyquist filter.

At the receiver end it is necessary toensure that the signal frequencies in theregion of the vestigial sideband do notappear with double amplitude afterdemodulation. This is obtained by the

Nyquist slope, the selectivity curve of thereceiver rising or falling linearly about thevision carrier frequency (Fig 16).

In accordance with CCIR, 7 MHz bandsare available in the VHF range and 8 MHzbands in the UHF range for TV broadcast-ing. The picture transmitter frequencyresponse and the receiver passband char-acteristic are also determined by CCIRstandards (Fig 17). In most cases, bothmodulation and demodulation take place

at the IF, the vision IF being 38.9 MHz andthe sound IF 33.4 MHz.

The modulation of the RF carrier by theCVS is in the form of negative AM, wherebright picture points correspond to a low

picture transmitterfrequencyresponse

receiverpassbandcharacteristic

demod. CVSfrequencyresponse

0 f

fvision

fvision

Nyquistslope

carrier amplitude and the sync pulse tomaximum carrier amplitude (Fig 18).

Fig 17

CCIR standard curves for picture transmitter

frequency response (top) and receiver passband

characteristic(bottom).

Fig 18

Negative amplitude modulation of RF vision carrier

by CVS.

A residual carrier (white level) of 10% isrequired because of the intercarriersound method used in the receiver. Oneadvantage of negative modulation is opti-

mum utilization of the transmitter, sincemaximum power is necessary only brieflyfor the duration of the sync peaks and atthe maximum amplitude occurring peri-odically during the sync pulses to serve asa reference for automatic gain control inthe receiver.

-1 1 2 3 4 5 6 MHz

5.5 MHzfvision fsound

-1.2

5

-0.7

5

-1 1 2 3 4 5 6 MHz

5.5 MHzfvision fsound

1.0

0.5

100%

10%

0

sync levelblack level

white level

carrier zero


10/51


IFosc.

IFmod.

VSBfilter

38.9 MHzpicture transmitter

mixer driveroutputstage

equa-lizer

AM

CVS

con-verterosc.

IFosc.

adder mixer driveroutputstage

vision/sound

diplexer

toantenna

IFosc.

33.4 MHz

33.15 MHz

sound 2

sound 1

FM

FMsound transmitter

channeltuning

fixedtuning

3.2 Sound signal transmission

In TV broadcasting the sound signal istransmitted by frequency-modulating theRF sound carrier. In accordance with the

relevant CCIR standard, the sound carrieris 5.5 MHz above the associated visioncarrier.

The maximum frequency deviation is50 kHz. Due to certain disturbances incolour transmission, the original sound/vision carrier power ratio of 1:5 wasreduced to 1:10 or 1:20 [2]. Even in thelatter case no deterioration of the soundquality was apparent if the signal wassufficient for a satisfactory picture.

As mentioned above, the intercarriersound method is used in most TV receiv-ers. The difference frequency of 5.5 MHzis obtained from the sound and vision car-rier frequencies. This signal is frequency-

modulated with the sound information.The frequency of the intercarrier sound isconstant and is not influenced by tuningerrors or variations of the local oscillator.

More recent studies have shown furtherpossibilities of TV sound transmission, inparticular as to transmitting severalsound signals at the same time. A secondsound channel permits, for instance, mul-tilingual transmission or stereo operation.

With the dual-sound carrier method, anadditional sound carrier 250 kHz abovethe actual sound carrier is frequency-modulated, its power level being 6 dB

lower than that of the first sound carrier.A multiplex method offers further possi-bilities by modulating an auxiliary carrierat twice the line frequency or using thehorizontal or vertical blanking intervalsfor pulse code modulation.

3.3 TV transmitter and monochrome

receiver

The RF television signal can be producedby two different methods.

If the modulation takes place in the out-put stage of the picture transmitter(Fig 19), the RF vision carrier is firstbrought to the required driving power

and then, with simultaneous amplitudemodulation, amplified in the output stageto the nominal vision carrier outputpower of the transmitter. The modulationamplifier boosts the wideband CVS to thelevel required for amplitude modulationin the output stage. The sound carrier isfrequency-modulated with a small devia-tion at a relatively low frequency. Thefinal frequency and the actual frequencydeviation are produced via multiplierstages. The picture and sound transmitteroutput stages are fed to the commonantenna via the vision/sound diplexer.

When using IF modulation (Fig 20), firstthe IF vision carrier of 38.9 MHz is ampli-tude-modulated. The subsequent filterproduces vestigial sideband AM. One ortwo sound carriers are also frequency-modulated at the IF. Next, mixing with acommon carrier takes place both in thevision and in the sound channel so thatthe vision/sound carrier spacing of5.5 MHz is maintained at the RF. Linear

amplifier stages boost the vision andsound carrier powers to the requiredlevel.

The advantage of the second method isthat the actual processing of the RF tele-vision signal is carried out at the IF, thusat a lower frequency, and band- andchannel-independent. However, for fur-ther amplification, stages of high linearityare required, at least in the picture trans-mitter.

Fig 20

Block diagram of TV transmitter using IF modulation in picture and sound transmitters.

osc.multi-plier

driveroutputstage

equa-lizer

mod.ampl

CVS

AM

osc.multi-plier

outputstage

soundFM

sound transmitter

vision/sound

diplexer

toantenna

picture transmitteroutput stagewith VSB filter

driver

picture transmitter

Fig 19Block diagram of TV transmitter using output stage modulation in picture transmitter.


11/51


5.5 MHzsoundIF ampl.

FMdemod.

AFampl.

VHF/UHFtuner

IFampl.

videodemod.

fromantenna

videodriver

videooutputstagecontrol

voltage

gen.

syncsep.

syncpulsesep.

vert.osc.

vert.outputstage

phasediscr.

hor.osc.

hor.output

stage HV

S

VSHS

HB

B

SI

CVS

sound IF

VB

Reproduction of the image on thereceiver screen is based on proper ampli-fication of the RF signal arriving at theantenna (Fig 21). To this effect, theincoming signal is converted into the IF inthe VHF/UHF tuner, where also standardselection by the Nyquist filter and therequired amplification are provided. Thesubsequent demodulator generates the

CVS and the 5.5 MHz sound IF.The latter is limited in amplitude, ampli-fied and frequency-demodulated. TheCVS is applied to the video amplifier; afterseparation of the sync component, thesignal is taken to the control section ofthe CRT via the video output stage. Thesync signal brought out from the videoamplifier by way of a sync separator is fedto the horizontal deflection system via adifferentiating network and to the verticaldeflection system via an integrating net-work.

The line deflection frequency is producedin the horizontal oscillator and comparedto the incoming horizontal sync pulses ina phase discriminator. A control circuitensures that the correct frequency andphase relation to the transmitter sync sig-nal is maintained. In the horizontal outputstage, the required deflection power isproduced and the high voltage for theCRT is obtained from the line retracepulses. The vertical oscillator is synchro-

nized directly by the vertical sync signal.The blanking pulses required for thebeam retrace are derived from the hori-zontal and vertical output stages.

3.4 TV standards

The characteristics of the television sig-nals mentioned in sections 1, 2 and 3refer to the CCIR standard. Various otherstandards are in use; for the differencesin the standard specifications see Tables1 and 2.

