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I recorded sounds using a plug in microphone on a Windows 7 laptop. I used the Windows Sound recorder to record the sounds. It was only one microphone, but when I load the sound file into Audacity, it loads 2 channels. This is to be expected, but the sound files are slightly different (subtracting them from each other in MATLAB doesn't result in a zeroed signal).

Shouldn't they be copies of one another? I didn't use any special effects etc.

Thanks

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What microphone are you using? Is it stereo? What do you get when you subtract one signal from the other? Is it still a signal or is it just noise? – endolith Nov 9 '12 at 19:42

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I am not %100 sure, but it sounds like you are recording the file in stereo. If this is the case, then Windows Sound recorder is taking stereo input, which means it's accessing 2 AD converters which have the same input applied to them. The difference between the two signals in this case would be the noise and any offset between the two converters.

I would see if Windows Sound Recorder could make a mono recording, or else use another program to make the recording.

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Yes, well I should have used Audacity etc. to do the recording. What do you mean by "AD converters"? If that's the case, how could I convert it to mono? Should I just remove one of the channels? – shim Nov 9 '12 at 6:07
@shim An analog-to-digital (AD) converter samples the incoming analog signal and converts it to an digital representation. Is your laptop even capable of recording stereo signals? If the situation is like Bjorn supposed (which is likely) you could convert the signal to mono by taking the mean of both channels which even provides some simple noise reduction or just take one of the channels. – Deve Nov 9 '12 at 8:21
@shim Wait, be careful with taking the mean. If there's a phase shift between the two channels you could end up with weird results. – Deve Nov 9 '12 at 10:29
Yes, just removing one channel is probably best. – Bjorn Roche Nov 9 '12 at 16:35
OK, so looking carefully at it, and actually both tracks are shifted up from the 0 line, but one is centred around 0.4 while another is at 0.2. How do I know which one to get rid of? Why are they different? – shim Nov 16 '12 at 0:25
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