Comparing Scratch [4] to Pencil Code [3] for teaching computing through music immediately reveals a major difference: Scratch represents notes as MIDI numbers, while Pencil Code represents notes as letters using ABC notation [1]. To someone who reads music, Pencil Code is clearly preferable because it is far easier to map, for example, a major third to C and E than 60 and 64. But to those who generate music algorithmically, ABC is not as easy as MIDI. For example, a major third in the key of D is not D and F, it is D and F#. Using MIDI values, however, the interval is always 4 semitones: 62 (D) + 4 = 66 (F#), just as before.

The Problem: Pencil Code is written in CoffeeScript [4], "a little language that compiles into JavaScript." Thus, one would think to use the String.fromChar-Code function to convert numeric MIDI to alphabetic ABC. Unfortunately, the conversion is not that straightforward, because ABC notation is based on a C scale, with A and B above G. That throws out a simple conversion. The 4th octave, the one that begins on middle C, is represented by all capital letters: C D E F G A B. For the 5th octave, apostrophes are added: C' D' E' F' G' A' B'. For the 6th octave, two apostrophes are added, and so on to the higher octaves. For the 3rd octave, the one below middle C, commas are added: C, D, E, F, G, A, B. And, as you can probably guess from the pattern, for the 2nd octave, two commas are added and so on to the lower octaves.

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A Solution: If one wants both the clarity of ABC notation and the algorithmic power of MIDI values, a conversion is indeed necessary. The code below runs in Pencil Code and plays the chromatic scale from C3 to C6 by passing MIDI values 48 (C3) through 84 (C6) to a MIDItoABC function that is generalized to convert any MIDI value to its proper representation in ABC notation and play it for a specified duration.

A Note on ABC Duration Notation: Unrelated to MIDI but germane to ABC, duration strings are based on quarter notes, which have a duration of 1. A half note has a duration of 2, and a whole note a duration of 4. Going the other way, an eighth note has a duration of 1/2, designated as "/2". Similarly, a 16th note has a duration of "/4", and a 32nd note has a duration of "/8".

The code below includes a test routine to play the chromatic scale from C3 to C6 with random note durations from a half note ("2") to a 32nd note ("/8").

References

1. ABC Notation; http://abcnotation.com. Accessed 22 February 2016.

2. CoffeeScript; http://coffeescript.org. Accessed 22 February 2016.

3. PencilCode; http://pencilcode.net. Accessed 22 February 2016.

4. Scratch; http://scratch.mit.edu. Accessed 22 February 2016.

Author

Jesse M. Heines
UMass Lowell—Computer Science
One University Avenue
Lowell, Massachusetts 01854 USA
heines@cs.uml.edu

Copyright held by author.

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