Audio recorded through my soundcard (Terratec Aureon 5.1 PCI) sounded a bit "metallic" and distorted, so I carried out a test; I burned an 800Hz test tone to CD, connected a CD player to the analogue line in and recorded it back.

The resulting distortion is clearly shown here:

I am using the latest drivers and control panel for the sound card. I also get the same thing if I use the Microsoft Sound Recorder instead of Sound Forge.

Just to be sure, using the same CD, player, cables, etc. I captured through the motherboard's on-board line in. This time there was no distortion. (Although I have other issues with the on-board audio, but that's another story...)

Any idea what causes this and how I can fix it?

That doesn't look good, more like a hardware glitch. I'd write an email to Terratec.

Oh and I see that the distortion occurs in a certain fixed interval, try to find out the duration of that interval, and then calcutate the frequency. May lead you to the cause of the glitch, maybe a certain hardware part or even some problems with your motherboard's PCI bus.

Could you please upload that recorded wave? May be just a little part of it.

@Fandango: I'm still waiting to hear back from Terratec. The manual does say to select a PCI slot "where possible, that is not directly next to an existing card because some cards, e.g. graphics cards, may emit signals that could interfere with your sound card". So I'll try moving the sound card to another slot, to see if that helps.

@rutra80: OK, I'll upload a bit of the wav to the upload forum if I can.

Certainly looks like a timing error of some description.... like somewhere, a clock is resyncing when it realises it's getting out of sync, or something like that.

Well, the most visible distortion (looking at the wave) occurs every 20 samples. At a sampling rate of 48kHz this means the distortion occurs in a frequency of 2.4kHz.



Check your board(s) for a oscillator component with a 24(xx) or 2.4 printed on it... maybe it's just that.

But I don't know... there are prominent overtones (thus the bell sound), and you didn't say whether this test sample was a pure sine wave or not, so I don't know if this is intentional or something to worry about:



You can see the 2.4kHz distortion here at detail, its intesity varies too:




Also note that either your line in or your cd player (cables?) seems to have a problem on the right channel as you can see on this pan view here:




Well, besides the nice pictures I can't help you that much since I know nothing about electronics. But maybe someone else find the info useful.

It's like if the software was specifying a sample rate while the soundcard specifies another, and there was "nearest neighbour" resampling between them.
Check that no audio is playing while you record, and that you have not a given sample rate setup in the soundcard while you are recording in a "new file" of another sample rate.