[opendtv] Re: PBS National Datacast
- From: "John Shutt" <shuttj@xxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 27 Feb 2006 11:22:11 -0500
----- Original Message -----
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
> 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.
Not the correct phrasing again, Bert. There are not three techniques used
by National Datacast. There are three techniques that customers developed
to to deliver data within an existing NTSC transmission. National Datacast
only provided the contract.
The "dispute" was over what PBS may or may not have thought about the
performance of Docast vs. ATSC, implying that PBS "chose" Dotcast. PBS
doesn't care.
> 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).
Dotcast is injected at a very low level in comparason to the NTSC visual
carrier, hence the Grade A contour requriement.
>> 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.
You are probably correct, if indeed Moviebeam is using QPSK modulation.
However, I would guess that by the time the Moviebeam box needs to be
'upgraded', you will trade it in for an IP based one that either connects to
cable or requires broadband internet access.
John
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