[opendtv] Re: The "real" problem with OFDM in the U.S.

  • From: Tom Barry <trbarry@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sun, 06 Mar 2005 16:04:33 -0500

John Willkie wrote:
 > Rember, in this context, that 8-VSB is supposed to be the equivalent 
of NTSC, not the better of it.

Yes, I guess that is true but it's such a bad decision I wish we 
wouldn't perpetuate it.

 > Upshot:  for all it's superiority, permitting COFDM would have 
provided a superior signal level to that of NTSC, and -- aside from 
narrow technical modulation issues -- would have unwound the carelfully 
(politically) negotiated replacement of NTSC with an equivalent service 
level from 8-VSB.  To permit COFDM, broadcasters would have had to pay a 
heavy price (or prices).

Sorry, you completely lost me on that one.  What heavy price are you 
talking about?

- Tom
> Several weeks back, mono-note Dermot, while continuing his consistent praise 
> of OFDM modulation over VSB, spilled the beans on why OFDM was inappropriate 
> as a replacement for NTSC in the U.S.
> I don't know if my friend Dermot fully realized the upshot of his comments, 
> and I suspect that only a few list members caught the import of his comments. 
>  It took me a week or two of pondering to realize the implications.
> 
> What the heck am I talking about?  Well, we were discussing probablity eg, 
> f(50,50) of reception with various schemes.  Rember, in this context, that 
> 8-VSB is supposed to be the equivalent of NTSC, not the better of it.
> 
> Dermot bragged how goold fashioned COFDM routinely offered F(50,90) reception 
> probability over entire coverage areas, and with -H, it was F(50,99) (or was 
> it F(99,99)?
> 
> These figures sound great.  For the uninitiated, that means that at fifty 
> percent of the reception locations, at least 90% of the time, a sufficient 
> signal level was received.  (Analog also imposes a grade of service figure 
> before these reception levels are relevant, but I'll table that here.)
> 
> Unfortunately, as any any good and many bad engineers know, NTSC reception 
> specifies a contour signal level and probability level.  Unfortunate because 
> the best (at the city-grade contour) that NTSC has to deliver is F(50,80).  
> At the grade A and grade B points, the probability is F(50,50).  And, the 
> inferference contour is F(50,10), wich a desired to undesired dB figure 
> specified.
> 
> Upshot:  for all it's superiority, permitting COFDM would have provided a 
> superior signal level to that of NTSC, and -- aside from narrow technical 
> modulation issues -- would have unwound the carelfully (politically) 
> negotiated replacement of NTSC with an equivalent service level from 8-VSB.  
> To permit COFDM, broadcasters would have had to pay a heavy price (or prices).
> 
> At least, back then.  I don't recall anybody offering to pay the government 
> money to permit COFDM operations.  Might make sense now, if it's so superior.
> 
> John ("i'm glad to see Dermot supporting my 'political' position on COFDM in 
> the U.S. with empirical data) Willkie
> 
> P.S.:  I saw a posting requesting information about the origins of 30-foot 
> reception criteria.  I'm suprised M. Schubin didn't respond; the 30-foot 
> rule, as I've pointed out here several times) comes from the FCC's R-6602 
> (Longley-Rice) study, which, if memory serves me correctly, was first 
> published in 1946 or so.
> 
> P.P.S.  I've also seen postings about Ibiquity/IBOC.  Sounds great, an 
> engineer of long tenure in the industry told me, except for it to work, 
> Shannon's law has to be repealed, and he pointed out, the folks at Ibiquity 
> are "real pearls" to work with.
> 
> 
> 
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>  
>  
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