[mythtv-users] OT: Cautionary tale of marginal 8VSB reception using Air2PC HD5000 and Sony HD TV

Larry Jones larry at foxgulch.com
Fri Jan 26 17:17:57 UTC 2007

I have a Sony Bravia KDL-40S20L1 ATSC 8VSB HD capable TV running Linux, 
and an ATrpms install of Myth .20 on a P4 3.2 GHz Fedora 6 box using an 
air2pc HD5000 pci board(1) for HD reception.  I only live 25 miles south 
of the two local HD/Digital TV transmitter sites near Missoula, Montana. 
However, I am shadowed by at least one intervening mountain.  Analog SD 
reception is satisfactory with some "ghosting".   But not digital!

Digital (SD and HD) were hit and miss.  If I moved the antenna slightly, 
I would lose one or more stations while the others were still received 
satisfactorily.  Early in the morning, I could receive all stations but 
later in the afternoon, the weakest of them would blink out.  Over a 
period of weeks and many antenna/preamplifier iterations later, here is 
my distilled wisdom:

1.  If you see ghosts in your old analog reception, beware!  Despite its 
vaunted error correction capability (Reed Soloman coding and all), ATSC 
8VSB isn't very tolerant to "ghosts" or as that phenomena is known, 
multi-path. (Multi-path is the reception of two signals one strong and 
one weaker simultaneously. The weaker is delayed by taking a slightly 
longer path to your receivers antenna.  Multi-path shows itself as a 
"ghost" image offset to the right in the analog TV picture. With digital 
TV, multi-path caused errors to be shown in the Sony on screen 
diagnostic menu and reception to be intermittent with pixelation.)

2.  I think the best chance to defeat multi-path is a high gain 
directive antenna mounted outside with an in-line low noise high gain 
pre-amp to over come the coax losses incurred while transporting the 
signals to your distribution amplifier.  In my case,  I used a four bay 
bow-tie UHF antenna(2)  and a small Yagi-Uda for VHF that I purchased 
from Radio Shack(3).  I was particularly impressed with the Channel 
Master 7778 preamp that has separate inputs for UHF and VHF. At my home, 
the Radio Shack antenna feeds the preamps VHF input while the bow-tie 
antenna feeds the UHF input. (Only one of the local stations is 
broadcasting digital in the high band VHF on Channel 7, the others are 
all UHF so I was forced to keep a separate VHF antenna for Channel 7).

3. There is a huge difference between outside and inside antennas.  
Initially I  attempted to get an attic mounted UHF/VHF yagi-uda Radio 
Shack(3) combo to work. I made countless trips up the ladder to the 
attic to adjust my attic mounted antennas.  Finally, I gave up and 
mounted a 4 bay bow-tie antenna outside near the peak of the garage 
roof. Finally I have good reception on all digital signals.

4. The Sony TV is more fault tolerant than the air2pc HD5000 card (1).  
I had ample opportunity to test them side by side under marginal 
conditions,  invariably the Sony would produce a picture where the 
HD5000 card failed, despite it's being a "third generation" tuner card.

5. In my location, the Sony diagnostics screen shows I need a minimum 
Signal to Noise ratio (SNR) greater than 14 dB.  Solid reception seems 
to demand a SNR greater than 18 - 20 dB.

6. This site and forum has excellent information regarding the OTA 
reception of ATSC.(http://www.highdefforum.com/forumdisplay.php?f=9) 
Kudos to the posts of forum member "tigerbangs" who, IMHO,  could "write 
the book" on HD OTA problems and cures.

(1) http://www.cyberestore.com/hdtv-tuner-cards-c-28.html
(2) http://www.cyberestore.com/multi-directional-hdtv-antenna-p-255.html

I hope this is helpful.
Larry J

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