Beam Beats tangible rhythm sequencer: work is in progress!

Beam Beats is an interactive and tangible rhythm sequencer that translates the geometry of beacons on the ground into rhythms and polyrhythms thanks to a rotating laser beam. This experimental MIDI instrument is about investigating self-similarities in polyrhythms, as described in this post.

Update: Here’s the video of a demo at Dorkbot at La Gaité Lyrique in Paris

BeamBeats demo at Dorkbot at la Gaité Lyrique from cyrille martraire on Vimeo.

Before I report more on this project with demos and videos, here are some pictures to illustrate the work currently in progress, thanks to lasercutting techniques in particular:

The brand new laser-cut acrylic structure
The prototype laser head and retro-reflection light sensor
The Arduino to detect the light variations and convert them into MIDI notes
Very early design of beacons each with their own light sensor

This prototype works, here’s a picture of the laser beam rotating:

The revolving laser beam reflected on the wall

And here’s a very short early video (no sound)

Beam Beats – revolving laser beam to detect beacons from cyrille martraire on Vimeo.

I’ve been working on the Arduino source code to add the ability to send MIDI clock in order to sync Beam Beats to other MIDI devices.

Now I mostly need to conclude on the design for the active  beacons(sensors stands) and the passive (retroreflective adhesive stands) beacons, and find out what to play with this sequencer…

Every post on this project:

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(Arduino) Interactive Generative Sequencer

Following my ongoing work on a theory of rhythms and a corresponding physical instrument using lasers, here is a version of the same idea implemented into an Arduino: a generative sequencer. The idea is to generate rhythms, and perhaps melodies, from one rhythm seed, then use mutated copies of it to create something more interesting, all this in realtime using knobs and buttons.

This is not ‘yet another step sequencer’, but really a generative music device, that builds a real time MIDI loop based on an underlying theory described in a previous post.

This is work in progress, and is shown ‘as is’ for the sake of getting feedback.


The approach is to generate a “seed” of rhythm that is then copied a few times into “instances”, each instance being distorted in its own way. The controls for the human player (or programmer) are therefore all about adjusting the deformations (transforms actually) to apply to each instance, and of course defining the seed.

Eventually each instance is wired to a MIDI note, channel etc. to remote control a synthesizer or a drum machine or any MIDI setup, to generate actual sounds.

Principle: one seed, many transformed instances
Principle: one seed, many transformed instances

The maths

Given a seed of rhythm of lengh length composed of pulses, each of duration m, then:

for each instance k of the seed, each pulse i,
pulse(k, i) happen at time t = phase(k)  + i . m . stretch(k), t < length
where phase(k) and stretch(k) are the phase and stretch settings for the instance k.

Realtime control of the sequencer is therefore all about setting the phase and stretch values for each instance, once the pulse number and the pulse duration of the seed have been globally set.

Inversely, for a given instance k, at time t, we have a pulse if:

there exists an i, such as t = phase(k) + i * m * stretch(k)
i.e. i = (t - phase(k))/(m * stretch(k))

In other words, if

(t - phase(k))/(m * stretch(k)) is integer
(i.e. numerator % denominator == 0)

Thinking in MIDI ticks (24 per quarters), in 4/4, for 1 bar, length = 4 * 24, phase is in [-24 .. 24] and stretch is in [4:1 .. 1:4] and m in [12 .. 48] by steps of 12 ticks.

The implementation is the very simple: for each instance of the seed, and given its phase and stretch settings, whenever the modulo condition above is true, then we emit its MIDI note, with the set velocity on the set MIDI channel.

As usual, the pace of the loop is primarily based on the value from the Tempo potentiometer.

Overview of the circuit
Overview of the circuit, with the switches and the knobs

Adding some swing

8th note swing
8th note swing

The EMU SP-1200, early Linn machines, Roland 909, Akai MPC and many other machines are famous for their swing quantize, a feature that delays every other note by a certain amount in order to create a groovy feeling (see Swung Note).

Different machines express the swing factor in different ways, we will stick to the MPC format, expressed in percent from 50% (no swing, play straight) to 75% (hard shuffle).

For a swing for 8th notes, this swing factor represents the ratio of the period of the first 8th note over the period of the second 8th note, in percent.

