type. SDID The SDID is only valid if the DID is less than 0x80. The SDID is nominally an 8-bit value, ranging from 0 to 255. It is encoded in the same fashion as the DID. DID/SDID words of 161 101 (hex) correspond to a DID of 61 hex, and a SDID of 1 (once the two high bits are removed); these values would indicate that the packet type is defined by SMPTE 334M, and contains DTV closed captions data. DBN The DBN is only valid if the DID is 80 hex or greater. It is (optionally) used to identify multiple packets of the same type within a field; each subsequent packet of the indicated type has a DBN which is one higher than the previous packet, wrapping around as necessary. The DBN is an 8-bit value, encoded in the same fashion as the SDID. DC The Data Count word is an 8-bit value, encoded in the same fashion as the DID, which indicates how many user data words are to follow. It can range from 0 to 255. UDW User data words are the "payload" present in the ANC packet. They are defined according to the packet type, SMPTE 291M does not define their use or impose any restrictions on the values which may be present in the UDW space. The only restriction is that the reserved values in the serial digital interface (0-3 and 1020-1023) may not be included in the UDW. Many ANC formats, though not all, are essentially 8-bit formats, and encode data in the same manner that the header words are encoded. Example SMPTE 352M (Video Payload ID) defines four UDW: Checksum The last word in an ANC packet is the Checksum word. It is computed by computing the sum (modulo 512) of bits 0-8 (not bit 9), of all the other words in the ANC packet, excluding the packet start sequence. Bit 9 of the checksum word is then defined as the inverse of bit 8. Note that the checksum word does not contain a parity bit; instead, the parity bits of other words are included in the checksum calculations. Usage Embedded audio Embedded audio is audio payload which is (typically) the soundtrack (music, dialogue, and sound effects) for the video program. Two standards, SMPTE 272M (for SD) and SMPTE 299M (for HD and 3G) define how audio is embedded into the ancillary space.