Auto-Correlation Vocoder
Manfred R. Schroeder, Thomas H. Crystal · The Journal of the Acoustical Society of America · 1960
In an auto-correlation vocoder system, samples of the short-time auto-correlation function of a speech signal are transmitted over a reduced bandwidth channel and converted into a time wave that is properly periodic and a replica of the auto-correlation function. By Wiener's theorem, the amplitude spectrum of the thus synthesized signal is the power spectrum of the analyzed speech signal. Phenomenologically, this spectrum squaring has four distinct effects: 1. Doubling of dynamic level fluctuations. 2. Doubling of formant level differences. 3. Deepening of spectral minima (“zeros”). 4 Reduction of formant bandwidths. The most serious of these spectral degradations have been removed by a variable equalizer preceding the auto-correlation analyzer. The remade speech quality produced by this technique exceeds that of known frequency-channel vocoders and of a previously described cross-correlation vocoder [J. Acoust. Soc. Am. 31, 852 (A) (1959)]. Bandwidth (or time) compression is about 10:1 (before multiplexing). Tape-recorded speech samples from the equalized auto-correlation vocoder will be played.