DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

SMPTE 260M describes a scheme,
now deprecated, to convey line
numbers in ANC packets where
DID=0. That value is now “undefined.”
Two types of ANC packet are differentiated by bit 7 of
the DID word. If DID 7 is asserted, the packet is Type 1;
DID is followed by data block number (DBN). There are
128 DID codes available for Type 1 packets. The DBN
value indicates continuity: If zero, it is inactive; otherwise,
it counts packets within each DID from 1 through
255, modulo 255.
If DID 7 is negated, the packet is Type 2: The DID is
followed by a secondary data ID (SDID), giving
127 · 255 (i.e., 32385) ID codes for Type 2 packets.
Three DID values, 004 h , 008 h , and 00C h indicate
a Type 2 ANC packet coded with 8-bit data; other DID
values in the range 001 h through 00F h are prohibited.
DID 80 h marks a packet for deletion. DID 84 h marks
the last ANC packet in a VANC or HANC region.
The data count (DC) word contains a value from 0
through 255 (protected by two parity bits), indicating
the count of words in the user data area. The DC word
spans all ten bits of the interface. Even if an 8-bit DID is
indicated, SMPTE standards imply that the two least
significant bits of the DC word are meaningful. (If they
were not, then the count of user data words could not
be uniquely determined.)
The checksum (CS) word provides integrity checking for
the contents of an ancillary packet. In every word from
DID through the last word of UDW, the MSB is masked
out (to zero); these values are summed modulo 512.
The 9-bit sum is transmitted in bits 8 through 0 of CS;
bit 9 is coded as the complement of bit 8.
SDI coding
In the obsolete Rec. 656 parallel interface that
I mentioned on page 128, a dedicated clock accompanied
the data. In the serial interface it is necessary for
a receiver to recover the clock from the coded
bitstream. The coded bitstream must therefore contain
significant power at the coded bit rate. Coaxial cable
attenuates high-frequency information; equalization is
necessary to overcome this loss. Because equalizers
involve high-frequency AC circuits, the coded bitstream
396 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES