The MPEG-4 Low Delay Advanced Audio Coding (AAC-LD) scheme has recently evolved into a popular algorithm for audio communication. It produces excellent audio quality at bitrates between 64kbit/s and 48kbit/s per channel. This paper introduces an enhancement to AAC-LD which reduces the bitrate demand by 25-33%. This is achieved by adding a combination of a delay-optimized version of the Spectral Band Replication (SBR) tool and by utilizing a dedicated low delay filterbank. The introduced techniques maintain the high audio quality and offer an algorithmic delay low enough for use in two way communication systems. This paper describes the coder enhancements including a detailed discussion of algorithmic delay issues, a performance assessment and possible applications.
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