of Microprocessors

552 MUSICAL ApPLICATIONS OF MICROPROCESSORS
DRAWING
STYLUS\
l,..----_/::::::::=t'l..,
\1~~""'~I~::;:I:ILlE
ELECTROSENSITIVE
DRUM
~ RECORDING DRUM
MAGNETIC
HEAD
I
HDRIVE
UMOTOR
TUNE
_AUDIO
INPUT
Fig. 16-4. Kay electric sound spectograph
The amplitude of the filter outputs is sampled periodically to provide the
spectral data. If the center frequencies and Qfactors of the filters are chosen
properly, all possible frequencies in the band of interest will excite at least
one filter.
Any number of analog methods for directly realizing or simulating such
a structure have been devised. Probably the most interesting is that utilized
by Kay Electric Company in their Sound Spectrograph. The basic parts of the
machine shown in Fig. 16--4 are the magnetic recording drum, the facsimile
image drum, and a single tunable bandpass filter. The recording drum is
mechanically coupled to the facsimile drum such that they rotate in unison.
In use, up to 2.4 sec of sound can be recorded on the surface of the
magnetic drum. To plot a spectrogram, a piece of electrosensitive paper
(turns dark when current is passed through it) is wrapped around the facsimile
drum and the machine is started in playback mode. The audio signal
from the drum goes through the bandpass filter and then directly to the
writing stylus. High-amplitude filter outputs create a darker trace than
low-amplitude outputs. A leadscrew causes the writing stylus to gradually
move along the length of the drum and at the same time increase the center
frequency of the filter. Thus, the audio signal is serially analyzed at a large
number of center frequencies outside of real time. An amplitude-quantizing
attachment is also available to replace the gray-scale plot with a contour plot.
Most analog spectrum analyzers, however, use a bank of bandpass
filters so that the spectrum analysis is performed in parallel in real time. The