teaching machines

Music Mouse, Part III

June 16, 2019 by Chris Johnson. Filed under electronics, music, public.

This post is part of a series of notes and exercises for a summer camp on making musical instruments with Arduino and Pure Data.

Music Mouse walks the horizontal axis in steps of one right now. If we were to interpret the x-position as a MIDI number and move along one halfstep at a time, we’d be walking the chromatic scale. It’s just like walking up a piano one key at a time.

We don’t normally play music in the chromatic scale. Let’s have Music Mouse walk the major scale instead.

Moving Right

When the joystick is pressed right, we want the Music Mouse to get to the next note in the major scale. We’re probably going to need a list of the offsets between the notes of the major scale.

Declare a global array named jumpUps that holds these offsets. When the joystick is pressed right, we want to apply one of these offsets the current x-position. But which offset?

We don’t know. We don’t have enough information to know where we’re at in the scale. So, let’s declare a global int named scale_index_x and assign it 0. We’ll use it to track our current position in the scale.

When we change the x-position, instead of adding dx, add on the offset for our current position in the scale:

x += jumpUps[scale_index_x]; 

Also, advance our position in the scale by adding 1 on to scale_index_x.

Upload and test your code. If you start at 60 and press right, you should advance to 62, 64, 65, 67, 69, 71, 72, and so on.

Wrap Around

When you advance past 72, you probably see some strange results.

if scale_index_x > 6
  scale_index_x = 0

scale_index_x = (scale_index_x + 1) % 7;

Moving Left

Okay, we’ve got the joystick’s right action working.

Declare a global array named jumpDowns. Assign it a list of seven numbers that describe how many halfsteps we should jump down to get to the previous note in the major scale.

The assignment to x needs to get a bit more complicated. We have to choose between jumping up and jumping down. This sounds like a conditional statement, and we’ll use dx to arbitrate between our choices. Update your code to match this pseudocode:

if old_dx and dx are different
  if we're not idling
    if we're going right
      x = x + jumpUps[scale_index_x]
      increment scale_index_x with wraparound
    else
      x = x - jumpDowns[scale_index_x]
      decrement scale_index_x with wraparound
  old_dx = dx

For the decrementing, you might have written this:

scale_index_x = (scale_index_x - 1) % 7;

Sadly, the mod operator doesn’t quite do what we want for negative numbers. In particular, it yields negative numbers, which are inappropriate for indexing into an array. Consider the following table that shows what values we want and what % actually yields:

scale_index_x = (scale_index_x - 1 + 7) % 7;