Iphone iOS如何从歌曲中获取音频样本（ipod库）？

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。我让用户播放ipod库中的歌曲，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中看到了应用程序，它可以分析ipod库中的歌曲。我该怎么做

您可以使用

AVAssetExportSession

将歌曲从用户库导出到本地文件。然后使用

ExtAudioRead

API打开文件并执行FFT操作。大意如下：

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;
        }
    }
}];

设法 h

然后您可以使用

[MPMediaQuery songsQuery]

要解析每首歌曲，请尝试以下代码，其中

[myPlayer nowPlayingItem]

是查询中的每个项目：

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);

我最终使用了。用它计算FFT，简单如下：

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];

self.audioManager=[Novocaine audioManager]；
self.audioManager.forceoutputospeaker=是；
self.fileReader=[[AudioFileReader alloc]
initWithAudioFileURL:inputFileURL
samplingRate:self.audioManager.samplingRate
numChannels:self.audioManager.numoutchannels]；
[self.fileReader play]；
__块体复合体_分割AA=A；
__块int nOver=nOver2；
FFT设置FFT设置2=FFT设置；
__块int log=log2n；
__块int n2=n；
__弱MainViewController*listener2=监听器；
__弱音频管理器*wself=self；
__块浮点*窗口=（浮点*）malloc（sizeof（浮点）*n2）；
vDSP_hamm_窗口（窗口，n2，0）；
__block BOOL songStarted=否；
__块浮点*数据=（浮点*）malloc（sizeof（浮点）*n2）；
[self.audioManager setOutputBlock:^（浮点*数据2、UInt32 numFrames、UInt32 numChannels）
{
[wself.fileReader retrieveFreshAudio:data2 numFrames:numFrames numChannels:numChannels]；
如果（！wself.fileReader.playing&&songStarted）
{
[听众2下声：无]；
[wself.audioManager setOutputBlock:nil]；
返回；
}
else if（wself.fileReader.playing&！songStarted）
songStarted=是；
vDSP_vmul（数据2，1，窗口，1，数据，1，n2）；
vDSP_ctoz（（复杂*）数据，2和AA，1，11月）；
vDSP_fft_zrip（fft设置2和AA，1，日志，fft_前向）；
//计算每个值的大小平方
vDSP_zvmags（&AA，1，AA.realp，1，nOver）；
float*tab_results=（float*）malloc（32*sizeof（float））；
对于（int i=0；这不是我要找的内容。你最终是如何计算音频文件的FFT的？我正在尝试做类似的事情，但不知道如何捕获单个音频样本进行分析。@hundley看我的答案。
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);

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];