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Filter: notch filter: remove unneeded value and pre-multiply for speed #24250

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Oct 16, 2023
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Filter: notch filter: remove unneeded value and pre-multiply for speed #24250

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20 changes: 15 additions & 5 deletions libraries/Filter/NotchFilter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -77,9 +77,18 @@ void NotchFilter<T>::init_with_A_and_Q(float sample_freq_hz, float center_freq_h
b0 = 1.0 + alpha*sq(A);
b1 = -2.0 * cosf(omega);
b2 = 1.0 - alpha*sq(A);
a0_inv = 1.0/(1.0 + alpha);
a1 = b1;
a2 = 1.0 - alpha;

const float a0_inv = 1.0/(1.0 + alpha);

// Pre-multiply to save runtime calc
b0 *= a0_inv;
b1 *= a0_inv;
b2 *= a0_inv;
a1 *= a0_inv;
a2 *= a0_inv;

_center_freq_hz = new_center_freq;
_sample_freq_hz = sample_freq_hz;
initialised = true;
Expand All @@ -100,16 +109,17 @@ T NotchFilter<T>::apply(const T &sample)
// sample as output and update delayed samples
signal1 = sample;
signal2 = sample;
ntchsig = sample;
ntchsig1 = sample;
ntchsig2 = sample;
need_reset = false;
return sample;
}

T output = sample*b0 + ntchsig1*b1 + ntchsig2*b2 - signal1*a1 - signal2*a2;

ntchsig2 = ntchsig1;
ntchsig1 = ntchsig;
ntchsig = sample;
T output = (ntchsig*b0 + ntchsig1*b1 + ntchsig2*b2 - signal1*a1 - signal2*a2) * a0_inv;
ntchsig1 = sample;

signal2 = signal1;
signal1 = output;
return output;
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4 changes: 2 additions & 2 deletions libraries/Filter/NotchFilter.h
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Expand Up @@ -41,9 +41,9 @@ class NotchFilter {
protected:

bool initialised, need_reset;
float b0, b1, b2, a1, a2, a0_inv;
float b0, b1, b2, a1, a2;
float _center_freq_hz, _sample_freq_hz;
T ntchsig, ntchsig1, ntchsig2, signal2, signal1;
T ntchsig1, ntchsig2, signal2, signal1;
};

/*
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2 changes: 1 addition & 1 deletion libraries/Filter/examples/TransferFunctionCheck/TransferFunctionCheck.cpp
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Expand Up @@ -61,7 +61,7 @@ class NotchHelper : public NotchFilterFloat {
hal.console->printf("NotchFilterFloat\n");
hal.console->printf("Sample rate: %.9f Hz, Center: %.9f Hz\n", _sample_freq_hz, _center_freq_hz);
hal.console->printf("Notch filter in the form: H(z) = (b0 + b1*z^-1 + b2*z^-2)/(a0 + a1*z^-1 + a2*z^-2)\n");
hal.console->printf("a0: %.9f, a1: %.9f, a2: %.9f, b0: %.9f, b1: %.9f, b2: %.9f\n", 1.0/a0_inv, a1, a2, b0, b1, b2);
hal.console->printf("a0: %.9f, a1: %.9f, a2: %.9f, b0: %.9f, b1: %.9f, b2: %.9f\n", 1.0, a1, a2, b0, b1, b2);
}
};

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