Press n or j to go to the next uncovered block, b, p or k for the previous block.
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// The BBC volumeTable[0] (loudest) is 0.25 (1.0 / 4 channels).
const speakerVolume = 0.5;
// Samples per output chunk handed to the onBuffer callback.
export const SoundBufferSamples = 512;
// The BBC's DC restoration circuit (Service Manual section 3.8: R8 10K, C2
// 4u7) cancels the SN76489's unipolar pedestal. Modelled as a first-order DC
// blocker with the same corner, 1/(2*pi*R*C). The generators must stay
// unipolar: sampled speech is amplitude modulation of a 125 kHz carrier's
// mean level, which a zero-mean output would silence (see issue #863).
const DcRestoreCornerHz = 1 / (2 * Math.PI * 10e3 * 4.7e-6);
// A chip with an event sink reports progress this often, so its events can
// stream through a long execute() rather than all arriving at its end.
const EventProgressCycles = 4000;
// The tone counters are 10 bits wide and wrap, so a period of 0 counts this many steps.
export const ToneCounterSteps = 1024;
export const volumeTable = new Float32Array(16);
(() => {
let f = 1.0;
for (let i = 0; i < 15; ++i) {
volumeTable[i] = f / 4; // Bakes in the per channel volume
f *= Math.pow(10, -0.1);
}
volumeTable[15] = 0;
})();
function makeSineTable(attenuation) {
const sineTable = new Float32Array(8192);
for (let i = 0; i < sineTable.length; ++i) {
sineTable[i] = Math.sin((2 * Math.PI * i) / sineTable.length) * attenuation;
}
return sineTable;
}
export class SoundChip {
/**
* @param {function(Float32Array): void} onBuffer called with each full
* SoundBufferSamples-sized buffer of output. Receives the same buffer
* every call, overwritten afterwards: copy the contents if they are kept.
* @param {object} [options]
* @param {function(object): void} [options.onEvent] called with each
* change to the chip's state, stamped with the cycle it takes effect at.
* A chip with an event sink does not render; something else renders
* from the events (see audio-renderer.js).
*/
constructor(onBuffer, { onEvent = null } = {}) {
this._onBuffer = onBuffer;
this._onEvent = onEvent;
/** The CPU's `replaying`, which asReplay sets. */
this.replaying = () => false;
this._hostBehind = false;
// 4MHz input signal. Internal divide-by-8
this.soundchipFreq = 4000000.0 / 8;
const sampleRate = this.soundchipFreq;
// Square wave changes every time a counter hits zero: A full wave needs to be 2x counter zeros.
this.waveDecrementPerSecond = this.soundchipFreq / 2;
// Each sample in the buffer represents (1/sampleRate) time, so each time
// we generate a sample, we need to decrement the counters by this amount:
this.sampleDecrement = this.waveDecrementPerSecond / sampleRate;
// How many samples are generated per CPU cycle.
this.samplesPerCycle = sampleRate / 2000000;
this.minCyclesWELow = 14; // Somewhat empirically derived; Repton 2 has only 14 cycles between WE low and WE high (@0x2caa)
this.registers = new Uint16Array(4);
this.counter = new Float32Array(4);
this.outputBit = [false, false, false, false];
this.volume = new Float32Array(4);
/** By channel, tones 0 to 2 then noise: a muted one is not run, so it is silent and resumes where it stopped. */
this.channelMuted = [false, false, false, false];
this.generators = [
this.toneChannel.bind(this),
this.toneChannel.bind(this),
this.toneChannel.bind(this),
this.noiseChannel.bind(this),
this.sineChannel.bind(this),
];
this.sineTable = makeSineTable(1 / this.generators.length);
this.sineStep = 0;
this.sineOn = false;
this.sineTime = 0;
this.lfsr = 0;
this.shiftLfsr = this.shiftLfsrWhiteNoise.bind(this);
this.enabled = true;
this.scheduler = { epoch: 0 };
this.lastRunEpoch = 0;
this.activeTask = null;
this.residual = 0;
this.position = 0;
this.buffer = new Float32Array(SoundBufferSamples);
this.dcAlpha = 1 - (2 * Math.PI * DcRestoreCornerHz) / sampleRate;
this.dcPrevIn = 0;
this.dcPrevOut = 0;
this.latchedRegister = 0;
this.slowDataBus = 0;
this.active = false;
this.toneGenerator = {
mute: () => {
this.catchUp();
this.sineOn = false;
this._emit("sine", 0);
},
tone: (freq) => {
this.catchUp();
this.sineOn = true;
this.sineStep = (freq / sampleRate) * this.sineTable.length;
this._emit("sine", freq);
},
};
this.eventHandlers = {
progress: () => {},
poke: (value) => this.poke(value),
sine: (freq) => (freq ? this.toneGenerator.tone(freq) : this.toneGenerator.mute()),
state: (state) => this.restoreState(state),
reset: (hard) => this.reset(hard),
};
}
_emit(kind, value) {
this._send({ cycle: this.scheduler.epoch, kind, value });
}
/**
* Sends `event` to the host unless the machine is replaying. After a replay held some back, the chip as it stands
* goes first, so that the host sounds on from where the replay left the machine.
