Sound format for iPhone app

Question

What kind of audio files are you using in your iPhone games/apps?

I have a game with 30MB of sounds in .wav format and I'm thinking of maybe converting to .mp3 to reduce the app size... Is there a major difference in performance? Any other issues?

Answer 1

Both AAC and CAF formats work fine and offer decent file sizes. For certain background looping tracks I found MP3 files getting too big, but YMMV. Experimenting with a decent sound editing app is the only way to find the right balance between size and quality. I've had pretty good luck with Audacity and Amadeus Pro.

Suggest listening to the output with a pair of really good noise-isolating headphones on the device itself. Most people won't be listening to your stuff with these but as you decrease sound quality to shrink file sizes you'll start getting static and hum artifacts. It's just a matter of balancing size vs. quality and what you're willing to live with.

Answer 2

I find mp3 fine, but keep in mind that decoding on the iPhone/Touch2G is only about 2.5x realtime speed.

Answer 3

I use a combination of WAV files (for sound effects) and MP3 (for music), which seems to work fine. You can have trouble if you try to play multiple MP3 files at the same time - drop outs, or performance degradation, depending on your AudioSession settings.

If I had to compress my sound effects, I'm not sure which codec has the least decoding overhead. Something like Apple Lossless would likely work well, and would cut the size roughly in half.

Answer 4

Keep in mind that certain codecs run in hardware and others in software. Therefore not all compressions will allow for simultaneous playback of more than one sound. For example, if you have a sound playing, a UI sound like a beep may not play if both were trying to use the same codec. For more info, see:

http://developer.apple.com/iphone/library/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/AudioandVideoTechnologies/AudioandVideoTechnologies.html#//apple_ref/doc/uid/TP40007072-CH19-SW6

iPhone Audio Hardware Codecs iPhone OS applications can use a wide range of audio data formats. Starting in iPhone OS 3.0, most of these formats can use software-based encoding and decoding. You can simultaneously play multiple sounds in all formats, although for performance reasons you should consider which format is best in a given scenario. Hardware decoding generally entails less of a performance impact than software decoding.

The following iPhone OS audio formats can employ hardware decoding for playback:

AAC ALAC (Apple Lossless) MP3 The device can play only a single instance of one of these formats at a time through hardware. For example, if you are playing a stereo MP3 sound, a second simultaneous MP3 sound will use software decoding. Similarly, you cannot simultaneously play an AAC and an ALAC sound using hardware. If the iPod application is playing an AAC sound in the background, your application plays AAC, ALAC, and MP3 audio using software decoding.

To play multiple sounds with best performance, or to efficiently play sounds while the iPod is playing in the background, use linear PCM (uncompressed) or IMA4 (compressed) audio.

To learn how to check which hardware and software codecs are available on a device, read the discussion for the kAudioFormatProperty_HardwareCodecCapabilities constant in Audio Format Services Reference.