iphone - iOS 如何从歌曲（ipod 库）中获取音频样本？

Question

目前在我的应用程序中，我正在对来自内置麦克风的音频样本进行 FFT 分析。我让用户从 ipod 库播放歌曲，使用扬声器、麦克风捕捉声音，我可以计算 FFT。我对这个解决方案不满意。我想直接从音频文件（ipod 库）中获取样本并计算 FFT。我知道这是可能的，因为我在 AppStore 中看到了可以分析 ipod 库中的歌曲的应用程序。我怎样才能做到这一点？

Answer 1

您可以使用AVAssetExportSession将歌曲从用户库导出到本地文件。然后使用 API 打开文件ExtAudioRead并执行 FFT 操作。有这样的效果：

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 2

我最终使用了novocaine。用它计算 FFT，就这么简单：

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 3

尝试.h

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

在.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.
}

然后您可以使用以下命令访问 iPod 库[MPMediaQuery songsQuery]

要解析每首歌曲，请尝试以下代码，[myPlayer nowPlayingItem]查询中的每个项目在哪里：

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);