When a transmitter is off, send absolutely nothing

This is done by shifting away the transmitter from center (DC) freqency.
Useless code was removed, since there is nothing to be transmitted.
pull/1/head
Andreas Eversberg 1 year ago
parent b8352af84d
commit 1bd471a703
  1. 22
      src/libfm/fm.c
  2. 51
      src/libsdr/sdr.c

@ -194,27 +194,11 @@ again:
mod->state = MOD_STATE_RAMP_UP;
break;
}
/* deviation is defined by the frequency value and the offset */
dev = offset + *frequency++;
/* just count, and add nothing */
frequency++;
power++;
length--;
if (fast_math) {
phase += 65536.0 * dev / rate;
if (phase < 0.0)
phase += 65536.0;
else if (phase >= 65536.0)
phase -= 65536.0;
*baseband++ += cos_tab[(uint16_t)phase] * amplitude * ramp_tab[ramp];
*baseband++ += sin_tab[(uint16_t)phase] * amplitude * ramp_tab[ramp];
} else {
phase += 2.0 * M_PI * dev / rate;
if (phase < 0.0)
phase += 2.0 * M_PI;
else if (phase >= 2.0 * M_PI)
phase -= 2.0 * M_PI;
*baseband++ += cos(phase) * amplitude * ramp_tab[ramp];
*baseband++ += sin(phase) * amplitude * ramp_tab[ramp];
}
baseband += 2;
}
break;
case MOD_STATE_RAMP_UP:

@ -295,6 +295,57 @@ void *sdr_open(const char __attribute__((__unused__)) *device, double *tx_freque
}
tx_center_frequency = (tx_high_frequency + tx_low_frequency) / 2.0;
/* prevent channel bandwidth from overlapping with the center frequency */
if (channels == 1 && !sdr->paging_channel) {
/* simple: just move off the center by two times half of the bandwidth */
tx_center_frequency -= 2.0 * bandwidth / 2.0;
/* Note: tx_low_frequency is kept at old center.
Calculation of 'low_side' will become 0.
This is correct, since there is no bandwidth
below new center frequency.
*/
PDEBUG(DSDR, DEBUG_INFO, "We shift center frequency %.0f KHz down (half bandwidth), to prevent channel from overlap with DC level.\n", bandwidth / 2.0 / 1e3);
} else {
/* find two channels that are aside the center */
double low_dist, high_dist, dist;
int low_c = -1, high_c = -1;
for (c = 0; c < channels; c++) {
dist = fabs(tx_center_frequency - sdr->chan[c].tx_frequency);
if (round(sdr->chan[c].tx_frequency) >= round(tx_center_frequency)) {
if (high_c < 0 || dist < high_dist) {
high_dist = dist;
high_c = c;
}
} else {
if (low_c < 0 || dist < low_dist) {
low_dist = dist;
low_c = c;
}
}
}
if (sdr->paging_channel) {
dist = fabs(tx_center_frequency - sdr->chan[sdr->paging_channel].tx_frequency);
if (round(sdr->chan[sdr->paging_channel].tx_frequency) >= round(tx_center_frequency)) {
if (high_c < 0 || dist < high_dist) {
high_dist = dist;
high_c = sdr->paging_channel;
}
} else {
if (low_c < 0 || dist < low_dist) {
low_dist = dist;
low_c = sdr->paging_channel;
}
}
}
/* new center = center of the two frequencies aside old center */
if (low_c >= 0 && high_c >= 0) {
tx_center_frequency =
((sdr->chan[low_c].tx_frequency) +
(sdr->chan[high_c].tx_frequency)) / 2.0;
PDEBUG(DSDR, DEBUG_INFO, "We move center freqeuency between the two channels in the middle, to prevent them from overlap with DC level.\n");
}
}
/* show spectrum */
show_spectrum("TX", (double)samplerate / 2.0, tx_center_frequency, tx_frequency, paging_frequency, channels);

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