In the last few decades, the world has seen the synthesizer rise from
an
experimental oddity to a mainstay in today's music world. This
electronic instrument
crosses over into almost every style of contemporary,
popular music from country
to jazz to techno. The synthesizer has brought
electronic music a long way from its
archaic beginnings when electronic
musicians played with vacuum-tube circuitry and
tedious tape splicing. The
invention of the synthesizer has made it easy for the
electronic musician
to create new and imitate familiar sounds with relative ease.
But just how
does the analog synthesizer create electronic sound and further more
how is
it amplified in recording situations? If you want to know the answers to
these questions... keep reading... we're about to find out.
Synthesizer technology is broken up into two categories "analog"
and "digital".
However, with the addition of the digital analog
modeling synth, things aren't as cut and
dry as it used to be. But... In
order to get a good basic understanding of how sound in a
synthesizer is
produced, the old fashioned analog system will be discussed here.
A typical analog synthesizer consists of a keyboard, usually at least 2
oscillators, a low
frequency oscillator (LFO), usually 2 envelope
generators (EG), a filter and a
voltage control amplifier (VCA). Playing
the synthesizer involves routing audio and
control signals from the
keyboard through the various components listed above (Yelton 49).
When a key is depressed on the keyboard a pitch control signal is sent
to the oscillators
(VCO- Voltage Control Oscillator). An oscillator is a
circuit that creates a single periodic
wave form at a desired frequency
(Moog 17). The oscillato generates the desired
frequency and wave form and
then routes an audio signal to the Voltage Control
Filter (VCF)(49) .
Before we contrinue on the route the audio signals thake within
the
synthesizer, let's discuss waveforms for a moment
.
Analog oscillators usually offer a limited variety of waveforms. The
most common are the
Sine, the Triangle, the Pulse, and the Sawtooth. Sine
waves are the simplest of waves.
They are pure and contain no harmonics.
Sine waves are known for their hollow and
woodwind-like sound. Triangle
waves rise linearly and fall linearly at the same rate (17).
Triangle waves
are brighter sounding than Sine waves because of the sharp edges in
their
wave form, but they are not as sharp sounding as the Pulse and Sawtooth waves.
Pulse waves rise abruptly and stay level for a period of time and then fall
abrubptly and
stay level. The percentage of time it takes for it to
complete its cycle is called its "pulse
width". The Pulse wave is known for creating a variety of tone colors.
Since it has periods
where the wave is flat, there are spots where certain
harmonics will be stronger than others
(17). Sawtooth waves rise linearly
anddrop abruptly every cycle. These waves have high
harmonic content and
are known for their bright, "buzzy" full sound.
Meanwhile, the keyboard has also sent control signals to another part
of the synthesizer,
the envelope generators. There are usually 2 envelope
generators in a synthesizer. One
controls the Voltage Control Filter (VCF)
and the other the Voltage Control Amplifier
(VCA). I''ll explain what these
two devices do in a moment, but first lets talk a minute
about the the type
of control signals that the keyboard is sending to these devices.
The first
control signal that is sent to the envelope generators is called a "trigger"
(Yelton 35). The "trigger" sends a message to the generators
telling them a key has been
pressed. This begins the envelope generator's
process of creating an envelope for the
wave form being generated by the
oscillators. As long as the key on the keyboard is
held down, another
control signal, the "gate" is sent to the envelope generators. The
"gate" signal tells the generators that the note is still being played
and the envelope
being generated will stay open until this "gate"
signal ceases. When the "gate" signal
stops, the envelope will go
into it's final stage (29). In some analog synths you will
find that the "gate"
signal will double as the "trigger" signal as well.
Envelope generators are used to help the synthesizer produce expressive
sounds (Moog 27). They allow you to shape sounds by sending a varying control
signal to the synth's VCA and VCF. Like I mentioned earlier there are
usually two
envelope generators in a typical analog synth. The output of
one generator is
hardwired tp the Control Voltage (CV) input of the VCF,
and the output is wired
to the CV input of the VCA. Having the generators
set up like this allows you to
have control over the brightness and
loudness of the sounds that the synth produces.
The envelope connected to
the VCA tells the amplifier when to open, how far to
open, and when to
close (Yelton29). The other sends a control signal to the VCF
telling it
how to filter the incoming audio signal. The most common type of
envelope
generators are the ADSR type (Attack, Decay,
Sustain,
Release). The Attack of the envelope determines how
much time
it will take for an audio signal level to go from zero to maximum
level when a
key is struck (29). For the envelope that is controlling the
filter, the
Attack
controls how long it will take the filter to open to it's
maximum brightness
(35). The Decay controls how long it will take
for the envelope
to fall to it's sustain level and determines how far to
close the cut off frequency
in the filter before it falls to it's sustain
level (29). The Sustain of the envelope
controls how much signal
can pass through the VCA when the note is being held
and tells the VCF to
hold brightness at a steady level until the key is released.
