[opendtv] Re: Bob likes COFDM

  • From: Bob Miller <bob@xxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sat, 19 Mar 2005 20:55:20 -0500

Manfredi, Albert E wrote:

>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
>  
>
Interesting points Bert but you know what, I don't want to have to deal 
with all the problems of 8-VSB. I just want a receiver that works and 
after seven years of promises I think I and the US public deserves one. 
We have been looking for a receiver for six years now that would meet 
even minimal standards to just get decent reception to a fixed location. 
Why are we still dealing with 8-VSB? Where are those who should be 
looking out for the US publics interest in DTV? Why are they not 
demanding that 8-VSB get fixed or that we switch modulations? I don't 
even hear a whisper. Congress does not even visit the subject of poor 
receivers.

The first 8-VSB receiver we tried didn't get the job done in 1999 and 
the last receiver we tried didn't get the job done in 2005. In between 
we tested one hopped up LG 5th gen receiver that they obviously do not 
want to make for cost considerations. I remember Frank Eory saying years 
ago that 8-VSB would climb a cost curve up while COFDM would follow one 
down because of multipath.

The first COFDM we tried was well past our wildest expectations and I 
can still remember the joy I felt when we first tried it. The last COFDM 
receiver we tried was much much better they tell me but I can't tell the 
difference. They just work and now they are very inexpensive so that 
more money can go for the PVR functionality. I can call any of a hundred 
companies that will make us good COFDM receivers. But instead  find 
myself on the phone with Korean manufacturers who are questioning that 
there is even a market for 8-VSB receivers. I have to convince them that 
anyone will even want these receivers before they will even discuss 
making them.

Everything is very interesting but we are not in some  debating society 
where we have the luxury of comparing all possibilities. The only reason 
Morse Code and AM are not in this debate is because no one with a big 
enough purse was proposing that they be used instead of 8-VSB.

Bob Miller



 
 
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