of Microprocessors

MUSIC SYNTHESIS PRINGPLES 9
VERY
SMALL
MIRROR
VERY
FINE
WIRE
/
'~~:rL
ROO
'-.... JEWELED
BEARING
MOVING
PHOTOGRAPHIC
FILM OR
PAPER
t
\
INCIDENT
SOUND
Fig. 1-1. Mechanical soundAwaveform-tracing apparatus
Steady, unchanging sounds fall into two classes, pitched and unpitched.
The two classes are not completely disjointed, however, since there
are some sounds that possess characteristics of both groups. As will be shown
later, all steady sounds can be described by a number of parameters that are
also steady. In all cases, these parameters are scalar quantities, that is, a
simple number with corresponding physical units. Changing sounds (all
sounds change to some degree--otherwise they would have no beginning or
end) are similar to steady sounds except that the parameters that describe
them change with time.
One way to visualize a sound is to show its waveform or the manner in
which air pressure changes with time. Before the discovery of electronics,
mechanical instruments similar to the one shown schematically in Fig. 1-1
were used. Movement of the air caused by sound vibrations would be picked
up by a diaphragm and cause a tiny mirror to rotate in synchronism with the
vibration. A light beam reflected from the mirror onto moving photographic
film would make a visibile waveform trace. The distance from the mirror to
the film acted as a long but massless lever arm to effectively amplify the
vibrations.
Of course, sound waveforms may now be photographed on the display
screen ofan oscilloscope. In order to do this, the sound is first directed into a