The Release
stage of the envelope starts when the gate signal to the
envelope generator
ceases. The amplifier's output is dropped down to zero at the rate
determined
by the Release parameter (29) and the VCF returns to it's original
cut
off frequency (35).Other types of envelope generators found in analog
synthesizers
are the AR (Attack & Release), DADSR (Delay, Attack,
Decay, Sustain, Release)
and ADS (Attack, Decay, & Sustain) found in
the Minimoog.
.
The basic concept of analog synthesizers is a technique called "subtractive
synthesis".
Subtractive synthesis is the act of manipulating tone
color by filtering complex wave
forms (Strange 33). Basically, it means
taking all of the audio signal that the oscillators
produce and
manipulating them to sound the way you want by "subtracting"
frequenceies
that are undesirable and "re-shaping" the wave form.
Almost all analog synthesizers
have atleast one VCF (Voltage Control
Filter), and they most often are low pass filters
(Yelton 29).
Low pass filters do exactly as the name implies-- they let the low
frequenciespass.
If you turn a lowpass filter's frequency all the way
down.. you close the filter and
nothing can ps. When you manually open it
up a little at a time, you sou slowly raise the
cut off frequency, and will
hear the lowest frequencies first (29). If the filter is already
wide open,
it will have no effect (35). Other types of filters sometimes found on analog
synths are highpass filters that filter out low frequencies, bandpass filters
that let a certain
band width pass and filters out low and high unwanted
frequency bands, and bad reject
filters that stop a narrow band width from
coming through.
It would seem at this point that the audio signal that the synth has
created is ready to be
passed out of the machine and on to human ears, but
there is still one more important
step. That is the use of the LFO (Low
Frequency Oscillator), also called the sweep
or modulation generator, to
modulate the signal. Modulation is the act of using one
waveform to shape
or texture a property of another waveform (Moog 31). The LFO
produces sine
and triangle wavesin the sub-audio range, usually below 9Jz, that contour
the
shape of the waveform they are sitting underneath. The LFO can be routed to
modulate almost any module of the synth (Leonard 36). An LFO routed to the
VCO
creates a change in pitch (vibrato). A LFO routed through the VCF will
alter the tone
color of the sound (trill). Modulating the VCA will change
the amplitude(tremolo) (36).
Other types of modulation modules that aren't
available in every synthesizer are: the ring
modulator, which acts on the
principle of amplitude modulation, and the random note generator, also called
sample & hold (58).
Once the waveform has been created by the oscillator, run through the
envelope
generators, VCF, VCA and LFO, the signal is ready to leace the
synth and become an
actual heard sound. Due to the electronic nature of the
synthesizer, microphones are not
used in recording it's sound. Generally,
synthesizers will plug directly into the mixing
console for recording.
Synthesizers are high impedance devices, and while it is possible
to plug
them into the mixing desk using 1/4" phone plugs, it is generally advisable
to
change the signal to low impedance through a device called a "direct
box" and then
plug the synth into the console with XLR cables. The
reason for this is that low
impedance outputs provide a cleaner signal and
are less likely to pick up extraneous
noise. This is especially important
for live performance situations.
In summary, we have seen how sound is created in an analog synthesizer.
When a
key on the keyboard is depressed, control signals are sent to the
VCO, and the two
envelope generators. The VCO sends out a specific waveform
that is modulated by the LFO and it goes to the VCF that is controlled by the
envelope of one of the envelope generators. The VCF controlls the timbre of the
wave. The audio signal is then sent to the
VCA which is controlled by the
envelope of tahe second envelope generator. The VCA controls the amplification
of the sound. Once the sound is created it can be plugged into
an external
amplifier or console using 1/4" phone jacks or the preferred low impedance
XLR cables by running the output through a "direct box".
By understanding the basic principles behind the analog synthesizer, how sound is produced and how to record it, the electronic musician can better manipulate electronic sound. It also gives him/her a solid base in which to expand on his knowledge of other synthesizer concepts, such as digital synthesis. The more familiar a synthesist is with how his/her instrument works, the more creative he/she can be, giving the synthesizer a chance to be a popular instrument (read world domination here!) in modern music for many years to come.
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