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.