switch points (set RP168 figure 2), the first allowed lines are 12 and 275 (for 525-line/59.94 Hz systems) or 8 and 321 (for 625-line/50 Hz systems). This conflicts with SMPTE 125M, and does not address requirements for carrying DVITC (Digital Vertical Interval TimeCode) and video index packets. (SMPTE 125M section 3.6.2): VANC should appear only in lines 1-13, 15-19, 264-276, and 278-282, with lines 14 and 277 reserved for DVITC and video index data. This conflicts with SMPTE 334M, and does not address 625-line/50 Hz systems. Packet format All ANC packets must start with a start sequence; for component interfaces (the only kind of serial digital interface in widespread use today), the start sequence is 0x000 0x3FF 0x3FF. This sequence is otherwise illegal in the serial digital interface. (In the obsolete composite versions of SDI, the ANC start sequence is a single word, 0x3FC). Three words immediately follow the start sequence in the header. The first word after the start sequence is the Data Identifier or DID, followed by either a 'Secondary Data Identifier (SDID) or a Data Block Number (DBN), followed by a Data Count (DC). After the Data Count word are 0 - 255 (inclusive) User Data Words (UDW), followed by a Checksum (CS) word. DID The Data Identifier word (along with the SDID, if used), indicates the type of ancillary data that the packet corresponds to. Data identifiers range from 1 to 255 (FF hex), with 0 being reserved. As the serial digital interface is a 10-bit format, the DID word is encoded as follows: Bits 0-7 (bit 0 being the LSB), are the raw DID value. Bit 8 is the even parity bit of bits 0-7. Bit 9 is the inverse of bit 8. Thus, a DID of 0x61 (01100001) would be encoded as 0x161 (0101100001), whereas a DID of 0x63 (01100011) would be encoded as 0x263 (1001100011). Note that this encoding scheme ensures that the reserved values in the serial digital interface (0-3 and 1020-1023) are never used. If the DID is equal to 128 (0x80) or greater, then the packet is a Type 1 packet, and the DID is sufficient to identify the packet type, and the following word is a Data Block Number. If the DID is less than 128, it is a Type 2 packet, and the following words is the Secondary Data Identifier; the DID and SDID together identify the packet