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flt_biquad: Allow recalculating the filter values at runtime if a component value can be altered or changed through some means. [Lord Nightmare]

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This commit is contained in:
Lord-Nightmare 2020-11-19 07:04:33 -05:00
parent 40646cd9dd
commit e76a5e0217
2 changed files with 65 additions and 36 deletions
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85
src/devices/sound/flt_biquad.cpp
View File

@ -76,9 +76,17 @@ filter_biquad_device& filter_biquad_device::setup(biquad_type type, double fc, d
m_gain = gain;
return *this;
}
filter_biquad_device& filter_biquad_device::setup(filter_biquad_device::biquad_params p)
{
m_type = p.type;
m_fc = p.fc;
m_q = p.q;
m_gain = p.gain;
return *this;
}
// update an existing instance with new filter parameters
void filter_biquad_device::update_params(biquad_type type, double fc, double q, double gain)
// modify an existing instance with new filter parameters
void filter_biquad_device::modify(biquad_type type, double fc, double q, double gain)
{
m_stream->update();
m_type = type;
@ -87,7 +95,15 @@ void filter_biquad_device::update_params(biquad_type type, double fc, double q,
m_gain = gain;
recalc();
}
void filter_biquad_device::modify(filter_biquad_device::biquad_params p)
{
m_stream->update();
m_type = p.type;
m_fc = p.fc;
m_q = p.q;
m_gain = p.gain;
recalc();
}
//-------------------------------------------------
// Filter setup helpers for various filter models
@ -141,13 +157,12 @@ filter_biquad_device& filter_biquad_device::opamp_sk_lowpass_setup(double r1, do
/* Setup a biquad filter structure based on a single op-amp Multiple-Feedback low-pass filter circuit.
* NOTE: vRef is not definable when setting up the filter.
* If the analog effects caused by vRef are important to the operation of the specific filter
* in question, a netlist implementation may work better under those circumstances.
* NOTE2: a variant of this circuit has the c1 capacitor left off. if so, set c1 to 0
* which will form a controllable gain 1st order version of the circuit.
* NOTE3: There is a well known 'proper' 1st order version of this circuit where r2 is
* a dead short, and c1 is also ommitted as in NOTE2. Set both r2 to 0 and c1 to 0 to
* emulate a circuit of this type.
* If the analog effects caused by vRef are important to the operation of the specific
* filter in question, a netlist implementation may work better under those circumstances.
* NOTE2: There is a well known 'proper' 1st order version of this circuit where r2 is
* a dead short, and c1 omitted. set both c1 and r2 to 0 in this case.
* NOTE3: a variant of NOTE2 has only the c1 capacitor left off, and r2 present. if so,
* set c1 to 0 and r2 to its expected value.
*
* .--------+---------.
* | | |
@ -165,37 +180,40 @@ filter_biquad_device& filter_biquad_device::opamp_sk_lowpass_setup(double r1, do
*/
filter_biquad_device& filter_biquad_device::opamp_mfb_lowpass_setup(double r1, double r2, double r3, double c1, double c2)
{
filter_biquad_device::biquad_params p = opamp_mfb_lowpass_calc(r1, r2, r3, c1, c2);
return setup(p);
}
void filter_biquad_device::opamp_mfb_lowpass_modify(double r1, double r2, double r3, double c1, double c2)
{
filter_biquad_device::biquad_params p = opamp_mfb_lowpass_calc(r1, r2, r3, c1, c2);
modify(p);
}
filter_biquad_device::biquad_params filter_biquad_device::opamp_mfb_lowpass_calc(double r1, double r2, double r3, double c1, double c2)
{
filter_biquad_device::biquad_params r;
if ((r1 == 0) || ((r2 == 0) && (c1 != 0)) || (r3 == 0) || (c2 == 0))
{
fatalerror("filter_biquad_device::opamp_mfb_lowpass_setup() - only c1 can be 0 (and if c1 is 0, r2 can also 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. */
fatalerror("filter_biquad_device::opamp_mfb_lowpass_calc() - only c1 can be 0 (and if c1 is 0, r2 can also 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. */
}
double const gain = -r3 / r1;
double fc, q = (M_SQRT2 / 2.0);
if ((r2 != 0) && (c1 == 0)) // set C1 to 0 to run this filter in single pole mode where C1 was left off the filter entirely. Certain Williams boards seem to have omitted C1, presumably by accident.
r.gain = -r3 / r1;
r.q = (M_SQRT2 / 2.0);
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.
{
fc = (r1 * r3) / (2 * M_PI * ((r1 * r2) + (r1 * r3) + (r2 * r3)) * r3 * c2);
#ifdef FLT_BIQUAD_DEBUG_SETUP
logerror("filter_biquad_device::opamp_mfb_lowpass_setup() in degraded mode yields: fc = %f, Q = %f(ignored), gain = %f\n", fc, q, gain);
#endif
return setup(biquad_type::LOWPASS1P, fc, q, gain);
}
else if ((r2 == 0) && (c1 == 0)) // proper first order case, set both C1 to 0 and R2 to 0 for the first order
{
fc = 1.0 / (2 * M_PI * r3 * c2);
#ifdef FLT_BIQUAD_DEBUG_SETUP
logerror("filter_biquad_device::opamp_mfb_lowpass_setup() in 1st order mode yields: fc = %f, Q = %f(ignored), gain = %f\n", fc, q, gain);
#endif
return setup(biquad_type::LOWPASS1P, fc, q, gain);
r.fc = (r1 * r3) / (2 * M_PI * ((r1 * r2) + (r1 * r3) + (r2 * r3)) * r3 * c2);
r.type = biquad_type::LOWPASS1P;
}
else // common case, (r2 != 0) && (c1 != 0)
{
fc = 1.0 / (2 * M_PI * sqrt(r2 * r3 * c1 * c2));
q = sqrt(r2 * r3 * c1 * c2) / ((r3 * c2) + (r2 * c2) + ((r2 * c2) * -gain));
#ifdef FLT_BIQUAD_DEBUG_SETUP
logerror("filter_biquad_device::opamp_mfb_lowpass_setup() in 2nd order mode yields: fc = %f, Q = %f, gain = %f\n", fc, q, gain);
#endif
return setup(biquad_type::LOWPASS, fc, q, gain);
r.fc = 1.0 / (2 * M_PI * sqrt(r2 * r3 * c1 * c2));
r.q = sqrt(r2 * r3 * c1 * c2) / ((r3 * c2) + (r2 * c2) + ((r2 * c2) * -r.gain));
r.type = biquad_type::LOWPASS;
}
#ifdef FLT_BIQUAD_DEBUG_SETUP
logerror("filter_biquad_device::opamp_mfb_lowpass_setup(%f, %f, %f, %f, %f) yields:\n\ttype = %d, fc = %f, Q = %f, gain = %f\n", r1, r2, r3, c1*1000000, c2*1000000, static_cast<int>(r.type), r.fc, r.q, r.gain);
#endif
return r;
}
/* Setup a biquad filter structure based on a single op-amp Multiple-Feedback band-pass filter circuit.
@ -272,7 +290,6 @@ filter_biquad_device& filter_biquad_device::opamp_mfb_highpass_setup(double r1,
{
fatalerror("filter_biquad_device::opamp_mfb_highpass_setup() - no parameters can be 0; parameters were: r1: %f, r2: %f, c1: %f, c2: %f, c3: %f", r1, r2, c1, c2, c3); /* Filter can not be setup. Undefined results. */
}
// TODO: if c1 is 0/shorted, should the circuit should work with a gain of 1 in a first order mode?
double const gain = -c1 / c3;
double const fc = 1.0 / (2 * M_PI * sqrt(c2 * c3 * r1 * r2));

