[opendtv] Re: Bob likes COFDM
- From: "John Willkie" <JohnWillkie@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sat, 19 Mar 2005 18:10:49 -0800
What kind fairy-tale planet do you live on? Congress doesn't require tests
of things that it purchases. Pray tell why they should have, let alone have
"controlled" tests to deal with Zenith/LG claims?
Who will control the tests in your world? Raven?
We live in a country that operates -- with some exceptions -- under "buyer
beware" with the market controlled by the unseen hand.
Just what market failure do you think Congress needs to attend to?
John Willkie
----- Original Message -----
From: "Tom Barry" <trbarry@xxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Saturday, March 19, 2005 4:28 PM
Subject: [opendtv] Re: Bob likes COFDM
> Manfredi, Albert E wrote:
>
> Until recently, the question
> > was whether such equalizers were just pipe dreams.
> > Now we know they exist, and we also know that in
> > time these equalizers can get even better. Because
> > we have Moore's law on our side.
>
> I am not entirely sure we have had an existence proof of such
> equalizers. We have had one reported demo which apparently so far has
> not been reproduced. All else is speculation so far.
>
> I realize that Bob & Mark both saw this demo but given what has gone
> before and since with ATSC I think I am again waiting to be convinced.
>
> If it is truly just a Moore's Law delay then ATSC will eventually be
> fine. If not then maybe it won't.
>
> Frankly I believe Congress should ask Zenith/LG to prove it in
> controlled tests since they claim to now be able to do so.
>
> - Tom
>
>
> > Bob Miller wrote:
> >
> >
> >>Was this formula developed with the characteristics
> >>of COFDM propagation and reflection in mind? The
> >>more buildings the better from our experience.
> >
> >
> > Interesting point. What you're saying is that
> > reflections can help, if they're strong. True. There
> > are cases where reflections will add constructively
> > to perhaps give you more localized signal than even
> > free space propagation would provide at that distance.
> > But of course, beyond where that hot zone is, the
> > signal will be very weak now. Weaker than free space
> > propagation. That's where buildings hurt.
> >
> >
> >>I like COFDM, it bounces around, doesn't need a lot
> >>of power but allows you to work with short and
> >>multiple broadcast antennas and not interfere with
> >>the next station over. When you get to the wide open
> >>spaces you can have taller sticks and add power.
> >>Very versatile.
> >
> >
> > Yes, but moving beyond evangelism, the same can be
> > said for any system that uses equalizers or rake
> > filters. That's what these devices do for their
> > respective modulation types. They just need to be
> > good enough to get the job done.
> >
> > Follow this. If you have a train of symbols
> > transmitted with no spacing between them, for
> > maximum spectral efficiency, what happens with
> > multipath distortion? What happens is that the
> > symbols now spread out over one another. Parts of
> > a given symbol are delayed more than other parts,
> > so you get so-called inter-symbol interference
> > (ISI). Also parts of the symbol are distorted to
> > be taller or shorter than the original, due to
> > constructive or destructive interference with the
> > symbols from multiple paths. Bad news. The symbols
> > can't be deciphered.
> >
> > COFDM cleverly addresses this problem by creating
> > very slow symbols, and sticking a gap between each
> > symbol. As long as the spreading out of symbols
> > does not exceed that pre-determined gap, you're
> > good to go. Also COFDM cleverly creates many tiny
> > subbands to transmit these slow symbols in
> > parallel, and transmits redundant data among them,
> > so that deep notches in the spectrum that clobber
> > a few of these carriers can just be ignored.
> >
> > What do equalizers do instead? Equalizers look at
> > the incoming symbol train and analyze how the
> > distortion happened to create this symbol overlap,
> > across the 6 MHz spectrum (in our TV example).
> > Then, with multiple hundreds of knobs, the equalizer
> > compensates for the delay that caused the ISI by
> > delaying other parts of the symbol, allowing the
> > delayed parts to catch up. This action also works to
> > restore the notches across the spectrum, because
> > after all, it was delayed components of the symbol
> > that created those notches. As long as nulls aren't
> > complete, the symbol should be restorable. Or at
> > least, a close enough facsimile to allow for demod.
> >
> > So what happens downstream of the equalizer? You
> > once again have a train of symbols with no spacing
> > between them. Good deal. You've not had to give
> > up spectral efficiency, and you've survived the
> > multipath distortion.
> >
> > As I've already described some time ago, when these
> > equalizers are applied to COFDM, e.g. as
> > STMicroelectronics has done in their latest COFDM
> > demod, the gap between symbols and the multiple
> > active carriers used in COFDM are no longer
> > necessary for successful signal demod.
> >
> > So what you like about COFDM is true about any RF
> > modulation scheme that adequately addresses
> > multipath distortion. Until recently, the question
> > was whether such equalizers were just pipe dreams.
> > Now we know they exist, and we also know that in
> > time these equalizers can get even better. Because
> > we have Moore's law on our side.
> >
> > Whatever it is today that keeps *good* 8-VSB
> > receivers from store shelves would equally keep
> > good COFDM receivers from shelves. You can always
> > screw up a good design. That's easy. And that's
> > what you experienced with the receiver you tested
> > recently.
> >
> > Bert
> >
> >
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