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I'm trying to find the best way to play a seamless loop of audio, that the user can switch out for another at the shortest possible notice, with a decent number (30-150) of very short loops being available. Will OpenAL be sufficient for this, or do I need to delve into Audio Units? The Apple Documentation says that for real-time feedback like an instrument, Audio Units is the right choice.

I just want to get the community's opinion on this, and any links and sample projects would be greatly appreciated.

DanF
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4 Answers4

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You can use AVAudioPlayer to seamlessly loop a compressed audio file (numberOfLoops = -1). I suggest using IMA4-encoded CAF files, as these are rumored to benefit from hardware decompression (saving CPU cycles for other things).

To keep file size down, you can lower the bit rate (try 96 kbps) and/or use mono.

Note that AVAudioPlayer does not allow you to change the tempo or frequency of playback.

Glenn Barnett
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  • More info on OpenAL vs. Audio Units: http://stackoverflow.com/questions/1002838/audio-on-the-iphone – Glenn Barnett Feb 15 '11 at 23:13
  • I was initially skeptical about whether this would allow me to seamlessly stitch together different samples, but using its various NSTimeInterval commands (current time, total duration) I should be able to figure out what I need to know. Thanks a ton, and wish me luck! – DanF Feb 16 '11 at 06:07
  • Good luck. This is how I manage seamless looping / loop switching in Hexaphone - I use an NSTimer (scheduledTimerWithTimeInterval) to "shadow" the AVAudioPlayer (also useful for updating a visible beat indicator), because the AVAudioPlayerDelegate events weren't fast enough for my needs. – Glenn Barnett Feb 16 '11 at 14:55
  • Perhaps "seamless" is subjective. In my app (Hexaphone), the background drum loops use AVAudioPlayer, and the loops, to my ear, are seamless. At the code level, it's seamless to within a millisecond or two. Perhaps you're using a file encoding that isn't as performant on the hardware? – Glenn Barnett Apr 13 '11 at 13:42
  • ! You know what, in the latest releases, in fact you're totally correct. The infamous "gap" problem with AVAudioPlayer has been massively improved, perhaps eliminated in many if not most situations. You're 10000% correct and I was wrong. I did not realise it had been improved so much. Thank you for the tip! Awesome. – Fattie Apr 16 '11 at 14:19
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this probably doesn't really answer your question, but have you ever looked at Finch? Just looking at the source might provide some pointers.

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justin
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Johannes Fahrenkrug
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  • That's a very excitingly easy sound application to implement, but unfortunately I don't see anything about detecting a sample's ending, or anything else that would make looping easy. – DanF Feb 15 '11 at 21:56
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If you need to be able to switch to another audio sample with no playback delay, you'll need to use OpenAL. AVAudioPlayer has a delay before it starts playing.

You can minimize that delay by calling prepareToPlay, but it won't always eliminate the delay completely. As well, if you have 30 to 150 samples that the user selects for playback, you won't know beforehand which samples need to be preloaded.

Here's a rundown of the pros and cons between OpenAL and AVAudioPlayer: http://kstenerud.github.com/ObjectAL-for-iPhone/documentation/index.html#choosing_sec

Karl
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  • If the samples are small enough, I was thinking I could just preload all of them. Do you know the cache size? – DanF Feb 16 '11 at 18:24
  • Samples in OpenAL must be uncompressed PCM, 16 bit (technically 8 bit is allowed but I've never gotten it to work on iPhone). So if they are not in that format you'll have to either store them uncompressed or decompress on load. So long as you have enough free ram to load everything, you're fine. The total RAM limit on apps for older arm6 based iOS devices (like iPhone 3g) is around 24MB. After that you run the risk of getting shut down by the OS. With later devices, you have a lot more RAM to play with. – Karl Feb 16 '11 at 20:15
  • One other thing if you're thinking of AVAudioPlayer still: I don't think that you can have 30-150 AVAudioPlayers with preloaded audio. I haven't actually TRIED to, mind you, but that's not what AVAudioPlayer was designed for. – Karl Feb 16 '11 at 20:21
  • My issue with OpenAL for _looped_ samples is that I couldn't stop the playback without an audible pop. – Glenn Barnett Feb 18 '11 at 03:17
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I ended up using Cocos2D's audio library for this, and it was far more performant than I'd expected. AVAudioPlayer ended up being a good bit lower-level than I'd needed.

DanF
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