1220 the bell system technical joubnal, october 1951 



Conclusions 



It has been shown that distortionless transmission of instantaneously 

 compressed speech over a frequency band no wider than that required for 

 the uncompressed speech does involve the transmission of a broad-band 

 signal over a relatively narrow-band channel. This is made possible by the 

 use of an instantaneous sampler which serves to "fold" the spectrum of the 

 compressed speech at the transmitting end so that the entire spectrum is 

 contained within the desired bandwidth. The criterion for distortionless 

 transmission of these "folded" signals is shown to be one of recovering at 

 the receiving end the precise samples of compressed speech that were gener- 

 ated at the transmitter. To accomplish this distortionless recovery of the 

 transmitted pulses it is necessary, first, that the transmission medium cause 

 no intersymbol interference, and, second, that the signals at the receiver 

 must be sampled in synchronism with the sampling at the transmitter. 



It was also shown that the full reduction in bandwidth can be realized 

 only by pulse operation under minimum bandwidth conditions. It was esti- 

 mated that the accuracy of control of the steady state phase and attenuation 

 vs. frequency characteristics that would be required to maintain the inter- 

 symbol interference below an acceptable level would be hard to meet in 

 practice, primarily because of having to operate under such minimum band- 

 width conditions. 



