R & T S1

TV Systems

Content:CRT scanning

composite video signal

colour systems

satellite orbit

earth station

satellite TV

R & T S2

CRT principle

Electrostaticdeflection

R & T S3

CRT principleElectromagnetic deflection: use of magnetic energy to

deflect electron beam both vertically & horizontallypractically all TV display devices use electromagnetic

deflection

R & T S4CRT

principleDeflection yoke: two sets of coils for generating electromagnetic deflection both vertically & horizontally

R & T S5CRT

principleBeam focusing: an electromagnetic focusing coil is placed around the neck of a CRT

R & T S6

Scanning process

Deflection process: positions the electron beam on the inner surface of CRT

Scanning process: controls the deflections of electron beam so as reconstruct images on the screen

Rectilinear scanning: two separate scanning procedures occurs simultaneously,

vertical scanninghorizontal scanning

R & T S7Simplified

scanningSimplified scanning lines are scanned sequentially dashed lines represent beam retrace beam retrace occurs very rapidly & is blanked by disabling the beam simple but required wide BW not used in TV receivers

R & T S8Simplified

scanning

R & T S9

Interlaced scanning

Each completed picture is divided into two fields:ODD fieldEVEN field

Each field is scanned one after another: ODD field => EVEN field => ODD field….

Two interlaced fields make up one frame

frame rate: 60/2 or 30Hz (50/2 or 25Hz for H.K.)

Complicated scanning

required half BW of simplified scanning

R & T S10Interlaced

scanning

R & T S11

New development

Interlaced scanning Progressive scanning

Horizontal frequency: 50Hz => 100Hz

HDTV (High Definition) : double resolution

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Aspect ratioDefined as the width versus the height of raster typical values 4:3 & 16:9pictures then more pleasing to eyes

Standard 4:3aspect ratio

R & T S13Synchronizatio

n Why synchronization is needed ?Electron beam scanning the CRT surface of TV receiver must be exactly follow the video signal sent from a station.Sync pulses are included as part of a video signalTwo types of sync pulses:

horizontal sync pulsesvertical sync pulses

Sync pulses trigger the flyback of electron beam at the end of lines or end of a field

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Synchronization

Vertical syncHorizontal sync

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BW of video signal

Max BW is required when sending patterns of alternate black & white vertical lines

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BW of video signalNo. of lines =625; Aspect ratio 4:3Pixel per line = 625x4/3 = 833 Horizontal freq = 625x25=15,625HzScan one horizontal line needs 1/15625 sec. or 64µs

t=(64/833)x2µs=0.154µs

Thus BW=1/t = 6.5MHz (practical BW:5.5MHz)

R & T S17B&W composite video

signal

Horizontal sync pulse

Control beam intensity

R & T S18B/W TV block

diagram

R & T S19B/W TV block

diagramVideo detector : demodulates the video signal (AM)

Video amplifier: a wide band amplifier used to amplify the video signal to drive the cathode of CRT

Sync separator: separates the vertical sync and horizontal sync from the video signal

High voltage supply: generates an EHT voltage to drive the anode of CRT

Horizontal oscillator: controls the horizontal deflection of electronic beam

Vertical oscillator: controls the vertical deflection of electronic beam

R & T S20

Colour composite video signalY signalluminance signal (control brightness of picture)monochrome receiver uses Y to display B/W imageY=0.59G+0.3R+0.11B

R: Red colour voltage, G: Green colour voltage, B: Blue colour voltage

I signalformed by (R-Y)modulates 3.58MHz subcarrier directlyQ signalformed by (B-Y)modulates a 90º shifted 3.58MHz subcarrier

R & T S21

NTSC colour systemNTSC stands for National Television Systems CommitteeColour TV in US, Japan, Korea, and the Philippines used

NTSC systemCompatible with monochrome receiverConsists of :

luminance signal Y (control brightness)colour signal I-Q (chrominance)

3.58MHz subcarrier (suppressed at transmitter) line rate:15,750Hz field rate: 60Hz525 lines

R & T S22NTSC colour

system

R & T S23

Colour composite video signal

used as a reference for colour demodulation

R & T S24Frequency spectrum of colour

composite video signal

R & T S25

PAL colour system

Phase Alteration by LineTV broadcast standard developed in Germany

and used in the H.K.,U.K, and most of Europe, Africa, Australia,etc.

