S/PDIF

S/PDIF is the domestic cousin of the studio standard AES3 (AES/EBU): the same frame format, but 75-ohm coax with RCA plugs instead of balanced XLR, or optical over TOSLINK. The cable carries two channels of PCM, or compressed surround such as Dolby Digital and DTS. Tucked into the status bits is SCMS, the record industry's copy protection that allowed one digital copy but not a copy of the copy.

The clock travels inside the data stream itself via biphase-mark coding, and the receiver recovers it from the transitions. Elegant, but it makes the link sensitive to jitter: a worn cable or a poor receiver yields a wobbly clock even when every bit arrives intact. Studios therefore prefer a separate word clock. HDMI has taken over the living room, but the optical output is still there on plenty of TVs.