Difference between revisions of "Category talk:Knowledge Base"

Latest revision as of 02:25, 14 September 2010

Multipath

Multipath caused havoc with my reception of digital (8VSB)ATSC television. This note documents my experience in a location cursed with multipath.

At this location, we have a 2006 Sony Bravia KDL-40S20L1 ATSC 8VSB HD capable TV, 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 was satisfactory with some "ghosting". But not digital!

Digital (both SD DTV and HD DTV) 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, multipath. (Multipath 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. Multipath 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. Your 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. Here the Radio Shack antenna feeds the 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(2) out side 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
(3) http://www.radioshack.com/search/index.jsp?kwCatId=&kw=antenna&origkw=antenna&sr=1
LarryJ