How to apply a software compressor to the stereo output on a Mac?

Question

I'm thinking of getting a mac mini to replace my aging XBMC-only original XBOX but I'm tired of the ridiculously extreme volume disparities in modern movie audio masters. I'd like to be able to run a software compressor (or anything else for that matter) over the audio output.

I'm thinking something like JackRack but native.

Update: ischeriad's answer works brilliantly, except for the nasty fact that the volume buttons on the Mac stop working. Is it possible to do this without losing volume control from the Mac itself?

Solution

Here are instructions using Soundflower and AU Lab (part of the Developer Tools): http://www.macosxhints.com/article.php?story=200504261547021

As I understand, you can apply Audio Units like AUDynamicsProcessor to your audio stream.

Edit I have followed the instructions and made some screenshots, system is 10.6.4:

(Turn on some music)

Audio MIDI Setup:

System Preferences–Sound–Output: After changing the output device, there is no sound.

AU Lab (Developer Tools)–File–New:

Next...: Here you add the input, the descriptions for Tiger are different.

Next...:

Next...:

Done: Now you should hear sound again. Add your desired effects.

Comments

[Shabbyrobe]

I'm having a fair bit of trouble with that tutorial. I don't get sound at the point where it says I should be getting it. The guide is a bit out of date as it is for Tiger and I'm running Snow Leopard.

[ischeriad]

It works for me. I have added some screenshots, see above.

[Shabbyrobe]

The volume control doesn't seem to work any more. Is there a way I can get it back while using this method?

[studiohack]

+1 just for the time put in and the screenshots!

[ischeriad]

Thanks studiohack. Shabbyrobe, I have not found a way to get the volume control to work, it depends on the audio device if they are available (e.g. external audio interfaces typically cannot be controlled either). You have to adjust the volume from within AU Lab.

[vincent]

this answer deserves so much more credit

[Christian]

For the Input Channel configuration step I see the device's input channel in addition to the Soundflower ones. It is unclear to me what to do there to get the picture shown or why it's important. I also don't get my audio back when the tutorial says I should.