Sony MK7807V1 - Manual (Page 162)

160
Technical References
EDH – Error Detection and Handling
The need for EDH
Video installations never get any simpler. In fact, the growth in viewer expectations of seeing what
they want when they want, has resulted in a huge, world wide expansion in systems devoted to
the switching and routing of video signals. Continuous quality monitoring of every signal path is,
in many cases, impractical. Signals may arrive at a switching center, be routed to a destination,
and leave the center without any check being made to ensure that the signal going out is as good
as the signal coming in. When these signals are serial digital video, their inherent resistance to
distortion will generally mean that they do pass through a system unchanged. However, a digital
signal can have deteriorated to the point where a minor equipment fault or a few extra meters of
cable can cause a catastrophic failure. EDH has been developed as a way of avoiding this type
of situation and is supported by SMPTE RP165 as an on-line quality checking system.
The concept of EDH
The concept of EDH is very simple. The data bits making up the each frame of a serial digital
video signal are 'counted' to generate a check sum. This check sum is inserted in the blanking
interval of the same video frame. At a later point along the path of the video signal, the check sum
for each individual video frame is recalculated and compared to the check sum carried in its
blanking period. If the sums do not agree, then data corruption has occurred somewhere
between these two points.
In a practical system this basic concept is expanded and refined to become a valuable tool in an
operational environment. The ultimate use of EDH in a system is that every item of video
equipment has the ability to carry out this checking process so that the precise point of data
corruption can be located.
EDH in practice
Once the check sums have been attached to each frame of a video signal, identifying the point
where data errors have been introduced is carried out by a three-part, frame-by-frame, checking
process:
• The check sum for the frame is recalculated.
• This check sum is compared to the check sum in the blanking interval. If they do not agree, a
warning flag is generated to warn of data corruption earlier in the signal path.
• The recalculated check sum is inserted into the blanking period, replacing the previous one so
that the checking process can continue.
This recalculating and comparing process can happen as often as required, the more frequently it
is done the greater the accuracy in determining where data corruption has occurred in the signal
path.
Technical References: EDH
*SGC_0145-0161 02.3.19 5:37 PM Page 160