[opendtv] Re: Analog v Digital TV
- From: "John Willkie" <johnwillkie@xxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sat, 13 Jan 2007 23:52:33 -0800
Bert is confused about the difference between data and information, but Tom
is not.
If chapter 2 is duplicated in a book, that book doesn't contain any more
information than a book with only one instance of chapter 2.
The first book does contain more data.
The additional data in error correction doesn't increase the amount of
information.
John Willkie
> -----Original Message-----
> From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx]
> On Behalf Of Albert Manfredi
> Sent: Saturday, January 13, 2007 11:46 PM
> To: opendtv@xxxxxxxxxxxxx
> Subject: [opendtv] Re: Analog v Digital TV
>
> 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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