Table 1 Frequency ranges, vision/sound carrier spacing, channel width,sound modulation

Table 2 Composite video signal

Standard CCIR

B, GOIRT

DFrench VHF

EFCC (USA)

M

VHF, band l / MHz 47 to 68 48.5 to 100 50 to 70 54 to 88

VHF, band III / MHz 174 to 230 174 to 230 160 to 215 174 to 216

UHF, band IV/V / MHz 470 to 853 470 to 890

Vision/sound carrier spacing 5.5 MHz 6.5 MHz 11.15 MHz 4.5 MHz

Channel width 7 MHz (B) 8 MHz 13.15 MHz 6 MHz8 MHz (G)

Sound modulation, FM deviation FM, 50 kHz FM, 50 kHz AM FM, 25 kHz

Standard CCIR

B, GOIRT

DFrench VHF

EFCC (USA)

M

Number of lines 625 625 819 525

Field-repetition frequency 50 Hz 50 Hz 50 Hz 60 Hz

Line frequency 15 625 Hz 15 625 Hz 20 475 Hz 15 750 Hz

Video bandwidth 5 MHz 6 MHz 10.6 MHz 4.2 MHz

Line duration H 64 s 64 s 48.84 s 63.5 sField duration 20 ms 20 ms 20 ms 16.667 ms

Fig 21

Block diagram of monochrome TV reveiver.


12/51


4 Adding the colour information

To reproduce a colour image of the pat-tern, additional information on the colourcontent, i.e. the "chromaticity" of the indi-vidual picture elements, must be trans-mitted together with the brightness or

luminance distribution. This requires firstthe extraction of the colour informationand then a possibility of reproducing thecolour image.

4.1 Problem

The problem of colour transmission con-sists in maintaining the transmissionmethod of black-and-white television andbroadcasting the additional colour infor-mation as far as possible within the avail-able frequency band of the CVS. Thismeans for any colour TV system that acolour broadcast is reproduced as a per-fect black-and-white picture on a mono-chrome receiver (compatibility) and that acolour receiver can pick up a mono-chrome broadcast to reproduce a perfectblack-and-white picture (reverse compat-ibility).

Fig 22

Representation of coloured pattern by luminance

and chrominance components.

These requirements can be met only if

information on the luminance distribu-tion and

information on the colour content

are obtained from the coloured patternand then transmitted.

The chromaticity is characterized by thehue determined by the dominant wave-length of light, for instance for distinctcolours such as blue, green, yellow, red and by the saturation as a measure ofspectral purity, i.e. of colour intensitywith respect to the colourless (white)

luminance

chrominance

hue

saturationcolouredpattern

(Fig 22). The chrominance signal cannotbe obtained directly from the pattern.Instead the three primaries (red, green,blue) are used in accordance with thethree-colour theory (Helmholtz). The red,

green and blue signals are also requiredfor reproducing the colour picture. Thusthe scheme of compatible colour trans-mission is established by the luminancesignal Y and the chrominance signal F(Fig 23).

Fig 23

Principle of compatible colour transmission.

4.2 Chromatics and colorimetry

Light is that part of the electromagneticradiation which is perceived by thehuman eye. It covers the wavelengthsfrom about 400 nm (violet) to 700 nm(red). The light emitted by the sun con-

sists of a multitude of spectral coloursmerging into each other. Spectral coloursare saturated colours. Mixing with whitelight produces desaturated colours.

Coloured (chromatic) light can be charac-terized by its spectral energy distribution.The radiation of the wavelength causesthe sensations of brightness and colour inthe eye. The sensitivity to brightness ofthe human eye as a function of the wave-length is expressed by the sensitivity

characteristic or luminosity curve (Fig 24).

This characteristic indicates how brightthe individual spectral colours appear tothe eye when all of them have the sameenergy level. It can be seen from thischaracteristic that certain colours appeardark (e.g. blue) and others bright (e.g.green).

B&Wcamera

colourcamera

coder decoder

B&Wpicture tube

colour-picture tube

luminance signal Y

chrominance signal F

RGB

RGB

Fig 24

Brightness sensitivity characteristic of human eye.

In monochrome television, where onlythe luminance distribution of a colouredpattern is transmitted, this sensitivitycharacteristic of the eye has to be takeninto account. This is done by using thespectral sensitivity of the camera tubeand, if required, correction filters in con-nection with the colour temperature ofthe lighting.

Fig 25

Colour stimulus at different degrees of saturation.

The colours of objects are those coloursthat are reflected from the light to whichthe object is exposed. The colour stimuluscurve shows the associated spectral dis-

tribution (Fig 25). In most cases, theobject colours are not spectral colours butrather mixtures consisting of a number ofclosely spaced spectral colours or of sev-eral groups of spectral colours.

This is an additive process. White (colour-less) can also be produced by mixing.Fig 26 shows typical examples of additivecolour mixing.

h

1.0

0.8

0.6

0.4

0.2

0400 500 600 700 nm

blue cyan green yellow red

colo

urstimulus

400 500 600 700 nm

colourless(equal-energy white)

more

lesssaturated green


13/51


curves in an r-g diagram, the locus of allspectral colours is plotted (Fig 28).

Fig 28

Colour surface in r-g diagram.

Due to the negative portion of the r ()colour mixture curve, here again negativevalues are obtained. Coordinate transfor-mation referring to a new set of fictive,i.e. non-physical primaries X, Y and Z,yields a curve which comprises only posi-tive colour values [3]. When using the fic-tive primaries (standard reference stimuliX, Y, Z), the relationship according to theequation (4) still holds, expressed by thestandard reference summands x, y, z:

x + y + z = 1. (6)

Fig 29

Standard colour diagram; colour surface in

x-y diagram.

The two-dimensional representation ofchromaticity in the x-y coordinate systemis called the standard colour diagram inaccordance with CIE (Commission Inter-nationale de lEclairage) or, briefly colour

g

r

-2 -1 B 21

R

2

1

Z

X

Y

G

green

blue-greenWo

red

x

0.4 0.6 0.8

1.0

0.8

0.6

0.4

0.2

0

0.2

y

1.00

purple

line

spectrum locus

500

490

480

560

G

B

R

570580

600

520

W 700 nm

Investigations into the colour stimulussensitivity of the human eye have shownthat a colour sensation is produced bymixing the part sensations caused in theprimaries red, green and blue. This leadsto the conclusion that any colour appear-ing in nature can be obtained by combin-ing the corresponding portions of the pri-maries red, green and blue. In accord-

ance with the Helmholtz three-colour the-ory, Grassmann (1854) found the follow-ing law:

F = R (R) + G (G) + B (B). (3)

Fig 26

Additive colour mixing using three primaries R, G, B.

This means that a distinct colour stimulusF can be matched by R units of the spec-tral colour red (R), G units of the spectralcolour green (G) and B units of the spec-

tral colour blue (B).

Monochromatic radiations of the wave-lengths

R= 700 nm, G= 546.1 nm, andR= 435.8 nm

have been determined as the standardspectral colours, called primary colours orstimuli. None of the three primaries mustbe obtainable from the other two by mix-

ing.

Based on the equation (3), colour mixturecurves were plotted, showing the portionof each primary stimulus required for thedifferent spectral colours (Fig 27). Theordinate scale refers to equal-energywhite. As can be seen from the curves,negative amounts or tristimulus valuesare associated with some components.This means that for matching certainspectral colours a specific amount of a

red

green blue

white

yellow purple

cyan

primary stimulus must be added to thecolour stimulus.

In a colorimetric representation of a col-our stimulus, the three primaries yield aspace vector. The direction of the colourvector in space determines the chroma-ticity, its length being a measure of thebrightness.

