[opendtv] Re: Letter to TV Technology
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 8 Oct 2007 15:14:53 -0400
http://www.tvtechnology.com/pages/s.0079/t.8845.html
First of all, parenthetically, I'd like to point out again that there is
no technical similarity between cell phone networks and SFNs. On the
contrary, cell phone networks consist of a mesh of low-powered
transmitters on separate frequency bands, much more similar to a network
of translators than to a SFN.
Bob Miller wrote:
> One little problem with using SFN's with 8-VSB in the US.
>
> They only work with current generation receivers.
This is not completely true. In fact, I'd like to see how true it
actually is from test results.
As discussed in the distant past, if the towers of the SFN are
synchronized actively, e.g. per A/110, then in principle the loudest
echoes should always be lagging echoes, which even 2nd generation
receivers were quite capable of managing. Problems would manifest
themselves more in the event of weather-related anomalies, I would
expect, in which the more distant towers create the loudest signals at
the unfortunate location.
The New Jersey SFN idea, where there is one big stick assisted by
synchronized small sticks (what the TNT calls "umbrella configuration"),
should be good for even 2nd gen receivers. It's a relatively benign way
of doing SFNs, it seems to me, allowing the power of the small sticks to
be dialed down if excessive interference is witnessed. And, as is the
case for all SFNs, there will be locations where directional antennas
will be required.
The main problem with SFNs has been the misinterpretation of facts.
Journalists who heard "two plus two," and offered 20 as the sum. SFNs
are far from fool proof. There are down sides. They are not a silver
bullet.
> If we decide that we MUST use such SFN networks and in a
> sense then are switching to a new architecture that it all
> intents and purposes is a new modulation that makes all
> legacy receivers obsolete we MUST take advantage of this
> about face to consider all the latest modulations and
> advanced codecs at the same time.
Obviously, should anyone begin with a clean sheet today, they would
incorporate the latest codecs. But that's not in the cards.
Also, you will note that the Chinese and DVB-T2 have both gone to, or
are considering, training sequences and potent receiver equalizers,
rather than depending on guard intervals for echo tolerance.
Bert
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