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oberheim/dmx.cpp: Emulating pitch tunning. Optimizing pitch computati…
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…ons.

Also simplified and documented pitch computations.
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@m1macrophage
m1macrophage committed Jan 12, 2025
1 parent fb60da7 commit 740da1a
Showing 1 changed file with 182 additions and 58 deletions.
240 changes: 182 additions & 58 deletions src/mame/oberheim/dmx.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -45,7 +45,6 @@ educational tool.
Known audio inaccuracies (reasons for MACHINE_IMPERFECT_SOUND):
- No anti-aliasing filters (coming soon).
- No voice tuning (coming soon).
- Closed and Accent hi-hat volume variations might be wrong. See comments in
HIHAT_CONFIG (to be researched soon).
- No metronome yet.
Expand All @@ -62,7 +61,9 @@ Known audio inaccuracies (reasons for MACHINE_IMPERFECT_SOUND):
Usage notes:
- Interactive layout included.
- The mixer faders can be controlled with the mouse, or from the "Sliders" menu.
- The mixer faders can be controlled with the mouse, or from the "Slider
Controls" menu.
- Voices can be tuned using the "Sider Controls" menu.
- The drum keys are mapped to the keyboard, starting at "Q". Specifically:
Q - Bass 1, W - Snare 1, ...
A - Bass 2, S - Snare 2, ...
Expand Down Expand Up @@ -148,10 +149,14 @@ struct dmx_voice_card_config
class dmx_voice_card : public device_t, public device_sound_interface
{
public:
// Default value of pitch adjustment trimpot.
static constexpr const s32 T1_DEFAULT_PERCENT = 50;

dmx_voice_card(const machine_config &mconfig, const char *tag, device_t *owner, const dmx_voice_card_config &config, required_memory_region *sample_rom) ATTR_COLD;
dmx_voice_card(const machine_config &mconfig, const char *tag, device_t *owner, uint32_t clock = 0) ATTR_COLD;

void trigger(bool tr0, bool tr1);
void set_pitch_adj(s32 t1_percent); // Valid values: 0-100.

protected:
void device_add_mconfig(machine_config &config) override ATTR_COLD;
Expand All @@ -161,7 +166,9 @@ class dmx_voice_card : public device_t, public device_sound_interface

private:
void reset_counter();
void configure_pitch();
void init_pitch() ATTR_COLD;
void compute_pitch_variations();
void select_pitch();
void configure_volume();

bool is_decay_enabled() const;
Expand All @@ -173,8 +180,14 @@ class dmx_voice_card : public device_t, public device_sound_interface
required_device<timer_device> m_timer; // 555, U5.
required_device<dac76_device> m_dac; // AM6070, U8. Compatible with DAC76.

// Configuration. Do not include in save state.
const dmx_voice_card_config m_config;
required_memory_region *m_sample_rom = nullptr;
std::vector<float> m_cv; // 555 CV (pin 5) voltage variations.
std::vector<attotime> m_sample_t; // Sample period variations.
s32 m_t1_percent = T1_DEFAULT_PERCENT;

// Device state.

bool m_counting = false;
u16 m_counter = 0; // 4040 counter.
Expand Down Expand Up @@ -203,6 +216,7 @@ dmx_voice_card::dmx_voice_card(
, m_config(config)
, m_sample_rom(sample_rom)
{
init_pitch();
}

dmx_voice_card::dmx_voice_card(
Expand Down Expand Up @@ -235,14 +249,21 @@ void dmx_voice_card::trigger(bool tr0, bool tr1)
m_decaying = false;

if (m_config.pitch_control)
configure_pitch();
select_pitch();
else
configure_volume();

LOGMASKED(LOG_SOUND, "Trigger: (%d, %d) %d %f\n",
tr0, tr1, m_trigger_mode, m_volume);
}

void dmx_voice_card::set_pitch_adj(s32 t1_percent)
{
m_stream->update();
m_t1_percent = t1_percent;
compute_pitch_variations();
}

void dmx_voice_card::device_add_mconfig(machine_config &config)
{
TIMER(config, m_timer).configure_generic(FUNC(dmx_voice_card::clock_callback));
Expand All @@ -265,7 +286,7 @@ void dmx_voice_card::device_reset()
{
m_trigger_mode = 0;
reset_counter();
configure_pitch();
compute_pitch_variations();
configure_volume();
}

Expand Down Expand Up @@ -304,20 +325,14 @@ void dmx_voice_card::sound_stream_update(

void dmx_voice_card::reset_counter()
{
m_stream->update();
m_counter = 0;
m_counting = false;
}

void dmx_voice_card::configure_pitch()
void dmx_voice_card::init_pitch()
{
// TODO: A lot of the computations below can be cached at object
// construction time for all 3 trigger modes.

