Using AudioUnits to play and eq songs from music library

Question

I'm building an app that involves playing songs from the user's music library while applying an equalization (EQ) effect. I've only used AudioUnits to generate sound before, so I'm having a bit of trouble.

My current plan is to use AVAssetReader to get the samples, and though I'm a bit fuzzy on that, my question here is with regards to the correct AudioUnit design pattern to use from Apple's documentation: https://developer.apple.com/library/ios/documentation/MusicAudio/Conceptual/AudioUnitHostingGuide_iOS/ConstructingAudioUnitApps/ConstructingAudioUnitApps.html#//apple_ref/doc/uid/TP40009492-CH16-SW1.

My guess is that a render callback is needed to perform my EQ effect (I was thinking kAudioUnitSubType_ParametricEQ), so that leaves either the "I/O with a Render Callback Function" pattern or the "Output-Only with a Render Callback Function." If I'm reading data from the music library (potentially via AVAssetReader), which of these two patterns would be the best fit?

Answer 1

I think you would need to use an Output-Only with a Render Callback Function. The callback function should be responsible for reading/decoding the audio data, and applying the EQ effect.

By the way, I don't know if this might be useful in any way, but here it says that there's an already existing EQ audio unit that you could use.