*/
_send(event) {
if (!this._onEvent) return;
Iif (this.replaying()) {
this._hostBehind = true;
return;
}
Iif (this._hostBehind && event.kind !== "state")
this._onEvent({ cycle: this.scheduler.epoch, kind: "state", value: this.snapshotState() });
this._hostBehind = false;
this._onEvent(event);
}
/** Applies an event from another chip's onEvent, at the cycle this chip is rendering. */
applyEvent(event) {
const handler = this.eventHandlers[event.kind];
Iif (!handler) throw new Error(`Unknown sound event ${event.kind}`);
handler(event.value, event);
}
/** Renders `length` samples of output from `cycle`, for a chip that is driven by events. */
renderAt(cycle, out, offset, length) {
this.scheduler.epoch = this.lastRunEpoch = cycle;
this.generate(out, offset, length);
}
sineChannel(channel, out, offset, length) {
if (!this.sineOn) return;
for (let i = 0; i < length; ++i) {
out[i + offset] += this.sineTable[this.sineTime & (this.sineTable.length - 1)];
this.sineTime += this.sineStep;
}
while (this.sineTime > this.sineTable.length) this.sineTime -= this.sineTable.length;
}
_doChannelStep(channel, addAmount) {
const newValue = this.counter[channel] - this.sampleDecrement;
if (newValue < 0) {
this.counter[channel] = Math.max(0, newValue + addAmount);
this.outputBit[channel] = !this.outputBit[channel];
return this.outputBit[channel];
} else {
this.counter[channel] = newValue;
return false;
}
}
tonePeriod(channel) {
return this.registers[channel] === 0 ? ToneCounterSteps : this.registers[channel];
}
toneChannel(channel, out, offset, length) {
const reg = this.tonePeriod(channel);
const vol = this.volume[channel];
for (let i = 0; i < length; ++i) {
this._doChannelStep(channel, reg);
out[i + offset] += this.outputBit[channel] * vol;
}
}
shiftLfsrWhiteNoise() {
const bit = (this.lfsr & 1) ^ ((this.lfsr & (1 << 1)) >>> 1);
this.lfsr = (this.lfsr >>> 1) | (bit << 14);
}
shiftLfsrPeriodicNoise() {
this.lfsr >>= 1;
if (this.lfsr === 0) this.lfsr = 1 << 14;
}
noisePoked() {
this.shiftLfsr =
this.registers[3] & 4 ? this.shiftLfsrWhiteNoise.bind(this) : this.shiftLfsrPeriodicNoise.bind(this);
this.lfsr = 1 << 14;
}
addFor(channel) {
channel = channel | 0;
switch (this.registers[channel] & 3) {
case 0:
return 0x10;
case 1:
return 0x20;
case 2:
return 0x40;
case 3:
return this.tonePeriod(2);
}
}
noiseChannel(channel, out, offset, length) {
const add = this.addFor(channel),
vol = this.volume[channel];
for (let i = 0; i < length; ++i) {
if (this._doChannelStep(channel, add)) this.shiftLfsr();
out[i + offset] += (this.lfsr & 1) * vol;
}
}
generate(out, offset, length) {
offset = offset | 0;
length = length | 0;
for (let i = 0; i < length; ++i) {
out[i + offset] = 0.0;
}
if (this.enabled) {
for (let i = 0; i < this.generators.length; ++i) {
if (!this.channelMuted[i]) this.generators[i](i, out, offset, length);
}
}
const alpha = this.dcAlpha;
let prevIn = this.dcPrevIn;
let prevOut = this.dcPrevOut;
for (let i = 0; i < length; ++i) {
const x = out[i + offset];
prevOut = x - prevIn + alpha * prevOut;
prevIn = x;
out[i + offset] = prevOut;
}
this.dcPrevIn = prevIn;
this.dcPrevOut = prevOut;
}
/** The channel's attenuation as the chip was last told it, 0 (loudest) to 15 (silent). */
attenuation(channel) {
const volume = this.volume[channel];
let nearest = 0;
for (let i = 1; i < volumeTable.length; ++i)
if (Math.abs(volumeTable[i] - volume) < Math.abs(volumeTable[nearest] - volume)) nearest = i;
