[opendtv] Re: 20060616 Free Friday Fragments (Mark's Monday Memo)

----- Original Message ----- 
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>


> Somehow, A-VSB seems capable of working in a similar way, training on
> its new training sequence. I can't imagine how using the data symbols
> themselves for training would make the "look to look" issue any more
> severe. If anything, the opposite should be the case.

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.

The problem is that if you add additional training sequences, you break 
existing receivers.  A-VSB's claim to fame is that by creating the 
additional training signals in the way they did, the training sequences 
appear to be ordinary data packets to older generation receivers, thus it 
won't break them, while new receiver designed to find the extra training 
signals take advantage of them.

Even with more frequent training signals, adapting for severe changes in 
multipath is a recursive process that will span several training sequences. 
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.

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.  (Every tap raises the noise floor.)  A-VSB can do this trick as 
well, but it eats up even more data capacity.

We would have been much better off switching to DVB-T in 1999, since we 
would have ended up with as good or better multipath performance at a higher 
data payload than A-VSB or E-VSB.

John 


 
 
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