[opendtv] Re: Bob likes COFDM

  • From: Tom Barry <trbarry@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sat, 19 Mar 2005 22:19:58 -0500

John Willkie wrote:

 > 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?
 >
...
 > Just what market failure do you think Congress needs to attend to?

Congress is occasionally making noises about shutting off analog TV and 
selling the resulting spectrum out from under the American public.  It 
would seem that to do that and keep their jobs they should ensure there 
is a sufficient replacement available.

And just normal CYA common sense dictates they should verify there exist 
functional replacement receivers on this new 'fairy-tale' digital TV 
planet or else they might end up looking somewhat foolish.

Someone is sure to ask that question later.  I am just helping by asking 
it now, before further embarrassment.  Congress could in turn ask it of 
Zenith/LG if they chose.  If LG was standing there waving a small box 
they say could be made economically it would seem very sensible for 
someone in Congress to ask if those boxes actually work, and can 
demonstrably be shown to work NOW, this year.  If not, why not?

So again I ask, is indoor reception a criteria for analog turnoff? 
Inquiring voters will want to know.

- Tom


> 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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