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Added a frequency error calibration mode to rtl_fm #9

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Added a frequency error calibration mode to rtl_fm #9

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57 changes: 56 additions & 1 deletion src/rtl_fm.c
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Original file line number Diff line number Diff line change
Expand Up @@ -144,6 +144,9 @@ struct demod_state
pthread_cond_t ready;
pthread_mutex_t ready_m;
struct output_state *output_target;
double cal_sum;
int cal_cnt;
int calibrate;
};

struct output_state
Expand Down Expand Up @@ -198,6 +201,17 @@ void usage(void)
//"\t for fm squelch is inverted\n"
//"\t[-o oversampling (default: 1, 4 recommended)]\n"
"\t[-p ppm_error (default: 0)]\n"
"\t[-c] Do frequency error calibration\n"
"\t Frequency error calibration will only work for FM.\n"
"\t Use a higher sample rate, like -s170k, to handle\n"
"\t large frequency errors. A strong and noise free\n"
"\t signal is needed for good results. For example, use a\n"
"\t broadcast FM station for calibration. Let the\n"
"\t calibration run for some minutes until the values\n"
"\t have stabilized. Rerun with -p option using the\n"
"\t suggested ppm_error. Try this on a couple of stations.\n"
"\t Repeat until the error is stable at about zero.\n"
"\t Example: rtl_fm -f99.3M -s170k -r48k -c - | aplay -r48k -fS16_LE -\n"
"\t[-E enable_option (default: none)]\n"
"\t use multiple -E to enable multiple options\n"
"\t edge: enable lower edge tuning\n"
Expand Down Expand Up @@ -727,6 +741,28 @@ void arbitrary_resample(int16_t *buf1, int16_t *buf2, int len1, int len2)
}
}

void calc_fm_fq_offset(struct demod_state *fm)
{
int i;
for (i=0; i<fm->result_len; ++i)
{
/* Correct demodulated FM using rate_in * pcm / 2 and remove
* integer multiplier */
fm->cal_sum += fm->rate_in * (double)fm->result[i] / (1 << 15);
if (++fm->cal_cnt >= fm->rate_in)
{
double fq_offset = fm->cal_sum / fm->cal_cnt;
double fq_corr = -1000000.0 * fq_offset / dongle.freq;
int ppm_error = (int)round(fq_corr);
fprintf(stderr, "fq_offset=%+.0fHz ppm_error=%+d\n",
fq_offset, ppm_error);
fm->cal_sum = 0.0;
fm->cal_cnt = 0;
}
}

}

void full_demod(struct demod_state *d)
{
int i, ds_p;
Expand Down Expand Up @@ -763,6 +799,12 @@ void full_demod(struct demod_state *d)
if (d->mode_demod == &raw_demod) {
return;
}

if (d->calibrate)
{
calc_fm_fq_offset(d);
}

/* todo, fm noise squelch */
// use nicer filter here too?
if (d->post_downsample > 1) {
Expand Down Expand Up @@ -975,6 +1017,9 @@ void demod_init(struct demod_state *s)
pthread_cond_init(&s->ready, NULL);
pthread_mutex_init(&s->ready_m, NULL);
s->output_target = &output;
s->cal_sum = 0.0;
s->cal_cnt = 0;
s->calibrate = 0;
}

void demod_cleanup(struct demod_state *s)
Expand Down Expand Up @@ -1047,7 +1092,7 @@ int main(int argc, char **argv)
output_init(&output);
controller_init(&controller);

while ((opt = getopt(argc, argv, "d:f:g:s:b:l:o:t:r:p:E:F:A:M:h")) != -1) {
while ((opt = getopt(argc, argv, "d:f:g:s:b:l:o:t:r:p:E:F:A:M:ch")) != -1) {
switch (opt) {
case 'd':
dongle.dev_index = verbose_device_search(optarg);
Expand Down Expand Up @@ -1142,13 +1187,23 @@ int main(int argc, char **argv)
demod.deemph = 1;
demod.squelch_level = 0;}
break;
case 'c':
demod.calibrate = 1;
break;
case 'h':
default:
usage();
break;
}
}

if (demod.calibrate && demod.mode_demod != &fm_demod)
{
fprintf(stderr, "Error: Frequency calibration can only "
"be done using the FM demodulator\n");
exit(1);
}

/* quadruple sample_rate to limit to Δθ to ±π/2 */
demod.rate_in *= demod.post_downsample;

Expand Down