PAL produces interlaced 625-line, 25 frames/second

R & T S26Generation of PAL

system

R & T S27Launch

sequence

R & T S28

History of satellite Comm.Before 1960s, most long-range communications via HF

bandHF band was overcrowded & unreliableSatellite communication provides:

greater communication capacityhigher qualitybetter reliability

In 1960s, a series of passive satellites were launchedEcho satellites like large metal balloons that reflected

radio waves

R & T S29

History of satellite Comm.Placed in low orbits

Active satellites were then launchedan active broadband repeatersignal from earth station is converted to another freq &

sent down to the eartha stronger signal can be received at the earth compared

with passive satellitebut the satellite can’t be accessed at any time since it

was placed at low orbitToday, communication satellites are placed in

synchronous orbits

R & T S30

Satellite comm. systems

R & T S31Uplink &

downlink

R & T S32

Satellite orbitsWhen satellite is in orbit:

force due to gravity F1 = centrifugal force F2

R & T S33Freq. band for sat.

comm.

BandUplink

Frequency (GHz)Downlink

Frequency (GHz)C 6 4

Ku 14 11/12K 29/30 19/20

Notice that the downlink freq < uplink freq since attenuation depends on freq: lower freq => lower attenuation => lower tx power needed for downlink

R & T S34

Satellite orbits

Gravitational force is proportional to distance

Centrifugal force is proportional to distance & velocity

The farther from earth, the slower the orbital speed

The closer to earth, the faster the orbital speed

low-altitude sat travels at higher speed

low-altitude sat completes one orbit faster due to higher speed and shorter distance

low-altitude sat appears to be moving when viewed from the earth

R & T S35

Satellite orbits

R & T S36

Types of orbit

Low Earth Orbit (LEO)

height < 1600km

round the earth in less than 2 hours

complicated tracking mechanism

used in early days of sat due to limited launching power

applications: - maritime & aviation navigation, weather forecasting & surveillance

R & T S37

Types of orbit

Geostationary(Synchronous) Orbit (GEO)

height : 35,860km from equator

orbital period: 24 hours

no tracking of antenna required

sat appears to be stationary observed from the earth

good for telecommunication, e.g. voice

worldwide coverage by 3 sat

R & T S38Look

angleThe coordinates to which an earth station antenna must be pointed to communicate with a sat are called look angle:

azimuth (Az)elevation (El)

R & T S39

Earth station

Collection of equipment on the surface for communicating with the satellite, may be:fixedground mobilemaritimeaeronautical

R & T S40

Earth station

Desired characteristics:high gain in the direction of wanted signals low gain in the direction of unwanted signals low noise for receiving systemhigh antenna efficiencycontinuous satellite pointingminimum performance variations caused by

weatheravailability of power resources

R & T S41

Earth station

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AntennaA ‘dish’ to collect very weak microwave signals to a

focus point.

•size depends on of signal•made from steel, aluminium or fiberglass embedded reflective foil => highly reflective surface

R & T S43Transmitte

rVary from simple single transmitter of few Watts to multi-channel transmitters using 10-kW (water cooling needed)Klystron

500~5000W output powersmall bandwidth, 40MHzmedium cost

Travelling Wave Tube (TWT)100~2500W output powerlarge bandwidth, 500MHzhigh cost

R & T S44

Receivers

signal from sat is received via an antenna

signal is then amplified by LNA (Low Noise Amplifier)

down-converted to lower frequency immediately before sending to receiver equipment via coaxial cable for demodulation

R & T S45

used to separate the transmitted signal & received signal since the same antenna is used for transmission & receiving

dual polarization (vertical & horizontal) to allow frequency reuse

Diplexer

R & T S46

Tracking to ensure the precise pointing of a narrow beamwidth

antenna automaticdetermine velocity of sat small earth stations with large beamwidth

=> no tracking

R & T S47Advantages of sat

comm.Signals from sat cover large area (footprint)

determined by beamwidth of transmitting antenna

only 3 sat to cover almost entire earth surface

distance insensitive cost

much cheaper for long distance comm. & regional

broadcasting

R & T S48Advantages of sat

comm.high reliability

EM wave propagation only slightly affected by atmosphere

flexibility

provide multi-channel TV, thousands of telephone channels & data transmission

R & T S49Limitations of sat

comm. sat transmitter power

limited by available power (solar) in sat

limited by payload of launch vehicle

sat receiver sensitivity

sat antenna intercepts only a small radiated energy

sat availability

only sat at geostationary orbit can provide continuous service

R & T S50Limitations of sat

comm.

long transmission time delay:

time delay=distance/velocity of light

distance = 2 X 36,000km (round trip)

velocity of light = 3X108m/s

time delay = 0.25s

time delay causes echo in telephone communication.

Limited life-time: approx. 10 years

R & T S51

Sat TV system

R & T S52

Feedhorn installed at the focal point of dishcollect all incident microwave & couples to

the LNAprevents noise from entering the LNA

R & T S53

LNB (Low Noise Block Converter)

LNB: down-converts signal from antenna to I.F. (coaxial cable can then be used) before transmission to receiver for demodulation

R & T S54Sat

receiver

Limiter: Limits the amplitude of signal before demodulation

I.F. Amp 2: Selects difference component output from 2nd mixer