70 BELL SYSTEM TECHNICAL JOURNAL 



over the system a wide-band signal having a continuous spectrum such as thermal 

 noise. At the sending end a narrow-band elimination filter was inserted. At the 

 far end the noise was measured within that same band. 



The total noise so measured depends upon the signal energy levels at the input 

 and the output of the repeaters, the former controlling the effect of line and 

 resistance noise and the latter controlling the effect of modulation. These levels 

 in turn are a function of repeater spacing. The tests that have been made indicate 

 that it is practicable to keep this type of interference within desirable limits on 

 long telephone or television circuits. 



Due to the 60-cycle power supply used on the system, power frequency modula- 

 tion products require special attention. Sixty-cycle sidebands are produced on 

 all signals transmitted due to the traces of nonhnearity in the system. As these 

 are very small in magnitude and result mostly in a 120-cycle component they are 

 unimportant for telephony. However, in the television transmission system used, 

 this component is larger because of the presence of a strong carrier and one or 

 more pilot channels. Also, 120-cycle sidebands produce a very disturbing type of 

 horizontal bar pattern across the picture. This type of interference will increase 

 as the circuit length is increased, and may become more visible as receiving tubes 

 are improved. On systems so far available for test, however, it has been possible 

 to hold this type of interference within acceptable limits, on present day television 

 broadcast images. 



Distortion in Television Images 



Departures from ideal transmission in the line, equipment or in a radio path 

 produce distortion in the form of negative or positive fringes or "ghosts." These 

 occur when there is a lack of proportionality between phase shift and frequency 

 through the system. This trouble in television images is perhaps more easily 

 understood if one thinks of it as an actual difference in time of transmission of 

 various parts of the signal. In discussing this matter in this paper, we wiU use 

 the term "delay" to mean the time of transmission of the envelope of a modulated 

 wave. This quantity is often more accurately referred to as "envelope delay".^^ 

 If this quantity varies too widely there is an actual difference in the time of 

 transmission of various parts of the signal, producing distortion in the form of 

 fringes or "ghosts" which are exhibited by many television images today. 



Band Width 



A band width of about 3 mc is required to give equal resolution in the vertical 

 and horizontal directions in a 441-line, 30-frame interlaced image. Recent 

 experiments^- with out-of-focus moving pictures have shown not only that the eye 

 is quite insensitive in its requirement for equal detail in the two directions but also 

 that the loss of detail due to a narrowing of the frequency band from 4 mc to 2| 

 mc will pass unnoticed by many careful observers at normal viewing distance. 



Television on Coaxial Systems 



As mentioned above, no practical method has been found for transmitting 

 television over long-haul coaxial circuits in the video frequency range. 

 By the carrier method, however, the video frequency band may be raised 

 to a region suitable for transmission. To conserve frequency space, 



