My laptop uses AC'97 sound system. I installed SuSE 8.2 on it. Whenever sound plays, there is a distinct background crackling noise accompanying it.

Has anyone else experienced this? What could be done to rectify it?

Hey Ken,
try to mute all inputs (line-in etc.). What kernel version and modules are you using? There are two modules that work with the AC97. If the mute thing doesn't do the trick you should check which modules you are using. If you have the OSS module, then i would suggest you recompile you're kernel with the ALSA driver and vice versa. If that does not help, then a kernel upgrade to 2.5 could help.

I'll try that out. Ahh... I'm a complete newbie to Linux and don't know what module I'm on! But I know my kernel is 2.4. Hmm.... let me figure out how to upgrade the kernel. Aren't I stupid.....

Not much help, but...
I had crackling noises on my Epia M. It turned out that the hardware said it could play 44.1kHz, but it coudln't (not without cracklings). A newer ALSA module had the option to turn that off (and use 48kHz only) and solved the problem nicely.

Are you using ALSA or OSS? Any multi channel filter stuff in the way? xmms? esd? esound?

Hi,
I have the same problem as well, but i know that installing OSS drivers can solve the problem.
BUT, it is possible to modify a value named DXS_SUPPORT.

This is the copy of what i've found.
Hope it can help

*******************************************
VIA82xx chip has different revisions, and the behavior of the chip
depends on the revision.
basically, via chip has two different playback modes.
one is called DXS (direct sound) mode, and another is multi-channel
mode. the former supports only two-channel format but you can play
up to 4 streams at the same time. the sample rate of these 4 streams
must be identical, though.

the another mode, multi-channel mode, supports only one stream but you
can play from 2 to 6 (5.1) channels with this. obviously, this is
used for playback of AC3 decoding, etc.

in the recent ALSA via82xx driver, the DXS mode is used as default
(i.e. the first pcm device hw:0,0) on the chip models which support
it. the multi-channel mode is assigned to the secondary pcm (hw:0,1),
which corresponds to /dev/adsp in the OSS-emulation mode.

the chip with the revision 0x40, which is called VIA8233A, has no DXS
mode. other revisions, VIA8233, 8233C and 8235, do support it.
each DXS channel (from 0 to 3) has an indepedent stereo volume.
please make sure that these volumes, "VIA DXS Playback Volume", are
set to full (although they are initialized to full as default).

unfortuantely, some motherboards have the problems of DXS channels.
on some, DXS mode doesn't work at all, and on some, only 48kHz is
available (otherwise you'll hear click noises).
according to the people of VIA tech, this is because of the BIOS
(typically AWARD BIOS). so, one solution is to update the BIOS.
but it's not always a preferred solution.

in the recent driver version, a workaround is provided instead.
there is a module option "dxs_support", which defines how to handle
the DXS channels. if you have a problem with the first pcm device,
try dxs_support=2 or dxs_support=3.

when dxs_support=2 is given, the DXS mode is disabled and you'll have
only the multi-channel playback mode. this means, the chip is handled
as VIA8233A. of course, you can play only on stream. in this case,
dmix plugin would be a workaround.

when dxs_support=3 is given, only 48kHz is allowed for the DXS
channels. that means, the sample-rate conversion will be done in
alsa-lib or OSS-emulation for playing MP3. but it's cheaper than dmix
plugin. so, try this once, and if still doesn't work, try the
previous one.

how to add such an option? if you already have module options for
via82xx driver, you should see the line like following in
/etc/modules.conf

options snd-via82xx index=0

and just add dxs_support=2 (or 3) there.

options snd-via82xx index=0 dxs_support=2

then restart the driver.

Yup, that's about the text that helped me get it working on the Epia M.
I was kind of fragged before I found it, since I got it just to use as mp3 player. :-)

Note that the ALSA version needed is flagged too new by for instance Gentoo. (0.9.6?)

I've been trying to edit the /etc/modules.conf file just as Tancrede said.

Can anyone explain to me what OSS and ALSI are? And where I can get them? It'd be most appreciated.

OSS and ALSA are the two variants in Linux how soundcards is handled.
OSS is the old open sound system. It's nice, simple. Some people decided it was not enough, and hence ALSA. You have one of those if you can play sound at all.
What dist do you have? You can probably switch by just some rpm tool.

If you have alsa, you probably have a /proc/asound:

% cat /proc/asound/version
Advanced Linux Sound Architecture Driver Version 0.9.4.
Compiled on Jul 14 2003 for kernel 2.4.20 (SMP) with versioned symbols.

More about ALSA: http://www.alsa-project.org/

I usually recommend switching to ALSA with any problem. :-)
It can give some other problems, but it usually has all the solutions to them too.

Sorry all for asking the stupid question. I just read the fine manuals a bit and it and found out that ALSA = Advanced Linux Sound Architecture and OSS = Open Sound System.

But my distro's documentation doesn't explain what they really are. Can somebody briefly describe the purpose of and the difference between the two?




Thanks for putting up with this lazybones.

Thanks Mirar. I'll continue reading the fine manuals to learn more.

First, sorry about my english.

The noise was caused by the wrong PCM spectrum demodulation sample rate at front DAC output (it seems to happen only in some systems, I haven't reported the bug)

Try this:
First, download the latest 0.9.X alsa-driver release from http://www.alsa-project.org/, and expand it.
Edit ./alsa-kernel/include/ac97_codec.h, from the root directory of driver, find the string AC97_FRONT_DAC_RATE, and change the value 0x2c to 0x2e (the same value as AC97_PCM_SURR_DAC_RATE).
./configure; make; make install, and enjoy!.

Greetings from Argentina.
Sebastián Wilwerth.