// Pitch variations are accomplished by changing the playback sample rate.
// This is done by changing the control voltage (CV) on the 555 timer that
// generates the sampling clock.

// *** Compute CV input (pin 5) of the 555.
// Precompute all variations of CV (pin 5 of 555 timer).

// The CV equations were derived from Kirchhoff analysis and verified with
// simulations: https://tinyurl.com/26x8oq75
Expand All @@ -327,47 +342,113 @@ void dmx_voice_card::configure_pitch()
// should help with accuracy.
static constexpr const float VD = 0.48F;
static constexpr const float R_555 = RES_K(5);
static constexpr const float R3 = RES_K(1);
static constexpr const float R4 = RES_K(10);
static constexpr const float R5 = RES_K(3.3);
static constexpr const float T1 = RES_K(6); // TODO: 10K potentiometer.

float cv = VCC * 2 / 3; // The 555 default, if the CV pin is not connected.
const float r12 = m_config.r12;
if (m_trigger_mode == 1)
m_cv.clear();
m_cv.push_back(VCC * 2 / 3); // The 555 default, if pin 5 is floating.

if (m_config.pitch_control)
{
const float alpha = 1.0 + r12 / m_config.r17;
cv = (alpha * R5 + r12) * (2 * VCC - 3 * VD) /
(3 * alpha * R5 + 3 * r12 + 2 * alpha * R_555) + VD;
const float r12 = m_config.r12;

// For trigger mode 1.
const float alpha = 1.0F + r12 / m_config.r17;
m_cv.push_back((alpha * R5 + r12) * (2 * VCC - 3 * VD) /
(3 * alpha * R5 + 3 * r12 + 2 * alpha * R_555) + VD);

// For trigger mode 2.
m_cv.push_back((R5 + r12) * (2 * VCC - 3 * VD) /
(3 * R5 + 3 * r12 + 2 * R_555) + VD);

// For trigger mode 3.
m_cv.push_back(m_cv[0]);
}
else if (m_trigger_mode == 2)

for (int i = 0; i < m_cv.size(); ++i)
LOGMASKED(LOG_PITCH, "%s 555 CV %d: %f\n", tag(), i, m_cv[i]);

// m_sample_t will be populated by subsequent calls to configure_pitch().
m_sample_t.clear();
m_sample_t.resize(m_cv.size());
}

void dmx_voice_card::compute_pitch_variations()
{
static constexpr const float R3 = RES_K(1);
static constexpr const float R4 = RES_K(10);
static constexpr const float T1_MAX = RES_K(10);

// The baseline pitch (and sampling rate) for all voice cards is controlled
// by a 555 timer (U5). Users can adjust the pitch with a trimpot (T1).

// For voice cards configured for pitch control (m_config.pitch_control is
// true), pitch variations are accomplished by changing the Control Voltage
// (pin 5) of the 555 (see init_pitch()).

// Computing the timer period is a bit involved, because of the use of CV,
// and because the 555 is not configured in the typical astable mode.
// For an RC circuit, V(t) = Vstart + (Vend - Vstart) * (1 - exp(-t / RC)).
// Solving for t: t = -RC * log( (Vend - V) / (Vend - Vstart) ).
// Let t_high be the time interval for which the 555 output is high. This is
// the time it takes for the capacitor to charge from CV/2 to CV.
// Let t_low be the time interval for which the 555 output is low. This is
// the time it takes for the capacitor to discharge from CV to CV/2.
// The timer period is then: t_high + t_low. t_* can be computed by
// substituting appropriate values in the function for `t`, keeping in mind
// that RC, Vstart, Vend and V are different for charging and discharging.

