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windowing

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This commit is contained in:
cho45 2019-09-11 22:35:44 +09:00
parent a3b511f468
commit 472b895d5b
1 changed files with 57 additions and 1 deletions
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58
main.c
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@ -108,6 +108,28 @@ toggle_sweep(void)
sweep_enabled = !sweep_enabled;
}
float bessel0(float x) {
const float eps = 0.0001;
float ret = 0;
float term = 1;
float m = 0;
while (term > eps * ret) {
ret += term;
++m;
term *= (x*x) / (4*m*m);
}
return ret;
}
float kaiser_window(float k, float n, float beta) {
if (beta == 0.0) return 1.0;
float r = (2 * k) / (n - 1) - 1;
return bessel0(beta * sqrt(1 - r * r)) / bessel0(beta);
}
static
void
transform_domain(void)
@ -116,9 +138,43 @@ transform_domain(void)
// use spi_buffer as temporary buffer
// and calculate ifft for time domain
float* tmp = (float*)spi_buffer;
uint8_t window_size, offset;
switch (domain_mode & TDR_FUNC) {
case TDR_FUNC_BANDPASS:
offset = 0;
window_size = 101;
break;
case TDR_FUNC_LOWPASS_IMPULSE:
case TDR_FUNC_LOWPASS_STEP:
offset = 101;
window_size = 202;
break;
}
float beta = 0.0;
switch (domain_mode & TDR_WINDOW) {
case TDR_WINDOW_MINIMUM:
beta = 0.0; // this is rectangular
break;
case TDR_WINDOW_NORMAL:
beta = 6.0;
break;
case TDR_WINDOW_MAXIMUM:
beta = 13;
break;
}
for (int ch = 0; ch < 2; ch++) {
memcpy(tmp, measured[ch], sizeof(measured[0]));
for (int i = 101+1; i < 128; i++) {
if (beta != 0.0) {
for (int i = 0; i < 101; i++) {
float w = kaiser_window(i+offset, window_size, beta);
tmp[i*2+0] *= w;
tmp[i*2+1] *= w;
}
}
for (int i = 101; i < 128; i++) {
tmp[i*2+0] = 0.0;
tmp[i*2+1] = 0.0;
}