DIGITAL ERROR DETECTION OVERVIEW
In order to check if the digital video signal has
been received correctly a Cyclic Redundancy Check (CRC) can be made
on each frame in the generating equipment, this four digit number
is then placed in a 'packet' and put in the EAV-SAV space of one
line of each field. At the receiving equipment the incoming video
field also has a Cyclic Redundancy Check number calculated, this
value is then compared with the 4 digit number sent in the packet.
If the two numbers are not identical an error has occurred between
transmission and reception of the signal. 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.
