Brief Analysis of the Television System

In addition to the picture impulses, special signals are sent out by the television transmitter for the purpose of synchronizing the picture at the receiver with that being picked up by the camera.

At the television receiver, a single antenna picks up the picture and audio signals simultaneously. The voltages induced in the receiving antenna are fed into the r-f stage of the receiver, and the picture carrier and the sound carrier are converted by the super heterodyne conversion method into two separate i-f signals, one corresponding to the sound carrier and the other to the video or picture carrier with its associated sidebands. Two separate i-f amplifier channels are employed, one for the picture signal and the other for the sound signal.

The sound i-f signal, after suitable amplification, is demodulated by an FM detector. After sufficient amplification by the audio amplifier, the sound signal is reproduced by the loudspeaker in the usual.way.

The picture i-f signal is amplified by several stages having wideband frequency characteristics and is then fed into the video (picture) detector, where the i-f signal is then demodulated in the same fundamental manner as in an ordinary sound receiver. The video (picture) signal which appears in the output of the detector is then amplified in a video amplifier, which corresponds to the audio amplifier in a sound receiver except that it must pass a much wider range of frequencies.

In place of the loudspeaker used in the sound system, a device is used which converts the varying amplitude of the video signals into corresponding variations of light which reproduce the original scene.

The picture-reproducing device is a cathode-ray tube, similar in many respects to the ordinary cathode-ray tube used in service-shop oscilloscopes. The cathode-ray tube may be called a picture tube, because the desired picture is reproduced on the face of this tube. Without going into the details of the cathode-ray tube at this time, we shall assume that it consists of a glass envelope, a source of electrons which are formed into a beam, a control grid for varying the intensity of the electron beam, a deflection system for deflecting the beam, and a screen coated with a fluorescent material that emits light upon impact by the electron beam.

The fundamental action of the cathode-ray tube in reproducing a picture consists in the electron beam's moving horizontally and vertically simultaneously so as to cover the entire area of the picture-tube screen at the same time that the intensity of the spot is being varied by the video signal which is applied between the grid and cathode elements of the cathode-ray tube. The control grid of the picture tube controls the intensity of the beam which strikes the screen in exactly the same way that the control grid of an amplifier tube controls the plate current. In this way, each portion of the picture tube is made to have the proper degree of light or shade to reproduce the original scene.

The brief description of the action taking place in the television receiver omitted the necessary scanning and synchronizing action which locks or synchronizes the picture at the receiver with that at the television camera. However, in the receiver, the synchronizing and scanning action takes place between the output of the video amplifier and the picture tube as shown by the block marked "synchronizing and scanning".