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The Television Picture

Contents Picture Elements Horizontal and Vertical Scanning Motion Pictures Frame and field frequencies Horizontal and vertical frequencies Horizontal and vertical synchronization Horizontal vertical blanking The 3.58 MHz color signal The 6 MHz television broadcast channel

Picture ElementsPicture ElementsA still picture is fundamentally an arrangement of many small dark and light areas. Each small area of light or shade is a picture elementor picture details. All the elements contain the visual informationin the scene. If they are transmitted and reproduce in the same degree of light or shade as the original and proper position, the picture will be reproduce.

A Still Picture

Reproducing a picture by Duplicating its picture elements

Magnified view to show Picture elements

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The television picture is scanned in a sequential series of horizontal lines one under the other. This scanning makes it possible for one video signal to includeall the elements for the entire picture. The sequence for scanning all the picture elements is as follows:

The electron beam sweeps across one horizontal line, covering all the picture elements in that line

At the end of each line, the beam is returned very quickly to the left side to begin scanning the next horizontal line. The return time is called retrace or flyback. No picture information is scanned during retrace because both the camera tube and picture tube are blanked out for this period. The retraces must be very rapid since they are wasted time in terms of picture information.

When the beam is returned to the left side, its vertical position is lowered so that the beam will scan the next lower line and not repeat over the same line. This is accomplished by the vertical scanning motion of the beam, which is provided in addition to horizontal scanning.

Horizontal and vertical scanning

Typical H-scanning patternTypical H-scanning pattern

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Typical V-scanning patternTypical V-scanning pattern

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Number of scanning lines per Number of scanning lines per frameframe

Maximum number of alternate light and dark elements (lines) which can be resolved by the eye is given by

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vN= minimum resolving angle of the eye expressed in radians= viewing distance /picture height =D/HExperimentally it is found that D/H=4=one minute=1/60 degree

8604)601180(11 vN

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Number of scanning lines per Number of scanning lines per frameframe

6027.0860 kNN vrThe effective number of lines

The values of k lies in between 0.65 to 0.75. While viewing picture having motion eye can detect effective sharpness if line number is 500. Also the channel bandwidth increases with increasing line numbers.

Frame per secondThe vertical scanning is at the rate of 30 Hz for the frame frequency of 30 frames per second. The frame rate of 30 per second means that 625 lines for one complete frame are scanned in 1/30 second.

Standard commercial motion picture practice 25 frames per second. Persistence of vision

The impression made by light seen by the eye persists for a small fraction of a second after the light source is removed. Therefore, if many views are presented to the eye during this interval of persistence of vision, the eye will integrate them and give the impression of seeing all the images at the same time.

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Fliker and elimination of flikerFliker and elimination of fliker

The rate of 25 frames per second, however, is not rapid enough to allow the brightness of one picture to blend smoothly into the next during the time when screen is black between the frame. The result is a definite flicker of light as the screen is made alternatively bright and dark.

If it is possible to project each frame twice in screen the flicker can be eliminated.

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Interlaced ScanningInterlaced Scanning

To achieve this the horizontal sweep oscillator is made to work at a frequency of 15625 Hz (312.5 × 50 = 15625) to scan the same number of lines per frame (15625/25 = 625 lines), but the vertical sweep circuit is run at a frequency of 50 instead of25 Hz.In all then, the beam scans 625

lines (312.5 × 2 = 625) per frame at the same rate of 15625 lines (312.5 × 50 = 15625) per second. Therefore, with interlaced scanning the flicker effect is eliminated without increasing the speed of scanning, which in turn does not need any increase in the channel bandwidth.

What a Television Broadcasting Is?

Monochrome Television Transmitter

Simplified Cross-sectional View of a Simplified Cross-sectional View of a Vidicon TV Camera TubeVidicon TV Camera Tube

Two simultaneous motions of the beam, one from left to right and the other from top to bottom encounters a different resistance across the target plate.

Depending on the resistance of the photoconductive coating the current passes through a load resistance RL, which is connected to the conductive coating on one side and to a dc supply source on the other.

Depending on the magnitude of the current a varying voltage appears across the resistance RL and this corresponds to the optical information of the picture.

The electrical information obtained from the TV camera tube is video signal.

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Monochrome Television ReceiverMonochrome Television Receiver

The receiver is of the heterodyne type and employs two or three stages of intermediate frequency (IF) amplification. The output from the last IF stage is demodulated to recover the video signal.

Picture Reception

The signal that carries the picture information is amplified and coupled to the picture tube which converts the electrical signal back into picture elements of the same degree of black and white.

The picture tube is very similar to the CRT used in an oscilloscope. The glass envelope contains an electron gun structure that produces a beam of electrons aimed at the fluorescent screen. When the electron beam strikes the screen, light is emitted. The beam is deflected by a pair of deflecting coils mounted on the neck of the picture tube in the same way and rate as the beam scans the target in the camera tube. The amplitudes of the currents in the horizontal and vertical deflecting coils are so adjusted that the entire screen, called raster, gets illuminated because of the fast rate of scanning.The video signal is fed to the grid or cathode of the picture tube. When the varyingsignal voltage makes the control grid less negative, the beam current is increased, making the spot of light on the screen brighter. More negative grid voltage reduces the brightness. If the grid voltages is negative enough to cut-off the electron beam current at the picture tube there will be no light. This state corresponds to black. Thus the video signal illuminates the fluorescent screen from white to black through various shades of grey depending on its amplitude at any instant. This corresponds to the brightness changes encountered by the electron beam of the camera tube while scanning the picture details element by element. The rate at which the spot of light moves is so fast that the eye is unable to follow it and so a complete picture is seen because of the storage capability of the human eye.

Picture Reception

Sound Reception

The path of the sound signal is common with the picture signal from antenna to the video detector section of the receiver. Here the two signals are separated and fed to their respective channels. The frequency modulated audio signal is demodulated after at least one stage of amplification. The audio output from the FM detector is given due amplification before feedingit to the loudspeaker.

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Synchronization

To ensure perfect synchronization between the scene being televised and the picture produced on the raster, synchronizing pulses are transmitted during the retrace, i.e., fly-back intervals of horizontal and vertical motions of the camera scanning beam. Thus, in addition to carrying picture detail, the radiated signal at the transmitter also contains synchronizing pulses. These pulses which are distinct for horizontal and vertical motion control, are processed at the receiver and fed to the picture tube sweep circuitry thus ensuring that the receiver picture tube beam is in step with the transmitter camera tube beam.

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Television Broadcast Channels

The band of frequencies assigned for transmission of the and sound signals is a television channel. FCC assigned 6 MHz for a channel

Frequency Range

Channel No. Frequency Band (MHz)

1 Not used

Low band VHF 2-4 54-60, 60-66, 66-72,

72-76 MHz Air Navigation 72-76

Low band VHF 5,6 76-82, 82-88

88-108 FM band 88-108

High band VHF 7,8,9,10,11,12,13

174-180,180-186,186-192,192-198,198-204, 204-210,210-216

UHF 14-83 470-890

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