[opendtv] Re: 5th generation Test

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
  • Date: Tue, 15 Mar 2005 15:27:26 -0500

Bob Miller wrote:

Doug wrote:
> > It's now time to try the el-cheapo 5th generation box
> > with a really really good single-channel filter
> > (Blonder-Tongue?) in front of it for the difficult
> > channels. If it really is the front end, that should
> > make it work. It could also be something else, like
> > the chip allows different amounts of external memory,
> > or uses aid from the main box CPU, and these were
> > lacking in teh cheapo box.
> Not being an engineer I don't know but at Mark Schubin's
> you could count the cars going by by watching the
> monitor for drop outs on CBS for example. That sounds
> like multipath to me.

It does sound like multipath problems, but Doug might
still be on target. If you recall from the Rhodes
discussions, IM3 products occur in the passband, and
eat at your signal margin. So if you have a cheapo front
end, the IM distortion created from loud stations at
adjacent, or also bad news were the third over,
either above or below your selected station, the "louder
than one might think" IM product will erode the signal
margin you have upstream of the equalizer.

So, a cheapo front end that causes the signal margin to
fall below ~15.2 C/N, even with excellent equalizers, will
cause the thing to fail.

A good front end is the best and quickest defense. More
innovative FEC schemes will also play a part in the future,
by lowering the required margin closer to the Shannon
limit (10.47 dB).

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