SPEECH ENCODER USING GAIN NORMALIZATION THAT COMBINES OPEN AND CLOSED LOOP GAINS

摘要

A multi-rate speech codec supports a plurality of encoding bit rate modes by adaptively selecting encoding bit rate modes to match communication channel restrictions. In higher bit rate encoding modes, an accurate representation of speech through CELP (code excited linear prediction) and other associated modeling parameters are generated for higher quality decoding and reproduction. To support lower bit rate encoding modes, a variety of techniques are applied many of which involve the classification of the input signal. The encoder utilizes gain normalization wherein LPC (linear predictive coding) gain provides a smoothing factor for combining both open and closed loop gains. The lower the LPC gain, the greater the open loop gain contribution to a gain normalization factor. The greater the LPC gain, the greater the closed loop gain contribution. For background noise, the smaller of the closed and open loop gains are used as the normalization factor. The normalization factor is limited by the LPC gain to prevent influencing the coding quality.