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Merge pull request #144 from czietz/window_loss

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Compensate IFFT window / zero-padding loss in TD
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TT 2020-11-12 06:02:21 +09:00 committed by GitHub
parent bc1b57c3f0 4d64ef6e48
commit d02db797a7
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GPG key ID: 4AEE18F83AFDEB23
1 changed files with 30 additions and 16 deletions
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main.c
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@ -198,40 +198,54 @@ transform_domain(void)
// and calculate ifft for time domain // and calculate ifft for time domain
float* tmp = (float*)spi_buffer; float* tmp = (float*)spi_buffer;
// correct IFFT window and zero-padding loss
// assuming a 2*POINT_COUNT window
float wincorr = 1.0f;
float beta = 0.0;
switch (domain_mode & TD_WINDOW) {
case TD_WINDOW_MINIMUM:
beta = 0.0; // this is rectangular
// loss by zero-padding 202 to 256 points
wincorr = (float)FFT_SIZE / (float)(2*POINTS_COUNT);
break;
case TD_WINDOW_NORMAL:
beta = 6.0;
// additional window loss: 1/mean(kaiser(202,6)) = 2.01
wincorr = (float)FFT_SIZE / (float)(2*POINTS_COUNT) * 2.01f;
break;
case TD_WINDOW_MAXIMUM:
beta = 13;
// additional window loss: 1/mean(kaiser(202,13)) = 2.92
wincorr = (float)FFT_SIZE / (float)(2*POINTS_COUNT) * 2.92f;
break;
}
uint8_t window_size = POINTS_COUNT, offset = 0; uint8_t window_size = POINTS_COUNT, offset = 0;
uint8_t is_lowpass = FALSE; uint8_t is_lowpass = FALSE;
switch (domain_mode & TD_FUNC) { switch (domain_mode & TD_FUNC) {
case TD_FUNC_BANDPASS: case TD_FUNC_BANDPASS:
offset = 0; offset = 0;
window_size = POINTS_COUNT; window_size = POINTS_COUNT;
// window size is half the size as assumed above => twice the IFFT loss
wincorr *= 2.0f;
break; break;
case TD_FUNC_LOWPASS_IMPULSE:
case TD_FUNC_LOWPASS_STEP: case TD_FUNC_LOWPASS_STEP:
// no IFFT losses need to be considered to calculate the step response
wincorr = 1.0f;
// fall-through
case TD_FUNC_LOWPASS_IMPULSE:
is_lowpass = TRUE; is_lowpass = TRUE;
offset = POINTS_COUNT; offset = POINTS_COUNT;
window_size = POINTS_COUNT * 2; window_size = POINTS_COUNT * 2;
break; break;
} }
float beta = 0.0;
switch (domain_mode & TD_WINDOW) {
case TD_WINDOW_MINIMUM:
beta = 0.0; // this is rectangular
break;
case TD_WINDOW_NORMAL:
beta = 6.0;
break;
case TD_WINDOW_MAXIMUM:
beta = 13;
break;
}
for (int ch = 0; ch < 2; ch++) { for (int ch = 0; ch < 2; ch++) {
memcpy(tmp, measured[ch], sizeof(measured[0])); memcpy(tmp, measured[ch], sizeof(measured[0]));
for (int i = 0; i < POINTS_COUNT; i++) { for (int i = 0; i < POINTS_COUNT; i++) {
float w = kaiser_window(i + offset, window_size, beta); float w = kaiser_window(i + offset, window_size, beta);
tmp[i * 2 + 0] *= w; tmp[i * 2 + 0] *= w * wincorr;
tmp[i * 2 + 1] *= w; tmp[i * 2 + 1] *= w * wincorr;
} }
for (int i = POINTS_COUNT; i < FFT_SIZE; i++) { for (int i = POINTS_COUNT; i < FFT_SIZE; i++) {
tmp[i * 2 + 0] = 0.0; tmp[i * 2 + 0] = 0.0;