Image Processing Reference
calculation adds all the bytes together, throws away any carried-over bits, and then gen-
erates a value which, when added to the sum, yields a known constant value. This is very
like the parity protection at the byte level. The technique of cyclic redundancy checks
(CRC) is simple but has limited error-correction capabilities. It tells you that the packet is
corrupted and if you have other countermeasures, they can be exercised only when nec-
essary since they are too computationally intensive to apply to every packet.
Forward Error Correction
Forward error correction (FEC) involves a technique called Reed-Solomon that adds
enough data to the packet that one can verify the correctness and also repair some
errors. The number of errors that can be repaired depends on the amount of
Reed-Solomon data that is added and the requirement for that depends on the trans-
mission system. In a very noisy transport, adding more Reed-Solomon code may
improve the reliability but it may also reduce the throughput since more redundancy
has to be coded. That additional overhead does not convey any extra information; it just
secures the existing data against potential corruptions. Figure 15-2 shows how the data
is wrapped like an onionskin.
The error correction should be provided by the transport mechanism and this is not some-
thing you should have to concern yourself about when compressing video. You should
expect the output of the transport process to deliver substantially what you put in at the
head-end and any error-correction processes will be transparent as far as you are con-
Handling Signal Outages
During a live broadcast, the incoming signal may be interrupted momentarily. This also
happens when transferring files around your system but the copying process deals with
minor network glitches and any missing data is requested again. This is not possible with
a live feed.
So the number-one priority is to guarantee that your incoming video arrives unin-
terrupted. One possibility is to interpose some equipment that buffers the signals. This
buffer might also detect a loss of signal and substitute another recently delivered frame or
fragment of an image. A simple dual-ported frame store with the input being fed from the
live source and the output going to your compression system would do a lot to improve
things during lossy transmissions. The movement might be a little jerky but at least you
would have a whole picture to display.
The incoming video may be delivered from a live outside broadcast installation. That
is likely to provide a very reliable feed but it might be doing this across a landline provided
by a telecom provider or via a microwave link. These links suffer periodic interruptions.