 |
|
| |
 |
|
|
Analog
synthesis
Words by H.M Sowards |
|
| 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. |
|
 |
Category
| TechTalk: Analog synthesis Added: 09/02/07 |
|
| |
Home |
News |
Products
| Downloads
| Shop
| Tech
Talk | Support
| Distributors
| About us
| Contact us |
Links
| |
|
|
|
