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github-actions[bot] edited this page Apr 9, 2026 · 3 revisions

SID — Sound Synthesis

File: Hardware/SID.h / Hardware/SID.cpp
Namespace: Hardware::SID

Overview

The emulator includes a sound synthesis chip emulation inspired by the SID chip (like the MOS 6581 used in the Commodore 64). It has 3 independent oscillators, each with its own waveform generator and ADSR envelope. Audio is played back in real time via SDL3 AudioStream.

Internal architecture

SID
├── voices[0]  →  Oscillator + ADSREnvelope
├── voices[1]  →  Oscillator + ADSREnvelope
└── voices[2]  →  Oscillator + ADSREnvelope
         ↓
    GenerateAudio()
         ↓
    SDL3 AudioStream  →  Speaker

Oscillator struct

Each voice has an oscillator with the following fields:

Field Type Description
accumulator uint32_t Phase accumulator
frequency uint32_t Programmed frequency
pulseWidth uint16_t Pulse width (for square wave)
control uint8_t Control register: Gate · Sync · Ring · Test · Tri · Saw · Pulse · Noise
noiseShift uint32_t LFSR shift register for noise

Waveforms (control register bits)

Bit Waveform Description
0 Gate Enables/disables the envelope
1 Sync Synchronises with the previous oscillator
2 Ring Ring modulation with the previous oscillator
3 Test Test mode (halts the oscillator)
4 Triangle Triangle wave
5 Sawtooth Sawtooth wave
6 Pulse Square/pulse wave
7 Noise White noise (LFSR)

ADSREnvelope struct

The ADSR envelope controls the volume of each note:

State Description
IDLE No activity
ATTACK Attack: level rises from 0
DECAY Decay: level falls toward the sustain level
SUSTAIN Sustain: constant level while Gate=1
RELEASE Release: level falls to 0 when Gate=0

Envelope parameters:

Field Description
attackRate Attack speed
decayRate Decay speed
sustainLevel Sustain level (0.0–1.0)
releaseRate Release speed

Memory-mapped registers

SID registers are accessible from 65c02 code through Write/Read functions (integrated into the emulator via hooks):

Offset Name Description
0x00–0x06 Voice 1 Frequency, pulse width, control, ADSR
0x07–0x0D Voice 2 Same as Voice 1
0x0E–0x14 Voice 3 Same as Voice 1
0x18 Volume/Filter Master volume byte and filter

Real-time audio

The SID uses SDL3's audio system (SDL_AudioStream) at 44100 Hz, 16-bit samples, mono. Audio generation happens in an asynchronous callback:

SID::AudioCallback(void* userdata, SDL_AudioStream* stream,
                   int additional_amount, int total_amount);

Enable control

sid.EnableSound(true);            // Enable audio
sid.SetEmulationPaused(true);     // Pause generation (mute)
bool enabled = sid.IsSoundEnabled();

Frontend visualisation

The SID Viewer window in the GUI shows in real time:

  • The waveform and state of each oscillator
  • The envelope state (ATTACK/DECAY/SUSTAIN/RELEASE)
  • Frequency and pulse-width values

MIDI-to-SID tool

The tools/sid/ directory includes a Python script that converts MIDI files to 65c02 assembly code playable on the emulator.

./scripts/midi-to-bin.sh <midi_file>
# Output: output/midi/<song>.bin

The script has 8 compression optimisation modes (granularities from 1 ms to 100+ ms) and automatically selects the best one.

Included sample songs

Several demonstration MIDI files are provided in assets/midi/

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