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FM modulation: Never turn carrier completely off

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This prevents IIR filter to become very slow for some unknown reason.
This commit is contained in:
Andreas Eversberg
2017-11-19 10:19:58 +01:00
parent 7cbfaa2861
commit 44247ffb53
1 changed files with 24 additions and 2 deletions
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26
src/libfm/fm.c
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@@ -49,6 +49,7 @@ int fm_mod_init(fm_mod_t *mod, double samplerate, double offset, double amplitud
/* generate ramp up with ramp_length */ /* generate ramp up with ramp_length */
for (i = 0; i < mod->ramp_length; i++) 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[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 #ifdef FAST_SINE
mod->sin_tab = calloc(65536+16384, sizeof(*mod->sin_tab)); mod->sin_tab = calloc(65536+16384, sizeof(*mod->sin_tab));
@@ -177,10 +178,31 @@ again:
mod->state = MOD_STATE_RAMP_UP; mod->state = MOD_STATE_RAMP_UP;
break; break;
} }
frequency++; /* deviation is defined by the frequency value and the offset */
dev = offset + *frequency++;
power++; power++;
length--; 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; break;
case MOD_STATE_RAMP_UP: case MOD_STATE_RAMP_UP: