MULTICHANNEL AMPLIFIERS BELOW OVERLOAD 593 



may be treated as noise, and the usual noise requirements apply. If 

 a great many products are superimposed, the noise heard will be fairly 

 steady, and the average weighted noise power is a sufficient indication 

 of the interfering effect. In systems with a small number of channels, 

 large variations in the noise may occur, and it may be necessary to 

 consider the infrequent large bursts of modulation from exceptionally 

 loud talkers as a limiting factor. The allowance to be made can be 

 estimated by determining the complete distribution curve of modula- 

 tion noise. Computation of the required distribution function may 

 be carried out by methods similar to those described in the paper by 

 Holbrook and Dixon. ^ The fact that the products are not independent 

 introduces a difficulty which complicates the calculation. For systems 

 with a large number of channels, the requirements may be based on 

 average values with a considerable resulting simplification. 



If in addition to the sidebands due to speech, "carrier leaks" 

 (partially suppressed carrier waves) are present, modulation products 

 are produced which are sums and differences of carrier frequencies and 

 speech sidebands. Products of this sort may cause intelligible cross- 

 talk. For example the carrier frequency mp modulating with the 

 channel frequency np -\- q causes an {A + 5) -product of frequency 

 {m + n)p + 5- which is received in the channel with carrier frequency 

 {m 4- n)p as the original signal frequency q. Requirements on in- 

 telligible crosstalk are in general more severe than on unintelligible; 

 hence it is important that the carrier leaks be suppressed well below 

 the level of the speech channels. The intelligibility tends to disappear 

 as the number of channels is increased, since the number of super- 

 imposed products becomes larger thereby producing masking effects. 

 Carrier leak modulation is however more serious than modulation from 

 speech channels having the same power since carrier leaks are present 

 all the time, while speech sidebands occur only in active channels. 

 Similar considerations apply to pilot and control tones. 



Quantitatively, the various frequency components in modulation 

 noise must be weighted in terms of their interfering effect on reception 

 of speech. In practice the weighting is done by a noise meter designed 

 for that purpose. The noise meter readings are expressed in terms of 

 db above reference noise. A reading of zero, or reference noise, is 

 produced by a lOOO-cycle sinusoidal wave with mean power equal to 

 one micromicrowatt. The weighting incorporated in the noise meter 

 is determined by judgment tests of the relative interfering effects of 

 single frequencies and other reproducible noises. 



