“Omnidirectional Mic” is Apple speak for mono mic in

Drove myself nutty trying to get stereo microphone input under slackware on my macbook pro (MBP 5,5).

I made a screencast under OSX using the built in version of quicktime. I recorded screencapture in one process, my voice and face (from the iSight cam) in another process, and merged them together in the movie editor. The lame result is on youtube complete with utter failure at the end. Makes me laugh when I see it, and it is a word of caution to use a script, not go off the top of your head.

Anyway, that’s not the point of this post. My point is getting the macbook pro internal mic to work under Linux. I ran across several tips and howtos on using ffmpeg to record the screen and capture the iSight input, so I wanted to do that rather than boot into OSX. But for some reason, no matter what I did the recorded sound was in mono and only on the left channel. Immediately I unfairly suspected that my Linux software was to blame, but found nothing on the interwebs about such an issue. I recompiled the kernel, downloaded and installed PulseAudio and all its dependencies, and so on, with absolutely no effect. The only thing I could do was make the input sound worse by using arecord directly.

Turns out, there is no such thing as stereo input on the MBP 5,5 — at least, none that I can find. Under OSX, it appears that “stereo” is actually mono piped to two channels. To test this I made a little audio recording using QuickTime and snapped my fingers right and left. No difference in my headset during playback, just a snap right in the middle of my brain. And if you look up the specs on the Apple website, you’ll see “Omnidirectional microphone”

So the only problem with Linux is trying to mix the left channel to both channels.


2 thoughts on ““Omnidirectional Mic” is Apple speak for mono mic in

  1. As far as I know, all mics are mono. You need two or more mics to get stereo sound. Omnidirectional only means it picks up sound from any direction.

  2. Very true. I just *thought* that there were two mics in the hardware and that Linux was failing to pick them both up. I was wrong of course, and I’m blogging this because I kept typing in search terms like “mono microphone macbook pro linux” and getting nothing helpful on Google.

    I do own a tiny plug-in microphone that has two mics and records in stereo. But it doesn’t have a preamp, and so apparently it doesn’t work with the line-in/line-out port either. At least, I think that is the problem. I couldn’t get it to work in Linux or in OSX.

    Interestingly, I can get a whole lot of garbage noise to be recorded on both channels by using Alsa’s arecord, but higher level tools like ffmpeg don’t make that mistake.

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