我正在尝试使用以下代码的输出制作彩色波形。但是当我运行它时,我只获得某些数字(参见freq变量,它使用bin大小,帧速率和索引来产生这些频率)作为输出频率。我不是数学专家,尽管我从现有的代码和答案中拼凑了这些。
//
// colored_waveform.c
// MixDJ
//
// Created by Jonathan Silverman on 3/14/19.
// Copyright © 2019 Jonathan Silverman. All rights reserved.
//
#include "colored_waveform.h"
#include "fftw3.h"
#include <math.h>
#include "sndfile.h"
//int N = 1024;
// helper function to apply a windowing function to a frame of samples
void calcWindow(double* in, double* out, int size) {
for (int i = 0; i < size; i++) {
double multiplier = 0.5 * (1 - cos(2*M_PI*i/(size - 1)));
out[i] = multiplier * in[i];
}
}
// helper function to compute FFT
void fft(double* samples, fftw_complex* out, int size) {
fftw_plan p;
p = fftw_plan_dft_r2c_1d(size, samples, out, FFTW_ESTIMATE);
fftw_execute(p);
fftw_destroy_plan(p);
}
// find the index of array element with the highest absolute value
// probably want to take some kind of moving average of buf[i]^2
// and return the maximum found
double maxFreqIndex(fftw_complex* buf, int size, float fS) {
double max_freq = 0;
double last_magnitude = 0;
for(int i = 0; i < (size / 2) - 1; i++) {
double freq = i * fS / size;
// printf("freq: %f\n", freq);
double magnitude = sqrt(buf[i][0]*buf[i][0] + buf[i][1]*buf[i][1]);
if(magnitude > last_magnitude)
max_freq = freq;
last_magnitude = magnitude;
}
return max_freq;
}
//
//// map a frequency to a color, red = lower freq -> violet = high freq
//int freqToColor(int i) {
//
//}
void generateWaveformColors(const char path[]) {
printf("Generating waveform colors\n");
SNDFILE *infile = NULL;
SF_INFO sfinfo;
infile = sf_open(path, SFM_READ, &sfinfo);
sf_count_t numSamples = sfinfo.frames;
// sample rate
float fS = 44100;
// float songLengLengthSeconds = numSamples / fS;
// printf("seconds: %f", songLengLengthSeconds);
// size of frame for analysis, you may want to play with this
float frameMsec = 5;
// samples in a frame
int frameSamples = (int)(fS / (frameMsec * 1000));
// how much overlap each frame, you may want to play with this one too
int frameOverlap = (frameSamples / 2);
// color to use for each frame
// int outColors[(numSamples / frameOverlap) + 1];
// scratch buffers
double* tmpWindow;
fftw_complex* tmpFFT;
tmpWindow = (double*) fftw_malloc(sizeof(double) * frameSamples);
tmpFFT = (fftw_complex*) fftw_malloc(sizeof(fftw_complex) * frameSamples);
printf("Processing waveform for colors\n");
for (int i = 0, outptr = 0; i < numSamples; i += frameOverlap, outptr++)
{
double inSamples[frameSamples];
sf_read_double(infile, inSamples, frameSamples);
// window another frame for FFT
calcWindow(inSamples, tmpWindow, frameSamples);
// compute the FFT on the next frame
fft(tmpWindow, tmpFFT, frameSamples);
// which frequency is the highest?
double freqIndex = maxFreqIndex(tmpFFT, frameSamples, fS);
printf("%i: ", i);
printf("Max freq: %f\n", freqIndex);
// map to color
// outColors[outptr] = freqToColor(freqIndex);
}
printf("Done.");
sf_close (infile);
}
以下是一些输出:
2094216: Max freq: 5512.500000
2094220: Max freq: 0.000000
2094224: Max freq: 0.000000
2094228: Max freq: 0.000000
2094232: Max freq: 5512.500000
2094236: Max freq: 5512.500000
它只显示某些数字,而不是它可能应该的各种频率。还是我错了?你们可以看到我的代码有什么问题吗?颜色的东西被注释掉了,因为我还没有这样做。
FFT的频率分辨率受到您拥有的数据样本长度的限制。您拥有的样本越多,频率分辨率就越高。
在您的特定情况下,您选择了5毫秒的帧,然后将其转换为以下行中的多个样本:
// samples in a frame
int frameSamples = (int)(fS / (frameMsec * 1000));
这对应于指定的44100Hz采样率下的8个样本。具有如此小的帧大小的频率分辨率可以被计算为
44100 / 8
或5512.5Hz,相当差的分辨率。相应地,观察到的频率将始终为0,5512.5,11025,16537.5或22050Hz之一。
要获得更高的分辨率,您应该通过增加frameMsec
来增加用于分析的样本数量(如评论“用于分析的帧大小,您可能想要使用此”)所建议的。