[mythtv-users] Crackling and popping in non-PVR analog card -- I think I can say it's MythTV's fault

[David Ellis]

Lower your volume level on the card setup in mythsetup.........I think the
problem is with an input volume level that is too high.

David

-----Original Message-----
From: mythtv-users-bounces at mythtv.org
[mailto:mythtv-users-bounces at mythtv.org] On Behalf Of Sérgio Gomes
Sent: Friday, September 02, 2005 5:00 PM
To: mythtv-users at mythtv.org
Subject: [mythtv-users] Crackling and popping in non-PVR analog card -- I
think I can say it's MythTV's fault

Hello everyone!

I'm probably one of the very few people left with a non-PVR analog card 
(a simple Hauppauge WinTV Go, based on cx8800). The audio goes from the 
TV card to the sound card via a simple external stereo cable, as I'm 
sure is the case in many setups out there.

Now, I have been having a problem with audio on my MythTV since I 
installed it yesterday: I get a lot of crackling and popping in my 
recordings and in LiveTV. I've been running several tests.

Outside MythTV:
- The audio gets to the sound card in great quality: if I turn on 
playback of the line-in channel, the sound is perfect
- It is also not the recording hardware of the sound card that is 
ruining the sound, as both "record" and "arecord" record perfect sound
- "record" and "arecord" still record perfect sound with high CPU and 
disk loads
- "record" and "arecord" record perfect sound whether or not video is 
playing (with XawTV, for instance)
- If I start recording sound then start mythfrontend and select "Watch 
TV", it won't get any sound (understandable, the device is busy), but 
the "record" or "arecord" program will keep on recording, and the sound 
is, once again, perfect
- Recorded MythTV shows play with crackling in mplayer

Inside MythTV:
- The amount of crackling (or at least the loudness) goes up with the 
loudness of the broadcast; if it's quiet there's almost no crackling, 
but if there are a lot of people talking or loud sounds, there is a lot 
of crackling
- Happens with ALSA and OSS
- Happens with MJPEG and MPEG4
- Happens with or without "Agressive Sound Buffering"
- Happens with both WAV and MP3, at any of the available rates
- Happens with or without internal volume controls, at any level
- Music and videos play without any problems
- The amount of crackling goes up with the quality of the video; there 
is almost no crackling at 160x160, but a lot of it at 768x576

So, given all this I think we can rule out any interference on the sound 
from external sources, hardware or other software, and assume the 
problem is limited to the TV functionality in MythTV. My guess would be 
that the sound is being clipped due to volume adjustments inside MythTV, 
and is only then recorded, assuming that MythTV does some volume 
processing on the sound (even if just boosting it).

Now, this is all fine and well and a good theory until the last point. I 
don't understand why the problem gets worse with higher video 
resolutions. That completely ruins my theory. With that in mind, I guess 
the problem could only be some kind of multiplexing or demultiplexing 
bug (probably multiplexing, because for it to be a demultiplexing bug it 
would also have to be present in mplayer), but that wouldn't account for 
the problem getting worse on loud samples (since WAV is constant bitrate).

So, I'm hoping someone who knows MythTV better than me can help me out 
with this... I'll gladly provide recording samples if anyone wants them. 
Here are my PC specs:

- Athlon 64 3200+
- NForce 3 motherboard (ASUS)
- Gentoo Linux with 2.6.12-r5 gentoo-sources kernel
- video4linux CVS (due to some cx8800 problems with the release 
version). --> shouldn't be the problem because of all the above reasons
- WinTV Go 2 with cx8800 chip
- Audigy 2 ZS (snd-emu10k1 module), with hardware mixing
- MythTV 0.18.1 compiled from source in AMD64 (as the rest of my system)
- Using PAL BG


Thank you for bearing with me through this exhaustive description.


Sérgio Gomes