[opendtv] Re: Twang's Tuesday Tribune (Mark'sMondayMemo)2004April20
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 23 Apr 2004 13:00:09 -0400
> >In the absence of test data, I use ETSI EN 300 744 and
> >compare the theoretical C/N requirements and echo tolerance
> >with those measured with the 8-VSB receiver being tested.
> >=20
> But what do you base the 8-VSB C/N requirement on? In the absence of
> echoes, it's 15.5 dB. In the presence of echoes, it can be higher or
> lower. Even the 5th-generation Zenith needs 24 dB for the Brazil E
> ensemble.
The test results I use to base any trend on have to include a
list of measured C/N margins. And while ETSI EN 300 744 does not
go into specific Brazil profiles, unfortunately, it does specify
margins needed for Gaussian, Ricean, and Rayleigh channels.
Brazil E happens to be a particularly difficult case, because there
are three 0 dB paths to the receiver, each spaced 1 usec apart. I
don't know how COFDM behaves with that exact profile, but I do know
that in a generic Rayleigh channel, at 64-QAM, 3/4 FEC, 1/16 GI,
COFDM requires 21.7 dB of C/N.
So again, in terms of a trend, that's the right direction to go.
Not long ago, 8-VSB would fail miserably with Brazil E.
Also, it's interesting to note, in the April 2002 CRC report, that
the Linx "gen 4" receiver needs 22.8 dB of C/N for Brazil E, or
24.8 dB with reacquisition. And that in all other Brazil profiles,
that test showed C/N margins required to be between 12.5 and 19.4
dB. Not half bad.
Also, the Linx tests in Chicago, for example, included the C/N
margins achieved at every test site. When there was no direct path
to the transmitter, they seemed to manage solid reception at 19 dB
of C/N.
So without being able to say anything directly, we *can* conclude
that 8-VSB receivers are heading in the right direction, and are
starting to play in the same ballpark as COFDM receivers. Are they
worse, equal, or better in some specific cases? Don't know.
Bert
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