[opendtv] Re: SFN considerations (was Doug is Missing the Point)
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 21 Feb 2006 16:56:36 -0500
Doug McDonald wrote:
>
> Richard Hollandsworth wrote:
[ ... ]
>> You might think that you could detect when the excessive
>> Pre-Echo came out of a fade and very carefully right
>> shift everything in the equalizer....but that would
>> require an special correlation detector, because the
>> equalizer would only know that things are messed up and
>> not know exactly what to do about it.
>
> This can be done. Using the training signal it becomes
> trivial. I have actually implemented this is simulations
> and it works fine, no problem. The correlation is very
> easy using hardware-implemented FFTs, and they provide
> **excellent** diagnostics about what the sate of the
> multipath is. All this requires is a designed who is
> ready to build something other than what he learned in
> school.
But it seems to me that this approach would be somewhat
compromised for dynamic echo, because now the relatively
long interval between the PN511 sequences becomes a factor.
Whereas if the pre-echo is suppressed (actually moved) by
a matched FIR filter, the entire input signal gets shifted
to the right, linear in phase, strong pulses coming first,
and then the equalizer doesn't need to figure out what to
lock onto.
And the training would be done much more frequently, using
your flat frequency spectrum idea.
Maybe you need both, to manage cases where the matched
filter isn't up to the job (not enough taps), but the
pre-echo is mostly static? Such as you might see in an
aggressively designed SFN?
Bert
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