[opendtv] Re: Analog v Digital TV
- From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sun, 14 Jan 2007 02:45:40 -0500
Tom Barry wrote:
Error correction never really provides any extra information. It just
averages errors over a longer period making them more predictable
and able to be handled. But it always requires enough redundancy
to make up for the amount of errors over that long period.
Depends what kind of error correction you're talking about.
I'm talking about real error correction schemes here, like the convolutional
Viterbi or turbo codes, and the block codes like RS. They do provide more
information. They provide redundant information in the bit stream to allow
missed blocks of bytes to be reconstructed completely. Within certain
limits.
Beyond that limit, then you fall back to error *concealment* schemes, which
sounds like what you're referring to.
Nothing is free.
Indeed. So, for example, the Viterbi codes used in ATSC assign 1/3 of the
bit rate to redundancy. The Reed Solomon block code assigns almost 10
percent of the bit rate to redundancy.
What makes these schemes more effective than the accidental redundancy
available in NTSC is that they are specifically designed for the purpose.
So, while in NTSC you can still see an image when there's snow and ghost,
just because of the extra image info that isn't really essential, you still
do see the errors. Instead with DTV, within the range of effectiveness, the
errors are completely corrected, and then less perfectly concealed beyond
that range, and then all quits.
The Shannon limit for 19.39 Mb/s in a 5.38 MHz channel is 10.47 dB of SNR.
As the erorr correction is accomplished in ATSC receivers today, about 15 dB
of SNR is needed. So the question is, what does an NTSC quality image look
like if the signal is received with only 10.47 or 15 dB of SNR? And
remember, that ain't HDTV. A more fair comparison would be 4.5 Mb/s SDTV,
against NTSC, and then the SNR needed for DTV is under 2 dB (A-VSB,
diversity antenna). How would that look for NTSC??
Bert
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