Servo - April 2012

FIGURE 9. The Parallax line follower module contains eight modulated infrared emitters and IR receivers. Tunebot uses it instead as a piano keyboard — playing the keyboard operates the robot.

can use a constant like CTRL_INSTRUMENT which mnemonically helps you to remember what the value does.

Constants are defined in an Arduino sketch using either of two methods:

const int SOME_VALUE = 5;

or

#define SOME_VALUE 5

noteOn, or otherwise the notes will just congeal into one another. (This continues until the MIDI chip runs out of the internal resource space.)

The first argument of the noteOn/noteOff methods is the channel. I’m using Channel 0, but there’s nothing stopping you from using another channel. The second argument is the pitch; in this case, it’s middle C on the piano. The third argument is the velocity. I’m using the highest value possible: 127.

Both do the same thing: They define a symbol that evaluates to a number. When the sketch is compiled, the Arduino substitutes each instance of SOME_VALUE with the number 5. Constants are similar to variables in that both let you reference a value by name. Unlike variables, however, constant definitions don’t take up memory in the Arduino. Take notice that when using the #define statement, there’s no semi-colon at the end of the line.

Listing 2 demonstrates the same functionality as the previous sketch, but uses constants to refer to the command and data bytes. While the sketch is longer, it takes up the same amount of program space inside the wee innards of the Arduino.

Using Self-Descriptive Constants Playing Multiple Notes

Programming MIDI is made easier by using constants instead of raw hexadecimal or decimal values. Rather than trying to remember that 0xC0 switches instruments, you The VS10xx chips are polyphonic, meaning they can play more than one note at a time (conversely, monophonic means one note at a time). You can play multiple notes on the same channel or on a different channel. If the notes are from the same instrument, it’s easier to use the same channel. Simply repeat the noteOn statement for each pitch you wish to play. Conversely, if you wish to play notes from different instruments, you’ll need to specify an instrument on each channel. Then, call noteOn for each note, referencing the channel you wish to use.