[opendtv] Re: 20040722 Thundering Thursday Thanks (Mark's Monday Memo)

John Shutt wrote:

> You keep referring to this, but it is my understanding that
> the reason why
> most chip manufacturers ignored the training sequence and
> went with blind
> adaptation was the fact that the training signals were too few and far
> between to use in tracking any sort of dynamic multipath.

Doug McDonald now thinks otherwise, with respect to dynamic
multipath. From my recollection of his most recent post on the
subject.

In any event, if you look at difficult ensembles like Brazil E,
you will see that C/N margins with reacquisition are much higher
than C/N margins without. Something like 22 dB without, 25 with,
for the Lynx receiver. And this LG receiver also came in at 25
dB, for Brazil E. So what we're after here is not just dynamic
echo performance, but also symbol sync performance with static
echo.

If, for example, you have good lock on a Brazil E signal, and
then an airplane goes by, you now need about *twice* the signal
strength of a marginally receiveable signal to reacquire lock.
It would obviously be much better if you could reacquire lock
with no more signal strength than what you need to maintain
lock. Makes for a more solid reception experience.

> If I recall correctly, it was the blind equalization
> techniques that NxtWave
> and Motorola were referring to when they wrote their Summer 1999 press
> releases that claimed to "crack the code" of 8-VSB.

Yes for NxtWave. I don't remember what Motorola said. But
just because they took the blind equalization path as a
first cut at improving receivers certainly can't be
interpreted to mean they thought that was the best one
could do.

> I also seem to recall that Dr. McDonald also was of the
> opinion that if
> there were more training signals, then 8-VSB would have been
> much easier to
> equalize for dynamic multipath, but blind adaptation
> techniques are required with the current training signals.

Could be. Doug might have changed his mind on that, though.
He can speak up and take away the suspence. His last comment
was that using the existing sequence would go a long way, as
I recall. He also prefers a different PN sequence than the one
used now, but it takes up some extra bit rate because it's
twice or three times longer. Besides, it's incompatible with
existing receivers.

Bert
 
 
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