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Factor out the sample -> magnitude conversion code and make everything a little less sample-rate-dependent.

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Add optional noise measurement (cheaper than the old version)
Add optional DC filter (expensive, not really needed with rtlsdr input)
This commit is contained in:
Oliver Jowett 2015-06-15 22:14:37 +01:00
parent f58ff14d7c
commit 03b53c2d29
7 changed files with 454 additions and 156 deletions
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49
demod_2400.c
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@ -19,12 +19,6 @@
#include "dump1090.h"
//
// Measuring the noise power is actually surprisingly expensive on an ARM -
// it increases the CPU use of the demodulator by 1/3. So it's off by default.
// You can turn it back on here:
#undef MEASURE_NOISE
// 2.4MHz sampling rate version
//
// When sampling at 2.4MHz we have exactly 6 samples per 5 symbols.
@ -155,26 +149,20 @@ static int best_phase(uint16_t *m) {
// Given 'mlen' magnitude samples in 'm', sampled at 2.4MHz,
// try to demodulate some Mode S messages.
//
void demodulate2400(struct mag_buf *mag) {
void demodulate2400(struct mag_buf *mag)
{
struct modesMessage mm;
unsigned char msg1[MODES_LONG_MSG_BYTES], msg2[MODES_LONG_MSG_BYTES], *msg;
uint32_t j;
#ifdef MEASURE_NOISE
uint32_t last_message_end = 0;
#endif
unsigned char *bestmsg;
int bestscore, bestphase;
#ifdef MEASURE_NOISE
// noise floor:
uint32_t noise_power_count = 0;
uint64_t noise_power_sum = 0;
#endif
uint16_t *m = mag->data;
uint32_t mlen = mag->length;
double total_signal_power = 0.0;
memset(&mm, 0, sizeof(mm));
msg = msg1;
@ -185,19 +173,6 @@ void demodulate2400(struct mag_buf *mag) {
int initial_phase, first_phase, last_phase, try_phase;
int msglen;
#ifdef MEASURE_NOISE
// update noise for all samples that aren't part of a message
// (we don't know if m[j] is or not, yet, so work one sample
// in arrears)
if (j > last_message_end+1) {
// There seems to be a weird compiler bug I'm hitting here..
// if you compute the square directly, it occasionally gets mangled.
uint64_t s = TRUE_AMPLITUDE(m[j-1]);
noise_power_sum += s * s;
noise_power_count++;
}
#endif
// Look for a message starting at around sample 0 with phase offset 3..7
// Ideal sample values for preambles with different phase
@ -445,7 +420,7 @@ void demodulate2400(struct mag_buf *mag) {
int k;
for (k = 0; k < signal_len; ++k) {
uint64_t s = TRUE_AMPLITUDE(m[j+19+k]);
uint64_t s = m[j+19+k];
signal_power_sum += s * s;
}
@ -457,6 +432,8 @@ void demodulate2400(struct mag_buf *mag) {
Modes.stats_current.peak_signal_power = signal_power;
if (signal_power > 0.50119)
Modes.stats_current.strong_signal_count++; // signal power above -3dBFS
total_signal_power += signal_power_sum / MAX_POWER;
}
// Decode the received message
@ -480,18 +457,16 @@ void demodulate2400(struct mag_buf *mag) {
// where the preamble of the second message clobbered the last
// few bits of the first message, but the message bits didn't
// overlap)
#ifdef MEASURE_NOISE
last_message_end = j + (8 + msglen)*12/5;
#endif
j += (8 + msglen - 8)*12/5 - 1;
// Pass data to the next layer
useModesMessage(&mm);
}
#ifdef MEASURE_NOISE
Modes.stats_current.noise_power_sum += (noise_power_sum / MAX_POWER / noise_power_count);
Modes.stats_current.noise_power_count ++;
#endif
/* update noise power if measured */
if (Modes.measure_noise) {
Modes.stats_current.noise_power_sum += (mag->total_power - total_signal_power) / mag->length;
Modes.stats_current.noise_power_count ++;
}
}