EnvelopeGenerator.h

/*
 * This file is part of libsidplayfp, a SID player engine.
 *
 * Copyright 2011-2013 Leandro Nini <drfiemost@users.sourceforge.net>
 * Copyright 2007-2010 Antti Lankila
 * Copyright 2004,2010 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.
 */
 
#ifndef ENVELOPEGENERATOR_H
#define ENVELOPEGENERATOR_H
 
#include "siddefs-fp.h"
 
namespace reSIDfp
{
 
class EnvelopeGenerator
{
private:
    enum State
    {
        ATTACK, DECAY_SUSTAIN, RELEASE
    };
 
private:
    int lfsr;
 
    int rate;
 
    int exponential_counter;
 
    int exponential_counter_period;
 
    int attack;
 
    int decay;
 
    int sustain;
 
    int release;
 
    State state;
 
    bool hold_zero;
 
    bool envelope_pipeline;
 
    bool gate;
 
    unsigned char envelope_counter;
 
    short dac[256];
 
private:
    static const int adsrtable[16];
 
private:
    void set_exponential_counter();
 
public:
    void setChipModel(ChipModel chipModel);
 
    void clock();
 
    short output() const { return dac[envelope_counter]; }
 
    EnvelopeGenerator() :
        lfsr(0),
        rate(0),
        exponential_counter(0),
        exponential_counter_period(1),
        attack(0),
        decay(0),
        sustain(0),
        release(0),
        state(RELEASE),
        hold_zero(true),
        envelope_pipeline(false),
        gate(false),
        envelope_counter(0) {}
 
    void reset();
 
    void writeCONTROL_REG(unsigned char control);
 
    void writeATTACK_DECAY(unsigned char attack_decay);
 
    void writeSUSTAIN_RELEASE(unsigned char sustain_release);
 
    unsigned char readENV() const { return envelope_counter; }
};
 
} // namespace reSIDfp
 
#if RESID_INLINING || defined(ENVELOPEGENERATOR_CPP)
 
namespace reSIDfp
{
 
RESID_INLINE
void EnvelopeGenerator::clock()
{
    if (unlikely(envelope_pipeline))
    {
        --envelope_counter;
        envelope_pipeline = false;
        // Check for change of exponential counter period.
        set_exponential_counter();
    }
 
    // 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 constly be stepped.
    // This has been verified by sampling ENV3.
    //
    // Envelope is now implemented like in the real machine with a shift register
    // so the ADSR delay bug should be correcly modeled
 
    // check to see if LFSR matches table value
    if (likely(lfsr != rate))
    {
        // it wasn't a match, clock the LFSR once
        // by performing XOR on last 2 bits
        const int feedback = ((lfsr << 14) ^ (lfsr << 13)) & 0x4000;
        lfsr = (lfsr >> 1) | feedback;
        return;
    }
 
    // reset LFSR
    lfsr = 0x7fff;
 
    // 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)
    {
        // likely (~50%)
        exponential_counter = 0;
 
        // Check whether the envelope counter is frozen at zero.
        if (unlikely(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;
 
            if (unlikely(envelope_counter == 0xff))
            {
                state = DECAY_SUSTAIN;
                rate = adsrtable[decay];
            }
 
            break;
 
        case DECAY_SUSTAIN:
 
            // 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.
            //
            // For a detailed description see:
            // http://ploguechipsounds.blogspot.it/2010/11/new-research-on-sid-adsr.html
            if (likely(envelope_counter == (sustain << 4 | sustain)))
            {
                return;
            }
 
            // fall-through
 
        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.
            //
            if (unlikely(exponential_counter_period != 1))
            {
                // The decrement is delayed one cycle.
                envelope_pipeline = true;
                return;
            }
 
            --envelope_counter;
            break;
        }
 
        // Check for change of exponential counter period.
        set_exponential_counter();
    }
}
 
} // namespace reSIDfp
 
#endif
 
#endif