[opendtv] Re: Let the games begin

  • From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Thu, 09 Nov 2006 12:09:15 -0500

And it's interesting that those rates are comparable to what's being done in NTSC while also transmitting analog television.


TTFN,
Mark


Manfredi, Albert E wrote:
Tom Barry wrote:

That is, at a reasonable, typical, and equivalent (very bursty,
non-random) error rate, how many bits per second can be delivered
in how much channel bandwidth by the various methods?

They are comparable, Tom. You can check for yourself, for example by
comparing the A-VSB numbers in
http://www.iptv.org/dtv/2006/media_pres.cfm?ses=5&id=7&Track=T

(the presentation Craig pointed out yesterday) with the COFDM numbers in

http://www.ttv.com.tw/TVaas/file/En300744.V1.4.1.pdf

You should do this, really, otherwise you seem unable to be convinced.

The short version is this:

The most robust for of COFDM is QPSK, non-HM, which provides 3.7 to 4.5
Mb/s in a 6 MHz channel. (The range is caused by varying the GI.) This
is 0.617 b/s/Hz to 0.75 b/s/Hz.

The C/N margin needed for this varies from 3.1 to 5.4 dB (single
antenna).

The most robust form of A-VSB is 1/4 rate, which provides 4.85 Mb/s in a
6 MHz channel (actually 5.38 MHz channel). This is 0.808 b/s/Hz.

The C/N margin needed for this is 4.5 dB C/N (single antenna) or 1.7 dB
C/N (diversity antenna).

Summary: in the most robust mode, COFDM provides slightly less spectral
efficiency at potentially slightly higher robustness.

The Shannon limit for 4.85 Mb/s in a 5.38 MHz channel is -0.615 dB of
SNR. Which means that with diversity antenna, A-VSB is about 2 dB away
from theoretical best. Which means, you won't get a whole lot better. I
have no COFDM numbers to compare, but as I said, you won't get a whole
lot better no matter what.

Furthermore, if A-VSB is used in an HM-sort of situation, i.e. some
A-VSB and some regular 8-VSB, then the A-VSB part of the signal is just
as robust as it was in the non-HM case. This is not quite true in COFDM,
though. The difference can be small in some modes, but in general, the
robust channel becomes somewhat less robust when 16-QAM or 64-QAM is
transmitted along with QPSK.

So, please check the numbers out. They are comparable.

Bert
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