我已经阅读了以下问题:
如何使用苹果FFT和Accelerate Framework?
iOS FFT Accerelate.framework如何在播放过程中绘制频谱图?
它们都介绍了如何在加速框架中设置FFT。在他们的帮助下,我能够设置FFT并获得基本的频谱分析器。目前,我正在显示从FFT获得的所有值。然而,我只想显示10-15个或可变数量的条形图,分别代表某些频率。就像iTunes或WinAmp Level Meter一样。 1. 我需要平均一定范围内的幅度值吗?还是它们只为特定频率条提供幅度? 2. 此外,我需要将幅度值转换为db吗? 3. 如何将我的数据映射到特定的范围?针对声音位深度的最大db范围进行映射?获取一个bin的最大值将导致最大映射值跳跃。
我的RenderCallback:
static OSStatus PlaybackCallback(void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList *ioData)
{
UInt32 maxSamples = kAudioBufferNumFrames;
UInt32 log2n = log2f(maxSamples); //bins
UInt32 n = 1 << log2n;
UInt32 stride = 1;
UInt32 nOver2 = n/2;
COMPLEX_SPLIT A;
float *originalReal, *obtainedReal, *frequencyArray, *window, *in_real;
in_real = (float *) malloc(maxSamples * sizeof(float));
A.realp = (float *) malloc(nOver2 * sizeof(float));
A.imagp = (float *) malloc(nOver2 * sizeof(float));
memset(A.imagp, 0, nOver2 * sizeof(float));
obtainedReal = (float *) malloc(n * sizeof(float));
originalReal = (float *) malloc(n * sizeof(float));
frequencyArray = (float *) malloc(n * sizeof(float));
//-- window
UInt32 windowSize = maxSamples;
window = (float *) malloc(windowSize * sizeof(float));
memset(window, 0, windowSize * sizeof(float));
// vDSP_hann_window(window, windowSize, vDSP_HANN_DENORM);
vDSP_blkman_window(window, windowSize, 0);
vDSP_vmul(ioBuffer, 1, window, 1, in_real, 1, maxSamples);
//-- window
vDSP_ctoz((COMPLEX*)in_real, 2, &A, 1, maxSamples/2);
vDSP_fft_zrip(fftSetup, &A, stride, log2n, FFT_FORWARD);
vDSP_fft_zrip(fftSetup, &A, stride, log2n, FFT_INVERSE);
float scale = (float) 1.0 / (2 * n);
vDSP_vsmul(A.realp, 1, &scale, A.realp, 1, nOver2);
vDSP_vsmul(A.imagp, 1, &scale, A.imagp, 1, nOver2);
vDSP_ztoc(&A, 1, (COMPLEX *) obtainedReal, 2, nOver2);
vDSP_zvmags(&A, 1, obtainedReal, 1, nOver2);
Float32 one = 1;
vDSP_vdbcon(obtainedReal, 1, &one, obtainedReal, 1, nOver2, 0);
for (int i = 0; i < nOver2; i++) {
frequencyArray[i] = obtainedReal[i];
}
// Extract the maximum value
double fftMax = 0.0;
vDSP_maxmgvD((double *)obtainedReal, 1, &fftMax, nOver2);
float max = sqrt(fftMax);
}
播放一些音乐,我得到从-96db到0db的值。在此绘制一个点:
CGPointMake(i, kMaxSpectrumHeight * (1 - frequencyArray[i]/-96.));
我的曲线看起来相当圆滑:
如果我不把它转换为分贝,我可以通过将数组值乘以10000来绘制出漂亮的峰值。
我是不是做错了什么?如何显示可变数量的条形图?