Fig 27

Colour mixture curves b(), g(), r(), referred toR, G, B.

However, a three-dimensional coordinatesystem is not convenient for graphic rep-resentation. But since brightness andchromaticity are independent of eachother, the tristimulus values can bestandardized to the luminance compo-nent:

= 1 (4)

or, using the chromaticity coordinates:

r + g + b = 1. (5)

These reduced values no longer containany luminosity information but merely thechromaticity.

Since, however, the sum of r, g and b isalways unity, one of the three coordi-nates can be omitted when specifying thechromaticity so that a two-dimensionalsystem, the colour surface, is obtained.When entering the chromaticity coordi-nates found from the colour-mixture

colourstimulus

500 600 700 nm

0.4

0.3

0.2

0.1

0

-0.1400

_b()

_r()

_g()

FR G B+ +---------------------------- =

R R( )R G B+ +----------------------------= G G( )

R G B+ +---------------------------- B B( )

R G B+ +----------------------------+ +


14/51


triangle (Fig 29). The area of colour stimulithat can be realized by additive mixing isenclosed by the spectrum locus and thepurple line. The line connecting the whitepoint W (equal- energy white) of x = 0.33and y = 0.33 to the position of any colourF yields the dominant wavelength, i.e. thehue, when extended to its point of inter-section with the spectrum locus.

The ratio of the distance between the hueF and the white point W to the distancebetween the spectrum locus and thewhite point W on the connecting linepassing through the colour position givesthe colour saturation. The closer the col-our point to white, the weaker the coloursaturation. The position of a mixture col-our is situated on the line joining the lociof the two colours mixed or, if three col-ours are added, within the triangleformed by the connecting lines.

Fig 30

Colour coordinates of receiver primaries and

representable colour range.

When defining the tristimulus values in acolour TV system, it is essential to con-sider the question of how the primary

stimuli can be realized at the receiverend. On the one hand, the requirementsregarding the receiver primaries aredetermined by the necessity of providingas wide a range of representable mixturecolours as possible, i.e. the chromaticitycoordinates of the receiver primariesshould be located on the spectrum locus.On the other hand, primaries of especially

x

0.4 0.6

0.8

0.6

0.4

0.2

00.2

y

0.80

spectrum locus

G

B

R

Gr

Br

Rr

W

C

range ofobjectcolours

high luminance which can be producedeconomically are required. In accord-ance with recent EBU (European Broad-casting Union) specifications, thereceiver primaries Rr, Grand Brare used;their colour coordinates are given in Fig30.

The colour mixture curves plotted with

the aid of the primary stimuli R, G and Bare based on equal-energy white W. Incolour TV technology, standard illuminantC is used as reference white, correspond-ing to medium daylight with a colour tem-perature of about 6500 K, the standardtristimulus values being:

xC= 0.310, yC= 0.316, zC= 0.374.

If now the tristimulus values of thereceiver primaries Rr, Grand Brare foundfor all spectral colour stimuli of equalradiation energy and plotted as standard-ized tristimulus values as a function of thewavelength (maximum of the curvereferred to 1), the colour mixture curvesused in TV engineering are obtained(Fig 31).

Fig 31

Colour mixture curves br (), gr (), rr (), referred toreceiver primaries Rr, Gr, Br.

Here again negative colour values appear

due to the colour stimuli located outsideof the triangle formed by Rr, Grand Br.

Therefore, the colour mixture curves areslightly modified for practical purposes(dashed line). The signals produced bythe camera tubes in the red, green andblue channels of the colour camera mustbe matched with these colour mixture

colou

rstimulus

500 600 700 nm

1.0

0.75

0.5

0.25

0

-0.25400

_br()

_rr()

_gr()

curves using their spectral sensitivity andadditional colour filters (see Fig 23). Theoutput voltage of the colour camera in thethree channels must have the same rela-tionship as the tristimulus values. Withstandard illuminant C, the three outputsignals must be equal, even at differentluminance values.

4.3 Luminance and chrominance sig-nals, colour difference signals

For reasons of compatibility, the colourcamera has to deliver the same signal-from a coloured pattern, i.e. the lumi-nance signal to the monochromereceiver, as the black-and-white camera.The spectral sensitivity of a black-and-white camera corresponds to the bright-ness sensitivity curve of the human eye toensure that the black-and-white picturetube reproduces the different colour stim-uli as grey levels of the same brightnessas perceived by the eye.

Fig 32

Brightness sensitivity of human eye to receiver pri-

maries.

The colour camera, however, deliversthree signals with spectral functionsmatching the colour mixture curves. Toobtain one signal whose signal spectralfunction corresponds to the sensitivitycurve of the eye, coding is required. Tothis end, the three colour values repre-

sented by the functions rr(), gr() andbr () are multiplied by the relative lumi-nosity coefficients hr, hgand hband thenadded up. Except for the proportionalityconstant k, the result must be identical tothe sensitivity function of the human eyeh().

h() = k[hr x rr() + hg x gr()+ hb x br()]. (7)

h

500 600 700 nm

1

0.5

400

0.2

(540/0.92)

(610/0.47)

(465/0.17)

Br Gr Rr


15/51


The relative luminosity coefficients hr, hgand hbare obtained by normalizationfrom the corresponding values h(Rr), h(Gr)and h(Br)of the eye (Fig 32):

h(Rr) = 0.47h(Gr) = 0.92h(Br) = 0.17

h = 1.56

The following figures are calculated forthe relative luminosity coefficients:

Due to normalization:

hr+ hg+ hb= 1. (8)

Thus for equation (7)

h() = k [0.30 x rr() + 0.59 x gr()+ 0.11 x br()]. (7)

is obtained or, written in a simplified way,for the luminance signal Y correspondingto the sensitivity curve of the eye

Y = 0.30 x R + 0.59 x G + 0.11 x B. (9)

This equation is one of the most impor-tant relations of colour TV engineering.

Technically the luminance signal VYisobtained from the tristimulus signals VR,VGand VBvia a matrix (Fig 33). The sig-nals listed in Table 3 below result for apattern consisting of eight colour bars(standard colour bar sequence) the

three primaries plus the associated com-plementary colours and the colourlessstripes white and black.

hr

h Rr( )h

--------------- 0.471.56----------- 0.30= ==

hg

h Gr( )h

--------------- 0.921.56----------- 0.59= ==

hb

h Br( )h

--------------- 0.171.56----------- 0.11= = =

Table 3 Signals of standard colour barsequence

To reproduce a colour pattern, the threetristimulus signals R, G and B arerequired. For compatibility, however, col-our transmission uses the luminance sig-nal Y and the chrominance signal F. Thelatter cannot be obtained directly fromthe tristimulus values but only by way ofthe colour difference signals:

R Y, G Y, B Y.

These are the colour values minus theluminance component.

Fig 33

Producing luminance signal VYfrom tristimulus

voltages VR, VG, VBand compatibility relationship.

The chrominance signal carries informa-tion on hue and saturation. Therefore twocolour difference signals are sufficient todescribe the chrominance component.

