[opendtv] Re: Letter to TV Technology
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 8 Oct 2007 15:32:33 -0400
Bob Miller wrote:
> This has always been the problem with "fixing" 8-VSB. As
> soon as the fix makes legacy receivers obsolete in any
> way you open the door to the consideration of changing
> both the modulation and codec.
All the fixes I have seen have been backward compatible, in the sense
that exisitng receivers were never expected to stop working, even if
they couldn't make use of the new features.
> With DTMB, the Chinese standard, you are talking higher
> data-rate reception than 8-VSB
You are? Can you be specific? Is it more than 3.3 b/s/Hz?
With a backward compatible 8-VSB upgrade, you cannot easily achieve
spectral efficiency greater than the limits imposed by the 2/3 trellis
code. (That's not absolutely true, if you start considering the
possibility of multiple transmit antennas in a MIMO sort of scheme, each
one transmitting a different symbol stream. But that gets hard to do
with one-way broadcast anyway, and especially so if there are locations
where reflections are not available from some of the transmit antennas.
So let's ignore the highly unlikely.)
So that could be a problem. If, for instance, equalizers become so
effective that a useful DTT systems could be based on a 3/4 FEC or even
less than that, then 8-VSB would not be able to benefit from those
improvements.
The same holds for DVB-T's COFDM, however. Existing receivers would not
work very well if spectral efficiency were dialed up using less strong
FEC, and especially not if the GI were taken up with new training
sequences, to permit the higher spectral efficiency. Which is why DVB-T2
would create a need for new receivers to be deployed.
Bert
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