[opendtv] Re: Interview: ARE MANY TRANSMITTERS BETTER THAN ONE?
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 19 Sep 2007 12:07:51 -0400
Craig Birkmaier wrote:
> The advisory committee, the FCC and the ATSC were all
> focused on a DTV transmission system based on the "Big
> Stick" model employed for NTSC. This was the easiest
> approach for the FCC to deal with, and the cheapest
> route for broadcasters. There was NO INTEREST in SFNs
> in 1994 when COFDM was being evaluated, however, this
> was being promoted by the DVB folks as a major advantage
> of their system. So an economic argument was created to
> mitigate this advantage...
Not really. The SFN feature, which was never a feature but rather a
potential interesting side-effect of having good echo tolerance, was for
the most part completely misunderstood, blown out of proportion, and
over-simplified by most people who ballyhooed it. Sorry to say, but we
have been over this for enough years that it makes no sense to keep
singing the same old tune.
Most advocates of SFNs were, to put it bluntly, quite thoroughly
uninformed. In 1998 (note: well past 1994), Paul Scott wrote an
excellent paper in which he discusses, among other things, SFNs.
http://www.bbc.co.uk/rd/pubs/papers/pdffiles/ptrev_278-stott.pdf
In Section 6, he goes into the considerations that need to be made,
including the loss of spectral efficiency, and concludes as follows:
"Since, for most practical purposes, the case of the 0 dB echo appears
to be more or less the worst one, this is very encouraging for the
planning and developing of SFNs."
This is a far cry from any notion that COFDM was developed for SFNs, and
a far cry from:
> No. I am saying that it was already obvious that there would
> be multiple technologies available to solve the synchronization
> problem in 1994,
COFDM's advantage is primarily in being able to deploy passively
synchronized SFNs. But more practical SFNs have to be actively
synchronized anyway, at which point the need for a multicarrier scheme
becomes greatly reduced.
The DVB-T2 effort is not there for no reason, Craig. All of this really
does fit together.
Bert
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