For this purpose, the quantities R Y andB Y were selected [4].

referring to the voltage of the luminancesignal derived from the coder yields thesetwo colour difference signals as:

Pattern R G B Y

White 1 1 1 1.00

Yellow (R + G) 1 1 0 0.89

Cyan (G + B) 0 1 1 0.70

Green 0 1 0 0.59

Purple (R + B) 1 0 1 0.41

Red 1 0 0 0.30

Blue 0 0 1 0.11

Black 0 0 0 0

B&Wcamera

B&W picturetube

colourcamera

colour-picturetube

VRVGVB

V Vy

1|0.89|0.30 1|0.89|0.30

1|1|11|1|01|0|0

patternwhite | yellow | red

The equation

VY = 0.30 x VR+ 0.59 x VG+ 0.11 x VB(10)

VRVY= 0.70 x VR0.59 x VG0.11 x VB (11)

andVBVY=0.30 x VR0.59 x VG

+ 0.89 x VB (12)

The colour difference signals only carryinformation on the colour content. For amonochrome scene (VR= BG= VB) theyare equal to zero.

The amplitude of the colour differencesignals shows the departure of the huefrom the colourless, which is a measureof colour saturation.

The hue is determined by the amplituderatio and by the polarity sign of the colourdifference signals. Transformation of theorthogonal coordinate system (B Y andR Y) into polar coordinates (Fig 34) yieldsthe colour saturation from the vectorlength A:

(13)

and the hue from the angle :

(14)

Fig 34

Representation of chromaticity as a function

of colour difference signal.

A B Y( )2 R Y( )2+=

arc tan R Y( )

B Y( )

---------------------=

R-Y

A

B-Y


16/51


Investigations have shown that the reso-lution of the eye is lower for coloured pat-tern details than for brightness varia-tions. Therefore it is sufficient to transmitonly the luminance signal with the fullbandwidth of 5 MHz. The bandwidth ofthe chrominance signal can be reducedto about 1.5 MHz by taking the two colourdifference signals via lowpass filters.

Fig 35

Producing luminance signal VYplus colour

difference signals VRVYand VBVY.

In the coder, the signals VR, VGand VBproduced by the colour camera are con-verted into the luminance component V

Y

and the colour difference signals VRVYand VBVY(Fig 35) and, in this form,applied to the reproducing system. How-ever, the tristimulus values are requiredfor unbalancing the red, green and bluebeams. Two different methods are com-monly used for restoring these colour val-ues:

1. Driving colour picture tube with

RGB voltages (Fig 36)

The tristimulus voltages VR, VGand VBare

produced from the luminance componentVYand the two colour difference signalsVRVYand VBVYvia matrices andapplied directly to the control grids of thecolour picture tube, the cathodes being atfixed potential.

Fig 36

Restoring tristimulus values when driving colour

picture tube with RGB.

colourcamera

VRVGVB

Coder

VY=0.30xVR+0.59xVG+0.11xVBVR-VY=0.70xVR0.59xVG0.11xVBVB-VY=0.30xVR0.59xVG+0.89xVB

Rmatrix

VY

0

VRVYVR

Gmatrix

Bmatrix

VBVY

VB

VG

2. Driving colour picture tube with

colour difference signals (Fig 37)

The third colour difference signal VGVYis obtained in a matrix from the two quan-tities VRVYand VBVY, based on the fol-lowing equations

VY = 0.30 x VR+ 0.59 x VG+ 0.11 x VBVY= 0.30 x VY+ 0.59 x VY+ 0.11 x VY

VYVY= 0.30 x (VRVY)+ 0.59 x (VGVY)+ 0.11 x (VBVY) = 0 (15)

or, after rewriting,

VGVY= 0.51 x (VRVY)0.19 x (VBVY) (16).

Fig 37

Restoring tristimulus values when driving colour

picture tube with colour difference signals.

The colour difference signals are taken tothe control grids of the deflection sys-tems; the negative luminance signal isapplied to the cathodes so that the tris-timulus signals are obtained as controlvoltages at the three systems, forinstance:

Ucontr R= (URUY) (UY) = UR. (17)

-VY

VRVY

VR

G-Ymatrix

VBVY

VB

VG

The advantage of this method is that thebandwidth of the final amplifier stagesfor the colour difference signals is smallerthan that of the stages for the tristimulussignals and that a black-and-white pic-ture appears if the colour difference sig-nals fail. The disadvantage, is that highervoltages must be produced in the finalstages associated with the colour differ-

ence signals. Fig 38 shows signals for thestandard colour bar pattern.

Fig 38

Tristimulus values, luminance component andcolour difference signals for standard colour bar

pattern.

R

1.0

0

pattern white yellow cyan green purple red blue black

G

1.0

0

B

1.0

0

Y

1.0

0

1.0 0.89 0.70 0.590.41 0.30

00.11

R-Y

0-0.70 -0.59

0.59 0.70

0-0.110 0.11

B-Y

00 -0.89

0.30-0.59

0.59

-0.30 0

0.89

G-Y

00 0.11

0.30 0.41

-0.41 -0.30 0-0.11

1.40

1.78

0.82


17/51


line

1

3

5

7

line(7)

2

4

6

line 11

33

55

77

1st fieldin 3rd field

line 22

44

66

2nd fieldin 4th field

fSC=n x fhe.g. n=3

5 Transmission of chrominance signalwith colour subcarrier

As explained in the preceding chapter,the colour TV signal is transmitted in theform of the luminance signal Y and thetwo colour difference signals R Y and B

Y for reasons of compatibility. To transmitthe complete picture information lumi-nance plus chrominance a triple trans-mission channel would be required. Hereone might think of making multiple use ofthe TV transmission channel either by fre-quency or time multiplex. However, nei-ther method is compatible with the exist-ing black-and-white transmissionmethod.

Fig 39

Detail of CVS spectrum.

A decisive thought for the colour trans-mission method actually selected isderived from spectral analysis of the lumi-

nance signal or CVS. It becomes apparentthat only certain frequency componentsoccur in the CVS spectrum, these compo-nents being mainly multiples of the linefrequency due to the periodic scanningprocedure.

The varying picture content amplitude-modulates the line-repetitive pulsesequence producing sidebands spaced atmultiples of the field frequency from thespectral components of the line pulse.

Fig 39 shows a detail of the CVS spec-trum. Essentially the spectrum is occu-pied only at multiples of the line fre-quency in their vicinity. Between thesefrequency and groups the spectrumexhibits significant energy gaps.

Since the colour information is also line-repetitive, the spectrum of the chromi-nance signal consists only of multiples ofthe line frequency and the correspondingsidebands. Therefore it is appropriate toinsert the additional colour information

amplitude

f

m x fh (m+1) x fh (m+2) x fhfSC fh fSC

fv fv

into the gaps of the CVS frequency spec-trum. This is done by modulating thechrominance signal onto a colour subcar-rier whose frequency fSC, and thus also

the spectrum of the line-repetitive modu-lation products, is located between thefrequency components of the CVS(Fig 40).

Fig 40

Spectrum of CVS and modulated colour subcarrier.

5.1 Determining the colour subcarrier

frequency

One condition for determining the coloursubcarrier frequency results from thesymmetrical interleaving of the CVS andchrominance signal spectra: the fre-quency fSCshould be an odd multiple ofhalf the line frequency fh:

(18)

In this way the half-line offset isobtained.

Generally, the interference which a col-our subcarrier produces on the black-and-white picture can be explained as fol-lows, if it is assumed that a sinewave in

amplitude

f

m x fh (m+1) x fh (m+2) x fhfF fh fF

colour subcarrier

fSC 2n 1+( )xfh2----=

the frequency region of the CVS producesa bright-dark interference pattern on thescreen.

