[opendtv] Re: AVSForum getting antsy over 8-VSB reception
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 25 Feb 2005 18:16:12 -0500
Tom Barry wrote:
> He was technically correct because ATSC was designed
> for directional rooftop antennas. That was an
> incredibly stupid decision but supposedly true.
I don't think this was true. From what I've read, ATSC
was supposed to have the same coverage as NTSC.
Anything less than that was a limitation.
The fact that directional antennas usually worked best
was simple: 8-VSB receivers didn't like multipath
distortion. It wasn't by design, nor was there any
design goal to restrict tolerance of multipath
distortion, ever.
There has never been *any* reason for 8-VSB to require
careful antenna aim *other than* the unintentional
lack of tolerance for multipath. Same goes for indoor
reception. It all boils down to multipath tolerance.
One of the indications I saw that the multipath
question was not well understood, by the way, was a
paper from Harris Corporation, from many years ago:
http://download.harris.com/app/public_download.asp?fid=3D505
(Remember to remove the "3D" after the equals sign.)
In that paper, they show the 8-VSB constellation. But
what they show is random energy in the Q axis, which is
correct, but discrete energy clumps in the 8 levels
along the I axis. So this implies that the real-only
equalizer would work just fine.
The truth is, when the incoming RF symbol is distorted
by obstacles, and some of its energy migrates to the
Q axis, the level on the real axis, I, is changed too.
The energy on the real axis becomes the transmitted
level * cos(theta), where theta is a random angle of
the symbol vector away from the I axis. So the real
component of that symbol is also randomized, and the
whole contellation now looks very messy. No wonder
line of sight was the only way these guys would work
half-way reliably.
Long time ago, Frank Eory explained that complex
equalizers were needed for both QAM and 8-VSB. That's
because complex equalizers will restore focus along the
I axis of the 8-VSB constellation, even if the Q axis
energy would ideally disappear dowmnstream of the
equalizer.
And that's why there were such high expections for 2nd
gen receivers. Unfortunately, aside from the problem of
needing cmplex equalizers, there was also the problem
of equalizing pre-echo without creating a lot of self
induced noise.
And that is what the 4th gen Linx first addressed.
All in all, COFDM is the best thing that ever happened
to 8-VSB.
Bert
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