Split-band APC System for low bit-rate encoding of speech

We describe a split-band adaptive predictive coding system for digital transmission of speech signals. In this system, the prediction residue signal obtained after spectral prediction is filtered into 2 or more frequency bands. Each of the filtered signals is reduced further by pitch prediction and is quantized by a 15-level noise feedback quantizer. The input to the quantizer is severely center-clipped to produce a quantized signal with low entropy. The division of the prediction residue signal into many frequency bands results in more accurate pitch prediction - particularly, at low frequencies. The split-band system uses separate quantizers for each frequency band. The step size of the quantizer and the center-clipping threshold can thus be adjusted to optimize speech quality in each band.