Secret Telephony as a Historical Example of Spread-Spectrum Communication

The spread-spectrum properties of the X-System for secret telephony developed by Bell Telephone Laboratories for use in World War II are examined. In this system, the bandwidth of the speech signal was reduced by a vocoder, the vocoder signals were sampled and quantized to base six, and a random, never-reused, six-valued key stream was added modulo six to obtain a public message which was undecipherable without the key. It is believed that this was the first practical example of digital speech transmission. Examples of its effectiveness are described, and a number of humaninterest type anecdotes are related.