[mythtv-users] Shuttle SK41G sound problem

Joseph A. Caputo jcaputo1 at comcast.net
Thu Dec 18 13:15:22 EST 2003


On Thursday 18 December 2003 11:59, Steven Marcotte wrote:
> ARRRGHHHH!  I need help.
>
> I'm setting up a new Myth box and I have a problem with the sound. 
> The TV card is a generic BT8x8 card without a DSP, so the internal
> audio out on the TV card to the CD in on the motherboard.  The
> hardware seems to works fine; I can use aplay to play sounds through
> the ALSA driver, I can use xawtv to watch TV with sound.  I can not,
> however, start xawtv, mute the CD in, and successfully execute 'aplay
> /dev/dsp'.

This may be for one of 3 reasons:
- You need to set the CD as the capture input in your mixer
- CD is the capture device, but 'muting' it sets the volume to 0.  Look 
for a separate slider for the 'Capture' volume.
- Your card/driver doesn't support full duplex (ie, simultaneous record/
playback

For the first 2 issues, see below.  For the third, you either need new 
drivers, a new sound card, or both.

>
> This is where it gets weird.  If I start watching Live TV, and I set
> the volume to 50%, the CD to only 5%, and unmute the CD in, it is
> distorted and LOUD, VERY VERY LOUD!  I'm talking blood from the ears
> loud!  The audio that is played is the raw audio feed, it is not in
> sync with the video.
>
> xawtv doesn't have this problem, I can set the volume to 50% and the
> CD to 100% and it is a nice respectable listening level; in fact, it
> is almost too quite.
>
> I originally tried Gentoo with a few different kernels and the ALSA
> 0.98 drivers, with no luck.  I thought maybe the problem was my
> aggressive CFLAGS so I installed KnopMyth R4.  Still no luck.
>
> Anyone have any suggestions?
>
> HARDWARE
> Shuttle SK41G
> AthlonXP 2400+
> 512MB PC2700
> Maxtor 120 GB HDD
> Generic BT8x8 based card
>
> SOFTWARE
> KnopMyth R4
> via82xx ALSA driver
> bttv video driver


Okay, here's what MIGHT be going on:

When using Xawtv, 3 mixer controls affect the overall volume:  'CD' and 
'Master Volume', plus the output volume of your tuner card (technically 
not a mixer slider, but you get the idea).  When *recording* with 
MythTV, the controls 'CD' (possibly plus a separate 'Capture' slider, 
depending on your sound card/driver/mixer app) and the output volume of 
your tuner card affect the volume of the encoded file.  When *playing 
back* with Myth (either playing back a recorded program or LiveTV), 
there is a change:  the 'CD' (and 'Capture') controls do not affect the 
output volume of the playback, but 'Master Volume' and 'PCM' do.

To properly adjust your volume, you should adjust the playback and 
recording volumes separately.  To adjust your playback volume, use 
something like mplayer + a DVD or ripped movie (a known good file), and 
adjust the 'PCM' and 'Master Volume' while it's playing.  Make a note 
of the settings.  Mine are 'Master Volume' 100% and 'PCM' 75% for good 
volume without distortion.  Keep in mind that mythfrontend may try to 
reset these volumes on startup; this can be adjusted in 
Setup->TV->Playback.  You can have mythfrontend adjust your levels to 
these values whenever it starts up, or you can have it leave them 
alone.  Your choice.

Now that you've got your playback levels set, you'll need to adjust your 
recording levels.  Here you just need to play with the output volume of 
your tuner card + the 'CD' slider (on some setups, the 'CD' slider 
volume will be '0%' for mute, and there is a separate slider to control 
the capture volume).

To summarize:
(note: for most people, replace 'CD' with 'Line In')

XawTV:
	Tuner LineOut -> CD Volume -> Master Volume -> sound you hear

MythTV:
	Recording:
	Tuner LineOut -> CD (or Capture) Volume -> encoded file/ringbuffer

	Playback:
	decoded file/ringbuffer -> PCM Vol -> Master Vol -> sound you hear

LiveTV = Recording -> Playback

Hope that helps.

-JAC



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