Vocoder and its Application to the Transmission of High-Quality Speech over Narrow-Band Channels

Abstract

A vocoder for frequencies between 3 kc and 10 kc is described. It requires a transmission band width of 300 cps. This covoder is used with a 3-kc uncoded speech band to which the compressed frequency components are added by standard multiplexing methods. Thus, high-quality speech may be transmitted over conventional 3.5-kc voice channels. As a result of this technique, the reproduced speech for the same transmission band width not only appreciates in quality but is more intelligible. The simple spectra[ structure of many speech sounds above 3 kc permits a particularly effective application of voice coding principles. A description by as few as 2 to 6 parameters, depending on the kind of encoding, is sufficient. On the other hand, in order to insure a faithful reproduction of the plosive sounds the band width necessary in the individual channels is approximately twice as great as in vocoders with more modest quality requirements. The frequency compression ratio is approximately 20:1 for a channel-type coding and 70:1 for resonance coding. In developing a vocoder for the transmission of high-quality speech a number of details must be considered which may be neglected in vocoders for intelligible speech. Among these we mention the mixed quasi-periodic and aperiodic excitation as it occurs in voiced fricatives, the exact reproduction of individual pitch period lengths, and a reasonable preservation of the peak factor of the synthesized speech. The techniques employed to solve these problems will be discussed. A magnetic tape recording demonstrating the quality of this vocoder will be played.