834 Chapter 28
28.9 Embedded Digital Audio in the Digital Video Interface .......................................
So far, we have considered the interfacing of digital audio and video separately.
Manifestly, there exist many good operational reasons to combine a television picture
with its accompanying sound “ down the same wire. ” The standard that specifi es the
embedding of digital audio data, auxiliary data, and associated control information into
the ancillary data space of the serial digital interconnect conforming to SMPTE 259M in
this manner is the proposed SMPTE 272M standard.
The video standard has adequate “ space ” for the mapping of a minimum of 1 stereo
digital audio signal (or two mono channels) to a maximum of 8 pairs of stereo digital
audio signals (or 16 mono channels). The 16 channels are divided into 4 audio signals
in 4 “ groups. ” The standard provides for 10 levels of operation (suffi xed A to J),
which allow for various different and extended operations over and above the default
synchronous 48-kHz/20-bit standard. The audio may appear in any and/or all the line
blanking periods and should be distributed evenly throughout the fi eld. Consider the case
of one 48-kHz audio signal multiplexed into a 625/50 digital video signal. The number of
samples to be transmitted every line is:
()(),48000 15625/which is equivalent to 3.072 samples per line. The sensible approach is taken within the
standard of transmitting 3 samples per line most of the time and transmitting 4 samples
per line occasionally in order to create this noninteger average data rate. In the case
of 625/50, this leads to 1920 samples per complete frame. (Obviously a comparable
calculation can be made for other sampling and frame rates.) All that is required to
achieve this “ packeting ” of audio within each video line is a small amount of buffering
Figure 28.13 : Gennum Corp. serializer chip.
(Photos courtesy of Gennum Corporation.)