[opendtv] Re: Patent application on 8-VSB equalizer
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 7 Jul 2006 15:50:47 -0400
John Shutt wrote:
> I've always wondered how you can use the data segment
> sync bits to estimate multipath when you first have to
> demod the 8-VSB before you can read the data segment
> sync bits, which means you have to equalize out
> multipath, which means you need to estimate the multipath.
> All very chicken and egg to me.
Not really chicken and egg. The sync symbols, four of them, are not
8-VSB. They're binary, +5, -5, -5, +5. That's why they can be detected
down to ~0 dB of C/N. The sync symbols are just like the PN511 and PN63
symbols AND also the 24-bit VSB level ID sequence, in this regard. What
you need is a training strategy that works at much lower C/N margin than
you expect to need for data signal demod. That way, you know "for sure"
that you can sync to any data signal that has any prayer of being
demodulated.
Here's the way I would train.
1. Using the averaged spectrum of the incoming signal, you adjust for
amplitude flatness across the 5.38 MHz spectrum. You may be off in
phase, though.
2. Detect the sync symbols (and/or the actual training segment itself).
Use these to set absolute phase and amplitude, across the spectrum. Sync
symbols are only of +5 and -5 amplitude, so that sets the reference.
Adjusting for zero phase across the spectrum will change the original
amplitude adjustment of each tap, but by amounts that can be calculated
as you adjust the phase. So it's a one step adjustment, each time.
3. After this process has settled down, i.e. you repeat it for some
number of segment sync sequences, demod the data symbols. Across the
spectrum, you check for Q energy in the signal. Since you know that any
phase information other than 0 phase is distortion, you adjust the
equalizer taps to restore a zero phase across the spectrum. This is a
continual process, and makes use of a known quantity, the phase, to your
advantage.
Bert
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