Some Systems for Speech-Band Compression

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Abstract
Speech‐band compression aims to reduce the frequency band width without impairing the desired informational content. Physical compression systems are of two types: spectrum‐alteration systems and vocoder (voice coding) systems. Spectrum alteration is not restricted to speech, is relatively simple to perform, but achieves only a compression factor of 3 to 5. Considerably higher ratios (10 to 20) are obtainable by vocoder‐type systems. Here, by a spectrum analysis on the transmitter side, a “pitch” frequency is extracted and a system function is derived. Both are then transmitted to a receiver where speech is synthesized. At the A. F. Cambridge Research Center, the following vocoder‐type systems are being developed and investigated: the scan vocoder with envelope scanning, the pulse vocoder in which the envelope is coded, and the formant vocoder in which the envelope is represented by three formant frequencies. By means of alterations and additions, the scan vocoder can be transformed into a pulse or a formant vocoder.

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