multi track mp3 playback for iOS application - objective-c

Question

I am doing an application that involves playing back a song in a multi track format (drums, vocals, guitar, piano, etc...). I don't need to do any fancy audio processing to each track, all I need to be able to do is play, pause, and mute/unmute each track.
I had been using multiple instances of AVAudioPlayer but when performing device testing, I noticed that the tracks are playing very slightly out of sync when they are first played. Furthermore, when I pause and play the tracks they continue to get more out of sync. After a bit of research I've realized that AVAudioplayer just has too much latency and won't work for my application.
In my application I basically had an NSArray of AVAudioPlayers that I would loop through and play each one or pause/stop each one, I'm sure this is what caused it to get out of sync on the device.
It seemed like apple's audio mixer would work well for me, but when I try implementing it I get a EXC_BAD_ACCESS error that I can't figure out.
I know the answer is to use OpenAL or audio units but It just seems unnecessary to spend weeks learning about these when all I need to do is play around 5 .mp3 tracks at the same time. Does anyone have any suggestions on how to accomplish this? Thanks

Answer 1

thanks to admsyn's suggestion I was able to come up with a solution.
AVAudioPlayer has a currentTime property that returns the current time of a track and can also be set.
So I implemented the startSynchronizedPlayback as stated by admsyn and then added the following when I stopped the tracks:
-(void) stopAll
{
int count = [tracksArr count];
for(int i = 0; i < count; i++)
{
trackModel = [tracksArr objectAtIndex:i]
if(i = 0)
{
currentTime = [trackModel currentTime]
}
[trackModel stop]
[trackModel setCurrentTime:currentTime]
}
{
This code basically loops through my array of tracks which each hold their own AVAudioPlayer, grabs the current time from the first track, then sets all of the following tracks to that time. Now when I use the startSynchronizedPlayback method they all play in sync, and pausing unpausing keeps them in sync as well. Hope this is helpful to someone else trying to keep tracks in sync.

Answer 2

If you're issuing individual play messages to each AVAudioPlayer, it is entirely likely that the messages are arriving at different times, or that the AVAudioPlayers finish their warm up phase out of sync with each other. You should be using playAtTime: and the deviceCurrentTime property to achieve proper synchronization. Note the description of deviceCurrentTime:
Use this property to indicate “now” when calling the playAtTime: instance method. By configuring multiple audio players to play at a specified offset from deviceCurrentTime, you can perform precise synchronization—as described in the discussion for that method.
Also note the example code in the playAtTime: discussion:
// Before calling this method, instantiate two AVAudioPlayer objects and
// assign each of them a sound.
- (void) startSynchronizedPlayback {
NSTimeInterval shortStartDelay = 0.01; // seconds
NSTimeInterval now = player.deviceCurrentTime;
[player playAtTime: now + shortStartDelay];
[secondPlayer playAtTime: now + shortStartDelay];
// Here, update state and user interface for each player, as appropriate
}

Answer 3

If you are able to decode the files to disk, then audio units are probably the solution which would provide the best latency. If you decide to use such an architecture, you should also check out Novocaine:
https://github.com/alexbw/novocaine
That framework takes a lot of the headache out of dealing with audio units.