ABSTRACT
This chapter focuses on one of the non-backwardly-compatible audio systems that will be used in advanced television — Dolby digital audio compression (AC-3). The importance of the data in each sub-band to the re-creation of the sound is determined in a bit allocator. Following the bit allocation and quantization processes, the bit allocation values and the quantized coefficents are multiplexed and sent to the decoder. If there is a forward adaptive bit allocation method, there must be a Backward Adaptive Bit Allocation scheme. The Dolby AC-3 system uses a hybrid of forward and backward adaptive bit allocation. The forward adaptive bit allocation system has the advantages that the psychoacoustic model resides only in the encoder and that it can be improved over time as better models of the human auditory system are developed. In the actual implementation of the AC-3 system, the downwards masking function is not realized in order to simplify the system.
