[opendtv] Re: Opinion: Mobile TV's New Free Market Economy
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 20 Mar 2009 16:43:25 -0400
Tom Barry wrote:
> I do not have access to DVB/COFDM to test. Do you now
> believe that you can get equivalent after-FEC throughput
> (spectral efficiency) with 8vsb using non-directional
> antennas at the same power and spectrum width?
With 8T-VSB set up as it is in A/53, we get 3.23 b/s/Hz (calculated as
described below). That is better than just about all deployed DVB-T
setups, in some cases by just a tad.
The most spectrally efficient COFDM used in DVB-T that I know of would
be the French 64-QAM, 2/3 FEC, 1/32 GI mode, which offers 24.13 Mb/s. To
be completely fair about expressing this in b/s/Hz, I'll use the total
channel bandwidth, not the actual bandwidth used by the signal. Which
will favor COFDM in 8K mode, because it's shoulders are steeper than
those of 8-VSB, meaning it can fill in more of the 8 MHz channel than
8-VSB can fill of its 6 MHz channel.
So in 8 MHz of RF bandwidth, 24.13 Mb/s comes out to 3.02 b/s/Hz. In
that mode, the C/N margin required in a gaussian channel is 16.5 dB.
8T-VSB provides 19.39 Mb/s in 6 MHz, or 3.23 b/s/Hz, for 15.2 dB of C/N
in a gaussian channel.
Now, I'm sure that most DVB-T receivers out there now are better at this
than most 8T-VSB receivers are, but I know that I can take my DB4
antenna downstairs, in the den, indoors, and wave it around the room,
and receive a solid signal for at least some of the broadcast channels.
I happened to try this on the local Fox 5 station, which fortuitously is
one of the stations that gives me a strong signal. I know for a fact
that other stations won't allow this sort of behavior, though, e.g. the
Baltimore stations. Still, the Fox 5 transmitter is more than 12 miles
away as the crow flies, and I'm down in a valley.
Honestly, I believe that today there are far more interesting and
intriguing things to wonder about in the future of TV distribution, than
modulation standard.
Bert
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