Digital audio that is multiplexed and carried within an SDI connection, so simplifying cabling and routing. The standard (ANSI/SMPTE 272M-1994) allows up to four groups each of four mono audio channels. Generally VTRs only support Group 1 but other equipment may use more, for example Quantel's Clipbox server connection to an edit seat uses groups 1-3 (12 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 genlocked to the associated video (8,008 samples per five frames in 525/60, 1,920 samples per frame in 625/50). 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).