By choosing carefully the waveform used, you can start with a harmonic structure that looks somewhat like the sound you want. To refine the sound, you can add another aspect of sound quality available on your Commodore 64 called filtering, which we'll discuss later in this section.

THE ENVELOPE GENERATOR

The volume of a musical tone changes from the moment you first hear it, all the way through until it dies out and you can't hear it anymore. When a note is first struck, it rises from zero volume to its peak volume. The rate at which this happens is called the ATTACK. Then, it fails from the peak to some middle-ranged volume. The rate at which the fall of the note occurs is called the DECAY. The mid-ranged volume itself is called the SUSTAIN level. And finally, when the note stops playing, it fails from the SUSTAIN level to zero volume. The rate at which it fails is called the RELEASE. Here is a sketch of the four phases of a note:

Each of the items mentioned above give certain qualities and restric- tions to a note. The bounds are called parameters.

The parameters ATTACK/DECAY/SUSTAIN/RELEASE and collectively called ADSR, can be controlled by your use of another set of locations in the sound generator chip. LOAD your first example program again. RUN it again and remember how it sounds. Then, changing line 20 so the program is like this: