Pictures
Keyboard Circuit Board (top)
Keyboard Circuit Board (bottom)
Theory
The AK-900 electronically creates sounds that closely resemble those
from traditional musical instruments. A quick look at the schematic shows
that the IC 9037 is the heart of the product. The IC 9037 is specifically
designed for electronic keyboard applications. It generates a 523.252 kHz
signal which is used as a master timing reference for all operations. The
37 keyboard keys represent musical notes C3 (130 Hertz) to C6 (1047
Hertz). These are connected directly to the IC 9037 and may be played
anytime. These tones are produced by dividing the 523.252 kHz reference
down to the appropriate frequency and the adjusting the levels of the
overtones based on which instrument is being played. When you play a note
on an instrument the sound produced is initially rather loud then
decreases with time. There are two sets of the circuits just described to
allow two notes to be played at the same time; one additional notes are
played the earlier notes will be discontinued. Since the two most recent
notes will be the loudest, this simplification from traditional
instruments (such as a piano where the strings continue to vibrate until
played again) will not be easily noticed by the listener. More complex
electronic instruments will have more circuitry to simulate more notes at
the same time as well as more advanced techniques for producing overtones.
All of the button selections are handled using ten control lines in a
matrix. A matrix is similar to the rows and columns of a table. On
power-up, the rhythm is stopped, rhythm selection is set to Pop, timbre
selection is set to Piano, Tempo is set to medium, and Vibrato is OFF.
Percussion (drum-like) sounds are created by playing a short pattern
stored in the IC 9037's electronic memory. All four percussion sounds may
be played at the same time. If the demo or one of the rhythms is selected
then a much longer pattern is played from the part’s memory in a similar
manner. This will include both the tone and percussion circuitry just
described and will be repeated until the user presses the stop button. The
tempo is adjusted by changing the rate at which this tone pattern is
played from memory (by dividing the 523.252 kHz timing reference
differently). The vibrato effect is created by varying the divide ratio to
the tone circuitry slightly, which will vary the tone frequency and hence
the pitch of the sound.
The outputs from the two tone circuits and the percussion circuits are
combined. The two volume control switches adjust the strength of the tone
and percussion signals before combination; weaker electrical signals will
result in weaker sound waves from the speaker. The combined result is
smoothed (to remove unwanted higher frequency signals that were created
along with the desired tone and percussion signal), increased in strength
(amplified), and applied to the speaker where it is converted from
electrical waves to sound waves.
Problems
The only issue came in the form of incorrectly formed keys. The inside
contacts were not large enough to reach the triggers one the circuit
board. A related issue came in trying to add width to the contacts, and by
trial and error going from too little, to too much, and then to the final
form, that of a working keyboard. |