Embedded audio

Audio that is carried within a video stream such as SDI – so simplifying cabling and routing. The standard SMPTE 272M allows embedding up to four groups each of four mono audio channels in SD-SDI. For HD the standard is SMPTE 299M. Both can carry up to 16 channels of audio (8 pairs). SMPTE 299-2:2010 extends audio capacity over 3G SDI to 32 channels.

48 kHz synchronous audio sampling is pretty well universal in TV but the standard also includes 44.1 and 32 kHz synchronous and asynchronous sampling. ‘Synchronous’ means that the audio sampling clock is locked to the associated video. For example in SD this means 1920 samples per frame in 576/50I, or 8008 samples per five frames in 480/59.94I. Up to 24-bit samples are allowed but mostly only up to 20 are currently used.

48 kHz sampling means an average of just over three samples per line, so three samples per channel are sent on most lines and four occasionally – the pattern is not specified in the standard. Four channels are packed into an Ancillary Data Packet and sent once per line (hence a total of 4 x 3 = 12 or 4 x 4 = 16 audio samples per packet per line).

See also: 1000/1001