// Compute RC time constant for charging.
assert(m_t1_percent >= 0 && m_t1_percent <= 100);
const float r_charge = m_t1_percent * T1_MAX / 100 + R4;
const float rc_charge = r_charge * m_config.c2;

// Compute Vend and RC time constant for discharging, taking into account
// the atypical 555 configuration.
const float rc_discharge = RES_2_PARALLEL(r_charge, R3) * m_config.c2;
const float ve_discharge = VCC * RES_VOLTAGE_DIVIDER(r_charge, R3);

// Optimization: when m_config.pitch_control is true,
// m_sample_t[0] = m_sample_t[3]. So skip index 0 and copy after the loop.
const int start_i = m_config.pitch_control ? 1 : 0;

for (int i = start_i; i < m_sample_t.size(); ++i)
{
cv = (R5 + r12) * (2 * VCC - 3 * VD) /
(3 * R5 + 3 * r12 + 2 * R_555) + VD;
const float cv = m_cv[i];
const float half_cv = 0.5F * cv;

// Time for C2 to charge from CV/2 (Vstart) to CV (V). Vend = VCC
const float t_high = -rc_charge * logf((VCC - cv) / (VCC - half_cv));
assert(t_high > 0);

// Time for C2 to discharge from CV (Vstart) to CV/2 (V).
const float t_low = -rc_discharge * logf((ve_discharge - half_cv) / (ve_discharge - cv));
assert(t_low > 0);

m_sample_t[i] = attotime::from_double(t_high + t_low);
LOGMASKED(LOG_PITCH, "%s Pitch variation %d: %f (%f, %f)\n",
tag(), i, 1.0 / m_sample_t[i].as_double(), t_high, t_low);
}
const float cv_norm = cv / VCC;
LOGMASKED(LOG_PITCH, "S555 CV: %f\n", cv);

// *** Compute sampling rate.

// Compute time for which the output is high.
// TODO: T1 will not be a constant once tuning is supported.
const float r_charge = T1 + R4;
const float t_high =
-r_charge * m_config.c2 * logf((2 * cv_norm - 2) / (cv_norm - 2));
assert(t_high > 0);

// Compute time for which the output is low.
const float target_v_fraction = RES_VOLTAGE_DIVIDER(r_charge, R3);
const float effective_r = RES_2_PARALLEL(r_charge, R3);
const float t_low =
-effective_r * m_config.c2 * logf((cv_norm - 2 * target_v_fraction) /
(2 * (cv_norm - target_v_fraction)));
assert(t_low > 0);

const attotime period = attotime::from_double(t_high + t_low);
m_timer->adjust(period, 0, period);
LOGMASKED(LOG_PITCH, "Sampling frequency: %f\n", 1.0 / period.as_double());

if (m_config.pitch_control)
m_sample_t[0] = m_sample_t[3];

select_pitch();
}

void dmx_voice_card::select_pitch()
{
attotime sampling_t;
if (m_config.pitch_control)
sampling_t = m_sample_t[m_trigger_mode];
else
sampling_t = m_sample_t[0];

if (sampling_t == m_timer->period())
return; // Avoid resetting the timer in this case.

m_timer->adjust(sampling_t, 0, sampling_t);
LOGMASKED(LOG_PITCH, "Setting sampling frequency: %f\n",
1.0 / sampling_t.as_double());
}

void dmx_voice_card::configure_volume()
Expand Down Expand Up @@ -605,6 +686,7 @@ class dmx_state : public driver_device
DECLARE_INPUT_CHANGED_MEMBER(voice_volume_changed);
DECLARE_INPUT_CHANGED_MEMBER(metronome_volume_changed);
DECLARE_INPUT_CHANGED_MEMBER(master_volume_changed);
DECLARE_INPUT_CHANGED_MEMBER(pitch_adj_changed);

protected:
void machine_start() override ATTR_COLD;
Expand Down Expand Up @@ -1008,6 +1090,13 @@ DECLARE_INPUT_CHANGED_MEMBER(dmx_state::master_volume_changed)
LOGMASKED(LOG_FADERS, "Master volume changed: %d\n", newval);
}

DECLARE_INPUT_CHANGED_MEMBER(dmx_state::pitch_adj_changed)
{
// Using "100 -" so that larger values increase pitch.
m_voices[param]->set_pitch_adj(100 - newval);
LOGMASKED(LOG_PITCH, "Voice %d pitch adjustment changed: %d\n", param, newval);
}

INPUT_PORTS_START(dmx)
PORT_START("buttons_0")
PORT_BIT(0x01, IP_ACTIVE_LOW, IPT_OTHER) PORT_NAME("2") PORT_CODE(KEYCODE_2_PAD)
Expand Down Expand Up @@ -1094,44 +1183,79 @@ INPUT_PORTS_START(dmx)
// Fader potentiometers. P1-P10 on the Switch Board.

