The BSI consists of a 5-bit, bit-stream identification information, 3 bits of bit-stream mode indicating the type of service that the frame conveys, 3-bit audio coding mode, indicating the channel array configuration. The BSI (BitStream Information) is the process of unpacking of various types of information in the bit stream received at the decoder. If either of the CRC’s (crc1 and/or crc2) comes out to be good, the decoder shall be assumed to be in sync, and hence, one may proceed with decoding and reproduction of the audio data. Hence, it is preferable to check the entire frame size using crc2 after the entire frame is received. The first CRC (crc1) covers 5/8 th of a frame, hence the CRC1 results will be available only after receiving the 5/8 th of a frame. A 16-bit sync word is used so that there’s a low probability of a false frame detection. When a sync pattern is detected, the decoder may be estimated to be in sync and one of the two Cyclic Redundancy Check words (crc1 or crc2). Image Courtesy () Synchronization and error detection
Thereafter, the exponents and mantissas are converted back into the time domain to extract the decoded PCM time samples. Decoding of the spectral envelope results in the exponent’s values. At the decoder, the bit allocation module execution results in unpacking and de-quantization of the mantissa values. The bit allocation module is designed to calculate the number of bits required to encode each transform coefficient mantissa value on the encoder side. The decoder must synchronize with the encoded bit stream, check for errors, and de-format the various types of data such as the encoded spectral envelope and the quantized mantissa. It supports data rates ranging from 32 kbps to 640 kbps. The AC3 bitstream specification allows sampling rates of 48 kHz, 44.1 kHz, and 32 kHz. The six-channel configuration consists of the left channel, center channel, right channel, left surround, right surround, and subwoofer.
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