[linux-audio-user] How to turn 5.1 on ALC655?

Thomas Ilnseher illth at gmx.de
Tue Nov 22 11:19:50 EST 2005

ejdem schrieb:

>Hi there. 
>I have a little problem with my soundcard. It's onboard Realtek 
>ALC655 AC97 codec Chip on Gigabyte GA-7VT600 1394 (more infos on 
>201394.htm) and I'm using Mandrakelinux 10.1 (Official) with kernel 
>, KDE 3.4.1 (with arts) and ALSA 1.0.10 + Soundblaster 1024 
>Player (aka SB Live! Value [CT4832]) and I have two questions:
>1). how to turn on 5.1 surround sound system in ALC655 ? I've found 
>one site about it (http://alsa.opensrc.org/Alsa) and I've tried to 
>do this... but it doesn't work. 
what did you try? i have no idea about the satus of via chipset + 
Realtek AC97,
all i can tell you, i got 5.1 sound on a nvidia + CMI AC97 combination.

to achieve it, i used alsamixer (runs in Xterm, but has a ncurses 
"graphical" interface)
this gave me some toggles to switch line-in to line-out2 and mic-in to 

>2). and how to (is it possible anyway?) to use both sndcards 
>simultaneously as one card? I want to use ALC655 as a 5.1 sndcard 
>and my SB as 4 speakers (2 speakers + headphones) and mic-in. 
this is basically possible by editing the .asoundrc file. You can use 
google to look
for information about this topic,

BUT: the real problem is, that the clocks of the two soundcards are 

both cards have an own crystal oszillator that creates a frequency from 
which the 44100 kHz audio
sample frequency is derived. now the two crystall have some tollerance, 
like +- 10 ppm (ie +- 0,0001%)
(or maybe better). but the clue is:

one soundcard might not run at 44100 Hz, but on 44101 Hz, while the 
other runs, say, on on 44099 Hz.

To play some billions of samples (like some hours music) this can sum up 
to seconds of latency, which means
that after playing several hours of music, the channels may be out of 
sync for a second or so.

>I know it sounds crazy, but if someone can help me I'll be grateful
> :)
>Thanks, I hope someone can help me :)

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