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Add the ability to define a biquad filter using raw parameters, instead of only by using component values or by type/cutoff/bandwidth/gain. Fix some of the issues with the Multiple Feedback Bandpass filter. [Lord Nightmare]
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Lord-Nightmare
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@ -14,10 +14,11 @@
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* It uses the Q factor directly in the filter definitions, rather than the damping factor (1/Q)
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* It implements every common type of digital biquad filter which I could find documentation for.
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* The filter is Direct-form II instead of Direct-form I, which results in shorter compiled code.
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* Optional direct control of the 5 normalized biquad parameters for a custom/raw parameter filter.
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Possibly useful features which aren't implemented because nothing uses them yet:
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* More Sallen-Key filter variations (band-pass, high-pass)
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* Direct control of the 5 normalized biquad parameters for a custom/raw parameter filter.
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*/
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#include "emu.h"
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#include "flt_biquad.h"
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@ -84,6 +85,17 @@ filter_biquad_device& filter_biquad_device::setup(filter_biquad_device::biquad_p
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m_gain = p.gain;
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return *this;
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}
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filter_biquad_device& filter_biquad_device::setup_raw(double a1, double a2, double b0, double b1, double b2)
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{
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m_type = biquad_type::RAWPARAMS;
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m_a1 = a1;
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m_a2 = a2;
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m_b0 = b0;
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m_b1 = b1;
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m_b2 = b2;
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return *this;
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}
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// modify an existing instance with new filter parameters
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void filter_biquad_device::modify(biquad_type type, double fc, double q, double gain)
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@ -104,14 +116,30 @@ void filter_biquad_device::modify(filter_biquad_device::biquad_params p)
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m_gain = p.gain;
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recalc();
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}
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void filter_biquad_device::modify_raw(double a1, double a2, double b0, double b1, double b2)
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{
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m_stream->update();
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m_type = biquad_type::RAWPARAMS;
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m_a1 = a1;
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m_a2 = a2;
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m_b0 = b0;
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m_b1 = b1;
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m_b2 = b2;
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recalc();
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}
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//-------------------------------------------------
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// Filter setup helpers for various filter models
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//-------------------------------------------------
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// NOTE: if a resistor doesn't exist, pass a value of RES_M(999.99) or the like, i.e. an 'infinite resistor'
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// NOTE: if a resistor is a direct short, set its resistance to RES_R(0.001)
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// Sallen-Key filters
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/* Setup a biquad filter structure based on a single op-amp Sallen-Key low-pass filter circuit.
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* This is sometimes, incorrectly, called a "Butterworth" filter structure.
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*
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* .----------------------------.
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* | |
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@ -154,9 +182,6 @@ filter_biquad_device::biquad_params filter_biquad_device::opamp_sk_lowpass_calc(
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{
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fatalerror("filter_biquad_device::opamp_sk_lowpass_calc() - no parameters can be 0; parameters were: r1: %f, r2: %f, r3: %f, r4: %f, c1: %f, c2: %f", r1, r2, r3, r4, c1, c2); /* Filter can not be setup. Undefined results. */
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}
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// NOTE: if R3 doesn't exist (no link to ground), pass a value of RES_M(999.99) or the like, i.e. an 'infinite resistor'
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// NOTE: if R4 is a direct short, set its resistance to RES_R(0.001)
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// NOTE: if R3 doesn't exist AND R4 is a direct short, follow both rules above.
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r.type = biquad_type::LOWPASS;
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r.gain = 1.0 + (r4 / r3); // == (r3 + r4) / r3
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r.fc = 1.0 / (2 * M_PI * sqrt(r1 * r2 * c1 * c2));
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@ -179,6 +204,7 @@ filter_biquad_device::biquad_params filter_biquad_device::opamp_sk_lowpass_calc(
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* a dead short, and c1 omitted. set both c1 and r2 to 0 in this case.
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* NOTE3: a variant of NOTE2 has only the c1 capacitor left off, and r2 present. if so,
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* set c1 to 0 and r2 to its expected value.
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* TODO: make this compatible with the RES_M(999.99) and RES_R(0.001) rules!
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*
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* .--------+---------.
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* | | |
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@ -215,7 +241,7 @@ filter_biquad_device::biquad_params filter_biquad_device::opamp_mfb_lowpass_calc
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}
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r.gain = -r3 / r1;
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r.q = (M_SQRT2 / 2.0);
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if (c1 == 0) // if both R2 and C1 are 0, it is the 'proper' first order case. There do exist some unusual filters where R2 is not 0, though. In both cases this yields a single-pole filter with limited configurable gain, and a Q of ~0.707. R2 being zero makes the (r1 * r3) numerator term cancel out to 1.0.
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if (c1 == 0) // if both R2 and C1 are 0, it is the 'proper' first order case. If C1 is 0 (Williams...) the filter is 1st order. There do exist some unusual filters where R2 is not 0, though. In both cases this yields a single-pole filter with limited configurable gain, and a Q of ~0.707. R2 being zero makes the (r1 * r3) numerator term cancel out to 1.0.
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{
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r.fc = (r1 * r3) / (2 * M_PI * ((r1 * r2) + (r1 * r3) + (r2 * r3)) * r3 * c2);
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r.type = biquad_type::LOWPASS1P;
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@ -234,14 +260,14 @@ filter_biquad_device::biquad_params filter_biquad_device::opamp_mfb_lowpass_calc
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/* Setup a biquad filter structure based on a single op-amp Multiple-Feedback band-pass filter circuit.
