src/hardware/reSID/envelope.cpp

//  ---------------------------------------------------------------------------
//  This file is part of reSID, a MOS6581 SID 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.,
//  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
//  ---------------------------------------------------------------------------

#define __ENVELOPE_CC__

#include "envelope.h"

// ----------------------------------------------------------------------------
// Constructor.
// ----------------------------------------------------------------------------
EnvelopeGenerator::EnvelopeGenerator()
{
  reset();
}

// ----------------------------------------------------------------------------
// SID reset.
// ----------------------------------------------------------------------------
void EnvelopeGenerator::reset()
{
  envelope_counter = 0;

  attack = 0;
  decay = 0;
  sustain = 0;
  release = 0;

  gate = 0;

  rate_counter = 0;
  exponential_counter = 0;
  exponential_counter_period = 1;

  state = RELEASE;
  rate_period = rate_counter_period[release];
  hold_zero = true;
}


// Rate counter periods are calculated from the Envelope Rates table in
// the Programmer's Reference Guide. The rate counter period is the number of
// cycles between each increment of the envelope counter.
// The rates have been verified by sampling ENV3. 
//
// The rate counter is a 16 bit register which is incremented each cycle.
// When the counter reaches a specific comparison value, the envelope counter
// is incremented (attack) or decremented (decay/release) and the
// counter is zeroed.
//
// NB! Sampling ENV3 shows that the calculated values are not exact.
// It may seem like most calculated values have been rounded (.5 is rounded
// down) and 1 has beed added to the result. A possible explanation for this
// is that the SID designers have used the calculated values directly
// as rate counter comparison values, not considering a one cycle delay to
// zero the counter. This would yield an actual period of comparison value + 1.
//
// The time of the first envelope count can not be exactly controlled, except
// possibly by resetting the chip. Because of this we cannot do cycle exact
// sampling and must devise another method to calculate the rate counter
// periods.
//
// The exact rate counter periods can be determined e.g. by counting the number
// of cycles from envelope level 1 to envelope level 129, and dividing the
// number of cycles by 128. CIA1 timer A and B in linked mode can perform
// the cycle count. This is the method used to find the rates below.
//
// To avoid the ADSR delay bug, sampling of ENV3 should be done using
// sustain = release = 0. This ensures that the attack state will not lower
// the current rate counter period.
//
// The ENV3 sampling code below yields a maximum timing error of 14 cycles.
//     lda #$01
// l1: cmp $d41c
//     bne l1
//     ...
//     lda #$ff
// l2: cmp $d41c
//     bne l2
//
// This yields a maximum error for the calculated rate period of 14/128 cycles.
// The described method is thus sufficient for exact calculation of the rate
// periods.
//
reg16 EnvelopeGenerator::rate_counter_period[] = {
      9,  //   2ms*1.0MHz/256 =     7.81
     32,  //   8ms*1.0MHz/256 =    31.25
     63,  //  16ms*1.0MHz/256 =    62.50
     95,  //  24ms*1.0MHz/256 =    93.75
    149,  //  38ms*1.0MHz/256 =   148.44
    220,  //  56ms*1.0MHz/256 =   218.75
    267,  //  68ms*1.0MHz/256 =   265.63
    313,  //  80ms*1.0MHz/256 =   312.50
    392,  // 100ms*1.0MHz/256 =   390.63
    977,  // 250ms*1.0MHz/256 =   976.56
   1954,  // 500ms*1.0MHz/256 =  1953.13
   3126,  // 800ms*1.0MHz/256 =  3125.00
   3907,  //   1 s*1.0MHz/256 =  3906.25
  11720,  //   3 s*1.0MHz/256 = 11718.75
  19532,  //   5 s*1.0MHz/256 = 19531.25
  31251   //   8 s*1.0MHz/256 = 31250.00
};


// For decay and release, the clock to the envelope counter is sequentially
// divided by 1, 2, 4, 8, 16, 30, 1 to create a piece-wise linear approximation
// of an exponential. The exponential counter period is loaded at the envelope
// counter values 255, 93, 54, 26, 14, 6, 0. The period can be different for the
// same envelope counter value, depending on whether the envelope has been
// rising (attack -> release) or sinking (decay/release).
//
// Since it is not possible to reset the rate counter (the test bit has no
// influence on the envelope generator whatsoever) a method must be devised to
// do cycle exact sampling of ENV3 to do the investigation. This is possible
// with knowledge of the rate period for A=0, found above.
//
// The CPU can be synchronized with ENV3 by first synchronizing with the rate
// counter by setting A=0 and wait in a carefully timed loop for the envelope
// counter _not_ to change for 9 cycles. We can then wait for a specific value
// of ENV3 with another timed loop to fully synchronize with ENV3.
//
// At the first period when an exponential counter period larger than one
// is used (decay or relase), one extra cycle is spent before the envelope is
// decremented. The envelope output is then delayed one cycle until the state
// is changed to attack. Now one cycle less will be spent before the envelope
// is incremented, and the situation is normalized.
// The delay is probably caused by the comparison with the exponential counter,
// and does not seem to affect the rate counter. This has been verified by
// timing 256 consecutive complete envelopes with A = D = R = 1, S = 0, using
// CIA1 timer A and B in linked mode. If the rate counter is not affected the
// period of each complete envelope is
// (255 + 162*1 + 39*2 + 28*4 + 12*8 + 8*16 + 6*30)*32 = 756*32 = 32352
// which corresponds exactly to the timed value divided by the number of
// complete envelopes.
// NB! This one cycle delay is not modeled.


// From the sustain levels it follows that both the low and high 4 bits of the
// envelope counter are compared to the 4-bit sustain value.
// This has been verified by sampling ENV3.
//
reg8 EnvelopeGenerator::sustain_level[] = {
  0x00,
  0x11,
  0x22,
  0x33,
  0x44,
  0x55,
  0x66,
  0x77,
  0x88,
  0x99,
  0xaa,
  0xbb,
  0xcc,
  0xdd,
  0xee,
  0xff,
};


// ----------------------------------------------------------------------------
// Register functions.
// ----------------------------------------------------------------------------
void EnvelopeGenerator::writeCONTROL_REG(reg8 control)
{
  reg8 gate_next = control & 0x01;

  // The rate counter is never reset, thus there will be a delay before the
  // envelope counter starts counting up (attack) or down (release).

  // Gate bit on: Start attack, decay, sustain.
  if (!gate && gate_next) {
    state = ATTACK;
    rate_period = rate_counter_period[attack];

    // Switching to attack state unlocks the zero freeze.
    hold_zero = false;
  }
  // Gate bit off: Start release.
  else if (gate && !gate_next) {
    state = RELEASE;
    rate_period = rate_counter_period[release];
  }

  gate = gate_next;
}

void EnvelopeGenerator::writeATTACK_DECAY(reg8 attack_decay)
{
  attack = (attack_decay >> 4) & 0x0f;
  decay = attack_decay & 0x0f;
  if (state == ATTACK) {
    rate_period = rate_counter_period[attack];
  }
  else if (state == DECAY_SUSTAIN) {
    rate_period = rate_counter_period[decay];
  }
}

void EnvelopeGenerator::writeSUSTAIN_RELEASE(reg8 sustain_release)
{
  sustain = (sustain_release >> 4) & 0x0f;
  release = sustain_release & 0x0f;
  if (state == RELEASE) {
    rate_period = rate_counter_period[release];
  }
}

reg8 EnvelopeGenerator::readENV()
{
  return output();
}