[mythtv] TV Anywhere: Black video, no audio, only on certain channels

[Brian Rogers]

Hello all,

For a while, and with a near-100% failure rate, MythTV keeps recording 
black video and no audio for certain channels. However, if I go to live 
TV and flip the channel a couple times, I can get it to tune these 
channels just fine. It seems like switching to these channels while 
recording works, but tuning to them straight away doesn't. I used to 
think my tuner was going out, but I eventually realized that mplayer 
always tunes them correctly.

The channels in question are 12 and 13, NTSC, us-cable. And also some 
other channels I watch less frequently. Channels 3 and 8 are reliable.

In trying to debug this, I set a breakpoint to determine what was going 
on when I got the 'strange error flushing buffer' message, because it 
seemed to be correlated to the tuning failures. However, I couldn't find 
the cause. Though the message appears sometimes when recording is 
successful, I never saw an instance of the problem not accompanied by 
the message. Thinking a problem with LAME could be the culprit, I 
switched to uncompressed audio, but the problem persisted.

So I wrote some workarounds in the channel changing code:

1. Tell the card to tune the channel multiple times.
2. Tune channel 8, wait a second, then tune the selected channel. 
(Trying to simulate the manual work that makes the channel come in)
3. Don't set the frequency at all if it's already on the correct channel.

None of these helped me get video except for my buggy first version of 
workaround 2 that just left the tuner on channel 8.

The problem persists regardless of my choice of MPEG4 or RTjpeg, MP3 or 
Uncompressed, or 640x480 or 320x240. I also tried removing my 
deinterlacing and denoise filters during recording.

My tuner card is the 'MSI TV Anywhere Plus' with the saa7134 driver on 
Ubuntu Hardy's 2.6.24 kernel. I've noticed if I run 'cat /dev/video0' or 
'touch /dev/video0' while MythTV is recording, the recording blacks out. 
I don't know if this is normal behavior or not, but the result looks the 
same as the channel tuning problem and makes me think that something's 
poking at the driver and causing this effect.

Does anybody have suggestions on debugging this? Or any stories of a 
tuner behaving like this?