ref: ca5618deb1a747f3b46f6240a2b232e632c37b0e
dir: /tests/calc_snr.c/
/* ** Copyright (c) 2002-2016, Erik de Castro Lopo <[email protected]> ** All rights reserved. ** ** This code is released under 2-clause BSD license. Please see the ** file at : https://github.com/libsndfile/libsamplerate/blob/master/COPYING */ #include "src_config.h" #include "util.h" #if (HAVE_FFTW3) #include <stdio.h> #include <stdlib.h> #include <string.h> #include <math.h> #include <fftw3.h> #define MAX_SPEC_LEN (1<<18) #define MAX_PEAKS 10 static void log_mag_spectrum (double *input, int len, double *magnitude) ; static void smooth_mag_spectrum (double *magnitude, int len) ; static double find_snr (const double *magnitude, int len, int expected_peaks) ; typedef struct { double peak ; int index ; } PEAK_DATA ; double calculate_snr (float *data, int len, int expected_peaks) { static double magnitude [MAX_SPEC_LEN] ; static double datacopy [MAX_SPEC_LEN] ; double snr = 200.0 ; int k ; if (len > MAX_SPEC_LEN) { printf ("%s : line %d : data length too large.\n", __FILE__, __LINE__) ; exit (1) ; } ; for (k = 0 ; k < len ; k++) datacopy [k] = data [k] ; /* Pad the data just a little to speed up the FFT. */ while ((len & 0x1F) && len < MAX_SPEC_LEN) { datacopy [len] = 0.0 ; len ++ ; } ; log_mag_spectrum (datacopy, len, magnitude) ; smooth_mag_spectrum (magnitude, len / 2) ; snr = find_snr (magnitude, len, expected_peaks) ; return snr ; } /* calculate_snr */ /*============================================================================== ** There is a slight problem with trying to measure SNR with the method used ** here; the side lobes of the windowed FFT can look like a noise/aliasing peak. ** The solution is to smooth the magnitude spectrum by wiping out troughs ** between adjacent peaks as done here. ** This removes side lobe peaks without affecting noise/aliasing peaks. */ static void linear_smooth (double *mag, PEAK_DATA *larger, PEAK_DATA *smaller) ; static void smooth_mag_spectrum (double *mag, int len) { PEAK_DATA peaks [2] ; int k ; memset (peaks, 0, sizeof (peaks)) ; /* Find first peak. */ for (k = 1 ; k < len - 1 ; k++) { if (mag [k - 1] < mag [k] && mag [k] >= mag [k + 1]) { peaks [0].peak = mag [k] ; peaks [0].index = k ; break ; } ; } ; /* Find subsequent peaks ans smooth between peaks. */ for (k = peaks [0].index + 1 ; k < len - 1 ; k++) { if (mag [k - 1] < mag [k] && mag [k] >= mag [k + 1]) { peaks [1].peak = mag [k] ; peaks [1].index = k ; if (peaks [1].peak > peaks [0].peak) linear_smooth (mag, &peaks [1], &peaks [0]) ; else linear_smooth (mag, &peaks [0], &peaks [1]) ; peaks [0] = peaks [1] ; } ; } ; } /* smooth_mag_spectrum */ static void linear_smooth (double *mag, PEAK_DATA *larger, PEAK_DATA *smaller) { int k ; if (smaller->index < larger->index) { for (k = smaller->index + 1 ; k < larger->index ; k++) mag [k] = (mag [k] < mag [k - 1]) ? 0.999 * mag [k - 1] : mag [k] ; } else { for (k = smaller->index - 1 ; k >= larger->index ; k--) mag [k] = (mag [k] < mag [k + 1]) ? 0.999 * mag [k + 1] : mag [k] ; } ; } /* linear_smooth */ /*============================================================================== */ static int peak_compare (const void *vp1, const void *vp2) { const PEAK_DATA *peak1, *peak2 ; peak1 = (const PEAK_DATA*) vp1 ; peak2 = (const PEAK_DATA*) vp2 ; return (peak1->peak < peak2->peak) ? 1 : -1 ; } /* peak_compare */ static double find_snr (const double *magnitude, int len, int expected_peaks) { PEAK_DATA peaks [MAX_PEAKS] ; int k, peak_count = 0 ; double snr ; memset (peaks, 0, sizeof (peaks)) ; /* Find the MAX_PEAKS largest peaks. */ for (k = 1 ; k < len - 1 ; k++) { if (magnitude [k - 1] < magnitude [k] && magnitude [k] >= magnitude [k + 1]) { if (peak_count < MAX_PEAKS) { peaks [peak_count].peak = magnitude [k] ; peaks [peak_count].index = k ; peak_count ++ ; qsort (peaks, peak_count, sizeof (PEAK_DATA), peak_compare) ; } else if (magnitude [k] > peaks [MAX_PEAKS - 1].peak) { peaks [MAX_PEAKS - 1].peak = magnitude [k] ; peaks [MAX_PEAKS - 1].index = k ; qsort (peaks, MAX_PEAKS, sizeof (PEAK_DATA), peak_compare) ; } ; } ; } ; if (peak_count < expected_peaks) { printf ("\n%s : line %d : bad peak_count (%d), expected %d.\n\n", __FILE__, __LINE__, peak_count, expected_peaks) ; return -1.0 ; } ; /* Sort the peaks. */ qsort (peaks, peak_count, sizeof (PEAK_DATA), peak_compare) ; snr = peaks [0].peak ; for (k = 1 ; k < peak_count ; k++) if (fabs (snr - peaks [k].peak) > 10.0) return fabs (peaks [k].peak) ; return snr ; } /* find_snr */ static void log_mag_spectrum (double *input, int len, double *magnitude) { fftw_plan plan = NULL ; double maxval ; int k ; if (input == NULL || magnitude == NULL) return ; plan = fftw_plan_r2r_1d (len, input, magnitude, FFTW_R2HC, FFTW_ESTIMATE | FFTW_PRESERVE_INPUT) ; if (plan == NULL) { printf ("%s : line %d : create plan failed.\n", __FILE__, __LINE__) ; exit (1) ; } ; fftw_execute (plan) ; fftw_destroy_plan (plan) ; maxval = 0.0 ; for (k = 1 ; k < len / 2 ; k++) { /* ** From : http://www.fftw.org/doc/Real_002dto_002dReal-Transform-Kinds.html#Real_002dto_002dReal-Transform-Kinds ** ** FFTW_R2HC computes a real-input DFT with output in “halfcomplex” format, i.e. real and imaginary parts ** for a transform of size n stored as: ** ** r0, r1, r2, ..., rn/2, i(n+1)/2-1, ..., i2, i1 */ double re = magnitude [k] ; double im = magnitude [len - k] ; magnitude [k] = sqrt (re * re + im * im) ; maxval = (maxval < magnitude [k]) ? magnitude [k] : maxval ; } ; memset (magnitude + len / 2, 0, len / 2 * sizeof (magnitude [0])) ; /* Don't care about DC component. Make it zero. */ magnitude [0] = 0.0 ; /* log magnitude. */ for (k = 0 ; k < len ; k++) { magnitude [k] = magnitude [k] / maxval ; magnitude [k] = (magnitude [k] < 1e-15) ? -200.0 : 20.0 * log10 (magnitude [k]) ; } ; return ; } /* log_mag_spectrum */ #else /* ! (HAVE_LIBFFTW && HAVE_LIBRFFTW) */ double calculate_snr (float *data, int len, int expected_peaks) { double snr = 200.0 ; data = data ; len = len ; expected_peaks = expected_peaks ; return snr ; } /* calculate_snr */ #endif