[opendtv] Re: Doug is Missing the Point
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 13 Feb 2006 17:23:54 -0500
Stephen Long wrote:
> With COFDM you can implement Single Frequency Networks,
> using much lower power transmitters in locations much
> closer to the actual consumers/viewers.
True, but there are practical limits, such as cost, that
result in very few actual towers being installed. The most
I've seen is in the Paris SFN: the Eiffel tower and three
smaller, low power sticks. Coverage still often requires
outdoor antennas, due to the low power of the sticks.
(And in practice, only Channel 35 is operated as a single
SFN. For the other 4 multiplexes, the Eiffel Tower uses
differemnt freqs from the small sticks.)
As Doug says, such low power repeaters can also be used
with 8-VSB, as shown in Ottawa, but you need decent
receivers which accept pre-echo. They exist. Just not on
store shelves. Why not on store shelves? Who knows.
Maybe it's the same people that kept COFDM away in 2001?
> 8VSB requires big stick mentality,
Not necessarily. But even Paris and Berlin, with their
SFNs, adopt what the French TNT calls "umbrella"
configuration. One big stick, assisted by smaller sticks.
This sort of arrangement could be used with 8-VSB as well.
http://www.csa.fr/pdf/Rapport-GT2-Aspects_Radiofrequence_de_la_TNT.pdf
Page 19.
> Also, I have yet to hear any meaningful conversation
> about the wasted bandwidth (wasted channels) required
> by 8VSB to support translators in remote areas
> (following the current analog model of a big stick with
> remote translators on separate frequencies). With COFDM,
> a television station could transmit using their big
> stick and then have their repeaters (not translators now)
> use the same frequency.
Not quite. They are limited by the guard interval. In
practice, COFDM uses translators too, if they need coverage
outside the tight constraints of the guard interval (which
are typically 7 or 14 miles max distance between towers,
respectively for 1/16 and 1/8 GI). That's why, for example,
the Paris channels are on different frequencies from the
Mantes channels, where Mantes is the next town, 29 miles
west of the Eiffel Tower.
http://www.csa.fr/pdf/Frequences_TNT_phases_1_2.pdf
They *could* synchronize their towers, of course. It's
something no one seems to want to do. 8-VSB can also
synchronize towers. Synchronization is used to guarantee
that the signal level of one tower in the SFN becomes
very weak by the time it exceeds the GI with the next
tower. And that when both signals are strong, they are
assured to be within the GI.
The same would happen if we wanted to link upo the PBS
stations in the DC area. Aside from the fact that they
have different schedules, they are too far apart. To
tie themin an SFN requires synchronization.
I don't think there are major differences here.
Bert
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