Redirection playback output of avplayer item

Question

What I want to do is to take the output samples of an AVAsset corresponding to an audio file (no video involved) and send them to an audio effect class that takes in a block of samples, and I want to be able to this in real time.

I am currently looking at the AVfoundation class reference and programming guide, but I can't see a way of redirect the output of a player item and send it to my effect class, and from there, send the transformed samples to an Audio output (using AVAssetReaderAudioMixOutput?) and hear it from there. I see that the AVAssetReader class gives me a way to get a block of samples using

[myAVAssetReader addOutput:myAVAssetReaderTrackOutput];
[myAVAssetReaderTrackOutput copyNextSampleBuffer];

but Apple documentation specifies that the AVAssetReader class is not made and should not be used for real-time situations. Does anybody have a suggestion on where to look, or if I am having the right approach?

Answer 1

The MTAudioProcessingTap is perfect for this. By leveraging an AVPlayer, you can avoid having to block the samples yourself with the AVAssetReaderOutput and then render them yourself in an Audio Queue or with an Audio Unit.

Instead, attach an MTAudioProcessingTap to the inputParameters of your AVAsset's audioMix, and you'll be given samples in blocks which are easy to then throw into an effect unit.

Another benefit from this is that it will work with AVAssets derived from URLs that can't always be opened by other Apple APIs (like Audio File Services), such as the user's iPod library. Additionally, you get all of the functionality like tolerance of audio interruptions that the AVPlayer provides for free, which you would otherwise have to implement by hand if you went with an AVAssetReader solution.

To set up a tap you have to set up some callbacks that the system invokes as appropriate during playback. Full code for such processing can be found at this tutorial here.