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I am watching a long video on YouTube and the audio is terrible, mostly because there is a lot of high-frequency noise. I'm about to get a headache.

Is there any software that can capture my system audio out (like Airfoil can) and run it through filters, or even a simple multi-band equalizer?

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up vote 5 down vote accepted
  • Set Soundflower as the default output device (in order to capture output from applications that don't support custom output devices)
  • In the application that's used for the effects, set the input device to Soundflower and the output device to a normal output

The default output device can be changed from the Sound preference pane or Audio MIDI Setup:


The application that's used for the effects needs to support:

  • Custom input and output devices (Fission doesn't?)
  • Playthru with effects (Audacity doesn't?)

I added instructions for Garage Band and AU Lab, but any DAW or standalone version of an effect plugin would probably do as well.


Garage Band only allows you to make a new project by choosing a template. Anyway choose for example the Voice template:

Change the input and output devices in the preferences:

Focus an audio track, and enable playing back the input:

Add effects in the edit tab:


AU Lab was installed with Xcode before 4.3, but it can now be downloaded from https://developer.apple.com/downloads/.

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+1 for the clear instructions. – JRobert Aug 11 '11 at 19:30
    
My hero. Thank you for the clear instructions for both GarageBand and AU Lab. – Alan H. Aug 11 '11 at 20:46
1  
AU Lab is now available through apple.com/itunes/mastered-for-itunes – ktolis Dec 26 '12 at 22:00

Another viable solution is to use Airfoil, which can capture system audio and play it back locally (with a ~3-sec delay), and has an Effects menu which has a multi-band equalizer. (It’s not free but it’s a useful bit of software I already owned!)

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Soundflower is free and can capture your audio and send it through another application such as iTunes, which has an equalizer.

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Intriguing. iTunes can “stream” from Soundflower? How? – Alan H. Aug 11 '11 at 2:14
    
Ah, yes - I don't know that iTunes can. Try audacity.sourceforge.net instead. Or Garage Band, as Lri suggests. – JRobert Aug 11 '11 at 19:29
    
I couldn't figure out how to do this with Audacity either. You can hear the input from Soundflower by selecting Transport - Software Playthru (on / off) and recording, but I didn't find a way to apply any effects in real time. – user495470 Aug 12 '11 at 17:59

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