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Recreating the ZX Spectrum loader with Web APIs

Remy Sharp
October 06, 2017

Recreating the ZX Spectrum loader with Web APIs

This talk is about using new technology to replicate an old, reasonably useless, technology: replicating the ZX Spectrum tape loader audio and visuals (but without the tape…), and sharing what I learned along the way.

Remy Sharp

October 06, 2017
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  1. * 3.5Hmz * 1 t-state = 1/3,500,000 * 1T =

    1/2 pulse wave * 1 bit of data = 2 equal pulses
  2. * 3.5Hmz * 1 t-state = 1/3,500,000 * 1T =

    1/2 pulse wave * 1 bit of data = 2 equal pulses * binary 0 = 855T
  3. * 3.5Hmz * 1 t-state = 1/3,500,000 * 1T =

    1/2 pulse wave * 1 bit of data = 2 equal pulses * binary 0 = 855T * binary 1 = 1710T
  4. const T = 1/3500000; const SAMPLE_RATE = 44100; // 44.1Mhz

    const ONE = 1710 * 2; // 2 = HIGH + LOW const asHz = pulse => 1 / (T * pulse); const toRadian = hz => hz * Math.PI * 2; function generateSample(output) { const length = ONE * T * SAMPLE_RATE; for (let i = 0; i < length; i++) { const time = i / SAMPLE_RATE; const angle = time * toRadian(asHz(ONE)); // store a square wave output[i] = Math.sin(angle) < 0 ? -1 : 1; } }
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  6. this.node = ctx.createScriptProcessor(bufferSize, 1, 1); this.node.onaudioprocess = audioProcessingEvent => {

    const channel = 0; const inputBuffer = audioProcessingEvent.inputBuffer; const input = inputBuffer.getChannelData(channel); // then we'll read the values for own processing this.read(input, performance.now()); // copy the input directly across to the output const outputBuffer = audioProcessingEvent.outputBuffer; const output = outputBuffer.getChannelData(channel); inputBuffer.copyFromChannel(output, channel, channel); }; // constructor continues...
  7. * Draw image into canvas * export canvas as blob

    * File Reader API to read as binary string
  8. * Draw image into canvas * export canvas as blob

    * File Reader API to read as binary string * convert char to binary
  9. * Draw image into canvas * export canvas as blob

    * File Reader API to read as binary string * convert char to binary * TURN binary INTO audio
  10. function imageToBlob(img) { const c = document.createElement('canvas'); const ctx =

    c.getContext('2d'); c.width = img.width; c.height = img.height; ctx.drawImage(img, 0, 0); return new Promise(resolve => { c.toBlob(file => resolve(file)); }) }
  11. function imageToBlob(img) { const c = document.createElement('canvas'); const ctx =

    c.getContext('2d'); c.width = img.width; c.height = img.height; ctx.drawImage(img, 0, 0); return new Promise(resolve => { c.toBlob(file => resolve(file)); }) }
  12. function fileToBinary(blob) { return new Promise(resolve => { const reader

    = new window.FileReader(); reader.onloadend = () => { const binary = []; const result = reader.result; for (let i = 0; i < result.length; i++) { const char = result[i]; binary.push(charToBinary(char)); } resolve( binary.reduce((acc, byte) => { return acc.concat(byte.split('')); }, []) ); }; reader.readAsBinaryString(blob); }); }
  13. function fileToBinary(blob) { return new Promise(resolve => { const reader

    = new window.FileReader(); reader.onloadend = () => { const binary = []; const result = reader.result; for (let i = 0; i < result.length; i++) { const char = result[i]; binary.push(charToBinary(char)); } resolve( binary.reduce((acc, byte) => { return acc.concat(byte.split('')); }, []) ); }; reader.readAsBinaryString(blob); }); }
  14. function charToBinary(chr) { return char .charCodeAt(0) // R = 82

    .toString(2) // 82 = 1010010 .padStart(8, '0'); // 1010010 = 01010010 }
  15. * SAMple audio * detect edge * length = 1/2

    pulse * read 2nd pulse, ?=== len
  16. * SAMple audio * detect edge * length = 1/2

    pulse * read 2nd pulse, ?=== len * Bit = len == 855 ? 0 : 1
  17. * SAMple audio * detect edge * length = 1/2

    pulse * read 2nd pulse, ?=== len * Bit = len == 855 ? 0 : 1 * Make bytes from 8 bits
  18. * SAMple audio * detect edge * length = 1/2

    pulse * read 2nd pulse, ?=== len * Bit = len == 855 ? 0 : 1 * Make bytes from 8 bits * <Repeat>
  19. * SAMple audio * detect edge * length = 1/2

    pulse * read 2nd pulse, ?=== len * Bit = len == 855 ? 0 : 1 * Make bytes from 8 bits * <Repeat> * make image from bytes
  20. export function loadEdge1(buffer) { if (!buffer.length) { return null; }

    let last = null; let point = buffer.shift(); pulseBuffer.push(point); do { // search for when the buffer point crosses the zero threshold if (last !== null) { // important: when we hit an edge, the data doesn't include the edge if (edge(point, last)) { // create a new array and return that instead const res = Array.from(pulseBuffer); // reset the buffer pulseBuffer = []; return res; } } pulseBuffer.push(point); last = point; } while ((point = buffer.shift())); return null; // no edge found }
  21. PULSE = 1 / sample rate * (HI count) BIT

    = PULSE === 855 ? 0 : 1;
  22. 4b FF 80 32 00 a1 42 02 BB 80

    c3 20 d1 d9 ff 02