[opendtv] Re: 20040722 Thundering Thursday Thanks (Mark's Monday Memo)
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 23 Jul 2004 16:03:10 -0400
Eory Frank wrote:
> If you want to constructively add multiple signals, then the
> less directional antenna is superior. It may be that the
> "principal ray," if there is one, does not have enough SNR to
> meet threshold, so a narrow beamwidth antenna that selects
> only this ray does not result in reception. But an omni or
> much less directional antenna like a loop can collect energy
> from a multitude of different signals, that when
> constructively added, exceed the threshold SNR with margin to
> spare. These less directional antennas have the added
> advantage that they don't need to be re-aimed, or at least
> require much less re-aiming, to receive all available broadcasts.
Good points.
So, what you achieve this way is a much more reliable reception
of signals in which (S + I)/N is something like 14.9 dB. And
where S/(N + I), the number people usually quote, might start to
appear like its approaching or even violating the Shannon limit.
This by itself would be wonderful. Reliable reception just about
any time (S + I)/N is better than 15 dB, rather than only
occasionally.
The Shannon limit, which applies to (S + I)/N, 19.39 Mb/s, and a
5.38 MHz channel, is 10.48 dB. So the question now is whether it
makes sense to further develop receivers along the lines of my
last post, which would come closer to the Shannon limit but will
never quite reach 10.48 dB, or whether to go the way of E-VSB.
(Of course, "both" is also possible.)
E-VSB changes the Shannon limit by reducing the bit rate, and
also reduces the practical S/N margin required downwards. If we
are aiming to get beyond a practical 11 or 12 dB of (S + I)/N,
E-VSB is the only way to go.
Mark Schubin makes a good point about creating a more robust
audio signal. The way E-VSB has been defined now, this can be
done for a very low price. It would more effectively mimic
NTSC in a feature that, maybe unintentionally, was very
effective.
> This sounds like the familiar COFDM "trick" of using
> multipath to your advantage. Lynx demonstrated that it was
> also possible to do this with 8-VSB. Now perhaps LG is
> demonstrating this principle yet again. They are
> well-deserving of congratulations, not only for demonstrating
> that excellent 8-VSB reception is possible, but for actually
> building a real chip that can be used in actual consumer products.
Hear hear.
Bert
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