496 BELL SYSTEM TECHNICAL JOURNAL 



used up by peak instantaneous voltage when 994 channels are disconnected 

 and 6 carry full-load tones. 



If a group of carrier channels in frequency-division multiplex were trans- 

 lated to microwave frequencies, the overload distortion affecting the trans- 

 mission would be predominantly of the third-order class. To a first approxi- 

 mation the third order distortion follows a cube law and may be predicted 

 from the single-frequency compression. We assume here that the power 

 capacity of the repeater is the output at which the single frequency com- 

 pression occurring through the ccmplete system does not exceed 1 db.^ 

 This criterion applies roughly to systems of several hundred channels 

 capacity, and to present transmission standards. 



Instantaneous Companding Advantage in Time Division 



In time-division systems, as ordinarily understood and known in the 

 current literature, each channel successively is provided with its full-load 

 capacity, and thus a non-simultaneous load advantage does not accrue. 

 However, because of the sampling process, instantaneous compression may 

 be applied at the transmitting terminal before noise and distortion are en- 

 countered; when complementary expansion is applied at the receiving term- 

 inal the noise is suppressed. The expanded samples derived at the receiving 

 terminal then bear an improved relation to noise, particularly in the case of 

 weak samples. Such an instantaneous companding process applied without 

 sampling to a continuous speech wave requires a greatly increased trans- 

 mission band between compressor and expandor but, in a time division 

 system, no more bandwidth is needed to transmit the speech samples after 

 they have been compressed than before. An instantaneous compandor 

 currently being used experimentally to handle 12 channels in time division 

 has the noise performance characteristics shown'" in Fig. 2. It is shown as 

 applied to a telephone system in which the channel noise power (unweighted) 

 would be 45 db down from the power of a sine wave which employs the full 

 load capacity provided for the "loudest talker". Abrupt overloading is 

 assumed to take place when peak amplitudes exceed that of the full-load 

 tone. The location, at —7.5 on the load scale, for the power representing 

 the very loud talker (one in a thousand) conforms approximately to current 

 practice. The speech volumes, referred to the point of zero db transmission 

 level, are shown for the sake of completeness. 



' In a multi-repeater system the compression accumulates. This means that each re- 

 peater must be restricted to operate approximately 10 log n db below the 1 db compression 

 point of one repeater, (n denotes the number of repeaters.) See Section VI. 



'" Use of the same curve to represent the performance with tone or speech implies an 

 independence of wave form which is not rigorously valid. Calculations based on speech- 

 like signals have indicated that the curve for tone loading is a good approximation when 

 average power is used as the criterion in the manner shown. 



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