[linux-audio-user] noise from soundcard

Mark Knecht mknecht at controlnet.com
Thu Oct 16 18:06:45 EDT 2003


> This is a little off-topic, but I hope you don't mind.  I get quite a
> lot of noise from my soundcard.  There is a pulsing that sounds
> something like a very fast morse code at a consistent pitch, and it's
> unacceptably loud.  I notice variations in this noise based on how I
> move my mouse.
>
> At first I thought this was just because of my shoddy 5-year-old
> Soundblaster Live, but I have a new Audiophile 2496 and it does the same
> thing.
>
> Today I tried removing the stereo mini plug from my soundcard and
> touching it to the metal on my case.  The sound was nearly identical.
> >From the little that I know about electronics, I conclude that the
> problem is voltage fluctuations in the metal of the case.  Does this
> mean my case needs to be grounded somehow?
>
> What can I do to fix this problem?
>
> Josh

Josh,
   Not at all sure about the case grounding part. That's pretty strange.
Maybe your external speakers and your PC are on different power grounds?
Watch out if either is 2-prong. People get killed in the US every year due
to our unsafe power system.

   The mouse part of this is a great example of what happens in some systems
when the sound card IRQ is 'behind' the mouse IRQ, or possibly shared with
the mouse IRQ. The 'IRQ ordering' in a standard PCI (non-SMP, non-IOAPIC)
is:

0,1,8,9,10,11,12,13,14,15,3,4,5,6,7

(sometimes the #8 is replace by the #2 - don't worry about that.)

Do an lspci to see what IRQ's you mouse and sound card have.

The reason you'd get a noise when you move you mouse is that the mouse
interrupt handler is called, it takes time, and that delays the sound card
doing it's work. This delay empties a buffer or something and that causes a
problem.

A consistent sound could be some other device, like a disk drive, creating
interrupts and your sound card being on a bad IRQ.

If you put a sound blaster on IRQ 5, 6 or, then in my experience I have had
a lot of problems.

This could all be correct and solve your problems, or it could have nothing
to do with your system. ;-)

Hope this helps,
Mark





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