Telephony by Pulse Code Modulation*

An experiment in transmitting speech by Pulse Code Modulation, or PCM, is described in this paper. Each sample amplitude of a pulse amplitude modulation or PAM signal is transmitted by a code group of ON-OFF pulses. 2 n amplitude values can be represented by an n-digit binary number code. For a nominal 4 kc. speech band these n ON-OFF pulses are transmitted 8000 times a second. Experimental equipment for coding the PAM pulses at the transmitter and decoding the PCM pulses at the receiver is described. Experiments with this equipment indicate that a three-unit code appears to be necessary for a minimum grade of circuit, while a six- or seven-unit code will provide good quality.