Making a synthesizer
With the power of PWM we can output an audio signal directly to a ~pin. And then connect that to headphones or speakers or something.
Instead of using the limited tone() function, we can code our own wave and modulate it! Proper synthesis! It’s still very low-fi, the Uno has 8bit depth, but it works!
There is various tutorials out there but I recommend this one by Peter Knight
also because so many people have tried and improved on it
I’ll post Peter’s code here, because who knows for how much longer Google Code Archive will exist
// Auduino, the Lo-Fi granular synthesiser
//
// by Peter Knight, Tinker.it http://tinker.it
//
// Help: http://code.google.com/p/tinkerit/wiki/Auduino
// More help: http://groups.google.com/group/auduino
//
// Analog in 0: Grain 1 pitch
// Analog in 1: Grain 2 decay
// Analog in 2: Grain 1 decay
// Analog in 3: Grain 2 pitch
// Analog in 4: Grain repetition frequency
//
// Digital 3: Audio out (Digital 11 on ATmega8)
//
// Changelog:
// 19 Nov 2008: Added support for ATmega8 boards
// 21 Mar 2009: Added support for ATmega328 boards
// 7 Apr 2009: Fixed interrupt vector for ATmega328 boards
// 8 Apr 2009: Added support for ATmega1280 boards (Arduino Mega)
#include <avr/io.h>
#include <avr/interrupt.h>
uint16_t syncPhaseAcc;
uint16_t syncPhaseInc;
uint16_t grainPhaseAcc;
uint16_t grainPhaseInc;
uint16_t grainAmp;
uint8_t grainDecay;
uint16_t grain2PhaseAcc;
uint16_t grain2PhaseInc;
uint16_t grain2Amp;
uint8_t grain2Decay;
// Map Analogue channels
#define SYNC_CONTROL (4)
#define GRAIN_FREQ_CONTROL (0)
#define GRAIN_DECAY_CONTROL (2)
#define GRAIN2_FREQ_CONTROL (3)
#define GRAIN2_DECAY_CONTROL (1)
// Changing these will also requires rewriting audioOn()
#if defined(__AVR_ATmega8__)
//
// On old ATmega8 boards.
// Output is on pin 11
//
#define LED_PIN 13
#define LED_PORT PORTB
#define LED_BIT 5
#define PWM_PIN 11
#define PWM_VALUE OCR2
#define PWM_INTERRUPT TIMER2_OVF_vect
#elif defined(__AVR_ATmega1280__)
//
// On the Arduino Mega
// Output is on pin 3
//
#define LED_PIN 13
#define LED_PORT PORTB
#define LED_BIT 7
#define PWM_PIN 3
#define PWM_VALUE OCR3C
#define PWM_INTERRUPT TIMER3_OVF_vect
#else
//
// For modern ATmega168 and ATmega328 boards
// Output is on pin 3
//
#define PWM_PIN 3
#define PWM_VALUE OCR2B
#define LED_PIN 13
#define LED_PORT PORTB
#define LED_BIT 5
#define PWM_INTERRUPT TIMER2_OVF_vect
#endif
// Smooth logarithmic mapping
//
uint16_t antilogTable[] = {
64830,64132,63441,62757,62081,61413,60751,60097,59449,58809,58176,57549,56929,56316,55709,55109,
54515,53928,53347,52773,52204,51642,51085,50535,49991,49452,48920,48393,47871,47356,46846,46341,
45842,45348,44859,44376,43898,43425,42958,42495,42037,41584,41136,40693,40255,39821,39392,38968,
38548,38133,37722,37316,36914,36516,36123,35734,35349,34968,34591,34219,33850,33486,33125,32768
};
uint16_t mapPhaseInc(uint16_t input) {
return (antilogTable[input & 0x3f]) >> (input >> 6);
}
// Stepped chromatic mapping
//
uint16_t midiTable[] = {
17,18,19,20,22,23,24,26,27,29,31,32,34,36,38,41,43,46,48,51,54,58,61,65,69,73,
77,82,86,92,97,103,109,115,122,129,137,145,154,163,173,183,194,206,218,231,
244,259,274,291,308,326,346,366,388,411,435,461,489,518,549,581,616,652,691,
