[mythtv-users] OT: Cautionary tale of marginal 8VSB reception using Air2PC HD5000 and Sony HD TV
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.
I hope this is helpful.
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