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 > > > > > > ---------------------------------------------------------------------- > > You can UNSUBSCRIBE from the OpenDTV list in two ways: > > > > - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org > > > > - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line. > > > > > > > ---------------------------------------------------------------------- > You can UNSUBSCRIBE from the OpenDTV list in two ways: > > - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org > > - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line. > ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.