How to program a real-time accurate audio sequencer on the iphone?

Question

I want to program a simple audio sequencer on the iphone but I can\'t get accurate timing. The last days I tried all possible audio techniques on the iphone, starting from Audio

Answer 1

If constructing your sequence ahead of time is not a limitation, you can get precise timing using an AVMutableComposition. This would play 4 sounds evenly spaced over 1 second:

// setup your composition

AVMutableComposition *composition = [[AVMutableComposition alloc] init];
NSDictionary *options = @{AVURLAssetPreferPreciseDurationAndTimingKey : @YES};

for (NSInteger i = 0; i < 4; i++)
{
  AVMutableCompositionTrack* track = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
  NSURL *url = [[NSBundle mainBundle] URLForResource:[NSString stringWithFormat:@"sound_file_%i", i] withExtension:@"caf"];
  AVURLAsset *asset = [AVURLAsset URLAssetWithURL:url options:options];
  AVAssetTrack *assetTrack = [asset tracksWithMediaType:AVMediaTypeAudio].firstObject;
  CMTimeRange timeRange = [assetTrack timeRange];

  Float64 t = i * 1.0;
  NSError *error;
  BOOL success = [track insertTimeRange:timeRange ofTrack:assetTrack atTime:CMTimeMake(t, 4) error:&error];
  NSAssert(success && !error, @"error creating composition");
}

AVPlayerItem* playerItem = [AVPlayerItem playerItemWithAsset:composition];
self.avPlayer = [[AVPlayer alloc] initWithPlayerItem:playerItem];

// later when you want to play 

[self.avPlayer seekToTime:kCMTimeZero];
[self.avPlayer play];

Original credit for this solution: http://forum.theamazingaudioengine.com/discussion/638#Item_5

And more detail: precise timing with AVMutableComposition