If the colour subcarrier frequency is aninteger multiple of the line frequency, i.e.if there is no offset with respect to theline frequency, an interference pattern ofbright and dark vertical stripes appears,their number corresponding to the factorn (Fig 41). As a result of the half-line off-set the phase of the colour subcarrieralternates by 180 from line to line of afield. However, because of the oddnumber of lines, bright and dark dotscoincide after two fields. The interferencepattern occurring in a rhythm of f

v/4 =

12.5 Hz would thus be compensated overfour fields (Fig 42). Nevertheless, thecompensation of the interference patternon the screen is not perfect due to thenonlinearity of the picture tube character-istic and the inadequate integratingcapability of the human eye.

If a half-line offset is assumed betweenthe colour subcarrier frequency and theline frequency, the subjective annoyancecan be further reduced by selecting the

colour subcarrier frequency as high aspossible. In this way, the interferencepattern takes on a very fine structure.However, the colour difference signalsmodulate the colour subcarrier so that fortransmitting the upper sideband a certainminimum spacing of the colour subcarrierfrequency from the upper frequency limitof the CVS has to be maintained.

Fig 41

Interference pattern

caused by colour

subcarrier with integer

relationship between

colour subcarrier

frequency and line

frequency.


18/51


line

1

3

5

7

line(7)

line 11

33

55

77

1st fieldin 3rd field

line 22

44

66

2nd fieldin 4th field

fSC = (2n+1) xfh

e.g. n=3_2

6

4

2

The best compromise has proved to be acolour subcarrier frequency of about4.4 MHz. Thus the principle of compatiblecolour TV transmission is found using theluminance signal and the chrominance

signal modulated onto the subcarrier; thisis the basis of the NTSC system and itsvariants (Fig 43). For the CCIR-modifiedNTSC method, which will be discussed indetail later, a colour subcarrier frequencyof

= 4.4296875 MHz (19)

has been fixed.

A further development of the NTSCmethod is the PAL method, which isbeing widely used today. In the PAL sys-tem, one component of the subcarrier isswitched by 180 from line to line. How-ever, this cancels the offset for this sub-carrier component so that a pronouncedinterference pattern would appear in thecompatible black-and-white picture. Thisis avoided by introducing a quarter-lineoffset of the subcarrier plus an additionaloffset of f

v/2 = 25 Hz. Thus the CCIR

fSC 567xfh2---- 283.5 xfh= =

standard colour subcarrier frequency inthe PAL system is

fsc=283.75 x fh+ 25 Hz= 4.43361875 MHz (20)

The rigid coupling of the subcarrier fre-quency to the line frequency is ensuredby deriving the line frequency fhor twicethe line frequency 2fhfrom the frequencyfSCof the colour subcarrier (Fig 44).

Fig 44

Coupling of colour subcarrier frequency and

line frequency.

5.2 Modulation of colour subcarrier

The chrominance signal is transmitted bymodulating the colour subcarrier with thetwo colour difference signals. The modu-lation method must permit the colour dif-ference signals to be extracted separatelyat the receiver end.

SSBfSCf

2f

1135f

ff

8f

f265f

2fh

fV

fSCfV/2

In the NTSC and PAL methods, doubleamplitude modulation is used. The 0component of the subcarrier is ampli-tude-modulated by the (B Y) signal andits 90 component by the (R Y) signal,the subcarrier being suppressed at thesame time. This results in quadraturemodulation (Fig 45). The modulationproduct is a subcarrier frequency whose

amplitude and phase are determined bythe two colour difference signals. Ampli-tude and phase modulation take place atthe same time. Compared to the chromi-nance signal represented in Fig 34, thecolour saturation now corresponds to theamplitude SC and the hue to the phaseangle SCof the modulated colour sub-carrier. For this reason, the modulatedcolour subcarrier is also called thechrominance signal.

(21)

(22)

Fig 45

Producing chrominance signal by quadrature

modulation of colour subcarrier with two colour

difference signals.

A vector diagram of the chrominance sig-nal shows the position of the differentcolours on the colour circle (Fig 46). Simi-lar to the colour triangle in Fig 29, the

complementary colours are located onopposite sides of the coordinate zero (col-ourless). When transmitting a colourlesspicture element, the colour differencesignals and the amplitude of the coloursubcarrier equal zero. In this case, thesubcarrier does not cause any interfer-ence in the black-and-white picture.

To demodulate the chrominance signal,the unmodulated carrier of correct phaseis required.

SC B Y

( )

2R Y

( )

2+=

SC

arc tanR Y

B Y-----------=

0

(B-Y)mod.

90

90

(R-Y)mod.

+ chrominancesignal

B-Y

coloursubcarrier

R-Y

Fig 42

Compensation of

interference pattern with

half-line offset of colour

subcarrier frequency.

matrix add.

mod.

fSC

RGB

R-Y B-Y

B, S

SC

matrixsep.

demod.

fSC

RGB

SC

B, SCCVS

fSC

SC

Y

f

Y Y Fig 43

Principle of compatible

colour TV transmission

using luminance and

chrominance signals.


19/51


90

(R-Y)demod.

RY

(B-Y)demod.

BY

ref.carrierosc.

burstamp.

comp.

integr.fh

chrominance signal and burst

fSC, 90

Fig 46

Vector representation of chrominance signal:

colour circle.

Synchronous detection is used, evaluat-ing only the chrominance componentwhich is in phase with the reference car-rier. Since the actual subcarrier is nottransmitted, it must be produced as a ref-erence carrier at the receiver end. Forsynchronization with the subcarrier at thetransmitter end, a reference signal isinserted into each line in the H blankinginterval, i.e. the colour sync signal orburst. This signal consists of about tenoscillations of the subcarrier at the trans-mitter end and is transmitted on the backporch (Fig 47).

Fig 47

Colour sync signal (burst).

In the NTSC system, the burst phase is at180 compared to the 0 reference phaseof the colour subcarrier.

In the receiver the burst is separated fromthe chrominance signal by blanking. A

phase comparator produces a controlvoltage from the departure of the refer-ence carrier phase from the burst phase;this voltage controls the frequency andphase of the reference carrier oscillatorvia an integrating network. The controlvoltage becomes zero if the phase differ-ence is 90. Coming from the referencecarrier oscillator, the 90 component istaken directly to the (R Y) synchronousdetector and, after a negative 90 phaseshift, as the 0 component to the (B Y)synchronous detector (Fig 48).

(R-Y) axis

(B-Y) axis

red

yellow

purple

green

blue

cyan

SCSC

burst

tbh

H sync pulse

5.3 Composite colour video signal

The chrominance signal is combined withthe composite video signal (CVS) to formthe composite colour video signal (CCVS).The CCVS is amplitude-modulated ontothe RF vision carrier. The full level of thecolour difference signals would causeovermodulation of the RF vision carrier bythe chrominance signal for certain col-

oured patterns. This is shown for the sig-nals of the standard colour bar sequence(Table 4).

Overmodulation occurs in both directions(Fig 49). In particular, the periodic sup-pression of the RF carrier and its fallingshort of the 10% luminance level wouldcause heavy interference. For this reason,the chrominance signal amplitude has tobe reduced.s a compromise betweenovermodulation on the one hand anddegradation of signal-to-noise ratio onthe other an overmodulation of 33% inboth directions with fully saturated col-ours has been permitted since, in prac-tice, fully saturated colours hardly everoccur. This is ensured by using different

reduction factors for the two colour dif-ference signals, multiplying them by

0.49 for the (B Y) signaland 0.88 for the (R Y) signal

In this way the reduced colour differencesignals U and V are obtained:

U = (B Y)red = 0.49 x (B Y)= 0.15 x R 0.29 x G +0.44 x B (23)

V = (R Y)red = 0.88 x (R Y)= 0.61 x R 0.52 x G 0.10 x B (24)

(The values are rounded off.)

