[opendtv] Re: Analog v Digital TV
- From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sun, 14 Jan 2007 15:05:42 -0500
Tom Barry wrote:
I think we are just talking around each other here, with maybe no
disagreement. I said error correction never "provides" new information
but meaning that sufficient bits of extra information have to be sent
("provided") by the sender, not mathematically created somehow at
the receiving end. The sent extra information is the redundancy that
all schemes include so you can calculate the values of any missing bits.
Reed-Solomon is a good example, say a simple erasure code. There,
for each group of so many symbols you can add a few extra ones,
with carefully calculated redundancy. If you drop a few symbols, say
n, in delivery you can make up the complete group as long as you
have sent an equal number n of redundant symbols (not just
duplicates). But you still have to plan ahead to send a sufficient
number (at least n) of extra ones to match the max number the
receiver might miss.
Agreed, Tom. I was taking issue with your "no extra info is sent."
You send redundant info, so that even though MPEG encoding deleted much of
this redundancy info from the image itself, your FEC schemes put it back in.
Al Limberg mentioned this some time ago. As of now, receivers use the RS
code in regular 8T-VSB to fix up to 10 messed up bytes in any 188-byte data
block, using its 20 check bytes. But if receivers could use the results of
the Viterbi decode process to localize where in the MPEG-TS block the errors
occurred, instead of having the RS decode perform that localization
function, then you should be able to fix 20 messed up bytes.
And this sort of computational tweak should be able to be enhanced even
more, perhaps with multiple passes, to finally arrive at the corrected
block. Every time you improve this FEC system, you get closer to the Shannon
limit.
So even though we can receive "perfect" images today (for 19.39 Mb/s of
payload), with something around 15 dB of C/N, there's no reason to think
this is a hard limit.
Bert
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