CHARACTERISTIC POINTS OF SYNTHESIZERS
The function of the synthesizer is to electrically
synthesize and control the three elements of sound,
that is, interval, tone and volume as well as the timewise
change from the instant asound is generated to the
point it fades away.
The interval is controlled by the VCO, tone by the
VCF, volume by the VCA and the timewise variation
of sound by the EG, "VC" in VCO, VCF and VCA
stands for "voltage-controlled". Now, let us consider
what kind of function this voltage control has in the
synthesizer. The drawing at the right is the block
diagram showing the basic composition of asynthesizer.
•KEYBOARD CIRCUIT
This circuit generates the key's voltage (KEY
VOLT) that corresponds to the interval, and the
signal voltage (TRIGGER) that indicates that the
key is being depressed. Although the TRIGGER will
become "o" volt when the finger is released from
the key, the KEY VOLT that indicates the interval
will be memorized by the SAMPLE AND HOLD
(S/II) circuit until the next key is depressed.
The drawing at the right shows the relationship
between the key's interval and KEY VOLT voltage
together with the wave forms of the TRIGGER and
KEY VOLT signals mi (E) do (C) -* sol (G)
are depressed.
•VCO (Voltage control oscillator)
The VCO makes asound source of afrequency
that corresponds to the KEY VOLT of the key-
board circuit. It oscillates simultaneously such sound
source wave forms as saw-tooth waves and square
waves that have agreat number of harmonics and
sine waves of asingle frequency. Although the
oscillator frequency corresponds linearly to the KEY
VOLT value, if the oscillator should be controlled
while adding an AC voltage such as low frequency
oscillator (LFO) to the KEY VOLT voltage, it will
be possible to modulate the key interval with the
LFO, or the like. The function of adding this voltage
is carried out by an adder. Regarding this voltage
value that is used to control the VCO, it is possible
to freely control the VCO's oscillating frequency
in accordance with the voltage value since asyn-
thetic voltage formed by several AC wave forms is
used instead of afixed DC voltage.
The drawing at the right shows acase of VCO
control in which the sine waves of the LFO is
added to the KEY VOLT. Needless to say, it is
also possible to add an EG voltage in place of, or
simultaneously with, the LFO.
•VCF (Voltage controlled filter)
The VCF, which is the heart of the synthesizer,
VCO VCF VCA
KEYBOARD FOLLOW
EG
KEY VOLT
KEYBOARD
EG
TRIGGER
VOLTAGE KEY VOLT
C
PITCH
HOLD HOLD
mm TRIGGER
E| I[c
EG C
132
^KEYON
L-KEY OFF
-KEYON LKEY OFF
PITCH
VCO MOD
CONTROL VOLT
LFO
KEY VOLT
VCO OUT
