iOS How to get audio samples from song (ipod library)?

Question

Currently in my app, I'm doing FFT analyse on audio samples coming from built-in microphone. I let user play song from ipod library, using speakers, microphone capture the sound and I can calculate FFT. I'm not happy with this solution. I would like to get samples directly from audio file (ipod library) and calculate FFT. I know this is possible, because I saw apps in AppStore, which can analyse song from ipod library. How can I do that?

Answer 1

I ended up using novocaine. Calculating FFT with it, is simple as that:

self.audioManager = [Novocaine audioManager];
self.audioManager.forceOutputToSpeaker = YES;

self.fileReader= [[AudioFileReader alloc]
                   initWithAudioFileURL:inputFileURL
                   samplingRate:self.audioManager.samplingRate
                   numChannels:self.audioManager.numOutputChannels];


[self.fileReader play];

__block COMPLEX_SPLIT AA = A;
__block int nOver = nOver2;
FFTSetup fftSetup2 = fftSetup;
__block int log = log2n;
__block int n2 = n;
__weak MainViewController *listener2 = listener;
__weak AudioManager *wself = self;
__block float *window = (float *)malloc(sizeof(float) * n2);
vDSP_hamm_window(window, n2, 0);
__block BOOL songStarted = NO;
__block float *data = (float *)malloc(sizeof(float) * n2);
[self.audioManager setOutputBlock:^(float *data2, UInt32 numFrames, UInt32 numChannels)
 {
     [wself.fileReader retrieveFreshAudio:data2 numFrames:numFrames numChannels:numChannels];
     if(!wself.fileReader.playing && songStarted)
     {
         [listener2 nextSong:nil];
         [wself.audioManager setOutputBlock:nil];
         return;
     }
     else if(wself.fileReader.playing && !songStarted)
         songStarted = YES;


     vDSP_vmul(data2, 1, window, 1, data, 1, n2);
     vDSP_ctoz((COMPLEX*)data, 2, &AA, 1, nOver);
     vDSP_fft_zrip(fftSetup2, &AA, 1, log, FFT_FORWARD);
     // calculating square of magnitude for each value
     vDSP_zvmags(&AA, 1, AA.realp, 1, nOver);

     float *tab_results = (float *)malloc(32 * sizeof(float));
     for(int i=0;i<32;i++)
         tab_results[i]=AA.realp[i+5];//i+5
     [listener2 sendResults:tab_results];
     memset(data, 0, n2*sizeof(float));
 }];

[self.audioManager play];

Answer 2

You can use AVAssetExportSession to export songs from user's library to a local file. Then open the file using ExtAudioRead API and do your FFT thingy. Something to this effect:

AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset: asset
                                                                  presetName: AVAssetExportPresetPassthrough];
NSString *ext = [url pathExtension];
if ([ext isEqual: @"mp3"])
{
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    ext = @"mov"; // this is create an mov with mp3 data hidden inside it.. huhuhhaha
}
else if ([ext isEqual: @"m4a"])
{
    exporter.outputFileType = AVFileTypeAppleM4A;
}
else if ([ext isEqual: @"wav"])
{
    exporter.outputFileType = AVFileTypeWAVE;
}
else if ([ext isEqual: @"aif"])
{
    exporter.outputFileType = AVFileTypeAIFF;
}
exporter.outputURL = exportURL;
[exporter exportAsynchronouslyWithCompletionHandler: ^(void){
    int exportStatus = exporter.status;
    switch (exportStatus) {
        case AVAssetExportSessionStatusCompleted: {
            exportedURL = exporter.outputURL;
            break;
        }
    }
}];

Answer 3

Try to .h

#import <MediaPlayer/MediaPlayer.h>
#import <CoreAudio/CoreAudioTypes.h>
@interface ViewController : UIViewController {
MPMusicPlayerController *myPlayer;
}

In .m

- (void)viewDidLoad {
[super viewDidLoad];

myPlayer = [MPMusicPlayerController applicationMusicPlayer];
[myPlayer beginGeneratingPlaybackNotifications];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(itemUpdated) name:MPMusicPlayerControllerNowPlayingItemDidChangeNotification object:nil];
// Do any additional setup after loading the view, typically from a nib.
}

Then you can access to iPods library with [MPMediaQuery songsQuery]

To parse each song try this code, where [myPlayer nowPlayingItem] is each item in query:

NSString *artist = [[myPlayer nowPlayingItem] valueForKey:MPMediaItemPropertyArtist];
NSString *title = [[myPlayer nowPlayingItem] valueForKey:MPMediaItemPropertyTitle];
NSString *genre = [[myPlayer nowPlayingItem] valueForKey:MPMediaItemPropertyGenre];
NSString *duration = [[myPlayer nowPlayingItem] valueForKey:MPMediaItemPropertyPlaybackDuration];
int minutes = floor([duration floatValue]/60);
int seconds = round([duration floatValue] - minutes * 60);
NSLog(@"%@ - %@ (%i:%02d) Genre:%@",artist,title, minutes,seconds,genre);