5 MUSIC GENERATION
The production of music involves the creation of series of frequen-
cies which are pleasing to the human ear (setting critical evaluation
aside). This involves essentially mathematical relationships, making
the application ideal for digital devices. For example, the shifting up
or down in octaves is a multiplication or division by a power of 2,
which is a simple shift operation for most microprocessors.
Another factor in music generation is “communication”. The com-
poser must be able to convey his tune ideas so that a musician or
group of musicians can reproduce the composer’s ideas-often on
widely differing instruments. This concept involves “tuning” the
instruments to a standard set of frequencies and following a set
rhythm pattern. The tuning frequency most widely used is based on
the third octave note “A” of 440Hz, the “Equal Tempered Chromatic
Scale”.
Although it is easy to construct recognizable tunes using only one
note at a time, the simultaneous sounding of more than one note to
produce chords and counterpoint vastly increases the quality of the
sound. This feature is easily achieved in the PSG since three
channels are provided, each independently programmable.
Note
5 . 1
Since notes are formed by sustaining a particular frequency for a
Generation
preset period of time at a varying amplitude, the PSG performs this
function with a series of simple register loads. The method used in
many cases is to obtain register load values for first octave notes and
to shift to the correct octave at playtime.
The chart in Fig. 23 lists a full 8 octaves of notes from a low of Cl
(32.703Hz) to a high of B8 (7902.080Hz). Assuming an input clock
frequency of 1.78977MHz (one half the standard “color” crystal
frequency of 3579545MHz), and applying the formulas of Section 3.1
for calculating Tone Period register load values, results in the
register values shown. The nature of the PSG divider scheme
produces a high degree of accuracy for low frequencies, less for high
frequencies. This can be seen in the chart in the comparison of “ideal
frequencies” and “actual frequencies”, with the ideal frequencies
being those of the Equal Tempered Chromatic Scale, and the actual
frequencies being the “best fit” values from the formula calculation.
Summary of Contents for ay-3-8910
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