furnace/src/engine/platform/sound/sid2/envelope.h

//  ---------------------------------------------------------------------------
//  This file is part of reSID, a MOS6581 SID2 emulator engine.
//  Copyright (C) 2004  Dag Lem <resid@nimrod.no>
//
//  This program is free software; you can redistribute it and/or modify
//  it under the terms of the GNU General Public License as published by
//  the Free Software Foundation; either version 2 of the License, or
//  (at your option) any later version.
//
//  This program is distributed in the hope that it will be useful,
//  but WITHOUT ANY WARRANTY; without even the implied warranty of
//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//  GNU General Public License for more details.
//
//  You should have received a copy of the GNU General Public License
//  along with this program; if not, write to the Free Software
//  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
//  ---------------------------------------------------------------------------
#include "siddefs.h"
#include <stdint.h>

// ----------------------------------------------------------------------------
// A 15 bit counter is used to implement the envelope rates, in effect
// dividing the clock to the envelope counter by the currently selected rate
// period.
// In addition, another counter is used to implement the exponential envelope
// decay, in effect further dividing the clock to the envelope counter.
// The period of this counter is set to 1, 2, 4, 8, 16, 30 at the envelope
// counter values 255, 93, 54, 26, 14, 6, respectively.
// ----------------------------------------------------------------------------
class EnvelopeGenerator2
{
public:
  EnvelopeGenerator2();

  enum State { ATTACK, DECAY_SUSTAIN, RELEASE };

  RESID_INLINE void clock();
  void reset();

  void writeCONTROL_REG(reg8);
  void writeATTACK_DECAY(reg8);
  void writeSUSTAIN_RELEASE(reg8);
  void writeVOL(reg8);
  reg8 readENV();

  // 8-bit envelope output.
  RESID_INLINE reg8 output();

protected:
  reg16 rate_counter;
  reg16 rate_period;
  reg8 exponential_counter;
  reg8 exponential_counter_period;
  reg8 envelope_counter;
  bool hold_zero;

  reg4 attack;
  reg4 decay;
  reg4 sustain;
  reg4 release;

  reg8 gate;

  reg8 volume;

  State state;

  // Lookup table to convert from attack, decay, or release value to rate
  // counter period.
  static reg16 rate_counter_period[];

  // The 16 selectable sustain levels.
  static reg8 sustain_level[];

friend class SID2;
};


// ----------------------------------------------------------------------------
// Inline functions.
// The following functions are defined inline because they are called every
// time a sample is calculated.
// ----------------------------------------------------------------------------

#if RESID_INLINING || defined(__ENVELOPE_CC__)

// ----------------------------------------------------------------------------
// SID2 clocking - 1 cycle.
// ----------------------------------------------------------------------------
RESID_INLINE
void EnvelopeGenerator2::clock()
{
  // Check for ADSR delay bug.
  // If the rate counter comparison value is set below the current value of the
  // rate counter, the counter will continue counting up until it wraps around
  // to zero at 2^15 = 0x8000, and then count rate_period - 1 before the
  // envelope can finally be stepped.
  // This has been verified by sampling ENV3.
  //
  //if (++rate_counter & 0x8000) {
  //  ++rate_counter &= 0x7fff;
  //}

  if(rate_counter < rate_period)
  {
    rate_counter++;
  }

  if(rate_counter > rate_period)
  {
    rate_counter = rate_period; //so you can do alternating writes (e.g. writing attack 10-11-10-11-... results in the somewhat average envelope speed)
  }

  if (rate_counter != rate_period) {
    return;
  }

  rate_counter = 0;

  // The first envelope step in the attack state also resets the exponential
  // counter. This has been verified by sampling ENV3.
  //
  if (state == ATTACK || ++exponential_counter == exponential_counter_period)
  {
    exponential_counter = 0;

    // Check whether the envelope counter is frozen at zero.
    if (hold_zero) {
      return;
    }

    switch (state) {
    case ATTACK:
      // The envelope counter can flip from 0xff to 0x00 by changing state to
      // release, then to attack. The envelope counter is then frozen at
      // zero; to unlock this situation the state must be changed to release,
      // then to attack. This has been verified by sampling ENV3.
      //
      ++envelope_counter &= 0xff;
      if (envelope_counter == 0xff) {
        state = DECAY_SUSTAIN;
        rate_period = rate_counter_period[decay];
      }
      break;
    case DECAY_SUSTAIN:
        if (envelope_counter != sustain_level[sustain]) {
          --envelope_counter;
      }
      break;
    case RELEASE:
      // The envelope counter can flip from 0x00 to 0xff by changing state to
      // attack, then to release. The envelope counter will then continue
      // counting down in the release state.
      // This has been verified by sampling ENV3.
      // NB! The operation below requires two's complement integer.
      //
      //--envelope_counter &= 0xff;
      envelope_counter--;
      break;
    }
    
    // Check for change of exponential counter period.
    switch (envelope_counter) {
    case 0xff:
      exponential_counter_period = 1;
      break;
    case 0x5d:
      exponential_counter_period = 2;
      break;
    case 0x36:
      exponential_counter_period = 4;
      break;
    case 0x1a:
      exponential_counter_period = 8;
      break;
    case 0x0e:
      exponential_counter_period = 16;
      break;
    case 0x06:
      exponential_counter_period = 30;
      break;
    case 0x00:
      exponential_counter_period = 1;

      // When the envelope counter is changed to zero, it is frozen at zero.
      // This has been verified by sampling ENV3.
      hold_zero = true;
      break;
    }
  }
}

// ----------------------------------------------------------------------------
// Read the envelope generator output.
// ----------------------------------------------------------------------------
RESID_INLINE
reg8 EnvelopeGenerator2::output()
{
  return (reg8)((uint32_t)envelope_counter * volume / 0xf);
}

#endif // RESID_INLINING || defined(__ENVELOPE_CC__)