Mesen2/Utilities/Audio/ymfm/ymfm_ssg.cpp

// BSD 3-Clause License
//
// Copyright (c) 2021, Aaron Giles
// All rights reserved.
//
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// modification, are permitted provided that the following conditions are met:
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// 1. Redistributions of source code must retain the above copyright notice, this
//    list of conditions and the following disclaimer.
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//    this list of conditions and the following disclaimer in the documentation
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#include "ymfm_ssg.h"

namespace ymfm
{

//*********************************************************
// SSG REGISTERS
//*********************************************************

//-------------------------------------------------
//  reset - reset the register state
//-------------------------------------------------

void ssg_registers::reset()
{
	std::fill_n(&m_regdata[0], REGISTERS, 0);
}


//-------------------------------------------------
//  save_restore - save or restore the data
//-------------------------------------------------

void ssg_registers::save_restore(ymfm_saved_state &state)
{
	state.save_restore(m_regdata);
}



//*********************************************************
// SSG ENGINE
//*********************************************************

//-------------------------------------------------
//  ssg_engine - constructor
//-------------------------------------------------

ssg_engine::ssg_engine(ymfm_interface &intf) :
	m_intf(intf),
	m_tone_count{ 0,0,0 },
	m_tone_state{ 0,0,0 },
	m_envelope_count(0),
	m_envelope_state(0),
	m_noise_count(0),
	m_noise_state(1),
	m_override(nullptr)
{
}


//-------------------------------------------------
//  reset - reset the engine state
//-------------------------------------------------

void ssg_engine::reset()
{
	// defer to the override if present
	if (m_override != nullptr)
		return m_override->ssg_reset();

	// reset register state
	m_regs.reset();

	// reset engine state
	for (int chan = 0; chan < 3; chan++)
	{
		m_tone_count[chan] = 0;
		m_tone_state[chan] = 0;
	}
	m_envelope_count = 0;
	m_envelope_state = 0;
	m_noise_count = 0;
	m_noise_state = 1;
}


//-------------------------------------------------
//  save_restore - save or restore the data
//-------------------------------------------------

void ssg_engine::save_restore(ymfm_saved_state &state)
{
	// save register state
	m_regs.save_restore(state);

	// save engine state
	state.save_restore(m_tone_count);
	state.save_restore(m_tone_state);
	state.save_restore(m_envelope_count);
	state.save_restore(m_envelope_state);
	state.save_restore(m_noise_count);
	state.save_restore(m_noise_state);
}


//-------------------------------------------------
//  clock - master clocking function
//-------------------------------------------------

void ssg_engine::clock()
{
	// clock tones; tone period units are clock/16 but since we run at clock/8
	// that works out for us to toggle the state (50% duty cycle) at twice the
	// programmed period
	for (int chan = 0; chan < 3; chan++)
	{
		m_tone_count[chan]++;
		if (m_tone_count[chan] >= m_regs.ch_tone_period(chan))
		{
			m_tone_state[chan] ^= 1;
			m_tone_count[chan] = 0;
		}
	}

	// clock noise; noise period units are clock/16 but since we run at clock/8,
	// our counter needs a right shift prior to compare; note that a period of 0
	// should produce an indentical result to a period of 1, so add a special
	// check against that case
	m_noise_count++;
	if ((m_noise_count >> 1) >= m_regs.noise_period() && m_noise_count != 1)
	{
		m_noise_state ^= (bitfield(m_noise_state, 0) ^ bitfield(m_noise_state, 3)) << 17;
		m_noise_state >>= 1;
		m_noise_count = 0;
	}

	// clock envelope; envelope period units are clock/8 (manual says clock/256
	// but that's for all 32 steps)
	m_envelope_count++;
	if (m_envelope_count >= m_regs.envelope_period())
	{
		m_envelope_state++;
		m_envelope_count = 0;
	}
}


//-------------------------------------------------
//  output - output the current state
//-------------------------------------------------

void ssg_engine::output(output_data &output)
{
	// volume to amplitude table, taken from MAME's implementation but biased
	// so that 0 == 0
	static int16_t const s_amplitudes[32] =
	{
		     0,   32,   78,  141,  178,  222,  262,  306,
		   369,  441,  509,  585,  701,  836,  965, 1112,
		  1334, 1595, 1853, 2146, 2576, 3081, 3576, 4135,
		  5000, 6006, 7023, 8155, 9963,11976,14132,16382
	};

	// compute the envelope volume
	uint32_t envelope_volume;
	if ((m_regs.envelope_hold() | (m_regs.envelope_continue() ^ 1)) && m_envelope_state >= 32)
	{
		m_envelope_state = 32;
		envelope_volume = ((m_regs.envelope_attack() ^ m_regs.envelope_alternate()) & m_regs.envelope_continue()) ? 31 : 0;
	}
	else
	{
		uint32_t attack = m_regs.envelope_attack();
		if (m_regs.envelope_alternate())
			attack ^= bitfield(m_envelope_state, 5);
		envelope_volume = (m_envelope_state & 31) ^ (attack ? 0 : 31);
	}

	// iterate over channels
	for (int chan = 0; chan < 3; chan++)
	{
		// noise depends on the noise state, which is the LSB of m_noise_state
		uint32_t noise_on = m_regs.ch_noise_enable_n(chan) | m_noise_state;

		// tone depends on the current tone state
		uint32_t tone_on = m_regs.ch_tone_enable_n(chan) | m_tone_state[chan];

		// if neither tone nor noise enabled, return 0
		uint32_t volume;
		if ((noise_on & tone_on) == 0)
			volume = 0;

		// if the envelope is enabled, use its amplitude
		else if (m_regs.ch_envelope_enable(chan))
			volume = envelope_volume;

		// otherwise, scale the tone amplitude up to match envelope values
		// according to the datasheet, amplitude 15 maps to envelope 31
		else
		{
			volume = m_regs.ch_amplitude(chan) * 2;
			if (volume != 0)
				volume |= 1;
		}

		// convert to amplitude
		output.data[chan] = s_amplitudes[volume];
	}
}


//-------------------------------------------------
//  read - handle reads from the SSG registers
//-------------------------------------------------

uint8_t ssg_engine::read(uint32_t regnum)
{
	// defer to the override if present
	if (m_override != nullptr)
		return m_override->ssg_read(regnum);

	// read from the I/O ports call the handlers if they are configured for input
	if (regnum == 0x0e && !m_regs.io_a_out())
		return m_intf.ymfm_external_read(ACCESS_IO, 0);
	else if (regnum == 0x0f && !m_regs.io_b_out())
		return m_intf.ymfm_external_read(ACCESS_IO, 1);

	// otherwise just return the register value
	return m_regs.read(regnum);
}


//-------------------------------------------------
//  write - handle writes to the SSG registers
//-------------------------------------------------

void ssg_engine::write(uint32_t regnum, uint8_t data)
{
	// defer to the override if present
	if (m_override != nullptr)
		return m_override->ssg_write(regnum, data);

	// store the raw value to the register array;
	// most writes are passive, consumed only when needed
	m_regs.write(regnum, data);

	// writes to the envelope shape register reset the state
	if (regnum == 0x0d)
		m_envelope_state = 0;

	// writes to the I/O ports call the handlers if they are configured for output
	else if (regnum == 0x0e && m_regs.io_a_out())
		m_intf.ymfm_external_write(ACCESS_IO, 0, data);
	else if (regnum == 0x0f && m_regs.io_b_out())
		m_intf.ymfm_external_write(ACCESS_IO, 1, data);
}

}