[opendtv] Re: PBS National Datacast
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 27 Feb 2006 10:48:13 -0500
John Shutt wrote:
> Once more, this time with feeling.
Thanks for the elaboration, John. So there are three techniques used for
the National Datacast over NTSC. But really, none of what you explain
was in dispute, I don't think.
> Dotcast is said to provide about 4 Mbps of data,
That's the absolute maximum. The range is 1 to 3.9 Mb/s, according to
their tutorial.
The discussion, I thought, was about comparative coverage between
Dotcast and ATSC datacast.
Dotacast is limited to the Grade A contour of the analog signal. That
translates to areas where the analog signal provides ~40 dB of C/N.
Compared with datacast over 8-VSB, which should be capable of operating
at C/N margins lower than 15 dB (because it is not limited either by the
threshold of visibility which created that 15 dB magic number in the
first place, or by the two FEC techniques used in real time ATSC program
transmission).
> Any future data services will have to be opportunistic
> data that replaces stuffing bits, which would be about
> 1 Mbps on average.
Which might not be all that different from Dotcast.
Bert
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