The Gradation of Television Pictures

The requirements of adequate gradation and contrast range in television pictures are discussed in relation to what the human eye can perceive. Thus it is shown that reproduction of the average brightness of the transmitted scene on the receiver screen is unnecessary, besides being possible only at the expense of reduced signal-to-noise ratio and contrast range. Because of the presence of synchronizing pulses, a fraction (15 per cent for the American standard) of the transmitted direct-current component reaches the screen even through pure alternating-current couplings. A comparison is made between the usual assumption of a linear modulation characteristic of the transmitter and linear light-signal characteristic of the receiver and a suggested alternative in which a logarithmic modulation characteristic of the transmitter is combined with an exponential light-signal characteristic of the receiver. The advantages of the latter are higher transmitter economy, measured in signal-to-noise ratio; free choice of contrast in the receiver, the control affecting high lights and shadows evenly; less critical background-light control because it has no effect on contrasts; independence of the contrast ranges of receiver and transmitter; and simpler design of cathode-ray-tube guns.