How to disable speaker and internal mic compression

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Level 1

13 points

How to disable speaker and internal mic compression

When I first started using my MacBook Pro I noticed the playback was odd, different from my PowerBook. It sounded compressed. I sort of ignored this until I noticed (while demonstrating saturation to a class) I couldn't distort the signal using the internal mic: it was compressing automatically. Several students have also noticed this when recording in Garage Band. I've tried it with Logic, Audio Companion, Amadeus Pro, and there is definitely compression; you can even see the attack, at .05 milliseconds. Here is an image of the compression, when recording using the internal mic: http://fileserver.music.utah.edu/cottle/comp/comp.png

(I just used my voice for this, and I admit this is rather unscientific, but it's still pretty clear.)

How do you turn this off? I've disabled ambient noise reduction in System Preferences/Sound, doesn't help. I've adjusted the volume sliders in Audio/Midi Setup, no change.

I haven't done any controlled tests with the speakers (i.e. setting up a mic with a separate system and sending a test tone), but I'm convinced it's being compressed too.

MacBook Pro, Mac OS X (10.4.7)

Posted on Feb 25, 2007 11:56 AM

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Answer 1

iVmichael

Level 6

13,740 points

Feb 25, 2007 4:17 PM in response to David Cottle

It's not the volume you need to change in Audio MIDI Setup but the FORMAT ... try 44.1Hz at 16-bit ?

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Answer 2

David Cottle Author

Level 1

13 points

Feb 25, 2007 4:50 PM in response to iVmichael

I don't understand how bit depth would cause compression. Bit depth is bit depth. It doesn't change over the space of .05 milliseconds (i.e. the signal starts out distorting, then adjusts the level so as not to distort - how would bit depth cause that?), but I tried it, and it still compresses.

Look at these two snapshots. One was done using Audio Companion, the other Amadeus Pro (to rule out a setting in either of those programs):

http://fileserver.music.utah.edu/cottle/comp/compa.png
http://fileserver.music.utah.edu/cottle/comp/compb.png

The signal is a _steady state_ sine tone generated in SuperCollider. It begins by saturating, then ducks below a steady state (maybe this is the speakers kicking in with it's compression too?), then settles on a steady state; not saturated.

In short; you can't saturate the input of the internal mic because there is a compressor. How do you turn it off?

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Answer 3

iVmichael

Level 6

13,740 points

Feb 25, 2007 4:59 PM in response to David Cottle

I think the internal microphones 'natural state' is 96Hz at 24-bit and any other setting is going to be a modification of that.

I see what you mean about the saturated/unsaturated sound waves but can't see how that would be the speakers ...

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Answer 4

David Cottle Author

Level 1

13 points

Feb 25, 2007 7:20 PM in response to iVmichael

If what you suggest is correct, that by changing the sample rate or bit depth I'm causing compression, then it should happen with the internal mic and the line input. Here is a test tone recorded using the line in; both saturated (too hot) and at a normal level. Note that there is no CHANGE in the first few milliseconds.

http://fileserver.music.utah.edu/cottle/comp/lineinput.png

Here is the same signal recorded with a headset firmly pressed over the internal mic, to avoid a proximity error. I did it several times to make sure it was happening every time. Note the CHANGE in volume level.

http://fileserver.music.utah.edu/cottle/comp/micinputcomp.png

And I've also since confirmed that the speakers are compressing too. The same steady state sine tone, played through the speakers, recorded with a Neumann KM 100, MBox. I get the same effect with my MacBook Pro and an intel iMac.

http://fileserver.music.utah.edu/cottle/comp/Speakercomp.png

Again, note the RADICAL change in volume in the first few milliseconds. This can't be sample rate or bit depth. It has to be automatic gain control.

I'm not saying compression is wrong. They probably put it in as a "feature." But I want to be able to turn it off. I've triple checked the ambient noise reduction option in System Preferences/Sound

Here are the mp3s. You can clearly hear saturation on the linein, but not the mic. (Use headsets, because your speakers will compress it!)

http://fileserver.music.utah.edu/cottle/comp/linein

http://fileserver.music.utah.edu/cottle/comp/micinput

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Answer 5

Feb 25, 2007 7:26 PM in response to David Cottle

In your experiments you have found on both a MacBookPro and an Intel iMac:

The internal mic has AGC and there's no control panel to disable it

AND

The built-in speakers run through a compressor circuit

BUT

The headset in uncompressed

Did I get it all?

Thanks for this research.

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Answer 6

David Cottle Author

Level 1

13 points

Feb 25, 2007 7:57 PM in response to Robin Grant

Yes:

Headset (lineout), and linein has no AGC.

Speakers and mic input appears to have AGC.

I can't find a control panel to disable either.

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Answer 7

David Cottle Author

Level 1

13 points

Feb 25, 2007 7:59 PM in response to Robin Grant

I didn't test the iMac as thoroughly, but I've observed the same effect on the speakers and mic. (I first noticed it on an iMac.)

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Answer 8

Feb 26, 2007 6:55 PM in response to David Cottle

Who do we complain to about this? I need to know that the built-in audio has auto-compression.

I can understand Apple wanting to protect the speakers and themselves from customers that have blown laptop speakers, but I need to be warned that I am not hearing the actual audio levels.

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