[opendtv] Re: 20060616 Free Friday Fragments (Mark's Monday Memo)
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 16 Jun 2006 17:33:02 -0400
John Shutt wrote:
> A-VSB plays tricks with the data interleaver and
> the trellis encoder to create pseudo training
> signals that occur much more often than the
> original ATSC training sequence does. Doug and
> C.B. have long said that if ATSC had more
> training signals, it could better cope with
> dynamic multipath, but the existing training
> signals were so few and far between that most
> receivers ignore them altogether and instead go
> for blind adaptation.
Again, if you use the data symbols for your "training sequence," you
have a virtually continuous training sequence. Even if data symbols
aren't as "perfectly" randomized as a PN sequence, you can afford to
listen for many, many more symbols to do your training, because doing so
does not impact on spectral efficiency in any way. Nor does this change
the standard in any way.
In other words, if a given PN1023 sequence is very, very good for
training, then use 2000 data symbols instead. What's the big deal? Use
3000 symbols if it makes you happy.
Additionally, each segment in n-VSB starts with a 4-symbol data segment
sync, which is very robust. It can be decoded down to 0 dB of C/N. So if
A-VSB syncs up their trellis magic to some new sync signal, you should
be able to do the same thing using the existing data segment sync
sequences, along with the previous 2000 (or whatever you choose) symbols
of data.
What's nice about this is that it's all up to the receiver design. You
can make the receiver as robust as you please, without affecting the
standard in any way and without reducing spectral efficiency.
> The problem is that if you add additional training
> sequences, you break existing receivers.
Which is why I specifically did not propose using a new training
sequence. The old training segment is used *only* as a time tick, to set
the clock in the portable receivers.
> Even with more frequent training signals, adapting
> for severe changes in multipath is a recursive process
> that will span several training sequences.
Doug says it can be done otherwise. But even if you make it recursive,
you have an infinite supply of randomized data symbols at your beck and
call.
> And the amount of data payload you eat up in the extra
> sequences brings your total to something lower than the
> 1999 Sinclair Baltimore tests used for DVB-T, and that
> data rate was deemed "unacceptably low" by many ATSC
> advocates on this list.
Using the data symbols, the spectral efficiency stays at 3.3 b/s/Hz. But
it's possible that the info in the time slots allocated to portable
devices might be encoded with additional FEC, both convolutional and
block, as explained in E-VSB. This works to lower the C/N margin.
> E-VSB did absolutely nothing about dealing with multipath,
> and relied on lowering the C/N threshold to give more
> margin to aggressive blind equalizers.
With 5th gen receivers, E-VSB mainly lowers the C/N margin. With older
receivers, it also helps with multipath. But with this new scheme, the
multipath is attacked by the training-on-data-symbols idea, and the
extra FEC of E-VSB may be used in addition, to further lower the C/N
margin.
Bert
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