In the Arduino code of our generative sequencer, we chose to do a fixed swing for 8th notes only.

A big constraint is that we are limited to a resolution of 24 ticks per quarter note, which is not a lot! By contrast, MPC have a 96 ppq resolution. Because a hard swing of 70% translates into hardly 2 MIDI ticks at 24 ppq, doing the swing on top of the ticks will not be accurate at all!

The only solution is to vary the internal tempo before and after each 8th note. The drawback (or advantage) is that the MIDI clock being sent will also move, reflecting the same swing. Since the Swing knob value is actually between 0 and 25 (to be read from50% to 75%), the tempo before (t-) and the tempo after (t+), are given by:

t+/- = (50 +/- swing) * t / 50
where t is the base loop period without swing

Video Demo

Here is a video demo. There are only 3 instances, selected by the switches 1, 2 and 3; the first switch selects the GLOBAL settings: step duration (quarter, 8th etc.), swing factor, tempo. Each instance can be set its Phase, Stretch, MIDI note, velocity and MIDI channel. Here I have preset the MIDI channels, instance 1 on channel 1 (the microKorg) and instances 2 and 3 on channel 2 (the MPC with a drum kit).

The goal is to build a simple beat by only adjusting the parameters.

(Arduino) Interactice Generative Sequencer from cyrille martraire on Vimeo.

The code

You can download the Arduino project here: generativesequencer1; below is the same source code for convenience. The code includes the knob pagination described in a previous post.

Please note that some parts of the code are not used any more, such as the big constant arrays, and some comments are not up to date (e-g no prime number anymore).

All analog inputs are wired to simple knobs. Digital inputs 8, 9, 10 , 11 are the four buttons used to switch pages. Digital output 12 is the activity LED (showing when the knob is active within the knob pagination). MIDI out is on the Tx pin.

 * Generative rhythm sequencer, more details at:
 * Knob mapping according to a grid 2^n . prime^m, against the MIDI 24 ticks/quarter.
 * Knob pagination to multiplex the knobs several times with LED to show activity.
 * Memory-less event handling thanks to maths relationships.
 * MIDI note on output on every 16 channel and MIDI clock according to tempo.
 * Creative Commons License Cyrille Martraire
int debug = false;
//---------- USER INPUT AND PAGINATION -----------
#define PAGE_NB 4
#define KNOB_NB 6
#define PROTECTED -1
#define ACTIVE 1
#define SYNC_LED 12
// the permanent storage of every value for every page, used by the actual music code
int pageValues[PAGE_NB][KNOB_NB];
// last read knob values
int knobsValues[KNOB_NB];
// knobs state (protected, enable...)
int knobsStates[KNOB_NB];
// current (temp) value just read
int value = 0;
// the current page id of values being edited
int currentPage = 0;
// signals the page change
boolean pageChange = false;
//temp variable to detect when the knob's value matches the stored value
boolean inSync = false;
// ---------- KNOBS CALIBRATION AND MAPPING ---------
// rhythmic scale, to select globally
int scale2[] =  {1, 2, 3, 6, 6, 12, 12, 24, 24, 48, 48, 96, 192, 384, 768};
int scale3[] =  {1, 2, 3, 6, 9, 12, 18, 24, 36, 48, 72, 96, 144, 384, 768};
int scale5[] =  {1, 2, 3, 6, 12, 15, 24, 24, 30, 48, 60, 96, 120, 384, 768};
int scale7[] =  {1, 2, 3, 6, 12, 21, 24, 24, 42, 48, 84, 96, 168, 384, 768};
int scale9[] =  {1, 2, 3, 6, 12, 12, 24, 24, 27, 48, 54, 96, 108, 384, 768};
int scale11[] = {1, 2, 3, 6, 12, 12, 24, 24, 33, 48, 66, 96, 132, 384, 768};
//int scale13[] = {1, 2, 3, 6, 12, 24, 12, 24, 39, 48, 78, 96, 156, 384, 768};
int maxValue = 890;
int scaleLen = 15;
int *scale = scale3;
int step = 60;
int center = 30;
int coeff = 10;
//---------- GENERATIVE MUSIC CODE ---------
unsigned int cursor = 0;
int bars = 1;
int length = bars * 4 * 24;
int PHASE = 0;
int STRETCH = 1;
//int DIRECTION = 2;
int NOTE = 2;
int DURATION = 3;
int VELOCITY = 4;
int CHANNEL = 5;
// GLOBAL KNOBS (seed and global settings)
int seedDuration = 24;
int seedTimes = 8;
int instanceNb = 4;
int swing = 0;//0..25% (on top of 50%)
int loopPeriod = 125/6;//120BPM
int actualPeriod = loopPeriod;
//instance i
int phase = 0;
int stretch = 1;
int note = 48;
int duration = 24;
int velocity = 100;
int channel = 1;
void setup(){
   Serial.begin(19200); //debug
  } else {

