[opendtv] Re: U.S. mobile TV spec in the works
- From: "John Shutt" <shuttj@xxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 10 Apr 2007 18:40:15 -0400
Bert,
Now I understand what you meant by "backwards compatible." Interesting
idea, but I guess I don't have enough imagination to envision a scheme other
than retransmission of redundant data that could do what you propose.
I mean, if a mobile device could decode the same stream that legacy decoders
could, there wouldn't be a need for a more robust coding or modulation
scheme. And if the mobile devices couldn't decode the legacy stream, then
whatever "enhancements" aimed at mobile devices would be a de facto
simulcast of the data, would it not?
It has been my experience that there is a very narrow window where you can
recover only some of the 8VSB data that might be improved with redundant
data. Usually either you can demod and decode the entire bitstream, or you
get nothing. That cliff edge thing. At least that's what I've seen playing
around with a portable antenna and a bitstream analyzer. Redundant data is
useless if you can't recover the transport stream.
It would seem simpler to me to just have a regular stream aimed at fixed
displays, and a robust stream in an enhanced modulation scheme with lower
resolution video suitable to portable and mobile devices, simulcast in the
same channel.
Remember, E-VSB did little or nothing to deal with dynamic echoes, hence
Samsung's A-VSB and their "pseudo-training" sequences.
John
----- Original Message -----
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
Another is where the robust stream is decodable by all receivers, even
if legacy receivers don't benefit from any extra robustness. That is
what I would describe as "backward compatible."
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