PORT_START("fader_p1")
PORT_ADJUSTER(100, "BASS")
PORT_ADJUSTER(100, "BASS volume")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::voice_volume_changed), dmx_state::VC_BASS)

PORT_START("fader_p2")
PORT_ADJUSTER(100, "SNARE")
PORT_ADJUSTER(100, "SNARE volume")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::voice_volume_changed), dmx_state::VC_SNARE)

PORT_START("fader_p3")
PORT_ADJUSTER(100, "HI-HAT")
PORT_ADJUSTER(100, "HI-HAT volume")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::voice_volume_changed), dmx_state::VC_HIHAT)

PORT_START("fader_p4")
PORT_ADJUSTER(100, "TOM1")
PORT_ADJUSTER(100, "TOM1 volume")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::voice_volume_changed), dmx_state::VC_SMALL_TOMS)

PORT_START("fader_p5")
PORT_ADJUSTER(100, "TOM2")
PORT_ADJUSTER(100, "TOM2 volume")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::voice_volume_changed), dmx_state::VC_LARGE_TOMS)

PORT_START("fader_p6")
PORT_ADJUSTER(100, "CYMBAL")
PORT_ADJUSTER(100, "CYMBAL volume")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::voice_volume_changed), dmx_state::VC_CYMBAL)

PORT_START("fader_p7")
PORT_ADJUSTER(100, "PERC1")
PORT_ADJUSTER(100, "PERC1 volume")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::voice_volume_changed), dmx_state::VC_PERC1)

PORT_START("fader_p8")
PORT_ADJUSTER(100, "PERC2")
PORT_ADJUSTER(100, "PERC2 volume")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::voice_volume_changed), dmx_state::VC_PERC2)

PORT_START("fader_p9")
PORT_ADJUSTER(100, "MET")
PORT_ADJUSTER(100, "MET volume")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::metronome_volume_changed), 0)

PORT_START("fader_p10")
PORT_ADJUSTER(100, "VOLUME")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::master_volume_changed), 0)

// Tunning potentiomenters. One per voice card, designated as T1 and labeled
// as "PITCH ADJ."

PORT_START("pitch_adj_0")
PORT_ADJUSTER(dmx_voice_card::T1_DEFAULT_PERCENT, "BASS pitch")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::pitch_adj_changed), dmx_state::VC_BASS)

PORT_START("pitch_adj_1")
PORT_ADJUSTER(dmx_voice_card::T1_DEFAULT_PERCENT, "SNARE pitch")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::pitch_adj_changed), dmx_state::VC_SNARE)

PORT_START("pitch_adj_2")
PORT_ADJUSTER(dmx_voice_card::T1_DEFAULT_PERCENT, "HI-HAT pitch")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::pitch_adj_changed), dmx_state::VC_HIHAT)

PORT_START("pitch_adj_3")
PORT_ADJUSTER(dmx_voice_card::T1_DEFAULT_PERCENT, "TOM1 pitch")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::pitch_adj_changed), dmx_state::VC_SMALL_TOMS)

PORT_START("pitch_adj_4")
PORT_ADJUSTER(dmx_voice_card::T1_DEFAULT_PERCENT, "TOM2 pitch")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::pitch_adj_changed), dmx_state::VC_LARGE_TOMS)

PORT_START("pitch_adj_5")
PORT_ADJUSTER(dmx_voice_card::T1_DEFAULT_PERCENT, "PERC1 pitch")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::pitch_adj_changed), dmx_state::VC_PERC1)

PORT_START("pitch_adj_6")
PORT_ADJUSTER(dmx_voice_card::T1_DEFAULT_PERCENT, "PERC2 pitch")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::pitch_adj_changed), dmx_state::VC_PERC2)

PORT_START("pitch_adj_7")
PORT_ADJUSTER(dmx_voice_card::T1_DEFAULT_PERCENT, "CYMBAL pitch")
PORT_CHANGED_MEMBER(DEVICE_SELF, FUNC(dmx_state::pitch_adj_changed), dmx_state::VC_CYMBAL)
INPUT_PORTS_END

ROM_START(obdmx)
Expand Down

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