  pinMode(13, OUTPUT);


void setupKnobMapping(){
  step = maxValue / scaleLen;
  if (step * scaleLen < maxValue) {
  center = step / 2; // for phase only
  coeff = step / 8; // +/-3 ticks, for phase only

void loop () {

    //TODO partition inputs reading every other cycle if required by CPU load


    // parameters for each instance (pages 1 to 3)
    for(int index = 1; index < instanceNb; index++){

    cursor = cursor % length;
void poolGlobalSettings(){
    // global parameters
    seedDuration = mapC(pageValues[0][0], maxValue, 1, 4) * 12;
    seedTimes = mapC(pageValues[0][1], maxValue, 1, 16);
    instanceNb = 4;//mapC(pageValues[0][2], maxValue, 1, PAGE_NB);
    // = mapC(pageValues[0][3], maxValue, 1, PAGE_NB);
    swing = mapC(pageValues[0][4], maxValue, 0, 25);
    loopPeriod = mapC(pageValues[0][5], maxValue, 63, 2);// 12.5 ms - 62.5
    if (cursor % 24 <= 12){
      actualPeriod = (50 + swing) * loopPeriod / 50;
    } else {
      actualPeriod = (50 - swing) * loopPeriod / 50;
    //TODO prime number selection and scale switch
// custom map function, with min value always 0, and out max cannot be exceeded
long mapC(long x, long in_max, long out_min, long out_max)
  if (x > in_max) {
    return out_max;
  return x * (out_max - out_min) / in_max + out_min;
void processSeeInstance(int * params){
  phase = mapC(params[PHASE], maxValue, 0, 24);
  stretch = mapC(params[STRETCH], maxValue, 0, 4);
  stretch = pow(2, stretch);// 4:1 to 1:4, in fourth
  note = mapC(params[NOTE], maxValue, 36, 48);
  //duration = mapC(params[DURATION], maxValue, 6, 96);
  velocity = mapC(params[VELOCITY], maxValue, 0, 127);
  channel = mapC(params[CHANNEL], maxValue, 0, 4);

  if(isPulse(phase, stretch)) {
     noteOn(channel, note, velocity);
// for each instance, and for the given cursor, is there a pulse?
boolean isPulse(byte phase, byte stretch){
  int num = cursor - phase;
  int denum = seedDuration * stretch / 4;
  return num % denum == 0;
// Sends a MIDI tick (expected to be 24 ticks per quarter)
void midiClock(){
  Serial.print(0xF8, BYTE);
//  plays a MIDI note for one MIDI channel.  Does not check that
// channel is less than 15, or that data values are less than 127:
void noteOn(char channel, char noteNb, char velo) {
   midiOut(0x90 | channel, noteNb, velo);
//  plays a MIDI message Status, Data1, Data2, no check
void midiOut(char cmd, char data1, char data2) {
   Serial.print(cmd, BYTE);
   Serial.print(data1, BYTE);
   Serial.print(data2, BYTE);
void setupPagination(){
  pinMode(SYNC_LED, OUTPUT);
  for(int i=0; i < KNOB_NB; i++){
    knobsValues[i] = analogRead(i);
    knobsStates[i] = ACTIVE;
// read knobs and digital switches and handle pagination
void poolInputWithPagination(){
  // read page selection buttons
     value = digitalRead(i);
     if(value == LOW){
         pageChange = true;
         currentPage = i - FIRST_PAGE_BUTTON;
  // if page has changed then protect knobs (unfrequent)
    pageChange = false;
    digitalWrite(SYNC_LED, LOW);
    for(int i=0; i < KNOB_NB; i++){
      knobsStates[i] = PROTECTED;
  // read knobs values, show sync with the LED, enable knob when it matches the stored value
  for(int i = 0;i < KNOB_NB; i++){
     value = analogRead(i);
     inSync = abs(value - pageValues[currentPage][i]) < 20;

