The SD and HD standards provide for up to 16 channels of PCM audio, while 3G allows up to 32 channels, typically encoded in the AES3 format. In HD, the embedded audio data packets are carried in the HANC space of Cb/Cr (chroma) parallel data stream. In addition, both standards define audio control packets. The audio control packets are carried in the HANC space of the Y (luminance) parallel data steam and are inserted once per field at the second video line past the switching point (see SMPTE RP168 for switching points of various video standards). The audio control packet contains audio-related metadata, such as its timing relative to video, which channels are present, etc. Embedded audio packets are Type 1 packets. EDH EDH packets are used for error detection in standard definition interfaces (they are not necessary in HD interfaces, as the HD-SDI interface includes CRC checkwords built in). External links SMPTE: SMPTE 291M-1998: Ancillary Data Packet and Space Formatting SMPTE: ANSI/SMPTE 125M-1995: Component Video Signal 4:2:2; Bit-Parallel Digital Interface SMPTE: ANSI/SMPTE 334M-1995: Vertical Ancillary Data Mapping for Bit-Serial Interface SMPTE: RP168-2002: Definition of Vertical Interval Switching Point for Synchronous Video Switching SMPTE: SMPTE 299-1:2010: 24-Bit Digital Audio Format for SMPTE 292 Bit-Serial Interface SMPTE: SMPTE 299-2:2010: Extension of the 24-Bit Digital Audio Format to 32 Channels for 3 Gb/s Bit-Serial Interfaces SMPTE: Data Identification Word Assignments for Registered DIDs Film and video technology Serial digital interface SMPTE standards