of Microprocessors

734 MUSICAL ApPLICATIONS OF MICROPROCESSORS
machine with 88-key velocity-senstive wooden keyboard that has been fitted
with weighted throw-bars in an attempt to duplicate the feel of a real piano
keyboard. It has 12 standard orchestral sounds built in including grand
piano, string section, trumpet, acoustic guitar, and Hammond organ; 14
percussion instruments including all kinds of drums and cymbals; an
"endless glissando"; and a sine wave. Add-on ROM "voice modules" allow the
repertoire to be expanded with instruments like a full chorus, both male and
female. The control panel is devoted mostly to setting up various keyboard
algorithms and splits even to the point of each key controlling a completely
different sound. Splits can also overlap, which allows some keys to control
twO Ot more instruments playing in unison or at a constant interval. Limited
modulations of the stored sounds such as pitch bending, vibrato, and
"brightness" variations are also possible.
Internally, the Kurzweil 250 uses no fewer than 68 masked ROM chips
of 256K bits each for a total of over 2M bytes of storage for the standard
sound repertoire. A lO-MHz 68000 microprocessor controls everything.
Digi'tal circuitry resides on two very large (15 X 17 inches) circuit boards,
while a surprisingly large amount of analog circuitry fills another board of
similar size. Much of the analog circuitry is devoted to 12 completely
independent voice channels where each channel includes a 12-bit CMOS type
D-to-A converter, a sample-and-hold deglitcher, a programmable low-pass
filter, and a programmable gain amplifier. The sample rate of each channel is
also variable according to the instrument and note being played and ranges
from 10 to 50 ks/s. A typical instrument's range is broken up into
approximately 10 subranges with those at the extremes covering somewhat
more than an octave, while those in the middle are about one-half octave
wide. Different pitches within a range are produced by varying the sample
rate. Sounds are stored as an attack portion, a sustain portion, and sometimes
a decay portion. During exceptionally long sustains, the stored sustain
portion, which averages about 3;4 sec, is repeated. It appears that waveform
coding is used for data compression, and the compression ratio is estimated
to be in the 4-to-l range for instruments like the string section.
To the author's ears, the accuracy of emulation is, for practical
purposes, perfect. While the piano, violin, and other instruments actually
recorded to prepare the voicing ROMs may not be the best in the world, they
are probably better than what a potential buyer would otherwise have access
to. If one knows exactly what to listen for and devises a suitable test score,
some of the data compression artifacts can be detected. For example, with the
piano sound, one can tell where the subrange boundaries are, while with the
string section, repetition within long sustains can be detected.
The Kurzweil-250 seems to appeal mostly to people with a classical
music background who might otherwise purchase a grand piano or large
organ for their home. Aside from curiosity and challenging the company's
slogan, "you can't tell the difference," the usual keyboard synthesizer