[opendtv] Re: ATSC experience

  • From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Wed, 28 Dec 2005 14:20:08 -0500

Tom Barry wrote:

>If your antenna is aimable you may find it works best
>not aimed quite dead on for channel 11.  ATSC is just
>sort of quirky.  As I have often stated here, I'm not
>very satisfied with 8vsb but, if I put my mind to it, I
>can usually manage to get most necessary channels
>with a little tinkering.

Right, but the antenna is not easily aimable. And NBC
11 is not our local affiliate. And it provides a good test
case for marginal reception.

First, at night Baltimore NBC 11 is solid, it's during the
day that the dropouts occur. Looking at the signal
monitor, it hovers around 50 percent to maybe 60
percent, whatever that means. But more interesting,
the SNR hovers between 14.1 and maybe 16.7 or so.
So not bad at all. This would be a perfect test case
for improving the forward error correction, e.g. by
more cleverly using the convolutional trellis and
separate block Reed Solomon codes. Those SNR
numbers, assuming they are accurate, are excellent
for now.

Even more interesting is that the SD weather multicast
on 11-2 is quite solid. Only a very occasional dropout or
slight pixellation. I suppose that if more packets per
unit time are required for the HD 11-1, and if the
corrupted packets are evenly spread out, maybe that's
why the SD subchannel appears more solid. Interesting
phenomenon. Perhaps you do gain from SD after all,
even without changing the RF coding.

I played with the EPG. This is a good EPG. It looks
much like the grid you see in www.zap2it.com, and it
gets populated as you go. Which means, if you have
not used the EPG and you press the EPG button in
the remote, you will be told to hang in there as it
downloads the EPG. It downloads all channels that fit
in the screen for a 3 hour time slot. If you then scroll
either down or up for more stations, or to the right
for more time slots, it asks you to wait a few seconds
as it continues to populate the grid. Cool. Once
populated, you can scroll back and forth without
waiting. Evidently, this is not done when the unit
is idle, only when you have it fully powered up.

When proper 4th and 5th (and beyond) boxes come
out, it will be interesting to see how the reception
changes. As of now, though, I have to say this works
quite well. If you remember that the Shannon limit
for 19.39 Mb/s in 5.38 MHz is 10.47 dB of SNR, we
have a lot of good improvement still to achieve.

Also, as is, 8-VSB reception is excellent in cases
where the equivalent analog is truly bad. For
example, the Channel 56 case I mentioned.


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