return nearest;
}
catchUp() {
const cyclesPending = this.scheduler.epoch - this.lastRunEpoch;
if (cyclesPending > 0) this.advance(cyclesPending);
this.lastRunEpoch = this.scheduler.epoch;
}
setScheduler(scheduler_) {
this.scheduler = scheduler_;
this.lastRunEpoch = this.scheduler.epoch;
this.activeTask = this.scheduler.newTask(() => {
Eif (this.active) this.poke(this.slowDataBus);
});
if (this._onEvent) {
this.progressTask = this.scheduler.newTask(() => {
this._emit("progress", true);
this.progressTask.schedule(EventProgressCycles);
});
this.progressTask.schedule(EventProgressCycles);
}
}
render(out, offset, length) {
this.catchUp();
const fromBuffer = this.position > length ? length : this.position;
for (let i = 0; i < fromBuffer; ++i) {
out[offset + i] = this.buffer[i];
}
offset += fromBuffer;
length -= fromBuffer;
for (let i = fromBuffer; i < this.position; ++i) {
this.buffer[i - fromBuffer] = this.buffer[i];
}
this.position -= fromBuffer;
Iif (length !== 0) {
this.generate(out, offset, length);
}
}
advance(cycles) {
if (this._onEvent) return;
const num = cycles * this.samplesPerCycle + this.residual;
let rounded = num | 0;
this.residual = num - rounded;
// The buffer is deliberately reused for the chip's whole life: the
// previous transfer-then-reallocate pattern is miscompiled by a V8
// optimiser bug (Chrome 150, crbug.com/537801199) which allocates the
// replacement with length 0, even when the reallocation is reordered
// before the transfer, wedging this loop forever. Reuse is also
// cheaper, so this needn't be reverted once the crbug is fixed. The
// guard fails loudly if the buffer is ever detached or the accounting
// goes bad.
while (rounded > 0) {
const leftInBuffer = SoundBufferSamples - this.position;
const numSamplesToGenerate = Math.min(rounded, leftInBuffer);
if (numSamplesToGenerate <= 0 || this.buffer.length !== SoundBufferSamples)
throw new Error(
`Sound buffer accounting error (buffer=${this.buffer.length}, position=${this.position}, rounded=${rounded})`,
);
this.generate(this.buffer, this.position, numSamplesToGenerate);
this.position += numSamplesToGenerate;
rounded -= numSamplesToGenerate;
if (this.position === SoundBufferSamples) {
this._onBuffer(this.buffer);
this.position = 0;
}
}
}
poke(value) {
this.catchUp();
this._emit("poke", value);
let command;
if (value & 0x80) {
this.latchedRegister = value & 0x70;
command = value & 0xf0;
} else {
command = this.latchedRegister;
}
const channel = (command >> 5) & 0x03;
if (command & 0x10) {
// Volume setting
const newVolume = value & 0x0f;
this.volume[channel] = volumeTable[newVolume];
} else if (channel === 3) {
// For noise channel we always update the bottom bits.
this.registers[channel] = value & 0x0f;
this.noisePoked();
} else if (command & 0x80) {
// Low period bits.
this.registers[channel] = (this.registers[channel] & ~0x0f) | (value & 0x0f);
} else {
// High period bits.
this.registers[channel] = (this.registers[channel] & 0x0f) | ((value & 0x3f) << 4);
}
}
updateSlowDataBus(slowDataBus, active) {
this.slowDataBus = slowDataBus;
this.active = active;
// TODO(#835) this probably isn't modeled correctly. Currently the sound chip "notices" a new data bus value some
// fixed number of cycles after WE (write enable) is triggered. In reality, the sound chip likely pulls data off
// the bus at a fixed point in its cycle, iff WE is active.
