[opendtv] Re: Bolivia TV standard mystery

  • From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Sat, 23 Dec 2006 13:32:20 -0800

Craig wrote:

> >You could transmit all three channels (and others) synchronously from
> >each stick,

Bert wrote:

> No. And the reason is simple: the towers are too far apart to do this
> reliably.
>
> A true SFN depends on guaranteeing that by the time the signal of
> one tower , etc.............

Bert, that was an excellent analysis.

Dale

> -----Original Message-----
> From: opendtv-bounce@xxxxxxxxxxxxx
> [mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of Albert Manfredi
> Sent: Saturday, December 23, 2006 12:49 PM
> To: opendtv@xxxxxxxxxxxxx
> Subject: [opendtv] Re: Bolivia TV standard mystery
>
>
> Craig Birkmaier wrote:
>
> >Dale wrote:
> >>I may not have explained the Santa Barbara markets (121st
> >>ranking) transmitter site distribution clearly. The NBC affiliate
> >>is located in the extreme north end of the market and the
> >>ABC station is 100 miles to the south, with CBS being about
> >>in the middle. Santa Maria is centrally located and generally
> >>can receive all three, but it deteriorates rapidly as you move
> >>north, south, east or west due to intervening terrain.
> >
> >Sounds like most of the needed infrastucture is already in
> >place. In this case, it should be possible to use the three
> >sticks already in place.
>
> I agree with this part. Use the three sticks for transmitter
> antennas, and
> at best you can use the same frequency channels on the two most remote
> sticks, and a different set of frequencies in the centrally located stick.
>
> That would allow those bent on hype to claim "SFN," where in fact
> all you're
> doing is reusing frequencies in two towers that are far apart
> enough not to
> interefere with each other. At least, not to interfere where it
> matters to
> anyone.
>
> >You could transmit all three channels (and others) synchronously from
> >each stick,
>
> No. And the reason is simple: the towers are too far apart to do this
> reliably.
>
> A true SFN depends on guaranteeing that by the time the signal of
> one tower
> is lagging or leading the signals(s) from another tower(s) by too
> many usec
> for the given receivers, there will be a dominant signal strong enough to
> overpower the long lag or long lead signal(s). Let's say, a
> dominant signal
> that is 15 dB louder, or more, than any potential interering signals. In
> areas where the signal of multiple towers are close in strength,
> *all* have
> to be within the echo tolerance of the receivers.
>
> This is true for COFDM as for anything else.
>
> If towers that are spaced too far apart for this to be true, as
> these three
> are, then you can try to synchronize the towers to achieve that
> same effect.
> The problem is, you are designing this system based on some sort of
> predicted average propagation contour for each tower. You don't have any
> actual signal source where you can guarantee that a dominant signal will
> exist. You are hoping that the north tower's signal will have been
> attenuated so much before it will interfere with the central stick, for
> example.
>
> But this stuff is very variable. Weather, time of year, and terrain,
> conspire to screw up your fine-tuned calculations. So on a good day, you
> might succeed. On a rainy day, you might fail miserably. And
> worse, even on
> good days, terrain will make the fine-tuned calcualtions fail in
> some parts
> of the market area anyway.
>
> Bottom line: for SFNs, even with COFDM, the safe bet is to locate
> the towers
> of the SFN within a cluster in which no two towers are further apart than
> the GI allows. Check out real-world examples, in Paris and
> Berlin. In these
> cases, once you get out to the far field, say starting at 20
> miles from the
> SFN center, the SFN will behave very much like a single stick.
> Make it low
> and low powered, and you'll lose the fringe areas completely, just as you
> would with any small stick. And no, you can't use OCRs effectively at the
> fringes. Not unless terrain offers signal blocking where it's
> needed. Which
> is why, in those real-world examples, they make use of translators to
> provide signal to outlying areas.
>
> Bert
>
> _________________________________________________________________
> From photos to predictions, The MSN Entertainment Guide to Golden
> Globes has
> it all. http://tv.msn.com/tv/globes2007/
>
>
>
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