AN EVALUATION OF SPEECH COMPRESSION TECHNIQUES
BOLT BERANEK AND NEWMAN INC CAMBRIDGE MA
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The results of previous technical reports are summarized and the results of tests of various recently developed speech compression systems are presented and analyzed. The results can be summarized as follows 1 Semi- vocoders, operating at 9600 bitssec, and channel vocoders, at 2400 bitssec, will provide speech of adequate intelligibility and quality for most military communications. The voice quality of the semi-vocoders will usually be somewhat superior to that of the channel vocoders. 2 Results with as yet in complete systems suggest that the Tarasoff-Daguet technique and the narrow-band spectrum sampling technique will ultimately provide speech intelligibility and quality comparable to that of the semi-vocoder. These systems will also require about 9600 bitssec channel capacity during transmission. 3 Formant-tracking vocoders operating at about 1200 bitssec can probably be developed to the point where they will provide speech intelligibility comparable to that from the channel vocoders operating at 2400 bitssec. At the present time formant- tracking vocoders operating a bitssec do not provide for adequate speech intelligibility. 4 Transmitting a restricted set of speech patterns, from a larger set obtained from a digitized vocoder-type of speech analyzer- synthesizer, may provide a reduction in the information transmission rate normally required by that particular vocoder. Although this pattern matching technique is still in its early experimental stages it appears that the normal bit rate for a given vocoder system may, by this technique, be reduced to 23 and possibly to 12 its normal magnitude.
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