ios - 如何播放和阅读.caf PCM音频文件

Question

我有一个应用程序可以从 iPod 库中选择一首歌曲，然后将该歌曲作为“.caf”文件复制到应用程序的目录中。我现在需要播放并同时将该文件从 Accelerate 框架读入 Apples FFT，这样我就可以像频谱图一样可视化数据。这是 FFT 的代码:

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));
}

我的问题是如何在播放歌曲的同时循环遍历“.caf”音频文件以提供 FFT？我只需要一个 channel 。我猜我需要获取歌曲的 1024 个样本，在 FTT 中对其进行处理，然后进一步向下移动文件并获取另外 1024 个样本。但我不明白如何读取音频文件来做到这一点。该文件的采样率为 44100.0 hz，采用线性 PCM 格式，16 位，如果有帮助，我相信也是交错的...

Answer 1

试试 ExtendedAudioFile API(需要 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 */ }
}