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In a digital communication system each digit of a character may be represented by one function of an orthogonal set of functions. This function is multiplied by + 1 or Â¿ 1 if binary digits are being used. All functions of the character are added and transmitted in parallel. This sum of functions is correlated in the receiver with each function of the orthogonal set and voltages of amplitude + 1 or Â¿ 1 are obtained. No interference between the digits of a character occurs if the signal is not band limited. The crosstalk between the digits because of band limitation is computed for a system of orthogonal sine and cosine functions. The results show that a binary Teletype system transmitting 60 words per minute can be operated with about 25 cps (cycles per second) bandwidth. This figure decreases to the theoretical minimum of 15 cps per channel for a multichannel system. Tests of experimental equipment have shown very good agreement with the theory.