[opendtv] Re: Seeing Ghosts on a Single Frequency Network

From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
Date: Thu, 10 Feb 2011 17:24:27 -0600
John Shutt wrote:

> Why in the world do you continue to insist that the only possible
> implementation of an SFN involves scores of low power transmitters,
> but MFNs can use fewer, farther spaced, high power towers?

Because it's true. The distance between towers is dictated by the GI. The 
greater the GI, the lower the spectral efficiency. Qualcomm had to balance that 
equation, since they only had one frequency channel. Now sure, you can spread 
the towers further apart and not lose as much efficiency IF you use DVB-T2, or 
IF you create your 12 MHz variant. This would be trying to mitigate the effects 
of the kludge.

> News flash: MediaFLO used 50 KW ERP transmitters. Not exactly the
> up to 1 MW ERP allocated to some ATSC broadcasters, but it is the
> same ERP that we (WKAR-TV) are currently allocated on UHF channel
> 40.

PLENTY of ATSC broadcasters are in the 10s of KW ERP, in UHF, John. And you 
don't need a 10 or 12 of them to cover one market. The point I was trying to 
make is, if you use 50 KW towers in a MFN, you can get by with many fewer 
towers than you can when trying to make SFNs work, and you can get much better 
coverage area too. With SFNs, you have to have enough towers to give you the 
signal strength you want everywhere (also true with MFNs), *and* you also have 
to install extra towers in places where signal strength is plenty adequate. You 
don't need to do that last part with MFNs.

Why those extra towers where there may be adequate signal strength?

The point of the multi-tower approach is to cover a whole area with good signal 
strength. With SFNs like Qualcomm's, that attempt to cover large areas, you 
cannot avoid situations where several towers provide adequate signal strength 
in a location, but are beyond the max distance dictated by the GI. This will 
almost certainly occur here and there, as seasons or weather conditions change. 
In those locations, you have to install another tower, to overpower the 
interference. That's why the MediaFLO system gave spotty coverage in so many 
locations that would otherwise have been covered with the other 50 KW sticks. 
Look at the MD Eastern Shore, for example. No coverage.

And that's also why European SFNs have few towers, closely spaced, so that all 
towers are within the GI limit. Or the umbrella conguration in Paris (big stick 
plus gap fillers).

> Your coverage plan would be perfect if the intent were solely to
> deliver programming to homes.

Did you forget about the low power gap filler translators? With receivers that 
can make constructive use of two frequency channels, there's no problem with 
mobility. Think, for example, a regular DVB-T receiver, already good with 
dynamic echo, and now capable of combining the signals from two 6 MHz channels 
that transmit the same content. Or ATSC receivers with fast feedback loops, to 
keep the spectrum looking flat. And diversity antenna to kill the deep null 
problems.

> In fact, it IS the current implementation when you take independent
> station affiliations into consideration. Fixed receivers don't need
> to dynamically choose one MFN channel over another. They can choose
> one or the other, and be done with it. 

Fixed receivers have an easier time of it, of course. Even in a single-market 
situation, if you have the low power gap filler translators, as I had 
suggested, a fixed receiver could benefit from constructive use of the two 
channels. Time of year and weather conditions change ease of reception of any 
signal frequency, when you're close to the edge. Use the same scheme I layed 
out, limit to single market only, it can still work. Low power Ch 11 gap 
fillers in DC, say 100-200 W, can coexist with a 50 KW Ch 11 big stick in Balt, 
as long as the bulk of DC coverage is Ch 7.

So, I'm not talking about ATSC vs COFDM. Neither was Al. I'm talking MFNs vs 
SFNs. And I don't buy that you must go M/H for mobility with ATSC, either. 
Adequate signal strength, via MFNs, and equalizers that make use of the info 
they have available to them, would go a long way. But that's a whole 'nother 
discussion.

Bert
 
 
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:

- Using the UNSUBSCRIBE command in your user configuration settings at 
FreeLists.org 

- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.
Follow-Ups:
- [opendtv] Re: Seeing Ghosts on a Single Frequency Network
  - From: John Shutt
[opendtv] Re: Seeing Ghosts on a Single Frequency Network

Other related posts:
