[mythtv-users] audio problems

Joseph A. Caputo jcaputo1 at comcast.net
Tue Sep 14 13:08:45 EDT 2004

On Tuesday 14 September 2004 11:48, Michael Skaastrup wrote:
> Hi all
> I have a running myth setup, and love it. Thanks.
> I do have one problem. I use a bttv capture card with an audio loop
> cable to the soundcard (Pinacle PCTV RAVE)
> I do get sound from my setup, but it sounds distorted, almost like if
> it was 8bit/8000 hz
> When looking at /proc/asound/card0/codec97#0/ac97#0-0 i can se that
> the DSP runs at 44100 hz. (Corrected that from 8000 hz)
> I did it to test whether or not it was sampling rate related, but it
> didn't change anything.
> I'm kinda stuck for now, so i really would appreciate any info from
> you gurus.
> When i do aplay /dev/dsp it sounds crappy, when i unmute the line-in
> in alsamixer it sounds ok, so it isn't the capture card that sends
> distorted audio.
> I'm running FC2 and Myth 0.15. The sound card is an onboard intel
> one.

IIRC, some sound chips can only do full-duplex by cutting the sample 
resolution in half (i.e. 16 bit -> 8 bit).  If this is the case with 
your setup, you would probably notice the 'bad' sound in LiveTV or 
programs that you recorded while watching the in-progress or another 
recording.  Try 'arecord test.wav', then play it back, or try recording 
a program while nothing is using the sound card for output, and see if 
the sound quality is better.

If it turns out that this is what your onboard sound is doing (reducing 
the sample bit resolution for full duplex), you have several options:

- get a tuner card that doesn't require an audio loop cable (i.e., 
something that supports btaudio or a hardware MPEG encoder card)

- get a better sound card that does full duplex well

- resign yourself to never watching/listening to anything while a 
recording is in progress, or get crappy sound if you do.



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