[opendtv] Re: Analog or Digital: Ghosts Plague Reception
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Sun, 19 Sep 2010 18:43:07 -0500
> The next issue, of course, is what this actually buys you, given that
> we want single-market SFNs. The safe COFDM configuration costs you
> 1/4 of capacity. And the next market over will need different
> frequencies anyway. It could buy ease of reception, in the ideal case,
> as long as you aren't at the edges of coverage. In the outer
> perimeter, and in areas of weak signal inside the SFN, reception could
> be more difficult than with a big stick. That 22 percent coverage loss
> problem.
BTW, I consider the safer "same-frequency multitower" alternative to be low
power, passive gap fillers assisting the big stick. Both for DVB-T1 and for
ATSC.
For ATSC in particular, if you assume a worst-case (as far as multiple
transmitters go) flat terrain situation, the gap fillers are low power and the
signal directed mainly downstream of the big stick. This will create
pre-echoes. The stronger signal from the repeater is preceded by the weaker
signal of the big stick, close in to the repeater, which looks like a leading
echo. But the good repeaters are those that introduce less than 10 usec of lag,
which any of the 4th gen and up receivers can handle. As long as you're heading
downstream, that less than 10 usec figure will not increase. And obstruction by
terrain would possibly help here. In COFDM, a similar arrangement would avoid
having to rely on wastefully high GIs.
In the actual SFNs that we've read about in Europe, what you see is either this
sort of arrangement (Paris), or two or three tall towers of similar ERP, close
together. Plenty of overlapping coverage among these towers, rather than
attempting to spread them out to the limit of their range.
Bert
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