iOS: How to read an audio file into a float buffer

Question

I have a really short audio file, say a 10th of a second in (say) .PCM format

I want to use RemoteIO to loop through the file repeatedly to produce a continuous musical tone. So how do I read this into an array of floats?

EDIT: while I could probably dig out the file format, extract the file into an NSData and process it manually, I'm guessing there is a more sensible generic approach... ( that eg copes with different formats )

Answer 1

You can use ExtAudioFile to read data from any supported data format in numerous client formats. Here is an example to read a file as 16-bit integers:

CFURLRef url = /* ... */;
ExtAudioFileRef eaf;
OSStatus err = ExtAudioFileOpenURL((CFURLRef)url, &eaf);
if(noErr != err)
  /* handle error */

AudioStreamBasicDescription format;
format.mSampleRate = 44100;
format.mFormatID = kAudioFormatLinearPCM;
format.mFormatFlags = kAudioFormatFormatFlagIsPacked;
format.mBitsPerChannel = 16;
format.mChannelsPerFrame = 2;
format.mBytesPerFrame = format.mChannelsPerFrame * 2;
format.mFramesPerPacket = 1;
format.mBytesPerPacket = format.mFramesPerPacket * format.mBytesPerFrame;

err = ExtAudioFileSetProperty(eaf, kExtAudioFileProperty_ClientDataFormat, sizeof(format), &format);

/* Read the file contents using ExtAudioFileRead */

If you wanted Float32 data, you would set up format like this:

format.mFormatID = kAudioFormatLinearPCM;
format.mFormatFlags = kAudioFormatFlagsNativeFloatPacked;
format.mBitsPerChannel = 32;