[opendtv] Re: FCC Gives Official Nod to DTS
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 4 Dec 2008 19:44:23 -0500
> Receivers do not lock into any one transmitter and ignore
> the rest. Not unless "the rest" are so much lower in strength
> that they are totally overpowered by the one. The problem is
> always in areas of equal or near equal power density, from
> two or more towers. Unless, of course, there's some obstacle,
> at and near the contour lines, that makes reception
> unimportant.
More completely, I should have said that I was speaking mostly of SFNs
with towers far apart, which is what a lot of people think about when
they mention "SFN" and "increased area of coverage" in the same
sentence.
Even if you synchronize the towers, to try to keep the symbols lined up
in time in the contour lines, with more than two towers you will at best
be able to adjust the location of the difficult reception areas. Not
eliminate them. Directional receive antennas help, at least in devices
that can actually use them. And when far-apart towers are involved, you
can't forget how weather conditions can change the relative power
density of different towers at different locations.
On the other hand, if you cluster a few SFN towers close together enough
so as not to exceed the echo tolerance of receivers, then you will ease
reception near that cluster, but you will not gain anything compared
with a single big stick in terms of coverage area. At least, not with
ATSC, or the original DVB-T at reasonably spectrum-efficient settings of
GI.
Bert
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