An Experimental Multichannel Pulse Code Modulation System of Toll Quality

1 2 BELL SYSTEM TECHNICAL JOURNAL duction of a properly formed standard pulse, free of noise, to correspond with each received pulse, even though the latter may be considerably misformed. The sole proviso here is that before regeneration the level of noise and distortion in each link be kept below the comparatively large threshold value at which a mark cannot be distinguished from a space. If this holds good throughout the transmission path then literally the received pulses can be made as good as new. In contrast it is impossible fully to repair or to regenerate signals not involving standard values of amplitude and of time. With such signals distortion and noise in each span contribute to the total which therefore increases with the system length. To sum up, conversion of speech to a code of pulses and spaces permits telephony to assume certain new and desirable properties; ability to work with small signal-to-noise ratios, and ability to regenerate any number of times with no degradation of quality. These advantages do not accrue without certain penalties. Conversion of speech waves to pulse form and back to speech involves a certain degree of apparatus complexity at the terminals. This complexity is not decreased by the need to handle pulses at high speeds, of the order of a million per second. Here radar and television circuit techniques are helpful. Another characteristic is that a greater band width is occupied in the transmission medium. This arises through the operation of two factors, of which one is the use of double sideband in pulse transmission (as against single sideband in carrier telephony), and the second involves the number of pulses used in the code.