The signal values listed in Table 5 areobtained for the colour bar pattern with100% saturated colours. Fig 50 shows theline oscillogram of the CCVS for a 100%saturated colour bar sequence.

For measurements and adjustments oncolour TV transmission systems the testsignal used is the standard colour barsequence for which all chrominance sig-

RFvision carrier

amplitude

Y

100%

75%

10%0

0

1.0

white yellow cyan green purple red blue black

Fig 48

Synchronization of

reference carrier

oscillator at receiver

end and synchronousdetectors associated

with two colour dif-

ference signals.

Fig 49

Amplitude modulation of RF vision carrier by CCVS without reducing colour difference signals.


20/51


nals, except in the white bar, are reducedto 75% in accordance with the EBU (Euro-pean Broadcasting Union) standard. Inthis way, 33% overmodulation by the col-our subcarrier is avoided (Fig 51).

To determine the colour subcarrier phaseor the colour points in the (B Y) (R Y)plane, a vectorscope is used. This is an

oscilloscope calibrated in polar coordi-nates including two synchronous detec-tors for the U and V components. Thedetected colour difference signals aretaken to the X and Y inputs of the oscillo-scope. Fig 52 shows the vector oscillo-gram of the standard colour barsequence.

Fig 52

Vector oscillogram of standard colour bar sequence.

5.4 NTSC method

The NTSC, PAL and SECAM methods,which are mainly used for colour TVtransmission, differ only with respect tothe modulation of the colour subcarrier.The NTSC method, named after theNational Television System Committee,constitutes the basis for the improvedvariants PAL and SECAM.

The principle of the NTSC method has

basically been described in the chapterson modulation of the colour subcarrierand on the composite colour video signal.However, the original NTSC system (USstandard) does not transmit the reducedcolour difference signals U and V butinstead the I and Q components, whichare referred to a coordinate systemrotated counter-clockwise by 33 (Fig 53).

V

U

12090

60

150 30

180 0

210 330

240 300

270

greencyan

red

yellow

blue

purple

burst

Table 4 Overmodulation of RF vision carrier by standard colour bars.

Table 5 Modulation of RF vision carrier by standard colour bars with reduced colourdifference signals

Colour bar Y BY RY SC Y + SC YSCWhite 1 0 0 0 1 1

Yellow 0.89 0.89 + 0.11 0.89 1.78 0Cyan 0.70 + 0.30 0.70 0.76 1.46 0.06Green 0.59 0.59 0.59 0.83 1.42 0.24Purple 0.41 + 0.59 + 0.59 0.83 1.24 0.42Red 0.30 0.30 + 0.70 0.76 1.06 0.46Blue 0.11 + 0.89 0.11 0.89 1.00 0.78Black 0 0 0 0 0 0

Colour bar Y U V SCred Y + SCred Y SCredWhite 1 0 0 0 1 + 1

Yellow 0.89 0.44 + 0.10 0.44 1.33 + 0.45Cyan 0.70 + 0.15 0.62 0.63 1.33 + 0.07Green 0.59 0.29 0.52 0.59 1.18 0Purple 0.41 + 0.29 + 0.52 0.59 1.00 0.18Red 0.30 0.15 + 0.62 0.63 0.93 0.33Blue 0.11 +0.44 0.10 0.44 0.55 0.33Black 0 0 0 0 0 0

picture

signal

1.0

0


0.4

33%

-33%

1.00

0.890.70

0.59

0.410.30

0.110

1.33

0.45

1.33

0.07

1.18

1.000.93

0.55

0

-0.18

-0.33 -0.33

picturesignal

1.0

0


-0.4

1.00

0.66

0.520.44

0.300.22

0.080

1.00

0.33

1.00

0.05

0.870.75

0.70

0.42

0-0.13

-0.25 -0.25

Fig 50

Line oscillogram of CCVS for

standard colour bar sequence,

colour saturation 100%, colourdifference signals reduced.

Fig 51Line oscillogram of CCVS for

standard colour bar sequence,

colour saturation 75%

(EBU test signal).


21/51


In the colour triangle, the I axis corre-sponds to the axis for which the eye hasmaximum colour resolution (Fig 54). Thisensures better transmission of colourtransitions.

Fig 53

I and Q components of reduced colour difference

signals in original NTSC system.

The modulation signals are thus

l = V x cos 33 U x sin 33 (25)

Q = V x sin 33 + U x cos 33 (26)

or, using the tristimulus matrix equations,

l = 0.60 x R0.28 x G 0.32 x B (27)

Q = 0.21 x R 0.52 x G + 0.31 x B (28)

Fig 54

I-Q axes in colour triangle.

The two signals I and Q are transmittedwith different bandwidth, i.e.

the I signal with 1.3 MHzand the Q signal with 0.5 MHz.

Fig 55 shows the complete block diagramof an NTSC coder. Except for the 33phase shift, the functioning of the corre-sponding decoder is essentially explainedin Fig 48.

red purple

yellow

blue

green cyan

V

U

33

Q

J

red

purple

yellow

blue

green

cyan

Y

X

W

J axis

Qaxis

minimum

maximum

colour perception

The human eye reacts very strongly toincorrect hue. The hue of the colourimage reproduced by the picture tube isdetermined by the phase angle of thechrominance signal referred to the phaseof the burst. When producing the CCVS inthe studio, it may happen that thechrominance signal from differentsources has different delays and thus dif-

ferent phases with respect to the burst.To correct wrong hue resulting from staticphase errors in the transmission path, theNTSC colour TV receiver is provided witha control permitting the phase of the ref-erence carrier to be adjusted. In mostcases this is done by referring to the hueof a well-known picture detail, such asthe flesh tone.

However, this hue control does not allowcorrection of differential phase distortion.In accordance with DIN 45 061 differen-tial phase means the difference of phaseshifts through a four-terminal network attwo different points of the transfer char-acteristic at the subcarrier frequency. Thedifferential gain is defined in a similarway.

Due to the shift of the operating point onthe transmission characteristic as a func-tion of the Y components of the CCVS, thechrominance signal suffers a gain change(because of the change in slope of the

characteristic) and a phase change(because the transistor input capacitanceis dependent on the emitter current andthus on the operating point) when pass-ing, for instance, through an amplifierstage with preceding tuned circuit. Whilethe differential gain can be eliminated to

a large extent by negative feedback, thedifferential phase can be reduced only bylimiting the driving level. Fig 56 showsthis influence on the CCVS and the effectin the vectorscope representation.

5.5 PAL method

The effects of static and differentialphase errors are considerably reduced

with the PAL method, the occurrentphase errors being corrected with rela-tively little extra outlay. The PAL system isbased on the following concept: an exist-ing phase error can be compensated by aphase error of opposite polarity. This isrealized technically by alternating thephase of one of the two chrominance sig-nal components, for instance the SCVcomponent, by 180 from line to line. PALstands for phase alternation line.

