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Experimental de-emphasis option for FM-Stereo

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This commit is contained in:
Charles J. Cliffe 2016-07-26 23:34:49 -04:00
parent 5b4a348306
commit 604ce8cdf9
2 changed files with 45 additions and 9 deletions
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44
src/modules/modem/analog/ModemFMStereo.cpp
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@ -48,8 +48,8 @@ ModemArgInfoList ModemFMStereo::getSettings() {
std::vector<std::string> demphOptNames;
demphOptNames.push_back("None");
demphOptNames.push_back("50μs");
demphOptNames.push_back("75μs");
demphOptNames.push_back("50us");
demphOptNames.push_back("75us");
demphArg.optionNames = demphOptNames;
std::vector<std::string> demphOpts;
@ -130,6 +130,24 @@ ModemKit *ModemFMStereo::buildKit(long long sampleRate, int audioSampleRate) {
nco_crcf_reset(kit->stereoPilot);
nco_crcf_pll_set_bandwidth(kit->stereoPilot, 0.25f);
kit->demph = _demph;
if (_demph) {
float f = (1.0 / (2.0 * M_PI * double(_demph) * 1e-6));
float t = 1/(2* M_PI * f);
t = 1.0 / (2.0 * float(audioSampleRate) * tan(1.0 / (2.0 * float(audioSampleRate) * t)));
float tb = (1.0f + 2.0f * t * float(audioSampleRate));
float b_demph[2] = { 1.0f / tb, 1.0f / tb };
float a_demph[2] = { 1.0f, (1.0f - 2.0f * t * float(audioSampleRate)) / tb };
kit->iirDemphL = iirfilt_rrrf_create(b_demph, 2, a_demph, 2);
kit->iirDemphR = iirfilt_rrrf_create(b_demph, 2, a_demph, 2);
} else {
kit->iirDemphL = nullptr;
kit->iirDemphR = nullptr;
kit->demph = 0;
}
return kit;
}
@ -143,6 +161,8 @@ void ModemFMStereo::disposeKit(ModemKit *kit) {
firhilbf_destroy(fmkit->firStereoR2C);
firhilbf_destroy(fmkit->firStereoC2R);
nco_crcf_destroy(fmkit->stereoPilot);
if (fmkit->iirDemphR) { iirfilt_rrrf_destroy(fmkit->iirDemphR); }
if (fmkit->iirDemphL) { iirfilt_rrrf_destroy(fmkit->iirDemphL); }
}
@ -232,12 +252,24 @@ void ModemFMStereo::demodulate(ModemKit *kit, ModemIQData *input, AudioThreadInp
audioOut->data.resize(numAudioWritten * 2);
for (size_t i = 0; i < numAudioWritten; i++) {
float l, r;
float ld, rd;
firfilt_rrrf_push(fmkit->firStereoLeft, 0.568f * (resampledOutputData[i] - (resampledStereoData[i])));
firfilt_rrrf_execute(fmkit->firStereoLeft, &l);
if (fmkit->demph) {
iirfilt_rrrf_execute(fmkit->iirDemphL, 0.568f * (resampledOutputData[i] - (resampledStereoData[i])), &ld);
iirfilt_rrrf_execute(fmkit->iirDemphR, 0.568f * (resampledOutputData[i] + (resampledStereoData[i])), &rd);
firfilt_rrrf_push(fmkit->firStereoRight, 0.568f * (resampledOutputData[i] + (resampledStereoData[i])));
firfilt_rrrf_execute(fmkit->firStereoRight, &r);
firfilt_rrrf_push(fmkit->firStereoLeft, ld);
firfilt_rrrf_execute(fmkit->firStereoLeft, &l);
firfilt_rrrf_push(fmkit->firStereoRight, rd);
firfilt_rrrf_execute(fmkit->firStereoRight, &r);
} else {
firfilt_rrrf_push(fmkit->firStereoLeft, 0.568f * (resampledOutputData[i] - (resampledStereoData[i])));
firfilt_rrrf_execute(fmkit->firStereoLeft, &l);
firfilt_rrrf_push(fmkit->firStereoRight, 0.568f * (resampledOutputData[i] + (resampledStereoData[i])));
firfilt_rrrf_execute(fmkit->firStereoRight, &r);
}
audioOut->data[i * 2] = l;
audioOut->data[i * 2 + 1] = r;

4
src/modules/modem/analog/ModemFMStereo.h
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@ -14,6 +14,10 @@ public:
firfilt_rrrf firStereoRight;
iirfilt_crcf iirStereoPilot;
int demph;
iirfilt_rrrf iirDemphR;
iirfilt_rrrf iirDemphL;
firhilbf firStereoR2C;
firhilbf firStereoC2R;