The SD-SDI transmitter calculates two CRC values for each video field—one corresponding to the active picture, and corresponding to the entire field (excluding the switching lines)--and places them in an EDH ancillary data packet.
This feature makes it easier to determine which link, in a multi-link chain, was the source of the error.
EDH is used primarily to assist in identifying faulty equipment in a video chain so that it can be quickly replaced or repaired.
EDH is not used with high definition video, as the HD serial digital interface includes a mandatory embedded CRC for each line.
The SD-SDI EDH protocol is defined by SMPTE RP 165-1994 and the equivalent ITU standard ITU-R BT.1304 .