[opendtv] Re: Doug is Missing the Point
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 16 Feb 2006 11:46:33 -0500
Ian Mackenzie wrote:
> If you need multiple high power stations then
> It is necessary in some cases to have a
> microwave feed between the sites but you
> would need that in ATSC wouldn't you?
Right, because the symbols have to be synchronized between these towers.
> 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.
I take it the VHF is a big stick providing the bulk of coverage, and the
UHF are low power gap fillers.
I think that with ATSC as is, i.e. with the sort of obsolete receivers
that are still being sold in stand-alone STBs today, that arrangement
would work very well indeed. As long as the gap filler patterns did not
overlap excessively, *or* as long as the areas of UHF gap filler overlap
were also well covered by the main VHF signal. That would permit
receivers in places where the UHF was experiencing pre-echo
(interference between two UHF gap fillers) to opt for the VHF signal
instead.
Apparent pre-echo occurs when the main tower's signal just begins to
become overpowered by the gap filler's signal. This is bad for old
receivers, until the gap filler's signal is more than 10-15 dB stronger
than the direct signal. But using VHF for the main tower, as you are
doing, or simply using a different UHF frequency for the big stick,
effectively eliminates that problem. What's left is the pre-echo in
regions where the gap filler patterns overlap, if such regions exist.
Another way to mitigate the pre-echo problem is to use directional
antennas for the gap fillers.
The best way is to deploy 4th and 5th gen receivers.
> The nationwide SFN is ridiculous, just like
> the thought of covering the USA in ATSC.
Hear hear. I wish more people would make that point. Nationwide SFNs
seem one of the comon misconceptions of those bent on hype.
> In Australia anyone, anywhere can get the
> networks in digital if not FTA VHF or UHF
> Then on satellite.
Here too, of course.
Bert
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