[opendtv] SFN considerations (was Doug is Missing the Point)
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 16 Feb 2006 18:37:43 -0500
Ian Mackenzie wrote:
> The coverage of Sydney uses a SFN on UHF from
> Multiple transmission sites which are fed
> From the digital transmission on VHF off air.
> No microwaves, no fibres, etc.
By the way, here's some interesting stuff. It turns out that this French
DTT (TNT) document
http://www.csa.fr/pdf/Rapport-GT2-Aspects_Radiofrequence_de_la_TNT.pdf
covers lots of SFN design issue quite well, both for receiver and
transmitter considerations. Much like A/111 does. They discuss both
true, synchronized SFNs, as well as on-channel repeaters. The majority
of their discussion points apply equally to COFDM and 8-VSB. But a
couple that are really almost humorous are (starting p. 22):
1. After defining what a pre-echo is, they caution that the presence of
pre-echo can disupt reception on receivers that are so-called "1st or
2nd generation." That sounds familiar.
2. They mention that various synchronization strategies are possible for
receivers, but they don't all favor pre-echo tolerance. As long as all
echoes arrive within the GI, a strategy for pre-echo tolerance devised
by Philips is to sync up on the first received signal, and treat all
other echoes as trailing echoes. The interesting point here being that
this was the essence of what Linx did, in their 4th generation demod.
Linx discussed this in a white paper that disappeared along with their
web site. Linx filtered the incoming signal in such a way that the
equalizer was presented with a strong initial pulse followed by a long
series of lower level trailing echoes, which equalizers can handle well.
The TNT document also describes how SFNs are reasonably easy to
implement as long as the number of transmitters is no more than two or
three, because all you really have to do is ensure that the GI can't be
exceeded. Beyond that, you can't help but exceed the GI, so things start
getting messy. That's when you start having to introduce delays in the
repeated signals, use directional antennas for receivers, use natural
obstructions, etc. This is COFDM they're talking about. It's refreshing
to see real engineering instead of inane marketing-speak.
On dynamic echo tolerance, and the use of diversity antennas, Figure
3.2.2 is not, I don't think, beyond reason for ATSC. It seems to me that
the Samsung Gemini demod might actually be competitive here, going from
what dynamic results the CRC provided (no diversity antenna schemes were
tried by the CRC, though, but such schemes can only help).
Also intriguing: a 100 mW on-channel repeater for apartment building TV
redistibution (p. 32).
> The nationwide SFN is ridiculous, just like
> the thought of covering the USA in ATSC.
BTW, while I agreed with the former completely, the latter is not quite
as clear. I'm sure there are huge expanses not covered by terrestrial TV
in the US. But there are also huge areas that are coverered completely,
or at least should be. Up and down the East and West coasts, and along
the Gulf coast, and in the Midwest, there really should be no holes. To
achieve ubiquitous coverage like that, one can't seriously rely on SFNs.
In fact, no one does, no matter what the modulation. So there aren't any
major disconnects in any of this.
Bert
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