Design of an MPEG-4 general audio coder for improving speech quality

This paper proposes a design for an ISO/IEC MPEG-4 general audio encoder to improve the speech quality at low bit rates. The main contributions to the improvement are using i) a higher sampling rate to get higher time resolution for a given frame length and ii) adaptive preprocessing to reduce the bandwidth. Listening tests at 8 and 16 kbit/s showed that compared with a conventional audio coder and a speech-specific coder, the proposed coder provided better speech quality than the conventional audio coder (MP3) while keeping the quality for music. For speech signals, however, the speech-specific coder (MPEG-4 CELP) produced significantly better quality than the audio coders. The proposed design will be especially useful for low-bit-rate audio-visual delivery applications which may include both speech and music signals.