Well, I've spent several hours working on this but before I give up, I'd like to see if anyone has any futher suggestions:

First and foremost:
Athelon 3500+, 1G RAM
Nvidia GForce 6600
DVI -> DVI connection (no adaptors, breakout boxes, etc.)
Toshiba 65HDX82 1080i display
PVR-150 SD tuner
Air2PC HD-5000 tuner
Roof-top antenna for OTA ATSC HD

I cannot seem to get any HD channel to signal lock in LiveTV. After successfully adding the Air2PC card, I was able to run the channel scan which found several digital stations, but during the entire scan, the signal strength showed as 50%. But the channels were added nonetheless.

I made sure the starting channel was on a good channel. mythtv-setup does NOT complain about a bad starting channel when exiting the setup (though it did before I fixed this problem).

Watch TV mode defaults to my PVR-150. I can switch to the Air2PC by hitting "Y" but doing so results in a "Signal 50% (l) no lock". I can switch back to the PVR-150 by hitting Y again, but switching back takes a while and there after the card is "busy" (mythtv-setup says "failed to open the card" and the only way I've found so far to correct this is to reboot).

On the frontend, I set my ATSC Signal Threshold to 50% (the lowest it would go). I also increased the time for signal lock to 10000 msec. Still no luck.

In the past, I've used other OTA set top boxes and been able to get HD signals and display them quite nicely, so I'm pretty sure my antenna reception is good enough. Perhaps the Air2PC (or its drivers?) is just more picky about less than perfect signals? (Even the other set top boxes would sometimes produce some picture glitches.)

Anyway, before giving up completely, I thought I'd see if anyone had other suggestions (maybe I'm forgetting to do something?) or if I should at this point just assume I have a signal strength problem.

Thanks.

Update:

Well, I borrowed a different HD Antenna from a friend and tried that one. I'm still having the exact same problem (that is, WatchTV is still only showing a 50% signal with no lock". I reran the channel scan and it picked up several stations, but during the ENTIRE scan it showed 50% signal strength and 100% sig/noise. It seems odd that throughout the entire scan the signal strength didn't vary at all! Is that normal? I would think there'd be at least a little variation in the different channels' signal strengths. Hummm...

Anybody have any suggestions? I don't know what to try next. How can the channel scan "lock" on channels but then WatchTV cannot?

I've also noticed there's something called "DVB Transport Editor" under the Advanced Settings in the Channel Scan menu. What does this do? Should I be playing around with it?

Any advice/suggestions/ideas would be greatly appreciated.

Thanks!

well I haven't played with ota in a while, so I can't help too much, but I can tell you that it is not supposed to stay at 50/100 like you said. it is supposed to go way up when it finds a channel.

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Update:

So, apparently my Air2PC tuner IS working... sort of.

I used azap and with the -r option was able to cat the dvb output to a file. Oddly enough, it worked just fine - clear digital picture! (used mplayer to play the file)

Then I went back into mythfrontend to Watch live TV. Switched from my PVR-150 to my Air2PC card and there it was - a 50% signal lock and very clear HDTV.

Needless to say that confused me. To make sure I rebooted and went straight to Watch TV. This time switching to the DVB reported "no lock" and behaved the same as I've described in my previous posts. I exited the frontend, ran azap from xterm (which, BTW, reports 0000 signal strength and FFFF snr, still don't know why that happens) then went back into mythfrontend. And now it will Signal Lock at 50% and display a clear digital picture. How can this be? What is azap doing?