sound/flt_biquad: Added calculator functions for RC-based band-pass filters.

src/devices/sound/flt_biquad.cpp

@@ -442,6 +442,101 @@ filter_biquad_device::biquad_params filter_biquad_device::opamp_diff_bandpass_ca
 }
 
 
+/* RC-based band-pass filters:
+ *
+ * RR variation: the two resistors are connected to each other:
+ *
+ *   Vin -- C1 -- R1 -+-----+- Vout
+ *                    |     |
+ *                    R2    C2
+ *                    |     |
+ *                   GND  (V)GND
+ *
+ *
+ * CC variation: The two capacitors are connected to each other:
+ *
+ *   Vin -- R1 -+- C2 -+- Vout
+ *              |      |
+ *              C1     R2
+ *              |      |
+ *             GND   (V)GND
+ *
+ * (V)GND could be a virtual ground.
+ *
+ * BPF transfer function: H(s) = (A * s) / (s ^ 2 + B * s + C)
+ * In the RR configuration, we have:
+ *   A = 1 / (R1 * C2)
+ *   B = (R1 * C1 + R2 * C2 + R2 * C1) / (R1 * R2 * C1 * C2)
+ *   C = 1 / (R1 * R2 * C1 * C2)
+ * In the CC configuration, we have:
+ *   A = 1 / (R1 * C1)
+ *   B = (R1 * C1 + R2 * C2 + R1 * C2) / (R1 * R2 * C1 * C2)
+ *   C = 1 / (R1 * R2 * C1 * C2)
+ * From the standard transfer function for BPFs, we have:
+ *   A = gain * (w / Q)
+ *   B = w / Q
+ *   C = w ^ 2
+ * The calculations of Fc, Q and gain in the *_calc functions below are derived
+ * from the equations above, with some algebra.
+ */
+
+filter_biquad_device& filter_biquad_device::rc_rr_bandpass_setup(double r1, double r2, double c1, double c2)
+{
+	return setup(rc_rr_bandpass_calc(r1, r2, c1, c2));
+}
+
+void filter_biquad_device::rc_rr_bandpass_modify(double r1, double r2, double c1, double c2)
+{
+	modify(rc_rr_bandpass_calc(r1, r2, c1, c2));
+}
+
+filter_biquad_device::biquad_params filter_biquad_device::rc_rr_bandpass_calc(double r1, double r2, double c1, double c2)
+{
+	if ((r1 == 0) || (r2 == 0) || (c1 == 0) || (c2 == 0))
+	{
+		fatalerror("filter_biquad_device::rc_rr_bandpass_calc() - no parameters can be 0; parameters were: r1: %f, r2: %f, c1: %f, c2: %f", r1, r2, c1, c2);
+	}
+	const double x = sqrt(r1 * r2 * c1 * c2);
+	const double y = r1 * c1 + r2 * c2 + r2 * c1;
+	const double z = r2 * c1;
+	filter_biquad_device::biquad_params p;
+	p.type = filter_biquad_device::biquad_type::BANDPASS;
+	p.fc = 1.0 / (2.0 * M_PI * x);
+	p.q = x / y;
+	p.gain = z / y;
+	LOGMASKED(LOG_SETUP, "filter_biquad_device::rc_rr_bandpass_calc(%f %f %f %f) yields: fc = %f, Q = %f, gain = %f\n", r1, r2, c1, c2, p.fc, p.q, p.gain);
+	return p;
+}
+
+filter_biquad_device& filter_biquad_device::rc_cc_bandpass_setup(double r1, double r2, double c1, double c2)
+{
+	return setup(rc_cc_bandpass_calc(r1, r2, c1, c2));
+}
+
+void filter_biquad_device::rc_cc_bandpass_modify(double r1, double r2, double c1, double c2)
+{
+	modify(rc_cc_bandpass_calc(r1, r2, c1, c2));
+}
+
+filter_biquad_device::biquad_params filter_biquad_device::rc_cc_bandpass_calc(double r1, double r2, double c1, double c2)
+{
+	if ((r1 == 0) || (r2 == 0) || (c1 == 0) || (c2 == 0))
+	{
+		fatalerror("filter_biquad_device::rc_cc_bandpass_calc() - no parameters can be 0; parameters were: r1: %f, r2: %f, c1: %f, c2: %f", r1, r2, c1, c2);
+	}
+	const double x = sqrt(r1 * r2 * c1 * c2);
+	const double y = r1 * c1 + r2 * c2 + r1 * c2;
+	const double z = r2 * c2;
+	filter_biquad_device::biquad_params p;
+	p.type = filter_biquad_device::biquad_type::BANDPASS;
+	p.fc = 1.0 / (2.0 * M_PI * x);
+	p.q = x / y;
+	p.gain = z / y;
+	LOGMASKED(LOG_SETUP, "filter_biquad_device::rc_cc_bandpass_calc(%f %f %f %f) yields: fc = %f, Q = %f, gain = %f\n", r1, r2, c1, c2, p.fc, p.q, p.gain);
+	return p;
+}
+
+
 //-------------------------------------------------
 //  device_start - device-specific startup
 //-------------------------------------------------

