A silence compression algorithm for multi-rate/dual-bandwidth MPEG-4 CELP standard

This paper proposes a silence compression algorithm operating at multi-rates (MR) and with dual-bandwidths (DB), a narrowband and a wideband, for the MPEG-4 CELP standard. The MR/DB operations are implemented by a variable-frame-size/dual-bandwidth voice activity detection (VF/DB-VAD) module with domain conversions of the input signal, and a variable frame-size comfort noise generator (VF-CNG). The CNG module adaptively controls the power smoothing of the decoded signal to improve the coding quality. The algorithm also employs a dual-rate discontinuous transmission (DR-DTX) module to reduce an average transmission bit rate during silence periods. The subjective test results show that the proposed silence compression algorithm gives no degradation in coding quality for clean and noisy speech signals. These signals include about 20 to 30% nonspeech frames and the average transmission bit rates are reduced by 20 to 40%. The proposed algorithm has been standardized as a part of the ISO/IEC MPEG-4 CELP version 2.