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Voice-excited vocoders for practical speech bandwidth reduction

1962; Institute of Electrical and Electronics Engineers; Volume: 8; Issue: 5 Linguagem: Inglês

10.1109/tit.1962.1057767

1557-9654

Edward E. David, Manfred R. Schroeder, B. F. Logan, A. J. Prestigiacomo,

Advanced Adaptive Filtering Techniques

In spite of their great potential for bandwidth saving in long distance telephony, vocoders have not found wide-spread acceptance. Two major problems have retarded their application. First is their strong electrical accent. Second is the so-called "pitch problem;" namely, deducing the nature of the talker's vocal excitation from his speech waveform. The reliability of this deduction and measurement depends critically upon high input speech-to-noise ratio, particularly between 50 and 200 cps. In many communication situations, this requirement precludes satisfactory operation. This limitation can be removed by a new method known as "voice excitations' which eliminates the necessity for a decision-making pitch detector. The principal advantage of voice excitation is its insensitivity to input signal-to-noise ratio and equalization. A voice-excited vocoder (VEV) with a 720 cps (250-970 eps) baseband and 17 spectrum channels low-passed to 25 cps each, covering the band 970-3700 cps, has been built and evaluated. The test shows an average PB-word intelligibility of 86%, compared to 92% for input speech of the same bandwidth, both with an 18 db signal-to-noise ratio. Quality tests indicate that listeners rate VEV speech "as good as" the input in about 90% of the test utterances. 0nly 19% of conventional vocoder utterances were so considered. The vocoder performed about equally well for each of the 12 speakers in the quality test. Voice-discrimination tests indicate that voice identity is well preserved. Crucial factors influencing the remade speech quality are the accuracy of spectral flattening and the impulse response of the analyzer low-pass filters. These results indicate that the principle of voice excitation provides the key to practical speech bandwidth reduction.