sound/cem3394.cpp: implement self-oscillation when no waveforms are enabled

src/devices/sound/cem3394.cpp

@@ -45,6 +45,9 @@ static constexpr double TRIANGLE_VOLUME = SAWTOOTH_VOLUME * 1.27f;
 // external input is unknown but let's make it the same as the pulse
 static constexpr double EXTERNAL_VOLUME = PULSE_VOLUME;
 
+// filter volume when self-oscillating unknown, using PULSE_VOLUME arbitrarily
+static constexpr double FILTER_OSC_VOLUME = PULSE_VOLUME;
+
 
 // waveform generation parameters
 #define ENABLE_PULSE        1
@@ -156,6 +159,7 @@ cem3394_device::cem3394_device(const machine_config &mconfig, const char *tag, d
 	m_filter_frequency(1300),
 	m_filter_modulation(0),
 	m_filter_resonance(0),
+	m_filter_osc_position(0),
 	m_filter_in{0},
 	m_filter_out{0},
 	m_pulse_width(0),
@@ -420,10 +424,31 @@ void cem3394_device::sound_stream_update(sound_stream &stream)
 		if (ENABLE_EXTERNAL && BIT(input_mask, AUDIO_INPUT))
 			result += EXTERNAL_VOLUME * m_mixer_external * stream.get(AUDIO_INPUT, sampindex);
 
-		// compute the modulated filter frequency and apply the filter
-		// modulation tracks the VCO triangle
-		double filter_freq = m_filter_frequency * (1 + m_filter_modulation * triangle);
-		result = filter(result, filter_freq);
+		if (FILTER_TYPE)
+		{
+			// compute the modulated filter frequency and apply the filter
+			// modulation tracks the VCO triangle
+			const double filter_freq = m_filter_frequency * (1 + m_filter_modulation * triangle);
+			// need to call filter() and advance the filter's state, even if we don't use the result
+			const double filter_result = filter(result, filter_freq);
+
+			// advance the position of the oscillator used when the filter is self-oscillating
+			m_filter_osc_position += filter_freq * m_inv_sample_rate;
+			if (m_filter_osc_position >= 1.0)
+				m_filter_osc_position -= floor(m_filter_osc_position);
+
+			if (!m_wave_select && m_filter_resonance >= 1.0)
+			{
+				// when no waveform is selected and the filter is configured to self-oscilate,
+				// use a sine wave oscillator.
+				const double angle = 2 * M_PI * m_filter_osc_position - M_PI;
+				result = FILTER_OSC_VOLUME * sin(angle);
+			}
+			else
+			{
+				result = filter_result;
+			}
+		}
 
 		// apply AC coupling
 		if (ENABLE_AC_COUPLING)
@@ -460,6 +485,7 @@ void cem3394_device::device_start()
 	save_item(NAME(m_filter_frequency));
 	save_item(NAME(m_filter_modulation));
 	save_item(NAME(m_filter_resonance));
+	save_item(NAME(m_filter_osc_position));
 	save_item(NAME(m_filter_in));
 	save_item(NAME(m_filter_out));
 
@@ -578,7 +604,7 @@ void cem3394_device::set_voltage_internal(int input, double voltage)
 			}
 			else if (voltage > 2.0)
 			{
-				m_pulse_width = 100;
+				m_pulse_width = 1;
 				m_wave_select &= ~WAVE_PULSE;
 			}
 			else

src/devices/sound/cem3394.h

@@ -97,6 +97,7 @@ class cem3394_device : public device_t, public device_sound_interface
 	double m_filter_frequency;        // baseline filter frequency
 	double m_filter_modulation;       // depth of modulation (up to 1.0)
 	double m_filter_resonance;        // depth of modulation (up to 1.0)
+	double m_filter_osc_position;     // current filter self-oscillation position (always < 1.0)
 	double m_filter_in[4];            // filter input history
 	double m_filter_out[4];           // filter output history