Posted by Martin Heffels on 07/31/06 12:00

Here's another explanation about digital error-correction from the "Quantel
Digital Fact Book", 11th edition (Crowley and I are not crazy ;-) ).
Still no figures, but I gave up looking on them. Maybe I will find them
when I'm looking for something compltely different (as usual) :-)



<quote>
Error detection, concealment and correction

No means of digital recording is perfect. Both magnetic tape and disks
suffer from a few marginal areas where recording and replay is difficult or
even impossible. However the errors can be detected and some remedial
action taken by concealment or correction.
The former attempts to hide the problem by making it less noticeable
whereas the latter actually corrects the error so that perfect data is
output.

When the recorded data is an image, an error can simply be concealed by
using data from previous or following TV lines, fields or frames. The
result is not guaranteed to be identical to the original but the process is
relatively simple and, as important, quick.
If the stored information is from a database, a computer program or from
special image processing, then 100% accuracy of data is essential. This can
be ensured by recording data in a manner where any errors can be detected
and the correct data calculated from other information recorded for this
purpose. This is error correction.

A difference between computer systems and TV is that the latter is
continuous and cannot wait for a late correction. Either the correct result
must be ready in time or some other action taken – the show must go on –
placing a very tight time constraint on any TV-rate error correction. In
contrast, a computer can usually afford to wait a few milliseconds.

Digital VTRs monitor the error rate and provide warnings of excessive
errors, which although not immediately visible, may build up during
multiple passes.

Although error rates from disks are generally many times lower than those
expected from digital videotape, they can still occur. To protect against
this there is data redundancy and the replay of all data is checked. If an
error is detected there is sufficient additional information stored to
calculate and substitute the correct data.
The total failure of a disk drive can be covered and the missing data
re-generated and recorded onto a new replacement – making the system highly
accurate and very secure.
</quote>
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