Re: Kernel idle interference??
On Tue, Apr 02, 2002 at 09:29:06PM -0500, H. S. Teoh wrote:
> [Please Cc: replies to me, I'm not subscribed due to high volume.]
>
> Recently, I noticed this very odd phenomenon with my sound card. I'm
> using an SB16 with the OSS drivers, and there is this persistent
> background static hum, which is noticeably loud on my headphones. This hum
> also has buzzes which coincides with hard drive accesses and mouse
> movements. At first I thought it was due to bad hardware, or perhaps
> broken/incompatible sound drivers; but one day, I got an idea and ran the
> following program:
>
> #include <unistd.h>
> void main() {
> nice(19);
> for(;;);
> }
>
> I ran the program in the background, basically to eat up all idle CPU
> cycles. Amazingly, when this program is running, the static hum
> disappears. When I kill it, the hum returns.
>
> Does anyone have any idea what might be causing this? I suspect it might
> be something in the kernel idle loop that's causing this; but I've looked
> over my kernel config several times and can't see any option that might
> affect this.
>From the audio-quality howto (http://www.linuxdj.com/audio/quality/):
Here's a tip from the Sound-HOWTO. Does the noise seem to correspond
with system activity? (mouse movements, disk activity, etc.) Try
booting the kernel with the no-hlt option. The "hlt" instruction
tells the CPU to go into a low-power mode when it doesn't have
anything to do (which is normally pretty often). Usually this is a
good thing-- it saves a bit of power and keeps the CPU cooler
(over-clockers beware!). For picky sound people, the no-hlt feature
can be a disaster: the CPU going in and out of hlt mode all the time
dirties up the power supply very badly, and this gets into the
soundcard.
To disable hlt, add this to your /etc/lilo.conf file (either at the
beginning, or under each image section):
append=no-hlt
My previously-quiet soundcard got VERY noisy when I put it in a new
system. I was able to drastically reduce the noise by using no-hlt.
HTH
--
Note that I use Debian version 3.0
Linux mus 2.4.17mvz4 #1 Fri Mar 15 23:30:15 CET 2002 i686 unknown
Matijs
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