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* This is sometimes called a "modified Deliyannis" or "Deliyannis-friend" filter circuit,
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* or an "Infinite Gain Multiple-Feedback [band-pass] Filter", or "IGMF".
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* or an "Infinite Gain Multiple-Feedback [band-pass] Filter" aka "IGMF".
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* NOTE: vRef is not definable when setting up the filter, and is assumed to be grounded.
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* If the analog effects caused by vRef are important to the operation of the specific filter
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* in question, a netlist implementation may work better under those circumstances.
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* TODO: There is a documented modification to this filter which adds a resistor ladder between
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* ground and the op-amp output, with the 'rung' of the ladder connecting to the + input of
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* the op-amp, and this allows more control of the filter.
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* NOTE2: If r2 is not used, then set it to 0 ohms, the code will switch to an Infinite Gain MFB Bandpass
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* NOTE2: If r2 is not used, then set it to RES_M(999.99), the code will effectively be an Infinite Gain MFB Bandpass.
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*
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* .--------+---------.
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* | | |
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@ -259,27 +285,18 @@ filter_biquad_device::biquad_params filter_biquad_device::opamp_mfb_lowpass_calc
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*/
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filter_biquad_device& filter_biquad_device::opamp_mfb_bandpass_setup(double r1, double r2, double r3, double c1, double c2)
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{
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if ((r1 == 0) || (r3 == 0) || (c1 == 0) || (c2 == 0))
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if ((r1 == 0) || (r2 == 0) || (r3 == 0) || (c1 == 0) || (c2 == 0))
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{
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fatalerror("filter_biquad_device::opamp_mfb_bandpass_setup() - only r2 can be 0; parameters were: r1: %f, r2: %f, r3: %f, c1: %f, c2: %f", r1, r2, r3, c1, c2); /* Filter can not be setup. Undefined results. */
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fatalerror("filter_biquad_device::opamp_mfb_bandpass_setup() - no parameters can be 0; parameters were: r1: %f, r2: %f, r3: %f, c1: %f, c2: %f", r1, r2, r3, c1, c2); /* Filter can not be setup. Undefined results. */
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}
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double r_in, gain;
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double const r_in = 1.0 / (1.0/r1 + 1.0/r2); // TODO: verify
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// gain = (r2 / (r1 + r2)) * (-r3 / r_in * c2 / (c1 + c2)); // ??? wrong?
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double const gain = -r3 / (2.0 * r1);
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// q = sqrt(r3 / r_in * c1 * c2) / (c1 + c2); // ??? wrong?
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double const q = 0.5 * sqrt(r3 / r1);
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if (r2 == 0)
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{
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gain = 1;
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r_in = r1;
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}
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else
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{
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gain = r2 / (r1 + r2);
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r_in = 1.0 / (1.0/r1 + 1.0/r2);
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}
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double const fc = 1.0 / (2 * M_PI * sqrt(r_in * r3 * c1 * c2)); // technically this is the center frequency of the bandpass
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double const q = sqrt(r3 / r_in * c1 * c2) / (c1 + c2);
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gain *= -r3 / r_in * c2 / (c1 + c2);
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double const fc = 1.0 / (sqrt(r_in * r3 * c1 * c2)); // technically this is the center frequency of the bandpass
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#ifdef FLT_BIQUAD_DEBUG_SETUP
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logerror("filter_biquad_device::opamp_mfb_bandpass_setup() yields: fc = %f, Q = %f, gain = %f\n", fc, q, gain);
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#endif
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@ -434,6 +451,9 @@ void filter_biquad_device::sound_stream_update(sound_stream &stream, std::vector
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*/
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void filter_biquad_device::recalc()
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{
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if (m_type == biquad_type::RAWPARAMS)
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return; // if we're dealing with raw parameters, just return, don't touch anything.
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double const MGain = fabs(m_gain); // absolute multiplicative gain
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double const DBGain = log10(MGain) * 20.0; // gain in dB
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double const AMGain = pow(10, fabs(DBGain) / 20.0); // multiplicative gain of absolute DB
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@ -442,9 +462,6 @@ void filter_biquad_device::recalc()
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double const KoverQ = K / m_q;
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double normal = 1.0 / (1.0 + KoverQ + Ksquared);
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m_a1 = 2.0 * (Ksquared - 1.0) * normal;
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m_a2 = (1.0 - KoverQ + Ksquared) * normal;
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switch (m_type)
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{
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case biquad_type::LOWPASS1P:
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@ -24,7 +24,8 @@ public:
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NOTCH,
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PEAK,
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LOWSHELF,
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HIGHSHELF
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HIGHSHELF,
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RAWPARAMS
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};
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struct biquad_params
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@ -44,7 +45,12 @@ public:
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void modify(biquad_type type, double fc, double q, double gain);
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void modify(biquad_params p);
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// helper setup functions to create common filters representable by biquad filters:
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// set up the filter with raw biquad coefficients
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filter_biquad_device& setup_raw(double a1, double a2, double b0, double b1, double b2);
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void modify_raw(double a1, double a2, double b0, double b1, double b2);
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// Helper setup functions to create common filters representable by biquad filters:
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// (and, as needed, modify/update/recalc helpers)
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// Sallen-Key low-pass
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filter_biquad_device& opamp_sk_lowpass_setup(double r1, double r2, double r3, double r4, double c1, double c2);
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