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NFM demod: cleanup and adjustment of phase discriminators

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This commit is contained in:
f4exb 2015-12-16 08:49:09 +01:00
parent 2d929171bb
commit 5ca9004167
2 changed files with 39 additions and 36 deletions
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42
plugins/channel/nfm/nfmdemod.cpp
View File

@ -38,6 +38,8 @@ NFMDemod::NFMDemod() :
m_audioMute(false),
m_afSquelch(2, afSqTones),
m_audioFifo(4, 48000),
m_fmExcursion(4800),
m_fmScaling(384000/4800),
m_settingsMutex(QMutex::Recursive)
{
setObjectName("NFMDemod");
@ -59,7 +61,6 @@ NFMDemod::NFMDemod() :
m_agcLevel = 1.0;
m_AGC.resize(m_agcAttack, m_agcLevel);
m_magsq = 0;
m_ctcssDetector.setCoefficients(3000, 6000.0); // 0.5s / 2 Hz resolution
m_afSquelch.setCoefficients(24, 600, 48000.0, 200, 0); // 4000 Hz span, 250us, 100ms attack
@ -138,40 +139,9 @@ void NFMDemod::feed(const SampleVector::const_iterator& begin, const SampleVecto
qint16 sample;
//m_AGC.feed(abs(ci));
//ci *= (m_agcLevel / m_AGC.getValue());
m_AGC.feed(ci);
//m_magsq = m_AGC.getMagSq() / (1<<30);
// demod
/*
Real argument = arg(ci);
Real demod = argument - m_lastArgument;
m_lastArgument = argument;
*/
/*
// Original NFM
Complex d = conj(m_m1Sample) * ci;
Real demod = atan2(d.imag(), d.real());
demod /= M_PI;
*/
/*
Real argument1 = arg(ci);//atan2(ci.imag(), ci.real());
Real argument2 = m_lastSample.real();
Real demod = angleDist(argument2, argument1);
m_lastSample = Complex(argument1, 0);
*/
// Alternative without atan - needs AGC
// http://www.embedded.com/design/configurable-systems/4212086/DSP-Tricks--Frequency-demodulation-algorithms-
Real ip = ci.real() - m_m2Sample.real();
Real qp = ci.imag() - m_m2Sample.imag();
Real h1 = m_m1Sample.real() * qp;
Real h2 = m_m1Sample.imag() * ip;
Real demod = (h1 - h2) * 1; // 10000 (multiply by 2^16 after demod)
Real demod = phaseDiscriminator2(ci, 0.5);
m_m2Sample = m_m1Sample;
m_m1Sample = ci;
@ -240,8 +210,7 @@ void NFMDemod::feed(const SampleVector::const_iterator& begin, const SampleVecto
else
{
demod = m_bandpass.filter(demod);
demod *= m_running.m_volume;
sample = demod * 10;
sample = demod * m_running.m_volume;
}
}
else
@ -362,6 +331,7 @@ void NFMDemod::apply()
m_interpolator.create(16, m_config.m_inputSampleRate, m_config.m_rfBandwidth / 2.2);
m_interpolatorDistanceRemain = 0;
m_interpolatorDistance = (Real) m_config.m_inputSampleRate / (Real) m_config.m_audioSampleRate;
m_fmScaling = m_config.m_inputSampleRate / m_fmExcursion;
m_settingsMutex.unlock();
}
@ -371,6 +341,8 @@ void NFMDemod::apply()
m_settingsMutex.lock();
m_lowpass.create(301, m_config.m_audioSampleRate, 250.0);
m_bandpass.create(301, m_config.m_audioSampleRate, 300.0, m_config.m_afBandwidth);
m_fmExcursion = m_config.m_afBandwidth;
m_fmScaling = m_config.m_inputSampleRate / m_fmExcursion;
m_settingsMutex.unlock();
}

33
plugins/channel/nfm/nfmdemod.h
View File

@ -176,7 +176,9 @@ private:
AFSquelch m_afSquelch;
Real m_agcLevel; // AGC will aim to this level
Real m_agcFloor; // AGC will not go below this level
Real m_magsq;
Real m_fmExcursion;
Real m_fmScaling;
AudioVector m_audioBuffer;
uint m_audioBufferFill;
@ -186,6 +188,35 @@ private:
NFMDemodGUI *m_nfmDemodGUI;
QMutex m_settingsMutex;
/**
* Standard discriminator using atan2. On modern processors this is as efficient as the non atan2 one.
* This is better for high fidelity.
*/
Real phaseDiscriminator(const Complex& sample)
{
Complex d(std::conj(m_m1Sample) * sample);
m_m1Sample = sample;
return (std::atan2(d.imag(), d.real()) / M_PI_2) * m_fmScaling;
}
/**
* Alternative without atan at the expense of a slight distorsion on very wideband signals
* http://www.embedded.com/design/configurable-systems/4212086/DSP-Tricks--Frequency-demodulation-algorithms-
* in addition it needs scaling by instantaneous magnitude squared and volume (0..10) adjustment factor
*/
Real phaseDiscriminator2(const Complex& sample, Real msq)
{
Real ip = sample.real() - m_m2Sample.real();
Real qp = sample.imag() - m_m2Sample.imag();
Real h1 = m_m1Sample.real() * qp;
Real h2 = m_m1Sample.imag() * ip;
m_m2Sample = m_m1Sample;
m_m1Sample = sample;
return ((h1 - h2) / (msq * M_PI)) * m_fmScaling;
}
void apply();
};