Optimal bit reservoir control for audio coding

Abstract

Audio coding schemes such as MP3 or MPEG AAC can use a variable number of bits to encode each signal frame. However they can still be used on constant bit rate channels thanks to a variation buffer limited in size. This buffer allows the encoder to use more bits for high complexity frames in order to maintain quality, the buffer being decreased e.g. for silent frames, leading to a constant rate coding on average. The use of such a buffer aims at reducing quality fluctuations over time, thanks to reasonable instantaneous bit rate variations. In this paper, a new design for a buffer controller is presented. This controller aims at finding the appropriate number of bits to encode each audio frame, according to chosen perceptual criteria and reduces the perceived distortion over time. This process can take place in a two pass encoding process, the first pass aiming at measuring frames complexity and bit demand, the second consisting in allocating the appropriate number of bits for each. In order to cope with on-line coding application a single pass encoding with a reasonable additional delay is also investigated.