Sony MK7807V1 User Manual download pdf (Page 162)

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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

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Sony MK7807V1 User Manual Page 162