The period between the EAV and SAV markers can
be used to send embedded digital audio signals. This is known
as SMPTE 272M. Up to 16 separate audio signals may be sent in
a single video channel. These are organised as four GROUPS of
four signals, the four signals are often two stereo pairs.
Typically only one group will be used, giving two stereo pairs
of audio. The audio data is digitised in the sending equipment
to 20 bits of resolution, usually at a 48 KHz sample rate.
Often only 16 bits are used in practice. The digitised data is
arranged in packets which are placed in the EAV-SAV space.
A typical packet consists of:
The Header: (000, 3FF, 3FF) This always
precedes an audio packet.
Data ID: This contains the Audio Group number.
Block Number: AES blocks have 192 'frames' of
audio data
Data Count: This contains the number of words
that follow.
Audio Sample:
Audio Sample:
Audio Sample:
Audio Sample:
Check Sum: This is used to test for
transmission errors.
Each audio sample consists of a sample of all
four audio signals,
eg: Channel 1 left, Channel 1 right, Channel 2
left, Channel 2 right.
Each signal requires 3 words to hold all 20
bits data, thus each audio sample has 12 words in it.
Typically 3 or 4 audio samples are sent in each EAV-SAV
period. As with the video signal, words which consist of all
'1's or all '0's are not allowed.