forked from cellular-infrastructure/osmocom-analog
FM modulation: Never turn carrier completely off
This prevents IIR filter to become very slow for some unknown reason.
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7cbfaa2861
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@ -49,6 +49,7 @@ int fm_mod_init(fm_mod_t *mod, double samplerate, double offset, double amplitud
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/* generate ramp up with ramp_length */
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for (i = 0; i < mod->ramp_length; i++)
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mod->ramp_tab[i] = 0.5 - cos(M_PI * i / mod->ramp_length) / 2.0;
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mod->ramp_tab[0] = mod->ramp_tab[1] / 2.0; /* never be 0 */
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#ifdef FAST_SINE
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mod->sin_tab = calloc(65536+16384, sizeof(*mod->sin_tab));
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@ -177,10 +178,31 @@ again:
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mod->state = MOD_STATE_RAMP_UP;
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break;
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}
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frequency++;
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/* deviation is defined by the frequency value and the offset */
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dev = offset + *frequency++;
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power++;
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length--;
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baseband += 2;
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/* somehow we need to have some value, otherwise IIR filter will be very slow!
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* we still continue with a carrier, but it has very low amplitude.
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* the low amplitude is set in ramp_tab[0]
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*/
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#ifdef FAST_SINE
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phase += 65536.0 * dev / rate;
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if (phase < 0.0)
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phase += 65536.0;
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else if (phase >= 65536.0)
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phase -= 65536.0;
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*baseband++ += cos_tab[(uint16_t)phase] * ramp_tab[ramp];
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*baseband++ += sin_tab[(uint16_t)phase] * ramp_tab[ramp];
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#else
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phase += 2.0 * M_PI * dev / rate;
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if (phase < 0.0)
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phase += 2.0 * M_PI;
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else if (phase >= 2.0 * M_PI)
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phase -= 2.0 * M_PI;
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*baseband++ += cos(phase) * amplitude * ramp_tab[ramp];
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*baseband++ += sin(phase) * amplitude * ramp_tab[ramp];
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#endif
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}
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break;
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case MOD_STATE_RAMP_UP:
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