if (active) {
this.activeTask.ensureScheduled(true, this.minCyclesWELow);
}
}
snapshotState() {
return {
registers: this.registers.slice(),
counter: this.counter.slice(),
outputBit: [...this.outputBit],
volume: this.volume.slice(),
lfsr: this.lfsr,
latchedRegister: this.latchedRegister,
residual: this.residual,
sineOn: this.sineOn,
sineStep: this.sineStep,
sineTime: this.sineTime,
dcPrevIn: this.dcPrevIn,
dcPrevOut: this.dcPrevOut,
slowDataBus: this.slowDataBus,
active: this.active,
activeTaskOffset: this.activeTask?.offset() ?? null,
};
}
restoreState(state) {
this.registers.set(state.registers);
this.counter.set(state.counter);
this.outputBit[0] = state.outputBit[0];
this.outputBit[1] = state.outputBit[1];
this.outputBit[2] = state.outputBit[2];
this.outputBit[3] = state.outputBit[3];
this.volume.set(state.volume);
this.lfsr = state.lfsr;
this.latchedRegister = state.latchedRegister;
// Sync to current scheduler epoch to avoid a catch-up burst
this.lastRunEpoch = this.scheduler.epoch;
this.residual = state.residual;
this.sineOn = state.sineOn;
this.sineStep = state.sineStep;
this.sineTime = state.sineTime;
// Rebind the LFSR function based on noise register
this.shiftLfsr =
this.registers[3] & 4 ? this.shiftLfsrWhiteNoise.bind(this) : this.shiftLfsrPeriodicNoise.bind(this);
// Reset output buffer
this.position = 0;
this.buffer.fill(0);
// Older snapshots predate the DC blocker
this.dcPrevIn = state.dcPrevIn ?? 0;
this.dcPrevOut = state.dcPrevOut ?? 0;
// Older snapshots keep what the System VIA's restore drove onto the bus before this one ran.
if (state.slowDataBus !== undefined) {
this.slowDataBus = state.slowDataBus;
this.active = state.active;
this.activeTask?.restoreOffset(state.activeTaskOffset);
}
this.progressTask?.ensureScheduled(true, EventProgressCycles);
this._emit("state", this.snapshotState());
}
reset(hard) {
if (!hard) return;
this._emit("reset", true);
for (let i = 0; i < 4; ++i) {
this.counter[i] = 0;
this.registers[i] = 0;
// Real hardware would be volumeTable[0] but that's really quite loud and surprising...
this.volume[i] = volumeTable[8];
}
this.noisePoked();
this.lastRunEpoch = this.scheduler.epoch;
}
enable(e) {
this.enabled = e;
}
mute() {
this.enable(false);
}
unmute() {
this.enable(true);
}
}
/**
* AtomSoundChip -- Acorn Atom sound via a 1-bit speaker driven by the PPIA.
* Uses only the sine channel (shared with BBC for tape tones) and a speaker
* channel with DC-blocking filter.
*/
export class AtomSoundChip extends SoundChip {
constructor(onBuffer, { cpuSpeed = 1000000, onEvent = null } = {}) {
super(onBuffer, { onEvent });
this.samplesPerCycle = this.soundchipFreq / cpuSpeed;
this.secondsPerCycle = 1 / cpuSpeed;
// Replace the BBC tone/noise generators with just sine + speaker.
this.generators = [this.sineChannel.bind(this), this.speakerChannel.bind(this)];
// Recompute sine attenuation for the Atom's 2-channel mix
// (parent computed it for 5 BBC channels).
this.sineTable = makeSineTable(1 / this.generators.length);
this.speakerGenerator = {
mute: () => {
this.catchUp();
this.speakerReset();
},
pushBit: (bit, cycles, seconds) => {
this.catchUp();
this.updateSpeaker(bit, cycles, seconds);
},
};
this.bitChange = [];
this.speakerBit = 0.0;
this.currentSpeakerBit = 0.0;
this._speakerPrevIn = 0;
this._speakerPrevOut = 0;
this._speakerCycleOffset = 0;
Object.assign(this.eventHandlers, {
bit: (bit, { cycle }) => this.bitChange.push({ bit, cycles: cycle }),
speakerReset: () => this.speakerReset(),
});
}
reset(hard) {
super.reset(hard);
Eif (hard) this.speakerReset();
}
catchUp() {
this._speakerCycleOffset = 0;
super.catchUp();
this._speakerCycleOffset = 0;
}
renderAt(cycle, out, offset, length) {
this._speakerCycleOffset = 0;
super.renderAt(cycle, out, offset, length);
}
snapshotState() {
return { ...super.snapshotState(), speakerBit: this.speakerBit };
}
restoreState(state) {
this.bitChange = [];
this.speakerBit = this.currentSpeakerBit = state.speakerBit ?? 0.0;
// Held at its level, so the DC blocker has settled.