src/devices/sound/flt_biquad.h

@@ -82,6 +82,15 @@ class filter_biquad_device : public device_t, public device_sound_interface
 	void opamp_diff_bandpass_modify(double r1, double r2, double c1, double c2);
 	biquad_params opamp_diff_bandpass_calc(double r1, double r2, double c1, double c2);
 
+	// RC-based band-pass, resistors connected to each other.
+	filter_biquad_device& rc_rr_bandpass_setup(double r1, double r2, double c1, double c2);
+	void rc_rr_bandpass_modify(double r1, double r2, double c1, double c2);
+	biquad_params rc_rr_bandpass_calc(double r1, double r2, double c1, double c2);
+
+	// RC-based band-pass, capacitors connected to each other.
+	filter_biquad_device& rc_cc_bandpass_setup(double r1, double r2, double c1, double c2);
+	void rc_cc_bandpass_modify(double r1, double r2, double c1, double c2);
+	biquad_params rc_cc_bandpass_calc(double r1, double r2, double c1, double c2);
 
 protected:
 	// device-level overrides

src/mame/linn/linndrum.cpp

@@ -908,6 +908,8 @@ void linndrum_audio_device::update_volume_and_pan(int channel)
 	// Using -gain_*, because the summing op-amps are inverting.
 	mixer_input->set_route_gain(0, m_left_mixer, 0, -gain_left);
 	mixer_input->set_route_gain(0, m_right_mixer, 0, -gain_right);
+	LOGMASKED(LOG_MIX, "Gain update for %s - left: %f, right: %f\n",
+	          MIXER_CHANNEL_NAMES[channel], gain_left, gain_right);
 
 	if (channel == MIX_CLICK)
 	{
@@ -931,59 +933,36 @@ void linndrum_audio_device::update_volume_and_pan(int channel)
 		// For now, scale by some number to match the volume of other voices.
 		static constexpr const float CLICK_GAIN_CORRECTION = 5;
 
