Television. Television provides a means of viewing the images of objects that are out of sight, i

 

Television provides a means of viewing the images of objects that are out of sight, i. e. far removed from the ob­server.

The images of moving or stationary objects are converted into electric signals and these signals are transmitted by a television transmitter. The television receiver (TV-set) picks up these signals and performs the reverse conversion of electrical signals into the image displayed on the screen of a cathode ray tube (CRT). Television signals can be transmitted by means of transmission lines as well as by radio.

The transmission of video signals is more complicated than the transmission of audio signals by means of radio-waves. There is a difference between the perception of audio signals and video signals by the human being. No matter how complex the audio signal is, the human ear interprets it as the sum total of all its components, i, e. as a single sound. The human eye, on the other hand, can perceive many different objects at one and the same time. Modern televi­sion techniques have taken all the peculiarities of human sight into consideration.

The iconoscope camera tube was developed as far back as the early thirties. Later, other types of camera tubes came into use, such as the supericonoscope which is more common­ly known as the image iconoscope. The tube in the television receiver, that provides picture display, is called the picture tube or kinescope.

The image of an object is projected onto the camera tube. The electron beam of this tube scans the image point by point. The beam scanning is controlled by a scan unit. At the tube output, pulses corresponding to the image are generated. These signals are usually termed the picture signals.

These pulses are amplified and used to drive the televi­sion transmitter, where they modulate the transmitter carrier. Transmission is usually achieved by amplitude-modulation techniques. The resulting radio-frequency vision signals are transmitted by the aerial and picked up by the receiving aerial, in which they induce an e. m. f. corresponding in fre­quency and waveform to the transmitted signals. Received signals are fed to the video channel amplifier, that is essen­tially a pulse receiver. Here the signals are amplified and de­tected; the picture signals from the detector output are am­plified and used to drive the television tube brightness con­trol electrode.

The movement of the electron beam in the television tube must be strictly synchronous and in phase with the electron beam of the camera tube. This phasing is accomplished by transmitting special syncpulses, provided by a synchroniz­ation generator (timer). These syncpulses control the scan of the camera tube and are transmitted along with the picture signals. At the receiver, these syncpulses are extracted from the composite video signal and used to control the operation of the scan.

In television broadcasting, the sound signal is transmit­ted simultaneously with the video signal. The audio signal from a microphone is amplified and used to modulate the frequency sound channel carrier. Both transmitters feed one common aerial through a special coupling filter. The receiver aerial picks up the sound and vision radio frequency signals. After amplification, the sound signal is separated from the composite signal, amplified, and used to drive a loudspeaker.

 








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