CODEWITHSUNDEEP
iosavfoundation
jules testard
jules testard

Reputation: 313

Redirection playback output of avplayer item

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?

Upvotes: 3

Views: 2976

Answers (3)

yano
yano

Reputation: 4453

AVAssetReader is not ideal for realtime usage because it handles the decoding for you, and in various cases copyNextSampleBuffer can block for random amounts of time.

That being said, AVAssetReader can be used wonderfully well in a producer thread feeding a circular buffer. It depends on your required usage, but I've had good success using this method to feed a RemoteIO output, and doing my effects/signal processing in the RemoteIO callback.

Upvotes: 0

Chris McGrath
Chris McGrath

Reputation: 1936

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.

Upvotes: 4

e7mac
e7mac

Reputation: 553

There's a new MTAudioProcessingTap object in iOS 6 and Mac OS 10.8 . Check out the Session 517 video from WWDC 2012 - they've demonstrated exactly what you want to do.

WWDC Link

Upvotes: 0

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