Built in Mic recording high sound (no feedback!)

[freque]

Hi!

I have a weird situation. When i record something with my built in mic, I get a really high-pitched tone into my recordings. It's no feedback, cause there is no output at the time of recording.

Here is a recording of it:
http://dl.dropboxusercontent.com/u/968291/peep from built in mic.mp3

It is no dust, no powercable problem, no input from anything else, no usb ports are used... but what is it?

I tried a PRAM reset... no results.

Has anyone got a clue? Or how to "reset" my built in mic?

MacBook4,1 / Intel Core 2 Duo / 2,1 GHz

 

[gr8tfly]

Has it done it for a long time, or just suddenly started?

Does it start at a high level, then taper off by itself (like in your clip)?

It kind of sounds like inverter noise, though smoother (more sine, less square/pulse; even saw would sound rougher - though all this listening was done using the iPad 3's built-in speaker at normal levels).

Try recording with the display sleeping. The inverter should then be powered down.

If that helps or eliminates the sound, then it most likely narrows the problem down to the inverter board and/or associated cabling - especially the ground connection (the cable has a tab to fasten to the display assy with a grounding screw).

 

[freque]

Thanks for helping!

it started a while ago. usually i don't use the built in for recordings. so i don't know when this started. i noticed it when i was on skype with a friend. Well, he noticed it...

I tried to record when the monitor was asleep, but nothing changed in the sound.

it's fading by itself. but when there is a little input the sound is a bit fluctuating with the input. It's like a gate of ducker...

He is a picture of the signal highly enlarged!

 

[gr8tfly]

Thanks. Good tech info to work off of.

The waveform looks like something switching at around 1176 Hz. As you can see, it's not a sine wave, or at least it's highly distorted.

Looks like it's sampled at 22 kHz (I got 20 just eyeballing it, but the nearest standard is 22). I'm curious to see what it looks like at a higher sample rate - 44 kHz or higher. A screen shot of around the same length would be fine. I'm not sure I can identify it, but maybe seeing a bit more of the waveform would help.

Background is engineer s/w & electronics (ret now); have done some audio work too (mastered a band's CD), but lots and lots of lab time with lots of different 'scopes looking at RF and many kinds of modulation. Also, licensed Amateur Extra. I might not nail it, but I might have some ideas of where to look.


edit:

Thinking a bit more on this: switching supplies and even the Macbook inverter run at much higher frequencies, so there's less chance they're generating any noise in the audio frequency range (though, in the back of my mind I remember some reports a long time ago of audio noise in a notebook). However, there are some inverters that do operate at relatively low frequency, in particular, those for electroluminescent lighting (EL panel, wire, strips).

For instance, the Motorola Razer had an EL backlight and I personally went through a couple of them to find one quiet enough to use while the backlight was on. Going back a tad further in time, my Newton 130 has a nice whine to it when the backlight is on.

Add any new gadget recently that might fall into that category (or moved nearby the Macbook)? An experiment would be to move the Macbook into a different room and see if there was any change. It still wouldn't explain why the noise fades, but something to think about anyway.

When you're doing tests, try turning off the noise reduction feature (in Sound Pref pane). That might explain why the sound seems to ride up and down in volume along with a voice (kind of a long shot, but noise reduction can have some odd effects at times).