If a phase error exists in the transmissionpath, alternately positive and negativedepartures of the chrominance signalphase from nominal are produced in thereceiver after elimination of the line-to-line polarity reversal of the SCVcompo-nent generated at the transmitter end.Delaying the chrominance signal for theduration of a line (64 s) and subsequentaddition of the delayed and the unde-layed signals cause two phase errors ofopposite polarity to coincide and thus tocancel each other. It should be men-

tioned, however, that this method isbased on the assumption that the chro-maticity does not change within two con-secutively transmitted lines. If horizontalcolour edges exist, the eye hardly per-ceives a falsification of the colour transi-tion even in this case.

Fig 55

Block diagram of

NTSC coder.

33

Q mod.

1-sdelay

SCosc.

burstgen.

LP0.5 MHz

add.

BI, Ssignalgen.

123

add.

0.6-s

delayI mod.

LP

1.3MHzmatrix

CCVSNTSC

RGB

fSC, 33

fSC, 123

YI

Q

fSC, 0


22/51


Fig 57 shows the compensation of aphase error with the PAL method. Theassumed phase error affects thechrominance signal with respect to theburst on the transmission link.

After elimination of the SCVpolarityreversal (PAL switchover) and addition ofthe chrominance signals in two succes-

sive lines, the phase angle of the result-ing signal SCresis equal to that of thetransmitted chrominance signal, and theoriginal hue is thus maintained. Afterreducing the resulting signal to half itsamplitude, this signal exhibits only slightdesaturation.

An additional identification is transmittedwith the burst to ensure correct phasereversal to the SCVcomponent in thereceiver or of the reference carrier for the

(R Y) sync detector. To this effect, theburst is split into two components, onebeing transmitted at 180 and the otherat 90 alternating from line to line inphase with the SCVreversal. This yields aswinging burst of 180 45 (Fig 58). Theactual burst reference phase (180) isobtained by averaging.

Fig 58

Swinging burst with PAL method.

n = odd number in1st and 2nd fields

n = even number in3rd and 4th fields

line n

line n+1

135

225

The reference carrier oscillator in thereceiver adjusts, via the phase control, to90 with respect to the mean burstphase. The identification signal (burstflag) for synchronizing the PAL switch isderived from the burst phase discrimina-tor (see Fig 62). With PAL the reduced col-our difference signals U and V are directlytransmitted, the bandwidth being

1.3 MHz. Limiting the sidebands of themodulated colour subcarrier to differentwidths no longer has an annoying effectthanks to the phase error compensation.Fig 59 is a block diagram of a PAL coder.Compared with the NTSC coder, the 33phase shift of the colour subcarrier com-ponents is omitted, but reversal of thesubcarrier component for the (B Y) mod-ulator and generation of the swingingburst are added.

The technical realization of the PAL errorcompensation requires special explana-tion as against Fig 57. For this purpose itis best to start with the group delaydecoder included in the PAL decoder. Incontrast to the NTSC decoder, thechrominance signal is not simultaneouslyapplied to the two sync detectors in thePAL decoder but is first split into the SCUand SCVcomponents.

This is performed in the group delaydecoder (Fig 60). At its output, the incom-

ing chrominance signal is divided intothree components. It is taken to the twooutputs via a 64 s delay network (lineduration) directly and after a 180 phaseshift. At the outputs, signal addition takesplace. The chrominance signal of the pre-ceding line (SCn) and that of the ongoingline (SCn+1) are added at the SCUoutput.Successive lines contain the SCVcompo-nent with a 180 phase alternation sothat the SCVcomponent is cancelledevery two lines. Thus the SCUcomponent

of the chrominance signal is constantlyavailable at this output.

The input signal is taken to the SCVout-put with a 180 phase shift. Addition ofthe delayed chrominance signal cancelsthe SCUcomponent, and the SCVcompo-nent appears at this output, althoughwith a 180 phase alternation from line toline. Based on the vector diagrams inFig 61, the functioning of the group delaydecoder can be explained very easily.

U

V

yellow

redpurple

blue

green

cyan

t

V2

V1

t

line n

+SCv

SCu

SCn

ahead of transmission link

chrominance signal

after transmission link

SCn

after polarity reversalof SCvcomponent

SCn

+

line n+1

-SCv

SCu

SCn+1

SCn+1

SCn+1

x

SCn

SCres

SCn+1x

addition of chrominance signalsin lines n and n+1

Fig 56

Generation of differential gain and phase distortion.

Fig 57

Compensation of phase error with PAL.


23/51


The line-to-line phase alternation of theSCVcomponent can be disabled by a con-trolled switchover. However, it is easier toprovide for line-to-line phase reversal ofthe reference carrier in the (R Y) syncdetector. Within the complete PALdecoder this task is performed by the PALswitch, which is synchronized by theswinging burst (Fig 62).

Fig 63

Effect of phase error on signals with PALgroup delay

decoder.

A phase error in the transmission pathaffects both the SCUand the SCVcompo-nents in the same sense (Fig 63). Since,

however, only the component in phasewith the reference carrier is weighted inthe sync demodulators, the (B Y)demodulator delivers the signalU = SCU x cos and the (R Y)demodulator the signalV = SCV x cos . Both colour differ-ence signals are reduced by the same fac-tor so that the ratio V/U or (R Y)/(B Y)remains constant and the hue of thereproduced image is not affected. Desat-uration, which corresponds to the factor

cos , becomes significant only withlarge phase errors.

line n+1

line n+1

64 s

SCn

SCn+1

2 x SCn

SCn

SCn+1

2 x SCn

SCnSCn+1

+2 x SCv

SCnSCn+1

2 x SCv

signal at output SCU output SCV

signal at input

SCU

SCn+SCV

SCU

SCn+1-SCV

SCU

SCn+SCV

output SCU

2 x SCU

SCn

SCn+1

2 x SCU

SCn

SCn+1

output SCV

2 x SCV

SCn SCn+1

SCn+1 SCn

+2 x SCV

line n

line n+1

64 s

64 s

line n+2= line n^

add.

Vdemod.

64s

add.180

Udemod.

-90-90

0 /180

ref.-carrierosc.

phasediscr.

chrominance

signal

burst, 135 /225

from burst amp.

control voltage

synchronization

B-Y

R-Y

fSC, 0

fSC, 90

fh/2

fSC, 90

SCV

SCU

gen.

Fig 59

Block diagram of

NTSCcoder.

Fig 61

Division of chromi-

nance signal into

SCUand SCV

components in PAL

group delay

decoder.

Fig 62

PAL decoder with

reference carrier

generation.

input

outputSCu

SCn, SCn+1,SCn+2 = SCn,

, SCn+1+SCn, SCn+SCn+1,

, SCn+1+SCn, SCn+SCn+1,

180 SCv

= 64 s

Fig 60

PAL group delay decoder.

V mod.

0.4-sdelay

SCosc.

0 /180

LP

1.3 MHz

add.

BI, Ssignalgen.

-90

add.

burstgen.

U mod.LP

1.3MHzmatrix

CCVSPAL

RGB

fSC 90

fSC0

YUV

fSC90


24/51


5.6 SECAM method

As against the NTSC method, the SECAMmethod too brings an improvement withrespect to wrong hues caused by phaseerrors in the transmission path. Like thePAL method it is based on the assumptionthat the colour information does notessentially vary from line to line or thatthe human eye does not perceive any

annoyance if the vertical colour resolu-tion is reduced to a certain extent.

Therefore, the colour difference signals(B Y) and (R Y) characterizing the col-our information need not be transmittedsimultaneously. They can be sent sepa-rately in successive lines. In the receiver,the signal content of one line is stored for64 s via a delay line and processedtogether with the signal of the next line.The short form SECAM derived from"squentiel mmoire" indicates thatthis is a sequential colour system withmemory.

