Tuesday, 10 November 2009

Audio Weirdnesses

Yesterday I got poking around a perplexing audio recording issue; quite bizarrely stereo (2 channel) audio from the internal microphone would record perfectly, but mono (1 channel) would not. After working my way down through the driver and though ALSA I got to the point where I could not see how there could be any driver problem, so the issue had to be with the ALSA user space library, or the hardware (or both!).

My colleague spotted an interesting characteristic - using alsamixer to set the capture level to 100% left channel and 0% right channel or vice-versa made the mono recording work. Further investigation by using stereo recording of a pure sine wave (whistling into the microphone!) showed that the left channel was the complete reverse of the right channel. Then the penny dropped - the mono recording was essentially ALSA summing left and right channels, causing a near perfect zero recording since the left channel was the inverse of the right channel. Doh.

As it is, this very same issue has been discussed on LKML here with a suggested workaround described by Takashi Iwai here as follows:

Put the below to ~/.asoundrc
pcm.imix {
type plug

slave.pcm "hw"

ttable.0.0 0.5

ttable.0.1 -0.5


And record using:

arecord -Dimix -c1 test.wav

Urgh. Well, at least there is some kind of ALSA workaround.

So the moral of the story is to twiddle with left and right channel levels and do some stereo recording and look at the results before hacking through the driver. I should have applied Occams' razor - I assumed the complex part (the driver) was the problem.

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