-		float fc = 0;
-		float q = 1;
-		float gain = 1;
 		if (volume >= 100)
 		{
 			// HPF transfer function: H(s) = (g * s) / (s + w) where
 			//   g = C1 / (C1 + C2)
 			//   w = 1 / (R * (C1 + C2))
-			fc = 1.0F / (2 * float(M_PI) * r_voice_gnd * (C_CLICK_DCBLOCK + C_CLICK_WIPER));
-			gain = C_CLICK_DCBLOCK / (C_CLICK_DCBLOCK + C_CLICK_WIPER);
+			const float fc = 1.0F / (2 * float(M_PI) * r_voice_gnd * (C_CLICK_DCBLOCK + C_CLICK_WIPER));
+			const float gain = C_CLICK_DCBLOCK / (C_CLICK_DCBLOCK + C_CLICK_WIPER);
 			m_click_bpf->modify(filter_biquad_device::biquad_type::HIGHPASS1P1Z, fc, 1, gain * CLICK_GAIN_CORRECTION);
+			LOGMASKED(LOG_MIX, "- HPF cutoff: %.2f Hz, Gain: %.3f\n", fc, gain);
 		}
 		else if (volume > 0)
 		{
-			// BPF transfer function: H(s) = (A * s) / (s ^ 2 + B * s + C)
-			// Where:
-			//   A = 1 / (R1 * C2)
-			//   B = (C1 * R1 + C1 * R2 + C2 * R2) / (R1 * R2 * C1 * C2)
-			//   C = 1 / (R1 * R2 * C1 * C2).
-			// From the standard transfer function for BPFs, we have:
-			//   A = gain * (w / Q)
-			//   B = w / Q
-			//   C = w ^ 2
-			// The calculations of Fc, Q and gain below are derived from the
-			// equations above, with some algebra.
-
-			const float x = sqrtf(r0 * r_wiper_gnd * C_CLICK_DCBLOCK * C_CLICK_WIPER);
-			const float y = C_CLICK_DCBLOCK * r0 + C_CLICK_DCBLOCK * r_wiper_gnd + C_CLICK_WIPER * r_wiper_gnd;
-			fc = 1.0F / (2 * float(M_PI) * x);
-			q = x / y;
-			gain = r_wiper_gnd * C_CLICK_DCBLOCK / y;
-
-			// The transfer function above includes the effect of the volume
+			filter_biquad_device::biquad_params p = m_click_bpf->rc_rr_bandpass_calc(r0, r_wiper_gnd, C_CLICK_DCBLOCK, C_CLICK_WIPER);
+			// The filter params above include the effect of the volume
 			// potentiometer. But `gain_left` and `gain_right` already incorporate
 			// that effect. So it needs to be undone from the filter's gain.
-			gain *= 1.0F / RES_VOLTAGE_DIVIDER(r0, r_wiper_gnd);
-
-			m_click_bpf->modify(filter_biquad_device::biquad_type::BANDPASS, fc, q, gain * CLICK_GAIN_CORRECTION);
+			p.gain /= RES_VOLTAGE_DIVIDER(r0, r_wiper_gnd);
+			// See comments for CLICK_GAIN_CORRECTION.
+			p.gain *= CLICK_GAIN_CORRECTION;
+			m_click_bpf->modify(p);
+			LOGMASKED(LOG_MIX, "- BPF cutoff: %.2f Hz, Q: %.3f, Gain: %.3f\n", p.fc, p.q, p.gain);
 		}
 		// Else, if the volume is 0, don't change the BPF's configuration to avoid divisions by 0.
-
-		LOGMASKED(LOG_MIX, "Gain update for %s - left: %f, right: %f, %s cutoff: %.2f Hz, Q: %.3f, Gain: %.3f\n",
-				  MIXER_CHANNEL_NAMES[MIX_CLICK], gain_left, gain_right,
-				  (volume >= 100) ? "HPF" : "BPF", fc, q, gain);
 	}
 	else
 	{
 		// The rest of the voices just have a DC-blocking filter. Its exact cutoff
 		// will depend on the volume and pan settings, but it won't be audible.
 		m_voice_hpf[channel]->filter_rc_set_RC(filter_rc_device::HIGHPASS, r_voice_gnd, 0, 0, C_VOICE);
-		LOGMASKED(LOG_MIX, "Gain update for %s - left: %f, right: %f, HPF cutoff: %.2f Hz\n",
-				  MIXER_CHANNEL_NAMES[channel], gain_left, gain_right,
-				  1.0F / (2 * float(M_PI) * r_voice_gnd * C_VOICE));
+		LOGMASKED(LOG_MIX, "- HPF cutoff: %.2f Hz\n", 1.0F / (2 * float(M_PI) * r_voice_gnd * C_VOICE));
 	}
 }
 

src/mame/roland/roland_tr707.cpp

@@ -262,7 +262,6 @@ class tr707_audio_device : public device_t
 	static inline constexpr double VBE = 0.6;  // BJT base-emitter voltage drop.
 	static inline constexpr u16 MAX_CYMBAL_COUNTER = 0x8000;
 