16
src/devices/sound/flt_biquad.h
View File

@ -27,12 +27,22 @@ public:
HIGHSHELF
};
struct biquad_params
{
biquad_type type;
double fc;
double q;
double gain;
};
filter_biquad_device(const machine_config &mconfig, const char *tag, device_t *owner, uint32_t clock = 0);
// set up the filter with the specified filter parameters and return a pointer to the new device
filter_biquad_device& setup(biquad_type type, double fc, double q, double gain);
// update an existing instance with new filter parameters
void update_params(biquad_type type, double fc, double q, double gain);
filter_biquad_device& setup(biquad_params p);
// modify an existing instance with new filter parameters
void modify(biquad_type type, double fc, double q, double gain);
void modify(biquad_params p);
// helper setup functions to create common filters representable by biquad filters
// Sallen-Key low-pass
@ -42,6 +52,8 @@ public:
// Multiple-Feedback low-pass
filter_biquad_device& opamp_mfb_lowpass_setup(double r1, double r2, double r3, double c1, double c2);
void opamp_mfb_lowpass_modify(double r1, double r2, double r3, double c1, double c2);
biquad_params opamp_mfb_lowpass_calc(double r1, double r2, double r3, double c1, double c2);
// Multiple-Feedback band-pass
filter_biquad_device& opamp_mfb_bandpass_setup(double r1, double r2, double r3, double c1, double c2);