     // enable knob when it matches the stored value
        knobsStates[i] = ACTIVE;

     // if knob is moving, show if it's active or not
     if(abs(value - knobsValues[i]) > 5){
          // if knob is active, blink LED
          if(knobsStates[i] == ACTIVE){
            digitalWrite(SYNC_LED, HIGH);
          } else {
            digitalWrite(SYNC_LED, LOW);
     knobsValues[i] = value;

     // if enabled then miror the real time knob value
     if(knobsStates[i] == ACTIVE){
        pageValues[currentPage][i] = value;
void printAll(){
     Serial.print("page ");

     //printArray(knobsValues, 6);
     //printArray(knobsStates, 6);

     for(int i = 0; i < 4; i++){
       printArray(pageValues[i], 6);
void printArray(int *array, int len){
  for(int i = 0;i< len;i++){
       Serial.print(" ");

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Geometric Rhythm Machine

In the post Playing with laser beams to create very simple rhythms” I explained a theoretical approach that I want to materialize into an instrument. The idea is to create complex rhythms by combining several times the same rhythmic patterns, but each time with some variation compared to the original pattern.

Several possible variations (or transforms, since a variation is generated by applying a transform to the original pattern) were proposed, starting from an hypothetical rhythmic pattern “x.x.xx..”. Three linear transforms: Reverse (”..xx.x.x”), Roll (”x.x.xx..”) and Scale 2:1 (”x…x…x.x…..”) or 1:2 (”xxx.”), and some non-linear transforms: Truncate (”xx..”) etc.

Geometry + Light = Tangible transforms

The very idea behind the various experiments made using laser light beams and LDR sensors is to build an instrument that proposes all the previous transforms in a tangible fashion: when you move physical object, you also change the rhythm accordingly.

Let’s consider a very narrow light beam turning just like the hands of a clock. Let’s suppose that our clock has no number written around, but we can position marks (mirrors) wherever on the clock surface. Still in the context of creating rhythms, now assume that every time a hand crosses a mark (mirror) we trigger a sound. So far we have a rhythmic clock, which is a funny instrument already. But we can do better…

Going back to our rhythmic pattern “x.x.xx..”, we can represent it with 4 mirrors that we position on a same circle. On the illustration below this original pattern is displayed in orange, each mirror shown by an X letter.. If we now link these 4 mirrors together with some adhesive tape, we have built a physical object that represents a rhythmic pattern. The rotating red line represents the laser beam turning like the clock hands.

Illustration of how the geometry of the rhythmic clock physically represents the transforms
Illustration of how the geometry of the rhythmic clock physically represents the transforms

Now we have a physical pattern (the original one), we can of course create copies of it (need more mirrors and more adhesive tape). We can then position the copies elsewhere on the clock. The point is that where you put a copy defines the transform that applies to it compared with the original pattern:

  • If we just shift a pattern left or right while remaining on the same circle, then we are actually doing the Roll transform (change the phase of the pattern) (example in blue on the picture)
  • If we reverse the adhesive tape with its mirrors glued on, then of course we also apply the Reverse transform to the pattern (example in grey on the picture)
  • If we move the pattern to another (concentric) circle, then we are actually applying the Scale transform, where the scaling coefficient is the fraction of the radius of the circle over the radius of the circle of the original pattern (example in green on the picture)

Therefore, simple polar geometry is enough to provide a complete set of physical operations that fully mimic the desired operations on rhythmic pattern. And since this geometry is in deep relationship with how the rhythm is being made, the musician can understand what’s happening and how to place the mirrors to get any the desired result. The system is controllable.

To apply the Truncate transform (that is not a linear transform) we can just stick a piece of black paper to hide the mirror(s) we want to mute.

If we layer the clock we just described, with one layer for each timbre to play, then again changing the timbre (yet another non-linear transform) can then be done by physically moving the pattern (mirrors on adhesive tape) across the layers.

