diff options
| author | Andreas Eversberg <jolly@eversberg.eu> | 2017-11-19 10:19:58 +0100 |
|---|---|---|
| committer | Andreas Eversberg <jolly@eversberg.eu> | 2017-12-03 08:46:04 +0100 |
| commit | 44247ffb536ef25c1fc8c49c75b501d6f1808c52 (patch) | |
| tree | 99616901ed985c50732c8e65d18b0739bd9aec74 | |
| parent | 7cbfaa286151d479f6da802d7fc07f68fedbe0c5 (diff) | |
FM modulation: Never turn carrier completely off
This prevents IIR filter to become very slow for some unknown reason.
| -rw-r--r-- | src/libfm/fm.c | 26 |
1 files changed, 24 insertions, 2 deletions
diff --git a/src/libfm/fm.c b/src/libfm/fm.c index e633f33..a006990 100644 --- a/src/libfm/fm.c +++ b/src/libfm/fm.c @@ -49,6 +49,7 @@ int fm_mod_init(fm_mod_t *mod, double samplerate, double offset, double amplitud /* generate ramp up with ramp_length */ for (i = 0; i < mod->ramp_length; i++) mod->ramp_tab[i] = 0.5 - cos(M_PI * i / mod->ramp_length) / 2.0; + mod->ramp_tab[0] = mod->ramp_tab[1] / 2.0; /* never be 0 */ #ifdef FAST_SINE mod->sin_tab = calloc(65536+16384, sizeof(*mod->sin_tab)); @@ -177,10 +178,31 @@ again: mod->state = MOD_STATE_RAMP_UP; break; } - frequency++; + /* deviation is defined by the frequency value and the offset */ + dev = offset + *frequency++; power++; length--; - baseband += 2; + /* somehow we need to have some value, otherwise IIR filter will be very slow! + * we still continue with a carrier, but it has very low amplitude. + * the low amplitude is set in ramp_tab[0] + */ +#ifdef FAST_SINE + 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] * ramp_tab[ramp]; + *baseband++ += sin_tab[(uint16_t)phase] * 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]; +#endif } break; case MOD_STATE_RAMP_UP: |