of Microprocessors

768 MUSICAL ApPLICATIONS OF MICROPROCESSORS
One very inexpensive ($29) device marketed by Radio Shack is a twooctave
electronic keyboard with built-in battery and speaker. It has a number
ofshort single-note melodies already built into its memory that can be played
automatically plus provision for user playing and storage (until power is
switched off) of new note sequences. While the preprogrammed melodies
have proper rhythm and accedentals (sharps and flats), note durations are lost
when they are programmed by the user, and the keyboard has only white
keys. Thus, the unit is strictly a toy, although the tone, which is a
rectangular wave, is solid and robust. Only one IC, which is a reprogrammed
calculator chip, and a few other components are present on the single printed
circuit board, which also includes the keyboard contacts.
Casio is a company best known for calculators and digital watches. In
the mid 1970s, in an attempt to inject some excitement into the calculator
business, it began offering calculators with built-in digital clocks. The most
popular models also had melody-playing alarms and a mode in which each
keypress would play a different note and the "=" key would play our the
entire answer through a piezeoelectric speaker. This later evolved into a line
of keyboard musical instruments that ranged from a $50 monophonic
calculator/clock with a "real (miniaturized) music keyboard" and dynamic
speaker to a $500 8-voice polyphonic wood-cased unit with full-sized fouroctave
keyboard, plus a number of models in between. Although there were
no headlines in the popular press, these units stunned low-end synthesizer
makers and experimenters (such as the author) with the capability that was
being offered at prices less than one-quarter of prevailing levels.
Probably the most impressive member of the line, from a
price-performance standpoint, is the Casiotone MT30. This $150 unit
comes packaged in a 23-inch long plastic case with a three-octave, almost
standard keyboard (key spacing is 85% of normal), 4-inch speaker, and
ourpLit jack for an external speaker or amplifier. It is eight-note polyphonic
and has a repertoire of 22 preprogrammed timbres. Although there is no
provision for user creation of sounds, there are vibrato on/off and sustain on/
off switches. The more expensive models differ by providing standard-sized
keys, more keys, variable controls for effects like vibrato and sustain, and an
"automatic arpeggio feature," in which a short keyboard sequence can be
memorized and then repeated automatically while a melody is played over it.
Upon disassembling the unit, one finds a small (5 X 6 inch) circuit
board with seven integrated circuits and about 75 discrete components
mounted. One of the integrated circuits is an audio power amplifier, while
another is a dual op-amp. Four more are CMOS hex inverters, which leaves
just one 64-pin chip to do all of the work. Part of the keyboard switch
matrix, which includes a series diode for each key, is mounted on this board,
while the remainder is on another 3 X l1-inch board. The contacts are
actually conductive rubber pads that bridge two parallel traces when a key is
pressed. Both boards are single-sided.