For some time, auto tune is used as a starting point for creating special audio effects for example known by Skrillex or T-Pain. I never found such software for free for Mac Os X, but today I read that even old versions of Garage Band can do this job. I think many Apple user do have an old version of this software.
So I tried the auto tune ability of garage band (version 4.1.1). And that is what I have done. By the way I do not know this software very well.
First of all create a new project and audio track. For that you have to press the plus button at the left botton and choose Real Instrument Track. The next step is importing your audio file. One way is dragging the file into the new audio track. After that you have to open the Track Editor. Just click on the scissors button at the left botton.
We are almost complete. At the right of the transport control is a drop down. Please choose Project. Than set Enhance Tuning to 100 % and check Limit to Key. Finally select the appropriate scale and listen to the result.
The audio export is such simple. Go to the menu Share/Export song to disk and uncheck compress. Maybe you should change the preferences in order to get a 24 bit export. So go to GrageBand/Preferences/Advanced menu and set the Audio Resolution to Best. And here is my result.
The vocals without and with auto tune effect:
I think, the result is not bad. Unfortunately, there is just the major and minor scale implemented, but I think in most cases it will be enough. Of course this is not a plugin solution, on the other hand it’s free. Keep in mind that this is only a starting point for audio processing.
To be continued & best regards.