Sync frames

A Dolby AC-4 bitstream in the AC-4 sync frame transport format (as specified in ETSI TS 103 190) consists of Dolby AC-4 synchronization frames. The AC-4 sync frame transport format must be used for MPEG packed audio.

Each frame begins with a sync word and optionally ends with a CRC word. The sync word allows a decoder to easily identify frame boundaries and begin decoding. The CRC word allows a decoder to detect the occurrence of bitstream errors and to perform error concealment when it detects an error. The actual codec frame is referred to as the raw AC-4 frame.

This figure shows the general structure of the AC-4 frames as specified in ETSI TS 103 190.

Figure: The AC-4 sync frame transport format

Possible values for the sync word are:
  • 0xAC40: No CRC field is included.
  • 0xAC41: A CRC field is included.
The CRC is based on the raw AC-4 frame. The frame size and sync word fields are not part of the CRC calculation.

The frame size signals the size of the raw AC-4 frame and can be either two or five bytes in size. When a five-byte frame size field is used, the first two bytes are set as 0xFF and the actual size value is written into the remaining three bytes.

Each AC-4 sync frame element with the sync word 0xAC41 can contain a 16-bit CRC word. The CRC applies to the complete AC-4 sync frame element, not including the sync word. Decoding of the CRC word allows errors to be detected.