Hi!
I have just started using rtl-sdr in my application, SvxLink:
http://svxlink.org/. Thanks for your work on the RTL code. It opens up
the world of wideband receivers to everyone. The SvxLink RTL code is in
the rtl-branch on GitHub right now if anyone is interested:
https://github.com/sm0svx/svxlink/tree/rtl
I was missing some simple utility to calibrate the frequency error for a
dongle. The simplest way I came up with was to modify the rtl_fm utility
since it is very easy to find out the frequency error of an FM signal.
To get a measure of the frequency error, all that have to be done in the
FM demodulator is:
samp_rate * angle / (2*PI)
The angle is already compensated with PI in the current demodulator so I
only had to do the rest in my added calibration code.
I first filed this patch as a pull request on GitHub
(https://github.com/steve-m/librtlsdr/pull/9) but then saw that
contributions should be mailed to this mailing list.
The rtl_fm utility may now be used to calibrate the frequency error of a
DVB-T dongle using the "-c" command line switch:
| [-c] Do frequency error calibration
Frequency error calibration will only work for FM.
Use a higher sample rate, like -s170k, to handle
large frequency errors. A strong and noise free
signal is needed for good results. For example, use a
broadcast FM station for calibration. Let the
calibration run for some minutes until the values
have stabilized. Rerun with -p option using the
suggested ppm_error. Try this on a couple of stations.
Repeat until the error is stable at about zero.
Example: rtl_fm -f99.3M -s170k -r48k -c - | aplay -r48k -fS16_LE -
|
Regards,
Tobias
diff --git a/src/rtl_fm.c b/src/rtl_fm.c
index 0f7ac38..a58fb24 100644
--- a/src/rtl_fm.c
+++ b/src/rtl_fm.c
@@ -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
@@ -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"
@@ -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;
@@ -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) {
@@ -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)
@@ -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);
@@ -1142,6 +1187,9 @@ int main(int argc, char **argv)
demod.deemph = 1;
demod.squelch_level = 0;}
break;
+ case 'c':
+ demod.calibrate = 1;
+ break;
case 'h':
default:
usage();
@@ -1149,6 +1197,13 @@ int main(int argc, char **argv)
}
}
+ 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 Îθ to ±Ï/2 */
demod.rate_in *= demod.post_downsample;
