[opendtv] Re: China writes its own digital TV standard
- From: "Bob Miller" <robmxa@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 29 Jun 2006 20:36:49 -0400
On 6/28/06, Manfredi, Albert E <albert.e.manfredi@xxxxxxxxxx> wrote:
>
> Doug McDonald wrote:
>
> > If you have stretches of COFDM interleaved with
> > stretches of pseudorandom PN sequence, you can
> > proceed as follows. It is desirable to have
> > double-length PN sequences. What you do is
> > calculate the cross-correlation between the known
> > PN sequence and the data in the vicinity of the PN
> > sequence. This gives you, directly, the signature
> > of the multipath. You then keep the largest of the
> > multipath echoes, and use those to calculate what
> > the received signal would be with just the PN
> > sequence as transmitted, with the COFDM part zero.
> > You then subtract that from the received signal.
> > This gets rid of the major part of the noise
> > introduced into the COFDM. All this of course is
> > done using Fourier transforms, not traversal
> > filters. It's quite elegant.
>
> Good, thanks. This is the way a PN sequence-savvy receiver uses the new
> training sequence. My question was how these PN sequences, now stuffed
> in the GI of COFDM, would affect older receivers.
>
> > If multipath is static or slowly changing, you can
> > average different occurances of the PN seqence and
> > get, in the static case, essentially perfect
> > removal of the PN sequence interference with the
> > COFDM caused by multipath. In the static case, in
> > other words, even with severe multipath, the PN
> > sequence add NO NOISE to the COFDM data, after
> > processing.
> >
> > Oh yes ... pilot carriers are either not needed or
> > of reduced need.
>
> Yes, this is interesting indeed, and shows what's happening here.
> Convergence. Multicarrier and single carrier receivers working much the
> same way.
>
> The advantage of COFDM is still that it can more easily overcome deep
> and narrow notches, although deep and narrow notches are also easily
> mitigated with the use of diversity antennas.
>
> As long as we're redesigning RF schemes to improve reception robustness,
> here's another proposal for 8-VSB.
>
> I'm stuck on the idea of using the frequent segment sync to assist in
> training. These occur every 77.3 or so usec, so very frequently, and
> much more frequently than the GI. But they are short, only 4 symbols.
>
> What if the segment training sequences were not all identical, but
> instead were a known series of PN symbols. A number of these could be
> combined by a receiver, perhaps, to be used as a reasonably good
> training sequence. I don't know how many it would take, but even if you
> need to accumulate 511 of these symbols, that would still occur more
> than twice as often at the existing training segment.
>
> Bert
I have said in the past that if 8-VSB could be made to work I would like to
use it. The question is can it be made to work with current receivers. Both
E-VSB and A-VSB do not work with current receivers IMO.
Since the main argument against COFDM in 2000 was that we could not change
to or allow the use of a new additional modulation since current receivers
would be unable to receive this other modulation and would be rendered
partially or wholly obsolete that is the hurdle that ANY fix of 8-VSB must
jump.
Any talk of any fix to 8-VSB that renders current receivers obsolete
partially or totally opens the door to consideration and comparison to ALL
other modulations.
That would include ADTB-T (VSB) IMO.
I expect that in an open market like China's, can't beleive I am saying
that, where broadcasters have been or will be given the choice of using a
VSB or OFDM based modulation, that the OFDM is and will be the overwhelming
choice. And that even with a VSB that I would die to be able to use in the
US.
Any discussion of fixing 8-VSB for use in the US should, IMO, either be
limited to solutions that are totally compatible with all current receivers
or if not compatible include comparisons with all modulations that could be
made to work in the US. Or at least all such discussions would be more
informed if they started with a statement that this fix is either totally
compatible or not.
Allowing COFDM in the US would, like E-VSB and A-VSB, make some broadcast
services not receivable by current 8-VSB receivers.
So far I have heard of no fix to 8-VSB that would make it acceptable,
compatible with current receivers and also have any mobile capacity.
I have always been open to using a working 8-VSB. After all we have been the
fist or second to test virtually every 8-VSB miracle that has come down the
line. Mark usually gets there first. Who esle has shown more interest in any
new possibility of a working, compatible 8-VSB? And we have been talking to
such as Legend and other Chinese companies and Universities about their
modulations since day one.
Most probably think I am wedded to COFDM. Never have been. Only interested
in something that works. Right now the best, but as yet untested, seems to
be DMB-T/H.
Bob Miller
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