From theory to practice

Second prototype, with big accuracy issues
Second prototype, with big accuracy issues

Though appealing in principle, this geometric approach is hard to implement into a physical installation, mostly because accuracy issues:

  1. The rotating mirror must rotate perfectly, with no axis deviation; any angular deviation is multiplied by two and then leads to important position deviation in the light spot in the distance: delta = distance . tan(2 deviation angle)
  2. Each laser module is already not very accurate: the light beam is not always perfectly aligned with the body. To fix that would require careful tilt and pan adjustment on the laser module support
  3. Positioning the retroreflectors in a way that is accurate and easy to add, move or remove at the same time is not that easy; furthermore, even if in theory the retroreflectors reflect all the incoming light back to where it comes from, in practice maximum reflectance happens when the light hits the reflector orthogonally, which is useful to prevent missed hits

Don’t hesitate to check these pages for progress, and any feedback much appreciated.

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Rhythmic clock machine

The rotating mirror and the 4 laser beams
The rotating mirror and the 4 laser beams

I finally received ten retroreflectors, the experiments started in Playing with laser beams to create very simple rhythms”, we can now move on.

Once again, the rotating mirror is reflecting the four parallel laser beams so that they sweep a 180 degrees area, where some retroreflectors are positioned to hit the beams trajectories.

Every time a beam hits a reflector then it should trigger a sound on a MIDI synth (here it is my little microkorg playing the sounds).

Generative music (a.k.a mangled rhythm)

However in the first try I forgot to set a short loop perid (the period was set to 100ms). Given the velocity of the laser beam when it hits the reflectors there is very little time to catch the signal on the sensors, and with a measure every 100ms the Arduino is missing most hits.

This means we got a simple and regular theoretical rhythm that is mangled by the input sampling process, and this fuzzyness actually creates “interesting” generative music, as in the video:

Generative music installation with laser beams and low-frequency sensors sampling from cyrille martraire on Vimeo.

Note that it is not totally random… (actually it is not random at all, just the result of different frequencies that sometime are in sync and most times are not).

Laser beams on the wall
Laser beams on the wall

Regular rhythm at last

With a shorter Arduino loop period (10ms) it becomes reliable: every (almost) hit triggers a note, as illustrated in the next video where we can hear a steady rhythmic pattern.

The Arduino code is quite simple: for each of the 4 sensors, read analog value, compare to threshold, debounce (not to send multiple notes for the actual same hit), then send MIDI note.

Arduino code for the rhythmic clock project

Laser beams generate regular rhythm at least from cyrille martraire on Vimeo.

Any feedback welcome…

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Playing with laser beams to create very simple rhythms

Just like many arts, music arousal is considered to follow the well-known Wundt curve that defines the balance between attractiveness and boredom. Too much repetition is boring, not enough repetition is confusing and considered just noise.

What for?

Let us assert that idea to music, to generate rhythms. A very simple application of the Wundt curve principle is to consider one given rhythmic pattern (e-g. , “x.x.xx..”) then to build up a more elaborate polyrhythm by combining various repetitions of it, although each copy must be distorted a bit to make the combination more complex hence more attractive. In other word, given a rhythmic seed, make it grow a rhythmic tree.

The transforms to apply to the rhythmic patterns can be linear:

  • Reverse (“..xx.x.x”)
  • Roll (“x.x.xx..”)
  • Scale 2:1 (“x…x…x.x…..”) or 1:2 (“xxx.”)

or non-linear:

  • Truncate (“xx..”)
  • Switch timbre (not really a transform, just to put somewhere)

In practice

To put that into practice I have been trying simple Java programs long ago, but it was too slow a process, and since I did not build a genetic algorithm around it was driven at random.

To make it more fun to investigate, we have started a small project of building an instrument to program rhythms on using laser beams and small reflectors. Each reflector triggers a sound (on a MIDI controlled MPC500) when hit by a laser beam (you need the sound on to listen to the Clap sound being triggered):

Playing with the beams to create very simple rythms from cyrille martraire on Vimeo.

The Arduino board
The Arduino board

Then by having several reflectors linked to each other to make patterns, we expect to be able to program rhythms by moving reflectors sets in the playground, using its geometry to derive the transformations to apply to the patterns.

EDIT: here is another video after some progress:

Playing with dual beams for kick and clap beat from cyrille martraire on Vimeo.

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