Audio Engineering

Video Synchronization 835

either end and a small data overhead to “ tell ” the receiver whether it should expect 3 or 4
samples on any given line.

Figure 28.14 illustrates the structure of each digital audio packet as it appears on preferably
all, or nearly all, the lines of the fi eld. The packet starts immediately after the TRS word
for EAV (end of active line) with the ancillary data header 000,3FF,3FF. This is followed
by a unique ancillary data ID, which defi nes which audio group is being transmitted. This
is followed with a data-block number byte. This is a free-running counter counting from
1 to 255 on the lowest 8 bits. If this is set to zero, a deembedder is to assume that this
option is not active. The 9th bit is even parity for b7 to b0 and the 10th is the inverse of
the 9th. It is by means of this data-block number word that a vertical interval switch could
be discovered and concealed. The next word is a data count, which indicates to a receiver
the number of audio data words to follow. Audio subframes then follow as adjacent sets of
three contiguous words. The format in which each AES subframe is encoded is illustrated
in Figure 28.15. Each audio data packet terminates in a checksum word.

Group n

AES 1, left

Group n

AES 1, right

Data

header

2FFh: group 1

1FDh: group 2

1FBh: group 3

2F9h: group 4

Indicates

the number of

data words

to follow

Bit 7MSB–BITOLSB

Bit 8 is even parity

Bit 9 is Bit 8

Group n

AES 2,

left

Checksum

Group n

AES 2, right

0

0

(^0) n
3
F
Fn
3
F
Fn Data IDData blocknumberData count
C.S.
Figure 28.14 : Data format for digital audio packets in SDV bit stream.