574 BELL SYSTEM TECHNICAL JOURNAL 



(a) The Character of the Television Signal. 



(b) Requirements upon the Signal Wave Set by the Characteristics 

 of Available Transmission Channels. 



(c) Requirements which the Transmission Channels must meet in 

 order to carry Television Signals. 



(a) The Character of the Television Signal. As we have seen, the 

 voltage produced across a resistance in series with the photoelectric 

 cell is a fluctuating unidirectional potential. The generated signal 

 therefore has frequency components beginning at and including zero 

 frequency. The value of the voltage at any instant is roughly pro- 

 portional to the average reflected illumination at that instant from 

 an illuminated spot whose size depends upon the apertures in the 

 scanning disk. At any point where there is a sudden change in the 

 tone value of the subject there will also be a sharp change in the 

 generated voltage. It will, therefore, be seen that but for the limits 

 of speed of action of the photoelectric cell and its connected circuits 

 the generated signals would tend to include components over the 

 whole frequency range up to infinity. Since it is possible to effectively 

 transmit but a limited range of these components, the width and 

 location of the frequency band necessary for the acceptable repro- 

 duction of a given size and structure of image must be determined. 

 It is convenient to consider first the low frequency end of the band. 



In the early experimental work it was soon found that in attempting 

 to amplify the lower frequencies by the use of direct current amplifiers, 

 unstable conditions of operation were reached before sufficient amplifi- 

 cation was obtained to operate the receiving apparatus. Experi- 

 ments were then made with resistance-condenser coupled amplifiers 

 which showed that, if the efficiency of such an amplifier at the frequency 

 equal to the number of pictures sent per second was not more than 

 about two T U below its average efficiency for the transmitted range, 

 acceptable reproduction of the picture was secured together with 

 stable operation of the amplifiers. When the low frequency cut-off 

 of the amplifier was set much above this, spurious shadows were 

 introduced into the picture. That there will be a critical lower 

 frequency for the transmission of an unchanging scene is obvious 

 since the Fourier series into which the signal may be analyzed starts 

 with a constant term and the sine wave terms begin with the picture 

 frequency and include a vast number of its harmonics. If the constant 

 component (d-c.) is removed, the lowest frequency which remains to 

 be transmitted is therefore the picture frequency. 



The effect of removing the d-c. component of the signal can be 

 qualitatively traced in a simple manner. Imagine three types of still 



