ref: 06a021bb7ab5a85975a6e27b7cc5255768df3666
dir: libsamplerate/tests/calc_snr.c
/*
** Copyright (c) 2002-2016, Erik de Castro Lopo <erikd@mega-nerd.com>
** 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