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cem3394: switch to alternate filter that is more stable

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This commit is contained in:
Aaron Giles 2020-09-19 02:47:47 -07:00
parent 0d0175f59f
commit 3dd50371d4
1 changed files with 109 additions and 20 deletions
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129
src/devices/sound/cem3394.cpp
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@ -4,12 +4,7 @@
Curtis Electromusic Specialties CEM3394 µP-Controllable Synthesizer Voice
This driver handles CEM-3394 analog synth chip. Very crudely.
Still to do:
- adjust the overall volume when multiple waves are being generated
- filter internal sound
- support resonance (don't understand how it works)
This driver handles CEM-3394 analog synth chip.
***************************************************************************/
@ -19,6 +14,18 @@
#include <algorithm>
// various filter implementations to play with; currently SVTRAP works best
#define FILTER_TYPE_NONE (0)
#define FILTER_TYPE_SVTRAP (1)
#define FILTER_TYPE_ESQ1 (2)
#define FILTER_TYPE FILTER_TYPE_SVTRAP
// logging
#define LOG_CONTROL_CHANGES (0)
// use 0.25 as the base volume for pulses
static constexpr double PULSE_VOLUME = 0.25;
@ -147,33 +154,91 @@ cem3394_device::cem3394_device(const machine_config &mconfig, const char *tag, d
//-------------------------------------------------
// sound_stream_update - generate sound to the mix
// buffer in mono
// filter - apply the lowpass filter at the given
// cutoff frequency
//-------------------------------------------------
#if (FILTER_TYPE == FILTER_TYPE_NONE)
double cem3394_device::filter(double input, double cutoff)
{
// clamp cutoff to useful range, 20Hz-20kHz
cutoff = std::max(std::min(cutoff, 20000.0), 20.0);
return input;
}
#elif (FILTER_TYPE == FILTER_TYPE_SVTRAP)
double cem3394_device::filter(double input, double cutoff)
{
// clamp cutoff to useful range, 50Hz-20kHz
cutoff = std::min(std::max(cutoff, 50.0), 20000.0);
// clamp resonance to below 1.0 to prevent runaway behavior; when clamping,
// also apply an (arbitrary) scale factor to the output since we're close
// to resonance and the datasheet indicates there is an amplitude correction
// in this case
double outscale = 1.0;
double res = m_filter_resonance;
if (res > 0.99)
res = 0.99, outscale = 0.5;
// core filter implementation
double g = tan(M_PI * cutoff * m_inv_sample_rate);
double k = 2.0 - 2.0 * res;
double a1 = 1.0 / (1.0 + g * (g + k));
double a2 = g * a1;
double a3 = g * a2;
double v3 = input - m_filter_out[1];
double v1 = a1 * m_filter_out[0] + a2 * v3;
double v2 = m_filter_out[1] + a2 * m_filter_out[0] + a3 * v3;
m_filter_out[0] = 2 * v1 - m_filter_out[0];
m_filter_out[1] = 2 * v2 - m_filter_out[1];
// lowpass output is equal to v2
double output = v2 * outscale;
// catch any NaNs
if (std::isnan(output))
{
logerror("NAN - vco: %6.0f cutoff: %6.0f res: %.5f output: %.5f\n", m_vco_step / m_inv_sample_rate, cutoff, m_filter_resonance, output);
output = 0;
m_filter_out[0] = m_filter_out[1] = 0;
}
// if we go out of range, scale down to 1.0 and also scale our
// feedback terms to help us stay in control
else if (fabs(output) > 1.0)
{
double scale = 1.0 / fabs(output);
output *= scale;
m_filter_out[0] *= scale;
m_filter_out[1] *= scale;
}
return output;
}
#elif (FILTER_TYPE == FILTER_TYPE_ESQ1)
double cem3394_device::filter(double input, double cutoff)
{
// clamp cutoff to useful range, 50Hz-20kHz
cutoff = std::min(std::max(cutoff, 50.0), 20000.0);
// clamp resonance to 0.95 to prevent infinite gain
double r = 4.0 * std::min(m_filter_resonance, 0.95);
double r = 4.0 * std::min(res, 0.95);
// core filter implementation
double g = 2 * M_PI * cutoff;
double zc = g / tan(g/2 * m_inv_sample_rate);
double gzc = zc / g;
double gzc2 = gzc * gzc;
double gzc3 = gzc2 * gzc;
double gzc4 = gzc3 * gzc;
double r1 = 1 + r;
double a0 = r1;
double a1 = 4 * r1;
double a2 = 6 * r1;
double a3 = 4 * r1;
double a4 = r1;
double b0 = r1 + 4 * gzc + 6 * gzc2 + 4 * gzc3 + gzc4;
double b1 = 4 * (r1 + 2 * gzc - 2 * gzc3 - gzc4);
double b2 = 6 * (r1 - 2 * gzc2 + gzc4);
@ -184,15 +249,18 @@ double cem3394_device::filter(double input, double cutoff)
