[opendtv] PBS National Datacast
- From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 22 Feb 2006 21:26:54 -0500
John Shutt wrote:
>Moviebeam is using NTSC with dotcast. PBS has not allocated
>any DTV bits to any datacasting service at all at this time.
Well, this site:
http://www.pbs.org/digitaltv/dataNS.html
says that PBS either is, or soon will be, providing this datacast service
over their DTT transmitter. It's called PBS National Datacast. And it makes
sense, especially if the previous dotcast was limited in its coverage, as
the article explaining how it works said. (They gave the example of 1.7
million viewers in the LA area, which is a small fraction of that
population.)
But you're right, they don't mention Moviebeam by name. They say:
"The Digital Difference
"As PBS stations in the data broadcasting network convert to digital
broadcasts, PBS NATIONAL DATACAST will begin offering commercial datacasting
services over digital television transmissions."
It seems a natural fit for Moviebeam and other similar services.
This could work well, actually, and it's not cold fusion at all. Since the
Moviebeam folk are clever, they may well have added another layer of forward
error correction to the basic Viterbi 2/3 and RS[208,188] when transmitting
their proprietary file formats over DTT. They would achieve a C/N margin
improvement similar to what E8-VSB provides. Why not? After all, their
streams don't need to conform to any standard other than to fit in the
MPEG-2 TS frame, with appropriate non-conflcting headers. They could use
A/90, the LLC/SNAP header option to avoid any problems with conflicts, then
add more interleaving and more redundancy to the packet stream.
The extra FEC may be enough to make the indoor antenna work in most cases,
even if you get no usable signal when trying to watch the real-time DTT
channel of that same station.
>We are on the list to have our analog transmitter modified, but I can't
>believe they will still go through with it.
It has to be considerably better than Dotcast over NTSC. Did you read how
that works? It's similar to IBOC radio, when the radio channel is also
carrying the analog station. Except it's possibly even more compromised,
since it is fit right around the VSB carrier, in a relatively very narrow
band (not in the relatively much larger guard band IBOC depends on),
apparently heavily attenuated before transmisssion, and uses plain QAM
instead of COFDM. I would expect that for such a system to work reliably,
the Moviebeam clever lads have already figured out how to add in robustness
in the data packet stream.
Bert
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