A variable-band coding Scheme for speech encoding at 4.8 kb/s

Abstract

The standard fixed sub-band coding scheme has been modified to allow the center frequency of the two upper bands to vary in accordance with the dynamic movement of the vocal tract resonances F2 and F3. A relatively simple zero-crossing technique is used to measure the formants F2 and F3. Through the use of this variable band coder, it is possible to produce moderate-quality, intelligible speech at 4.8 kb/s (quality is slightly less than that of a 7.2-kb/s fixed sub-band coder and equal to that of about a 16-kb/s adm coder). The reasonably good intelligibility of the 4.8-kb/s variable-band coded speech can be attributed to the coders attempt to capture and encode those spectral components of the signal that are perceptually most significant (the region around the formants). The major advantage of the variable-band scheme is that its implementation is considerably less complex than other waveform coding schemes or vocoder systems that can produce intelligible, narrowband speech.