of Microprocessors

15
Percussive Sound
Generation
Up to this point, discussion has concentrated on the synthesis of basically
periodic tones. Percussive sounds, however, are quite important as well and
may in fact have even more variety than periodic types of sounds. Whereas
tones can be fairly well described by giving a few parameters, many percussive
sounds defy simple description. As is the case with tones, direct digital
techniques offer considerably more freedom in the synthesis of percussive
sounds than do analog techniques. Percussive sound generation is such a
large topic that only a brief introduction can be offered here. Nevertheless,
the techniques discussed should be suitable for a wide variety of percussive
sounds.
Types of Percussive Sounds
Out of the infinite variety of percussive sounds, it is possible to define
roughly four categories. Type 1 sounds are those that are basically sine wave
tones with a suitable amplitude envelope. Any of the synthesis techniques
covered previously are quite adequate for generation of the tone component of
the sound, while direct computation or table lookup is suitable for the
envelope. Nearly all sounds in this group have a moderate to strong sense of
pitch due to the periodic foundation. Familiar instruments producing sounds
in this group are wood blocks, daves, orchestral bells, and bongo drums.
Type 2 is similar, but the underlying "tone" consists of several,
nonharmonically related sine wave components. Most free (without snares)
drums produce sounds in this category when struck with moderate force by a
padded drumstick. Unlike sttings and metal bars, the various vibration
modes of a drumhead do not correspond to integrally related frequencies.
Again, the previous synthesis methods can be used to produce the basic tone
to which an amplitude envelope is added.
Type 3 sounds are best described as filtered, enveloped noise. In many
cases, the instrument physics are basically the same as for Type 2, but the
number offrequency components is so large that the sound resembles random
noise. In other cases, the instrument operates by means of scraping or rat-
527