How to play and read .caf PCM audio file - ios

Question

I have an app that selects a song from the iPod Library then copies that song into the app's directory as a '.caf' file. I now need to play and at the same time read that file into Apples FFT from the Accelerate framework so I can visualize the data like a spectrogram. Here is the code for the FFT:
void FFTAccelerate::doFFTReal(float samples[], float amp[], int numSamples)
{
int i;
vDSP_Length log2n = log2f(numSamples);
//Convert float array of reals samples to COMPLEX_SPLIT array A
vDSP_ctoz((COMPLEX*)samples,2,&A,1,numSamples/2);
//Perform FFT using fftSetup and A
//Results are returned in A
vDSP_fft_zrip(fftSetup, &A, 1, log2n, FFT_FORWARD);
//Convert COMPLEX_SPLIT A result to float array to be returned
amp[0] = A.realp[0]/(numSamples*2);
for(i=1;i<numSamples;i++)
amp[i]=sqrt(A.realp[i]*A.realp[i]+A.imagp[i]*A.imagp[i])/numSamples;
}
//Constructor
FFTAccelerate::FFTAccelerate (int numSamples)
{
vDSP_Length log2n = log2f(numSamples);
fftSetup = vDSP_create_fftsetup(log2n, FFT_RADIX2);
int nOver2 = numSamples/2;
A.realp = (float *) malloc(nOver2*sizeof(float));
A.imagp = (float *) malloc(nOver2*sizeof(float));
}
My question is how to I loop through the '.caf' audio file to feed the FFT while at the same time playing the song? I only need one channel. Im guessing I need to get 1024 samples of the song, process that in the FTT and then move further down the file and grab another 1024 samples. But I dont understand how to read an audio file to do this. The file has a sample rate of 44100.0 hz, is in linear PCM format, 16 Bit and I believe is also interleaved if that helps...

Answer 1

Try the ExtendedAudioFile API (requires AudioToolbox.framework).
#include <AudioToolbox/ExtendedAudioFile.h>
NSURL *urlToCAF = ...;
ExtAudioFileRef caf;
OSStatus status;
status = ExtAudioFileOpenURL((__bridge CFURLRef)urlToCAF, &caf);
if(noErr == status) {
// request float format
const UInt32 NumFrames = 1024;
const int ChannelsPerFrame = 1; // Mono, 2 for Stereo
// request float format
AudioStreamBasicDescription clientFormat;
clientFormat.mChannelsPerFrame = ChannelsPerFrame;
clientFormat.mSampleRate = 44100;
clientFormat.mFormatID = kAudioFormatLinearPCM;
clientFormat.mFormatFlags = kAudioFormatFlagIsFloat | kAudioFormatFlagIsNonInterleaved; // float
int cmpSize = sizeof(float);
int frameSize = cmpSize*ChannelsPerFrame;
clientFormat.mBitsPerChannel = cmpSize*8;
clientFormat.mBytesPerPacket = frameSize;
clientFormat.mFramesPerPacket = 1;
clientFormat.mBytesPerFrame = frameSize;
status = ExtAudioFileSetProperty(caf, kExtAudioFileProperty_ClientDataFormat, sizeof(clientFormat), &clientFormat);
if(noErr != status) { /* handle it */ }
while(1) {
float buf[ChannelsPerFrame*NumFrames];
AudioBuffer ab = { ChannelsPerFrame, sizeof(buf), buf };
AudioBufferList abl;
abl.mNumberBuffers = 1;
abl.mBuffers[0] = ab;
UInt32 ioNumFrames = NumFrames;
status = ExtAudioFileRead(caf, &ioNumFrames, &abl);
if(noErr == status) {
// process ioNumFrames here in buf
if(0 == ioNumFrames) {
// EOF!
break;
} else if(ioNumFrames < NumFrames) {
// TODO: pad buf with zeroes out to NumFrames
} else {
float amp[NumFrames]; // scratch space
doFFTReal(buf, amp, NumFrames);
}
}
}
// later
status = ExtAudioFileDispose(caf);
if(noErr != status) { /* hmm */ }
}