-	static filter_biquad_device::biquad_params rc_bpf(double r1, double r2, double c1, double c2, bool crrc);
 	static double mux_dac_v(double v_eg, u8 data);
 
 	void advance_sample_w(offs_t offset, u16 data);
@@ -602,7 +601,7 @@ void tr707_audio_device::device_add_mconfig(machine_config &config)
 	for (int i = 0; i < MC_COUNT; ++i)
 	{
 		FILTER_BIQUAD(config, m_voice_bpf[i]);
-		m_voice_bpf[i]->setup(rc_bpf(bpf_comps[i][0], bpf_comps[i][1], bpf_comps[i][2], bpf_comps[i][3], false));
+		m_voice_bpf[i]->rc_cc_bandpass_setup(bpf_comps[i][0], bpf_comps[i][1], bpf_comps[i][2], bpf_comps[i][3]);
 		voices[i]->add_route(0, m_voice_bpf[i], 1.0);
 
 		FILTER_VOLUME(config, m_level[i]);
@@ -622,8 +621,8 @@ void tr707_audio_device::device_add_mconfig(machine_config &config)
 	// making it to the left and right output sockets.
 	auto &left_bpf = FILTER_BIQUAD(config, "left_out_bpf");
 	auto &right_bpf = FILTER_BIQUAD(config, "right_out_bpf");
-	left_bpf.setup(rc_bpf(RES_K(1), RES_K(47), CAP_U(10), CAP_U(0.01), true));  // R114, R112, C79, C76
-	right_bpf.setup(rc_bpf(RES_K(1), RES_K(47), CAP_U(10), CAP_U(0.01), true));  // R113, R111, C80, C77
+	left_bpf.rc_rr_bandpass_setup(RES_K(1), RES_K(47), CAP_U(10), CAP_U(0.01));  // R114, R112, C79, C76
+	right_bpf.rc_rr_bandpass_setup(RES_K(1), RES_K(47), CAP_U(10), CAP_U(0.01));  // R113, R111, C80, C77
 	m_left_mixer->add_route(0, left_bpf, 1.0);
 	m_right_mixer->add_route(0, right_bpf, 1.0);
 
@@ -655,49 +654,6 @@ void tr707_audio_device::device_reset()
 		update_mix(channel);
 }
 
-// Biquad parameters for an RC-based bandpass filter.
-// crrc == true:
-//   Vin -- C1 -- R1 -+---+- Vout
-//                    |   |
-//                    R2  C2
-//                    |   |
-//                   GND  GND
-// crrc == false:
-//   Vin -- R1 -+- C2 -+- Vout
-//              |      |
-//              C1     R2
-//              |      |
-//             GND    GND
-// TODO: move to sound/flt_biquad.
-filter_biquad_device::biquad_params tr707_audio_device::rc_bpf(double r1, double r2, double c1, double c2, bool crrc)
-{
-	// BPF transfer function: H(s) = (A * s) / (s ^ 2 + B * s + C)
-	// In the C-R-R-C configuration, we have:
-	//   A = 1 / (R1 * C2)
-	//   B = (R1 * C1 + R2 * C2 + R2 * C1) / (R1 * R2 * C1 * C2)
-	//   C = 1 / (R1 * R2 * C1 * C2)
-	// In the R-C-C-R configuration, we have:
-	//   A = 1 / (R1 * C1)
-	//   B = (R1 * C1 + R2 * C2 + R1 * C2) / (R1 * R2 * C1 * C2)
-	//   C = 1 / (R1 * R2 * C1 * C2)
-	// From the standard transfer function for BPFs, we have:
-	//   A = gain * (w / Q)
-	//   B = w / Q
-	//   C = w ^ 2
-	// The calculations of Fc, Q and gain below are derived from the
-	// equations above, with some algebra.
-
-	const double x = sqrt(r1 * r2 * c1 * c2);
-	const double y = crrc ? (r1 * c1 + r2 * c2 + r2 * c1) : (r1 * c1 + r2 * c2 + r1 * c2);
-	const double z = crrc ? (r2 * c1) : (r2 * c2);
-	filter_biquad_device::biquad_params params;
-	params.type = filter_biquad_device::biquad_type::BANDPASS;
-	params.fc = 1.0 / (2.0 * M_PI * x);
-	params.q = x / y;
-	params.gain = z / y;
-	return params;
-}
-
 // Computes the output voltage of the mux DAC circuit, given the voltage at the
 // output of the envelope generator and the DAC data value.
 double tr707_audio_device::mux_dac_v(double v_eg, u8 data)