[opendtv] Re: Charles Rhodes on SFNs
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 26 Jun 2010 14:21:37 -0400
At 2:20 PM -0500 6/25/10, Manfredi, Albert E wrote:
Craig Birkmaier wrote:
1. Unless you deploy a Boston-to-Richmond regionwide SFN, you still
cannot reuse the same frequencies in these different markets. The
case today is that the same channels ARE used, in every other market
up the coast, using the intevening market as the interference zone.
Not exactly. Many frequencies - especially VHF channels - need much
wider protection, as in hundreds of miles.
Sorry Craig.
Things have changed some with the digital transition, because a lot
of VHF analog stations went and stayed in UHF. But when VHF was used
extensively, Richmond and Phildelphia shared VHF channels, Wash/Balt
and NYC did as well.
Richmond to Philadelphia = 202 miles
Washinton to New York City = 211 miles
But none of this is relevant to the current discussion.
But now, after the transition ended, your point is even more off base.
Channel 42 is used in Richmond and in Annapolis MD.
Distance is 113 miles - I can check, but strongly suspect that both
use emission masks to prevent interference.
Channel 7 is used in DC, and a low power Channel 7 transmitter also
in Phildelphia.
Irrelevant.
Channel 13 is used in Baltimore, and a low power Channel 13 in Philadelphia.
Equally irrelevant.
Channels 30, 32, and 42 are now used in DC/Annapolis and in Philadelphia.
Channel 39 is used in Richmond and in DC.
The above examples show that these UHF channels are satisfied to not
have continuous coverage between markets. For example, it would be
difficult for someone in Fredericksburg VA to receive Ch 39 from DC.
Maybe with a good directional antenna it's possible.
You won't do any better than that with single-market SFNs, Craig.
Not sure how you can conclude this. First, Richmond to DC is about
100 miles and emission masks are used to prevent interference (not
to mention that UHF does not cover as far as VHF).
The distances between antenna sites for SFNs that use the same
channel for different content can be significantly closer than 100
miles based on power levels and emission masks.
> With SFNs the potential for interference is much more limited and YES,
you can checkerboard frequencies from market to market, which means
that every market can have about half of the available channels, as I
have noted for years.
What I have been trying to get across is that this *IS* how the
frequencies are assigned now.
I will concede that with very dense SFNs, the interference zones
would be reduced. For instance, Channel 39 from DC would be easier
to receive in Fredericksburg if the demarkation line were made a lot
tighter. But that requires the kind of dense mesh of low towers that
Doug Lung mentioned, and by the way you seemed to agree with his
assessment when I pointed it out. None of this "3 or 4 medium
powered towers around the beltway" approach.
The European SFNs DO NOT employ such dense mesh topologies.
Nor do we need to. With the right modulation most markets can have
half the channels with less 5-10 transmitter sites.
Regards
Craig
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