5 February 2010

Audio 101 for iPhone Developers: Playing Audio Programatically

This post is also available in: Chinese (Simplified)

Screenshot from BasicSounds sample project

This article is the third in a three-part Audio 101 series covering audio topics of interest to the iPhone developer.

So far in this Audio 101 series we’ve talked about the difference between file and data formats and how to convert and record audio on your Mac. Now we’ll get to the fun part – actually playing audio on your phone!

There are many ways to play audio on the Mac – System Sound Services, AVAudioPlayer, Audio Queue Services, and OpenAL. Without outside support libraries, the two easiest ways by far are System Sound Services and AVAudioPlayer – so let’s talk about when you would (and wouldn’t) want to use those, and how you can use them.

System Sound Services

System Sound Services provides an extremely easy way to play audio files. All you have to do is the following:

NSString *pewPewPath = [[NSBundle mainBundle] 
    pathForResource:@"pew-pew-lei" ofType:@"caf"];
NSURL *pewPewURL = [NSURL fileURLWithPath:pewPewPath];
AudioServicesCreateSystemSoundID((CFURLRef)pewPewURL, &_pewPewSound);
AudioServicesPlaySystemSound(_pewPewSound);

Doesn’t get much easier than that. However there are some strong drawbacks to this method:

It only supports audio data formats linear PCM or IMA4.
It only supports audio file formats CAF, AIF, or WAV.
The sounds must be 30 seconds or less in length.
And more – see the iPhone Application Programming Guide, page 149.

AVAudioPlayer

So what if you have an audio file encoded with AAC or MP3 that you want to play as background music? Another easy way to play music is via the AVAudioPlayer class. For the most part, it again is quite simple:

NSError *error;
_backgroundMusicPlayer = [[AVAudioPlayer alloc]
    initWithContentsOfURL:backgroundMusicURL error:&error];
[_backgroundMusicPlayer prepareToPlay];
[_backgroundMusicPlayer play];

However, the drawback of AVAudioPlayer is it is extremely slow. If you tap a button and try to trigger a sound with AVAudioPlayer, there will be a noticeable small delay. But if that doesn’t matter to you (like for starting up background music), AVAudioPlayer is a fine choice.

And there are a couple other things to keep in mind:

If you’re playing background music, you should check to see if other audio (like the iPod) is playing first, so you don’t have two layers of music going on at once!
If a phone call arrives and the user chooses “Decline”, by default your AVAudioPlayer will stop. You can start it back up again by registering for the AVAudioPlayerDelegate and starting the music back up again in the audioPlayerEndInterruption method.

Sample Code

I put together some sample code showing how to use System Sound Services and AVAudioPlayer that you might want to check out. Not only does it demonstrate those APIs, it has some mighty funky beats and a cool spaceship to boot. Pew-pew!

OpenAL

If you’re writing a game or another app where you want fine grained control of audio with low latency, you probably don’t want to use the above methods. Instead, you might want to use OpenAL, a cross-platform audio library supported by the iPhone.

OpenAL can be a beast with a steep learning curve. Luckily, Alex Restrepo has extended a great example by gehacktes.net and made a nice Sound Engine library using OpenAL that you can either use in your projects or use as a reference.

Another option is the Cocos2D game library includes an extremely easy to use sound engine that makes playing audio a snap. You can learn how to use it in my tutorial on How To Make a Simple iPhone Game With Cocos2D.

And That’s a Wrap!

That’s about all I’m going to cover about audio topics in iPhone programming for now – but keep in mind I’ve barely scratched the surface. If you’re interested in more, I’d recommend reading Apple’s docs, especially the Core Audio Overview and Audio Session Programming Guide, and possibly digging into OpenAL a bit more.

I hope this Audio 101 series has been useful to other developers who may be new to audio concepts. Feel free to share any additional info you’re aware regarding audio programming that may be useful to others!

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