[opendtv] Re: From Broadcast Engineering - WRAL tests mobile DTV
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sun, 10 Aug 2008 19:19:58 -0400
Tom Barry wrote:
> So it seems you could have an acceptable C/N ratio averaged
> over time but the bursty error rate in multi-path conditions
> would still cause dropouts as the error rate in any short
> period exceeds the ability to error correct within that same
> limited period.
>
> How well have C/N ratios actually predicted 8vsb observed
> performance under multi-path, assuming failures are observed
> as drop-outs?
It's certainly true that the SNR varies over time. The Accurian will
give you a very real demonstration of that. Although from what I've
noticed, at any given session, the variation when receiving a signal
solidly is within less than 1 dB. And this can be very, very close to
the 15 dB margin. So, perhaps it will bounce between 15.1 and 16 dB, or
so, and you'll get solid reception. (Until, that is, some weather front
creates a problem.) This is how I received the Annapolis station, when I
had the outdoor antenna. Almost always solid, but close to the margin.
>> The way MPH was being used, the 8T-VSB wide channel was 14.9
>> Mb/s (15.2 dB of C/N) and the narrow channel should be
>> something less than 2.2 Mb/s if 1/2 rate were used, at an
>> undisclosed C/N margin.
>
> And yet, adjusting to pretend all of the the narrow channel was
> using 1/2 rate, only 1.2 Mb/s was actually observed, just
> slightly over half your prediction. Though Mark A. has indeed
> said the numbers were flaky.
Except that no one claimed the MPH channel was using the entire capacity
at its disposal, right? These numbers aren't magic. However, I don't
know the details of MPH. It's possible that the extra training etc.
takes a bigger bite out of the M/H capacity than I'm assuming here. But
somehow I doubt this would take up half the capacity above what the FEC
already takes up. If you take 4.5 Mb/s out of the 8T-VSB stream, three
384 Kb/s streams should just about fit in a 1/4 rate channel, maybe
slightly less than 384 Kb/s to account for the training etc. Or twice
that many in a 1/2 rate channel.
> In any event, all this is why I am skeptical of numbers that
> look good on paper but haven't yet been shown in real world
> experience.
But we have seen reports of real world tests of A-VSB, in Las Vegas, and
now MPH now. And I've been doing my own long term test, with my antenna
sitting casually in the fireplace, at ground level. The result is that
even in bad weather, reception of local channels is good to excellent
and glitch-free, depening how far the transmiiters are and their ERP.
But reception of Baltimore, for instance, drops off completely, in bad
weather.
Bert
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