小编Alb*_*ert的帖子

我正在开发iphone 3GS上的低音吉他音高检测应用程序.我发现使用RemoteIO无法获得低于150Hz的声音数据.然而,低音吉他可能会产生低于50hz的音调.根据报告"iPhone 4耳机输入频率响应",http://blog.faberacoustical.com/2010/iphone/iphone-4-audio-and-frequency-response-limitations/ 150以下急剧下降赫兹.

这里显示了我如何设置AudioUnit.

// set audio unit
{
    // create AudioUnit
    {
        AudioComponentDescription desc;
        desc.componentType = kAudioUnitType_Output;
        desc.componentSubType = kAudioUnitSubType_RemoteIO;
        desc.componentManufacturer = kAudioUnitManufacturer_Apple;
        desc.componentFlags = 0;
        desc.componentFlagsMask = 0;

        AudioComponent comp = AudioComponentFindNext(NULL, &desc);
        OSAssert(AudioComponentInstanceNew(comp, &m_AudioUnit));
    }

    //enable input on the remote I/O unit (output is default enabled, but input is not)
    {
        UInt32 one = 1;
        OSAssert(AudioUnitSetProperty(m_AudioUnit, kAudioOutputUnitProperty_EnableIO,
                                      kAudioUnitScope_Input, 1, &one, sizeof(one)));
    }

    //set render callback function
    {
        AURenderCallbackStruct callbackInfo;
        callbackInfo.inputProc=staticPerformThru;
        callbackInfo.inputProcRefCon=this;

        OSAssert(AudioUnitSetProperty(m_AudioUnit,
                                      kAudioUnitProperty_SetRenderCallback,
                                      kAudioUnitScope_Input, 
                                      0, …

我使用AVAssetExportSession导出ipod库中的mp3/m4a文件.此方法适用于iOS 5.0及更早版本.但是在将iOS升级到5.1之后,此方法不再适用于mp3,但仍适用于m4a.

这是源代码.

AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL: [mediaItem assetUrl] options:nil];

NSLog (@"compatible presets for songAsset: %@",[AVAssetExportSession exportPresetsCompatibleWithAsset:songAsset]);

AVAssetExportSession *exporter = [[AVAssetExportSession alloc]
                                  initWithAsset: songAsset
                                  presetName: AVAssetExportPresetPassthrough];

NSLog (@"created exporter. supportedFileTypes: %@", exporter.supportedFileTypes);
NSLog(@"output file type=%@",[mediaItem fileType]);
NSLog(@"export file path=%@",exportPath);
exporter.outputFileType =[mediaItem fileType];

NSError *error1;
error1=0;

if([[NSFileManager defaultManager] fileExistsAtPath:exportPath]) 
{

    [[NSFileManager defaultManager] removeItemAtPath:exportPath error:&error1];
    if(error1)
        NSLog(@"%@",error1);

}

NSURL* exportURL = [NSURL fileURLWithPath:exportPath];

exporter.outputURL = exportURL; 

// do the export
[exporter exportAsynchronouslyWithCompletionHandler:^{

    int exportStatus = exporter.status;

    switch (exportStatus) {

        case AVAssetExportSessionStatusFailed: …