this._speakerPrevIn = this.currentSpeakerBit * speakerVolume;
this._speakerPrevOut = 0;
this._speakerCycleOffset = 0;
super.restoreState(state);
}
speakerReset() {
this._emit("speakerReset", true);
this.bitChange = [];
this.speakerBit = 0.0;
this.currentSpeakerBit = 0.0;
this._speakerPrevIn = 0;
this._speakerPrevOut = 0;
this._speakerCycleOffset = 0;
}
speakerChannel(channel, out, offset, length) {
const fromCycle = this.lastRunEpoch + this._speakerCycleOffset;
this._speakerCycleOffset += length / this.samplesPerCycle;
let bitIndex = 0;
// DC-blocking high-pass filter: y[n] = x[n] - x[n-1] + alpha * y[n-1]
// The SoundChip runs at 500 kHz (4 MHz / 8). For a ~20 Hz cutoff:
// alpha = 1 - 2*pi*fc/fs = 1 - 2*pi*20/500000 ≈ 0.99975
const alpha = 0.99975;
for (let i = 0; i < length; ++i) {
while (
bitIndex < this.bitChange.length &&
this.bitChange[bitIndex].cycles <= fromCycle + i / this.samplesPerCycle
) {
this.currentSpeakerBit = this.bitChange[bitIndex].bit;
bitIndex++;
}
const input = this.currentSpeakerBit * speakerVolume;
this._speakerPrevOut = input - this._speakerPrevIn + alpha * this._speakerPrevOut;
this._speakerPrevIn = input;
out[i + offset] += this._speakerPrevOut;
}
if (bitIndex > 0) {
this.bitChange.splice(0, bitIndex);
}
}
updateSpeaker(value, microCycle, seconds) {
const cycles = microCycle + seconds / this.secondsPerCycle;
const bit = value ? 1.0 : 0.0;
this.speakerBit = bit;
if (this._onEvent) this._send({ cycle: cycles, kind: "bit", value: bit });
else this.bitChange.push({ bit, cycles });
}
}
/**
* InstrumentedSoundChip - wraps a real SoundChip and captures all writes
* for debugging purposes. Provides the same interface as SoundChip.
*/
export class InstrumentedSoundChip extends SoundChip {
constructor() {
super(() => {}); // no audio output callback needed in headless mode
this._capturedWrites = [];
this._capturing = false;
this._totalCycles = 0;
}
poke(value) {
// Bring _totalCycles up to now first: it only moves when the chip
// catches up, so without this each write would carry the time of the
// write before it.
this.catchUp();
Eif (this._capturing) {
this._capturedWrites.push({ cycle: this._totalCycles, value });
}
super.poke(value);
}
/** Start capturing SN76489 writes. Clears any previous capture. */
startCapture() {
this._capturedWrites = [];
this._capturing = true;
}
/** Stop capturing and return the captured writes. */
stopCapture() {
this._capturing = false;
return this._capturedWrites;
}
/** Track total cycles for timestamps. Called by the scheduler. */
advance(cycles) {
this._totalCycles += cycles;
super.advance(cycles);
}
/** Runs `run`, a replay of the machine's past, leaving out of the capture what it writes and the cycles it takes. */
withoutCapturing(run) {
// Caught up either side, so that the cycles owed to the chip are only those the machine runs forward.
this.catchUp();
const [writes, totalCycles] = [this._capturedWrites.length, this._totalCycles];
try {
return run();
} finally {
this.catchUp();
this._capturedWrites.length = writes;
this._totalCycles = totalCycles;
}
}
/** Read current SN76489 register state in a friendly format. */
getState() {
return {
tone: [this.registers[0], this.registers[1], this.registers[2]],
noise: this.registers[3],
volume: [this.attenuation(0), this.attenuation(1), this.attenuation(2), this.attenuation(3)],
lfsr: this.lfsr,
latchedRegister: this.latchedRegister,
};
}
}
export class FakeSoundChip {
reset() {}
enable() {}
catchUp() {}
mute() {}
unmute() {}
render() {}
withoutCapturing(run) {
return run();
}
updateSlowDataBus() {}
setScheduler() {}
constructor() {
this.toneGenerator = {
mute: () => {},
tone: () => {},
};
this.speakerGenerator = {
mute: () => {},
pushBit: () => {},
};
}
snapshotState() {
return {};
}
restoreState() {}
}
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