[opendtv] Re: The "real" problem with OFDM in the U.S.
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 18 Mar 2005 19:17:07 -0500
Craig Birkmaier wrote:
> > I'll accept that, if it's the FCC mandating such a
> > policy [minimizing market overlap].
>
> As for the FCC policy, the problem is that it was
> created nearly a century ago, when the tools needed
> to properly control market-into-market emissions were
> either crude, or non-existent.
Perhaps so. But also note that were it not for our
system of affiliated stations, an upgrade of this
policy would be unnecessary. If the networks owned
their own network of OTA transmitters, they could
build the system to be as efficient as it can be.
Which means, *use* the overlap to improve coverage,
and don't waste your time and money on bewilderingly
complicated infrastructures.
Just as they have done in Europe forever.
> As for synchronization, I think you are mistaken.
> The issue is the relative levels of the signals
> from the mains versus the gap fillers. At the
> distance we are talking about, even very low
> power repeaters will swamp the distant signal.
No, because you are covering the entire area. You
still think in terms of "pockets." The signal from
the tall towers will drop off and the signal from
the small stick will emerge somewhere, and in that
contour of equal power density people WILL live.
One of OTA's assets is that it ain't cable. It's
available all over the market. Undoing that would
be a big mistake.
> You glossed over one of my main comments in an
> earlier message. The one where I stated that each
> market could have 20-30 channels. That was not 5
> channels with 6 sub channels.
Because first we have to make sensible assumptions,
and your huge SFN is not one, IMO. In other words,
it is you who are glossing over what's important
here. Not me.
> This is a matter of system design. If you want to
> simply extend an existing signal a repeater
> should work fine, and it DOES NOT need to be
> synchronized with the mains.
>
> If you want to localize the content then a
> different channel can be used.
I think it's something different, Craig.
If you want to extend the main signal, a translator
is *far* simpler to use. The gap filler is a cool
idea, but works best in, you know, *gaps*. If you
want ubiquitous coverage, you want to have the gap
filler's power swamped all around its pattern, and
only used in that hole. The problem using repeaters
at the fringes is that in order to have the signal
degrade fast enough so as not to exceed the GI or
equalizer echo limit of receivers, the repeater
will have to be *very* weak. Get it?
Take 1/16 GI with COFDM. The GI translates to 75
usec of echo tolerance. Which means that the
repeater's signal has to be way down by the time
your round trip delay to/from the repeater has
taken 75 usec. At the speed of light, that means
that 7 miles from the repeater, the signal has to
be gone, basically. Since the main signal is
already very weak out there, how weak does the
repeater need to be to ensure this constraint is
met? So this is not a great way to extend range.
Works fine for a small cluster of houses, or
individual farms, but that takes a *lot* of these
repeaters to create your sharp boundaries.
More powerful edge transitters will certainly
have to be synchronized with the main SFN, and
even then good luck at creating ubiquitous
coverage. You can't, in truth, but at least you
have better control over the dead zones. And, of
course, translators have no such worries.
The real issue here is that your idea of creating
sharp boundaries is *very* costly, if at the same
time you need ubiquitous coverage. And that's
exactly the situation we have.
That's why a big stick that covers the market area,
supported by gap fillers used as God intended, is
a really attractive compromise.
Bert
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