Technical Details
The data layout of AC-3 is described by simplified "C-like" language in official specifications. An AC-3 stream is a series of frames; each with a fixed number of 6 audio blocks; each audio block contains 256 audio samples per channel. For example, a 5.1 AC-3 bitstream contains 1536 samples per audio block (6 channels × 256 samples/channel). Channel blocks can be either long, in which case the entire block is processed as single modified discrete cosine transform or short, in which case two half length transforms are performed on the block. Below is a simplified AC-3 header. A detailed description is in the ATSC "Digital Audio Compression (AC-3) (E-AC-3) Standard", section 5.4.
| Field Name | # of bits | Description |
|---|---|---|
| Syncword | 16 | 0x0B77, data transmission is left bit first: big endian |
| Cyclic redundancy check | 16 | |
| Sampling frequency | 2 | '11'=reserved '10'=32 kHz '01'=44.1 '00'=48 |
| Frame size code | 6 | |
| Bit stream identification | 5 | |
| Bit stream mode | 3 | '000'=main audio service |
| Audio coding mode | 3 | '010'=left, right channel ordering |
| Center mix level | 2 | |
| Surround mix level | 2 | |
| Dolby Surround mode | 2 | '00'=not indicated '01'= Not surround encoded '10'= Yes, surround encoded |
Read more about this topic: Dolby Digital
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