+ m_filter_in[0] * a1 + m_filter_in[1] * a2 + m_filter_in[2] * a3 + m_filter_in[3] * a4
- m_filter_out[0] * b1 - m_filter_out[1] * b2 - m_filter_out[2] * b3 - m_filter_out[3] * b4) / b0;
// sound_assert(!std::isnan(output));
// sound_assert(output >= -1.5 && output <= 1.5);
if (output < -1.5 || output > 1.5 || std::isnan(output))
// catch NaNs
if (std::isnan(output))
{
if (m_filter_out[0] > -1.5 && m_filter_out[0] < 1.5)
printf("cutoff: %6.0f res: %.5f output: %.5f\n", cutoff, m_filter_resonance, output);
logerror("NAN - vco: %6.0f cutoff: %6.0f res: %.5f output: %.5f\n", m_vco_step / m_inv_sample_rate, cutoff, m_filter_resonance, output);
output = 0;
}
if (std::isnan(output)) output = 0;
// if output goes significantly out of range, scale it down
else if (fabs(output) > 10.0)
output = 10.0;
// update memories
m_filter_in[3] = m_filter_in[2];
m_filter_in[2] = m_filter_in[1];
m_filter_in[1] = m_filter_in[0];
@ -203,6 +271,7 @@ double cem3394_device::filter(double input, double cutoff)
m_filter_out[1] = m_filter_out[0];
m_filter_out[0] = output;
// clamp to range and return
if (output < -1.0)
output = -1.0;
else if (output > 1.0)
@ -210,6 +279,18 @@ double cem3394_device::filter(double input, double cutoff)
return output;
}
#else
#error Unknown FILTER_TYPE
#endif
//-------------------------------------------------
// sound_stream_update - generate sound to the mix
// buffer in mono
//-------------------------------------------------
void cem3394_device::sound_stream_update(sound_stream &stream, std::vector<read_stream_view> const &inputs, std::vector<write_stream_view> &outputs)
{
auto &external = inputs[0];
@ -370,6 +451,7 @@ void cem3394_device::set_voltage(int input, double voltage)
case VCO_FREQUENCY:
temp = m_vco_zero_freq * pow(2.0, -voltage * (1.0 / 0.75));
m_vco_step = temp * m_inv_sample_rate;
if (LOG_CONTROL_CHANGES) logerror("VCO_FREQ=%6.3fV -> freq=%f\n", voltage, temp);
break;
// wave select determines triangle/sawtooth enable
@ -381,6 +463,7 @@ void cem3394_device::set_voltage(int input, double voltage)
m_wave_select |= WAVE_TRIANGLE | WAVE_SAWTOOTH;
else if (voltage >= 2.3 && voltage <= 3.9)
m_wave_select |= WAVE_SAWTOOTH;
if (LOG_CONTROL_CHANGES) logerror("WAVE_SEL=%6.3fV -> tri=%d saw=%d\n", voltage, (m_wave_select & WAVE_TRIANGLE) ? 1 : 0, (m_wave_select & WAVE_SAWTOOTH) ? 1 : 0);
break;
// pulse width determines duty cycle; 0.0 means 0%, 2.0 means 100%
@ -397,11 +480,13 @@ void cem3394_device::set_voltage(int input, double voltage)
m_pulse_width = MINIMUM_WIDTH + (MAXIMUM_WIDTH - MINIMUM_WIDTH) * m_pulse_width;
m_wave_select |= WAVE_PULSE;
}
if (LOG_CONTROL_CHANGES) logerror("PULSE_WI=%6.3fV -> raw=%f adj=%f\n", voltage, voltage * 0.5, m_pulse_width);
break;
// final gain is pretty self-explanatory; 0.0 means ~90dB, 4.0 means 0dB
case FINAL_GAIN:
m_volume = compute_db_volume(voltage);
if (LOG_CONTROL_CHANGES) logerror("TOT_GAIN=%6.3fV -> vol=%f\n", voltage, m_volume);
break;
// mixer balance is a pan between the external input and the internal input
@ -417,11 +502,13 @@ void cem3394_device::set_voltage(int input, double voltage)
m_mixer_internal = compute_db_volume(3.55 - 0.45 * (voltage * 0.25));
m_mixer_external = compute_db_volume(3.55 + voltage);
}
if (LOG_CONTROL_CHANGES) logerror(" BALANCE=%6.3fV -> int=%f ext=%f\n", voltage, m_mixer_internal, m_mixer_external);
break;
// filter frequency varies from -3.0 to +4.0, at 0.375V/octave
case FILTER_FREQENCY:
m_filter_frequency = m_filter_zero_freq * pow(2.0, -voltage * (1.0 / 0.375));
if (LOG_CONTROL_CHANGES) logerror("FLT_FREQ=%6.3fV -> freq=%f\n", voltage, m_filter_frequency);
break;
// modulation depth is 0.01*freq at 0V and 2.0*freq at 3.5V
@ -432,6 +519,7 @@ void cem3394_device::set_voltage(int input, double voltage)
m_filter_modulation = 1.99;
else
m_filter_modulation = (voltage * (1.0 / 3.5)) * 1.98 + 0.01;
if (LOG_CONTROL_CHANGES) logerror("FLT_MODU=%6.3fV -> mod=%f\n", voltage, m_filter_modulation);
break;
// this is not yet implemented
@ -442,6 +530,7 @@ void cem3394_device::set_voltage(int input, double voltage)
m_filter_resonance = 1.0;
else
m_filter_resonance = voltage * (1.0 / 2.5);
if (LOG_CONTROL_CHANGES) logerror("FLT_RESO=%6.3fV -> mod=%f\n", voltage, m_filter_resonance);
break;
}
}