Picture Transmission

The picture information is optical in character and may be thought of as an assemblage of a large number of bright and dark areas representing picture details. These elementary areas into which the picture details will be broken up are known as picture elements, which when viewed together, represent the visual information of the scene. Thus the problem of picture transmission is fundamentally much more complex, because, at any instant there are almost an infinite number of pieces of information, existing simultaneously, each representing the level of brightness of the scene to the reproduced. In other word the information is a function of two variables, time and space. Ideally then, it would need an infinite number of channels to transmit optical information corresponding to the entire pictures element simultaneously. Presently the practical difficulties of transmitting all the information simultaneously and decoding it at the receiving end seem insurmountable and so a method known as scanning is used instead. Here the conversion of optical information to electrical form and its transmission are carried out element by element, one at a time and in a sequential manner to cover the entire scene which is to be televised.

Picture Transmission

Scanning of the element is done at a very fast rate and this proses is repeated a large number of times per second to create an illusion of simultaneous pick-up and transmission of picture details.

A TV camera, the heart of which is a camera tube, is used to convert the optical information into a corresponding electrical signal, the amplitude of which varies in accordance with the variation of brightness. In Vidicon camera tube an optical image of the scene to be transmitted is focused by a lens assembly on the rectangular glass face-plate of the camera tube. The inner side of the glass face-plate has a transparent conductive coating on which is laid a very thin layer of photoconductive material. The photo layer has a very high resistance when no light falls on it, but decrease depending on the intensity of light falling on it Thus depending on the light intensity variation in the focused optical image, the conductivity of each element of the polyline changed accordingly. An electron beam is used to pick-up the picture information now available on the target plate in terms of varying resistance at each point. The beam is formed by an electron gun in the TV camera tube. On its way to the inner side of the glass face-plate it is deflected by a pair of deflecting coils mounted on the glass envelop and kept mutually perpendicular to each other to achieve scanning of the entire target area. Scanning is done in the same way as one reads a written page to cover all the word in one line and all the words in one line. To achieve this they deflecting coil are fed separately from two sweep oscillators which continuously generate saw-tooth waveform, each operating at a different desire frequency. The magnetic deflection caused by the current in one coil gives horizontal motion to the beam from left to right at a uniform and then brings it quickly to the left side to commence the trace of next line.

Picture Transmission

The other coil is used to deflect the beam from top to bottom at a uniform rate and for its quick retrace back to the top of the plate to start this process all over again. Two simultaneous motions are thus given to the beam, one from left to right across the target plate and the other from top to bottom thereby covering the entire area on which the electrical image of the picture is available. As the beam moves from element from element to element, it encounters a different resistance across the target-plate, depending on the resistance of the photoconductive coating. The result is a flow of current which varies in a magnitude as the element is scanned. This current passes through a load resistance Rl, connected to the conductive coating on one side and to 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.

If the scanning beam moves at such a rate that any portion of the scene content does not have time to move perceptibly in the time required for one complete scan of the image, the resultant electrical signal contains the true information existing in the picture during the time of the scan. The desired information is now in the form of a signal varying with time and scanning may thus be identified as a particular process which permit the conversion of information existing in space and time coordinate into time variation only. The electrical information obtained from the TV camera tube is generally referred to as video signal. This signal is amplified and then amplitude modulated with the channel picture carrier frequency. The modulated output is fed to the transmitter antenna for radiation along with the sound signal.

Related Posts