[mythtv-users] Another Audio Question, I need to figure this out

Poul Petersen petersp at MATH.ORST.EDU
Fri May 30 23:54:04 EDT 2003


> Ok I'm kinda lost when it comes to setting up this audio for myth. Is there 
> any way of figuring out which input to set in setup for the capture card? I 
> mean I have things like /dev/dsp, /dev/dsp1, /dev/dspW,  /dev/sound/dsp, 
> etc. Likewise how do I know which one to use for playback? People keep 
> saying to use the btaudio drivers for inputting into the Wintv 401 capture 
> card, but what input would that be, i'm so lost.
> 
> My main problem is when I'm in KMix and I select my line in for recording 
> (as I hear audio on that input), and then I select mute, and it does not 
> mute. So that is pretty useless, is there some other way to mute linein? Is 
> this why OSS drivers don't work?
> 

	The main reason to use ALSA drivers is that many (all?) OSS
drivers are *NOT* full-duplex. What "full-duplex" means is that you can
both write to and read from /dev/dsp. So, if you have a full-duplex
audio driver, you would capture and play back to /dev/dsp. This is why
you mute the Line-IN. If you don't mute it, your mixer will mix the
Line-IN channel into the sound output. At the same time, mythtv will be
recording the Line-IN and then playing it back, so you'll get double
sound. 

	If you use btaudio (modprobe btaudio), it will send the output
on a different dsp device. In order to determine what device it is using,
try typing:

	dmesg | grep dsp

	You should see a line that says something like

	btaudio0: [other stuff] using /dev/dsp2 [digital]
	btaudio0: [other stuff] using /dev/dsp3 [analog]

	Or something like that, I can't get into my box to cut and
paste. Anyways, that /dev/dsp2 (or whatever yours would be) is where you
would capture the audio from. Notice that if you load btaudio *before*
your sound driver, then btaudio will probably take /dev/dsp and your
sound card will get some other dsp device, so be wary of the order you
load these modules (you can do some magic with post-install and
pre-install in /etc/modules.conf to make sure the order goes the way you
want).

-poul


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