Gakken SX-150 Assembly And Use Manual - Page 4

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Operating Principles for

the Supplement's Synthesizer

The electric circuit in the supplement's Synthesizer is made up of a

combination of several circuits, each performing a simple operation.

Breaking the circuit down at this level would yield a configuration like

the one shown in the diagram below. Each of the blocks in the diagram

is referred to as a function block. The function blocks send requests to

each other to operate as a synthesizer. The requests that the function

blocks send to each other are called "signals." The signals used in the

supplement's Synthesizer can roughly be divided into two groups of

"sound signals" and "control signals."

This may seem a little bit difficult to understand, but it might be easier

if you think of it in terms of a piano. Roughly speaking, a piano is made

This part

corresponds to the

keyboard in a regular

synthesizer.

Controller

Gate

* Here, this explains the

electrical characteristics

of the supplement's

electric circuit. Please

read from page 76 for a

more general

description of the

Structure of the supplement's Synthesizer

Synthesizer.

EXT.SOURCE

F/V

LFO

LFO WAVE

Gate

TOUCH PEN

Controller

ATTACK

DECAY

PITCH ENV

Envelope generator

Written by: Gan

up of the following:

A mechanism for generating sound

(Strings wound with piano wire Sounding board)

A mechanism for controlling how sound is released

(Keyboard/hammers/dampers)

The supplement's Synthesizer is similar to this, where sound signals

are the mechanism for generating sound and control signals are the

mechanism for controlling how the sound is released.

LFO WAVE

LFO

LFO RATE

CUTOFF

RESONANCE

VCO

VCF

Amplifier

PITCH ENV

Sound signal

Envelope

ATTACK

Control signal

DECAY

generator

Electric circuit diagram for the supplement's Synthesize

LFO RATE

VCO

CUTOFF

OUTPUT

POWER

RESONANCE

VCF

Amplifier

So, what happens inside the supplement's Synthesizer once you

actually start playing sounds using the electrode?

Once you start playing sounds with the electrode, the controller

generates two types of control signals. One signal is referred to as the

control voltage (CV) and is a signal for controlling the pitch of the

sound. The other signal is referred to as a gate and is a signal for

controlling ON/OFF for the sound. The CV changes according to the

position of the electrode on the slider panel. The gate turns ON when

the electrode touches the slider panel. These two signals are sent to a

voltage controlled oscillator (VCO). The VCO generates a sound signal

while the gate signal is ON. The pitch of the sound is determined by

the CV. If the output of the VCO is left as is, the only sound that will be

produced is a monotone buzzer-like "bzzz" sound. To change this, this

signal is passed through a voltage controlled filter (VCF). Of the

frequency components of the sound signal, the VCF removes only

those from the high pitch region and modifies the tone. At this time,

the sound is not only merely processed, but application of the filter is

changed in response to the amount of time that passes since the gate

starts working, and the tone of the sound is changed. This variation in

time is controlled by a signal from an envelope generator (EG). The EG

How a VCO works

A VCO is made up of a part for charging and discharging

capacitors, a part for monitoring the charging and

Sound

discharging, and a discharge switch. Charging is done

output

slowly at a speed that is based on the CV. Once a certain

amount of electricity has built up in the capacitor, the

monitoring part responds and turns on the discharge

switch, and the electricity is discharged at high speed.

The waveform output is a sawtooth wave.

How a VCF works

The basic structure of a VCF is formed of transistors

and capacitors. It may be easier to understand how

a VCF works if the transistors are replaced with

resistors. Capacitors have a property that allows

them to pass signals with high frequencies more

easily, so, only those components of high pitch are

removed, out of all of the sound signals. If the

control signals applied to the transistors are

changed, the frequency at which the filter begins

to be applied will change. Sounds passed through

the VCF become softer and the waveforms become

more rounded. This supplement employs a Korg

MS-20-type filter.

Power supply

Sound signal

input

Considering what would happen if the

transistors were replaced with resistors...

Sound signal input

Speaker

generates a signal with a pattern that decays after it has risen once in

response to the gate being turned ON. You can adjust the rising times

and the decay times and use the results to produce different kinds of

variant patterns. The supplement's Synthesizer has one more control

signal generator, which is referred to as an LFO. The LFO will make the

pitch of the sound of the VCO vibrate by generating a control signal

with a repeating pattern. You can produce a variety of sounds using

combinations of these functions.

η Waveform output from the VCO.

In the low pitch

Control signal

Sound signal input

Sound signal output

region, signals do not

Sound signal

pass through the

output

capacitors very easily

Sound signals

pass through

unaffected

In the high pitch

Sound signal input

Sound signal output

region, signals pass

through the

Sound signal output

capacitors quite easily

Sound signals

decay

Analog Synthesizer

Discharge switch

Performing

charging

and

discharging

Monitoring

amount of

charge

Control signal

VCO output

How an Envelope Generator (EG) works

An envelope generator (EG) obtains a

control signal that varies with time through

charging and discharging of a capacitor.

Charging

Output

Discharging

Signal level

Time

Charging

Discharging