dsps_snr_f32.cpp File Reference

#include "dsps_snr.h"
#include "dsps_fft2r.h"
#include "dsp_common.h"
#include <math.h>
#include <limits>
#include "esp_log.h"

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Functions
float	dsps_snr_f32 (const float *input, int32_t len, uint8_t use_dc)
	SNR.

Variables
static const char *	TAG = "snr"

Function Documentation

dsps_snr_f32()

float dsps_snr_f32	(	const float *	input,
		int32_t	len,
		uint8_t	use_dc )

SNR.

The function calculates signal to noise ration in case if signal is sine tone. The function makes FFT of the input, then search a spectrum maximum, and then calculated SNR as sum of all harmonics to the maximum value. This function have to be used for debug and unit tests only. It's not optimized for real-time processing. The implementation use ANSI C and could be compiled and run on any platform

Parameters

input	input array.
len	length of the input signal
use_dc	this parameter define will be DC value used for calculation or not. 0 - SNR will not include DC power 1 - SNR will include DC power

Returns

• SNR in dB

Definition at line 24 of file dsps_snr_f32.cpp.

{
    if (!dsp_is_power_of_two(len)) {
        return 0;
    }
 
    float *temp_array = new float[len * 2];
    for (int i = 0 ; i < len ; i++) {
        float wind = 0.5 * (1 - cosf(i * 2 * M_PI / (float)len));
        temp_array[i * 2 + 0] = input[i] * wind;
        temp_array[i * 2 + 1] = 0;
    }
    dsps_fft2r_init_fc32(NULL, CONFIG_DSP_MAX_FFT_SIZE);
 
    dsps_fft2r_fc32_ansi(temp_array, len);
    dsps_bit_rev_fc32_ansi(temp_array, len);
 
    float min = std::numeric_limits<float>::max();
    float max = std::numeric_limits<float>::min();
    int max_pos = 0;
    for (int i = 0 ; i < len / 2 ; i++) {
        temp_array[i] = temp_array[i * 2 + 0] * temp_array[i * 2 + 0] + temp_array[i * 2 + 1] * temp_array[i * 2 + 1];
        if (temp_array[i] < min) {
            min = temp_array[i];
        }
        if (temp_array[i] > max) {
            max = temp_array[i];
            max_pos = i;
        }
        ESP_LOGD(TAG, "FFT Data[%i] =%8.4f dB", i, temp_array[i]);
    }
    int start_pos = 0;
    int wind_width = 7;
 
    if (use_dc == 0) {
        start_pos = wind_width;
    }
    float noise_power = 0;
    for (int i = start_pos ; i < len / 2 ; i++) {
        if ((i < (max_pos - wind_width)) || (i > (max_pos + wind_width))) {
            noise_power += temp_array[i];
            ESP_LOGD(TAG, "FFT Data[%i] =%8.4f dB, maX=%f, max_pos=%i, noise_power=%f", i, temp_array[i], max, max_pos, noise_power);
        }
    }
 
    delete[] temp_array;
    noise_power += std::numeric_limits<float>::min();
    if (noise_power < max * 0.00000000001) {
        return 192;
    }
    float snr = max / noise_power;
    float result = 10 * log10(max / noise_power) - 2; // 2 - window correction
    ESP_LOGI(TAG, "SNR = %f, result=%f dB", snr, result);
    return result;
}

References CONFIG_DSP_MAX_FFT_SIZE, dsp_is_power_of_two(), dsps_bit_rev_fc32_ansi(), dsps_fft2r_fc32_ansi, dsps_fft2r_init_fc32(), ESP_LOGD, M_PI, and TAG.

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Variable Documentation

TAG

const char* TAG = "snr"

static

Definition at line 22 of file dsps_snr_f32.cpp.