Technique for Coding Speech Signals for Transmission over a Reduced Capacity Digital Channel Free

The process of transmitting speech signals over digital channels requires sampling and quantizing the signals into a discrete number of levels occurring at discrete times. Speech signals quantized into 128 levels and sampled 8000 times a second are considered good quality for communications purposes and require a transmission rate of 56 000 binary pulses per second. It is the purpose of this paper to describe a coding technique that allows comparable quality speech to be sent over digital channels at pulse rates of from 20 000 to 40 000 binary pulses per second. The coding scheme consists of filtering the speech into two frequency bands and digitalizing the bands separately. The high‐frequency band can be quantized into fewer amplitude levels and the low‐frequency band requires fewer samples. The speech is remade at the receiving terminal by adding the outputs of the two channels. Because the average spectrum of speech signals falls off at high frequencies, the use of few levels in the high‐frequency band introduces only a small relative error in the total signal. A typical system might use a 1000‐cps dividing frequency and 64 and 4 levels and 2000 and 8000 samples, respectively, to produce a pulse rate of 28 000 binary pulses per second.