As the two colour difference signals aretransmitted separately, the type of modu-lation can be freely selected. SECAMuses frequency modulation, which is notvery interference-prone. However, thereference frequency of the FM demodula-tor must be kept very stable so that thedemodulated colour difference signalsare not falsified.

Fig 64 is a simplified block diagram of aSECAM coder and decoder. In the coder,the (B Y) and the (R Y) signals areapplied to the frequency modulator inalternate lines. To ensure that in thedecoder the demodulated colour differ-ence signals are in synchronism with thetransmitter end, identification pulses inthe form of the modulated colour subcar-rier are transmitted during nine lines ofthe field blanking interval.

When frequency-modulating the coloursubcarrier, the latter is not suppressed. Inparticular with colours of low saturation,this would produce an interference pat-tern on a black-and-white receiver inspite of the colour subcarrier offset.Therefore the colour subcarrier is attenu-

ated by preemphasis at the transmitterend and boosted by deemphasis at thereceiver end. The effect of noise isreduced by video frequency preemphasisand deemphasis.

SECAM has gone through several phasesof development. Its latest variant, SECAMIII b or SECAM III opt., is based on slightlydifferent colour subcarrier frequencies forthe (B Y) and (R Y) signals, furtherreducing the interference pattern causedby the colour subcarrier.

As against PAL, SECAM features somesystem-dependent weak points since fre-quency modulation is utilized at its physi-cal limits [3].

Fig 64

Simplified block diagram

of SECAM coder (above)

and decoder (below).

FMmod.

add.matrixRGB

Y

B-Y

R-Y

CCVSSECAM

fh/2 fSC B, S

FMdemod.

SCBYBY

FMdemod.

SCRYRY

fh/2

64s

, SCB-Y, SCR-Y


25/51


6 Colour picture pickup and reproduction

Previous explanation was based on anelectrical picture signal obtained from thepattern to be transmitted by optoelectri-cal conversion. Below, the convertersused at the TV transmitter and receiver

ends are briefly looked at and finally thereproduction of the colour picture by theTV receiver is explained.

Fig 65

Design of Vidicon camera tube.

6.1 Principle of colour picture pickup

An optoelectrical converter is used totranslate brightness variations into anelectrical picture signal. Different con-verter systems are available, but only thepickup tubes with a photosensitive semi-conductor layer are really important forTV technology.

In Vidicon tubes a semiconductor layer isused as the storage plate or target, itsblocking resistance varying with theintensity of the light falling upon it. Thecharacteristics of the converter differdepending on the composition of thesemiconductor target. Frequently a Plum-bicon is used; this has a target of leadmonoxide and, compared to the Vidiconusing an antimony-trisulphide layer, fea-tures higher sensitivity and less inertia.

Fig 65 shows a Vidicon pickup tube withits deflection and focusing coils. The Vidi-con works as follows: the electron beam,caused to emanate from the cathode byan electric field, negatively charges theside of the target facing the beam-pro-ducing system. Positive charge carriersare bound on the picture side of the tar-get with the aid of the positive target-plate voltage. At the points upon whichlight falls, the incident photons releaseelectrons in the semiconductor layer

focusing coil

glass disk with storage layer electron gun

signal electrode aligning coildeflection coil

incident

light

causing a charge compensation at thecorresponding picture elements due tothe resulting lower blocking resistance.During the next charging process, elec-trons are again bound at these places on

one side of the target plate and on theother side the same number of electronsare set free. These electrons flow acrossthe external circuit resistance causing asignal voltage to be produced. Fig 66shows the equivalent circuit of a picturepoint on the target, represented by acapacitor shunted by an exposure-dependent resistor..

Fig 66

Equivalent circuit for picture element on storage

plate of Vidicon camera tube.

In line with the basic principle of colour

transmission, three pickup tubes arerequired; the image to be televised is pro-

jected, using the primaries red, blue andgreen, onto three photosensitive semi-conductor targets via an optical beamsplitter, called colour splitter, and via cor-recting filters for matching the signals tothe spectral sensitivity of the semicon-ductor layers (Fig 67). To make the threepartial images coincide accurately withtheir rasters , high mechanical andelectron-optical precision is required.

Coincidence errors of the colour rasterswould cause a loss in definition for theluminance signal. For this reason, colourtelevision cameras with a separatepickup tube for the luminance signal arealso used. Progressive developments,already partly implemented in portablecolour TV cameras, point to a single-tubecolour TV camera producing the tristimu-lus signals in the red, green and bluechannels by a multiplex method.

signal electrode cathode

signalvoltage

R

C

RL

+ 1050V

Fig 67

Splitting of incident light into three primaries

in colour TV camera.

6.2 Colour picture reproduction using

hole or slot mask tube

For reproducing the brightness pattern,television uses picture tubes with a phos-phor screen which lights up in accord-ance with the intensity of the electronbeam falling upon it. The electron beam isdeflected within the raster with the aid ofmagnetic fields produced by the deflec-tion currents in the horizontal and verticaldeflection coils (Fig 68). The intensity ofthe electron beam is influenced by thevoltage across the control electrode.

Fig 68

Black-and-white picture tube with deflectionsystem.

Whereas a homogeneous, whitish-bluephosphor screen is used in picture tubesfor monochrome reproduction, the screenof the colour picture tube must emit theprimaries red, green and blue.

However, colour detail resolution shouldgo as far as the individual picture ele-ments. For this purpose each picture ele-ment is represented on the screen by

correcting filter

blueVB

redVR

greenVG

colour splitter pickup tubes

electron gun

deflectioncoils

phosphorscreen

HV connector, 20 kV


26/51


three luminescent dots in the colours red,green and blue, called colour triad (Fig69). About three times 400 000 lumines-cent dots are accommodated on thescreen area.

Fig 69

Arrangement of luminescent dots in colour

picture tube.

The colour picture tube includes threebeam-producing systems. In the deltacolour picture tube, which for manyyears was practically the only colour pic-ture tube in use, the beam-producing sys-tems are arranged at angles of 120 toone another. The intensity of the emittedelectron beams is controlled by the tri-stimulus signals that are applied.

B R

G RG R B

B BG RG R G

To obtain with common deflection of thethree electron beams a clear assignmentto the luminescent dots, a hole mask isplaced about 15 mm from the phosphorscreen (Fig 70).

Fig 70

Part of hole mask and phosphor screen in delta

colour picture tube.

More recent colour picture tubes are fit-ted with a slot mask. Accordingly, theluminescent areas on the screen areeither oblong or in the form of stripes.With thisin-line colour picture tubethethree beam-producing systems arearranged in one plane (Fig 71). This con-figuration yields high colour purity, i.e.

G B

R GBB

R

hole mask

electron beam phosphor screen

R, G, Bluminescent dots

B

GR

the electron beams only strike lumines-cent stripes of the correct colour associ-ated with the corresponding beam pro-ducing system.

Moreover, in conjunction with a specialdeflection field, excellent convergence isachieved, i.e. the correct luminescentareas associated with the picture ele-

ments are excited. With the delta colourtube, greater complexity of circuitry wasrequired for this purpose.

Fig 71

Part of slot mask and phosphor screen with in-line

colour picture tube.

slot mask

electron beam

phosphor stripes

B

G

R

R G

B

phosphorscreen

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