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SERIAL DIGITAL BASICS
This type of error detection is known as Error Detection and Handling or EDH and is defined
by SMPTE RP165. In practice two check sums are sent per frame, one for the active video
period and one for the full frame. A typical packet consists of:
The Header:
(000, 3FF, 3FF) This always precedes an EDH packet.
Data ID:
(1F4)
Block Number:
(200)
Data Count:
This contains the number of words that follow.
Active picture crc:
3 words
Full-field crc
3 words
Error flags:
3 words
Reserved: 7
words
Check Sum:
This is used to test for transmission errors.
EMBEDDED AUDIO OVERVIEW
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,
e.g.: 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.
