Issue with MTAudioProcessingTap on device - ios

Question

I'm trying to create an MTAudioProcessingTap based on the tutorial from this blog entry - http://chritto.wordpress.com/2013/01/07/processing-avplayers-audio-with-mtaudioprocessingtap/
The issue is working with various audio formats. I've been able to successfully create a tap with M4A's, but it is not working with MP3's. Even stranger to me - the code works on a simulator with both formats, but not on the device (only m4a works). I'm getting OSStatusError code 50 in my process block, and if I attempt to use the AudioBufferList data, I'll get a bad access. The tap setup and callbacks I'm using are below. The process block seems to be the culprit (I think) but I do not know why.
Update - It seems like it is very sporadically working on the first time after a bit of a break, but only the first time. I get the feeling there is some sort of lock on my audio file. Does anyone know what should be doing in the unprepare block for clean up?
Unprepare block -
void unprepare(MTAudioProcessingTapRef tap)
{
NSLog(#"Unpreparing the Audio Tap Processor");
}
Process block (will get OSStatus error 50) -
void process(MTAudioProcessingTapRef tap, CMItemCount numberFrames,
MTAudioProcessingTapFlags flags, AudioBufferList *bufferListInOut,
CMItemCount *numberFramesOut, MTAudioProcessingTapFlags *flagsOut)
{
OSStatus err = MTAudioProcessingTapGetSourceAudio(tap, numberFrames, bufferListInOut,
flagsOut, NULL, numberFramesOut);
if (err) NSLog(#"Error from GetSourceAudio: %ld", err);
}
Tap Setup -
NSURL *assetURL = [[NSBundle mainBundle] URLForResource:#"DLP" withExtension:#"mp3"];
assert(assetURL);
// Create the AVAsset
AVAsset *asset = [AVAsset assetWithURL:assetURL];
assert(asset);
// Create the AVPlayerItem
AVPlayerItem *playerItem = [AVPlayerItem playerItemWithAsset:asset];
assert(playerItem);
assert([asset tracks]);
assert([[asset tracks] count]);
self.player = [AVPlayer playerWithPlayerItem:playerItem];
assert(self.player);
// Continuing on from where we created the AVAsset...
AVAssetTrack *audioTrack = [[asset tracks] objectAtIndex:0];
AVMutableAudioMixInputParameters *inputParams = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:audioTrack];
// Create a processing tap for the input parameters
MTAudioProcessingTapCallbacks callbacks;
callbacks.version = kMTAudioProcessingTapCallbacksVersion_0;
callbacks.clientInfo = (__bridge void *)(self);
callbacks.init = init;
callbacks.prepare = prepare;
callbacks.process = process;
callbacks.unprepare = unprepare;
callbacks.finalize = finalize;
MTAudioProcessingTapRef tap;
// The create function makes a copy of our callbacks struct
OSStatus err = MTAudioProcessingTapCreate(kCFAllocatorDefault, &callbacks,
kMTAudioProcessingTapCreationFlag_PostEffects, &tap);
if (err || !tap) {
NSLog(#"Unable to create the Audio Processing Tap");
return;
}
assert(tap);
// Assign the tap to the input parameters
inputParams.audioTapProcessor = tap;
// Create a new AVAudioMix and assign it to our AVPlayerItem
AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
audioMix.inputParameters = #[inputParams];
playerItem.audioMix = audioMix;
// And then we create the AVPlayer with the playerItem, and send it the play message...
[self.player play];

Answer 1

This was apparently a bug in 6.0 (which my device was still on). Simulator was on 6.1
Upgraded device to 6.1.2 and errors disappeared.