732,776,822,871,923,978,1036,1097,1163,1232,1305,1383,1465,1552,1644,1742,
1845,1955,2071,2195,2325,2463,2610,2765,2930,3104,3288,3484,3691,3910,4143,
4389,4650,4927,5220,5530,5859,6207,6577,6968,7382,7821,8286,8779,9301,9854,
10440,11060,11718,12415,13153,13935,14764,15642,16572,17557,18601,19708,20879,
22121,23436,24830,26306
};
uint16_t mapMidi(uint16_t input) {
return (midiTable[(1023-input) >> 3]);
}
// Stepped Pentatonic mapping
//
uint16_t pentatonicTable[54] = {
0,19,22,26,29,32,38,43,51,58,65,77,86,103,115,129,154,173,206,231,259,308,346,
411,461,518,616,691,822,923,1036,1232,1383,1644,1845,2071,2463,2765,3288,
3691,4143,4927,5530,6577,7382,8286,9854,11060,13153,14764,16572,19708,22121,26306
};
uint16_t mapPentatonic(uint16_t input) {
uint8_t value = (1023-input) / (1024/53);
return (pentatonicTable[value]);
}
void audioOn() {
#if defined(__AVR_ATmega8__)
// ATmega8 has different registers
TCCR2 = _BV(WGM20) | _BV(COM21) | _BV(CS20);
TIMSK = _BV(TOIE2);
#elif defined(__AVR_ATmega1280__)
TCCR3A = _BV(COM3C1) | _BV(WGM30);
TCCR3B = _BV(CS30);
TIMSK3 = _BV(TOIE3);
#else
// Set up PWM to 31.25kHz, phase accurate
TCCR2A = _BV(COM2B1) | _BV(WGM20);
TCCR2B = _BV(CS20);
TIMSK2 = _BV(TOIE2);
#endif
}
void setup() {
pinMode(PWM_PIN,OUTPUT);
audioOn();
pinMode(LED_PIN,OUTPUT);
}
void loop() {
// The loop is pretty simple - it just updates the parameters for the oscillators.
//
// Avoid using any functions that make extensive use of interrupts, or turn interrupts off.
// They will cause clicks and poops in the audio.
// Smooth frequency mapping
//syncPhaseInc = mapPhaseInc(analogRead(SYNC_CONTROL)) / 4;
// Stepped mapping to MIDI notes: C, Db, D, Eb, E, F...
//syncPhaseInc = mapMidi(analogRead(SYNC_CONTROL));
// Stepped pentatonic mapping: D, E, G, A, B
syncPhaseInc = mapPentatonic(analogRead(SYNC_CONTROL));
grainPhaseInc = mapPhaseInc(analogRead(GRAIN_FREQ_CONTROL)) / 2;
grainDecay = analogRead(GRAIN_DECAY_CONTROL) / 8;
grain2PhaseInc = mapPhaseInc(analogRead(GRAIN2_FREQ_CONTROL)) / 2;
grain2Decay = analogRead(GRAIN2_DECAY_CONTROL) / 4;
}
SIGNAL(PWM_INTERRUPT)
{
uint8_t value;
uint16_t output;
syncPhaseAcc += syncPhaseInc;
if (syncPhaseAcc < syncPhaseInc) {
// Time to start the next grain
grainPhaseAcc = 0;
grainAmp = 0x7fff;
grain2PhaseAcc = 0;
grain2Amp = 0x7fff;
LED_PORT ^= 1 << LED_BIT; // Faster than using digitalWrite
}
// Increment the phase of the grain oscillators
grainPhaseAcc += grainPhaseInc;
grain2PhaseAcc += grain2PhaseInc;
// Convert phase into a triangle wave
value = (grainPhaseAcc >> 7) & 0xff;
if (grainPhaseAcc & 0x8000) value = ~value;
// Multiply by current grain amplitude to get sample
output = value * (grainAmp >> 8);
// Repeat for second grain
value = (grain2PhaseAcc >> 7) & 0xff;
if (grain2PhaseAcc & 0x8000) value = ~value;
output += value * (grain2Amp >> 8);
// Make the grain amplitudes decay by a factor every sample (exponential decay)
grainAmp -= (grainAmp >> 8) * grainDecay;
grain2Amp -= (grain2Amp >> 8) * grain2Decay;
// Scale output to the available range, clipping if necessary
output >>= 9;
if (output > 255) output = 255;
// Output to PWM (this is faster than using analogWrite)
PWM_VALUE = output;
}