Resonant Frequency: Article Archive

An analog synthesizer is basically a collection of many types of circuits. Even a complete monosynth (like an Arp Axxe, Roland SH-101, etc) uses the same "building blocks" that can be found in a modular system. Each circuit has it's own specific function in building the waveforms of the sounds you hear. I hope this is a good starting point in helping you understand the basics of analog synths.

The Keyboard:

Most analog synths have a 2 switch per note keyboard. What that means is there are 2 little switches underneath each key. Pressing the key will close the two switches simultaneously, and cause certain voltages to appear at the keyboards two outputs.

One switch usually connects to a resistor ladder. This series of resistors connects all the keys together, and puts out a specific voltage corresponding to each key. Usually the keyboard produces1 volt of output per octave. This voltage is called a keyboard control voltage, and gets connected to another part of the synth called the VCO.

The second switch under the key produces a trigger or gate voltage. This is basically a specific voltage that appears on the second output regardless of which key is pressed. This is used to notify the envelope generator of the key press.

VCO: Voltage Controlled Oscillator

This is one of the most important portions of the synthesis process. The VCO is a circuit that produces the pitch of the note played. Generally speaking, a VCO has keyboard input, where you'd connect the appropriate type of keyboard controller. The keyboard sends out a specific voltage for each key on the controller. The VCO interprets this voltage, and outputs a signal of the correct frequency for the key pressed. Most VCOs will have both coarse and fine tuning dials to calibrate the synthesizer to play at the correct pitch.

There are 2 different types of VCO's... some accept inputs that have a Volt/Hz response (A Linear VCO), while others will have the more common Volt/Octave response. Generally, Korg synths and the PAIA Fatman are linear VCO designs. Most other common synths use the V/Oct standard.

Also, many VCOs will output multiple waveforms. These can be a combination of any of the following: sine, pulse (square), triangle, sawtooth and reverse sawtooth. The wave output is often connected to a VCF.

ADSR: Envelope Generator

The output of the VCO in your synth is basically just a signal that is on or off. Connecting a VCO to your amplifier will basically just play a beeping or buzzing type of sound at the pitch you're playing on the keyboard. The envelope generator is the portion of the synth that controls the level of the VCOs output over time. The envelope generator gives the sound it's definition. The letters A-D-S-R correspond to the different portions of the envelope which are described below:

A - Attack: This is how quickly the volume of the note rises as you press the key. A quick attack is a more percussive or abrupt note-on, while a long attack makes the note fade in slowly the longer you press the key.
D- Decay: This is the immediate drop in level after the peak level is reached in the attack. A short decay means the note volumes drops off very quickly after being struck, while a long decay means the note will fade more slowly.
S - Sustain: This is basically the "ring" of the instrument as it sustains after the note is played.
R - Release: This is the amount of time for the final fade out of the note after the sustain.

An envelope generator usually has a knob for each of the 4 functions, although some are a little more limited, and may be simply an AD- or ADR-style with less control possibility. These simpiler types of envelope generators typically control filters, etc.

The Envelope generator will accept an input from your keyboard. Depending on what type of synth you have, this may be a trigger or gate signal. Whenever the ADSR receives a signal from the keyboard, it outputs the wave shape you defined with the 4 knobs. This wave or envelope is sent to the modulation input of the VCA, or the cutoff or resonance inputs on a VCF

VCA: Voltage Controlled Amplifier

The voltage controlled amplifier is the device that controls the volume of pitch developed by the VCO, by following the envelope generated by the ADSR to produce the actual synthesised sound.

The VCO is connected to the signal input, while the ADSR is connected to the modulation or control voltage input. The ADSR's waveform adjusts the level of the output of the VCA, much like you'd adjust it with a level control.

LFO: Low Frequency Ocsillator

This is similar to a VCO. It also an oscillator that produces a wave shape, but it will have a reduced bandwidth. A typical LFO will produce a wave with frequencies from somewhere below 1 Hz to possibly 100Hz. Again, like a VCO, LFO's will often have several output wave forms to produce different sounds.

An LFO generally isn't used to make a tone, but instead as a modulation source. Modulating basically means changing something over time. This is pretty essential in synthesis, otherwise the sounds will sound less interesting and more mechanical.

Examples of modulation:

If you were to modulate a VCO, you would be slightly varying the pitch. ;Since a VCO determined the pitch of a signal, this would be a vibrato effect. The depth of the modulation is set by the output level of the LFO. A higher voltage from the LFO produces a larger sweep of the pitch. A small signal may vary the signal only a few cents, while a larger signal may produce a sweep of an octave or more. The rate of the vibrato is determined by the frequency setting of the LFO. This determines how rapidly the VCO sweeps from the highest to lowest pitch.
LFOs can also be used to modulate VCAs, too. Since a VCA controls the overall volume of the sound, modulating it with a LFO will add some variation in volume, or a tremelo effect.
Your LFO can also modulate other circuits we haven't talked about yet; the most notable ones being your VCF's.

VCF: Voltage Controlled Filter

A filter is essentially the same as an equalizer circuit. They come is several varieties including band pass, low pass and high pass. As common sense would show, a low pass filter only allows low frequency signals through, while a band pass will let a specific mid-frequency range through, and a high pass will pass high frequencies.

They way a VCF varies from a normal equalizer is the fact that it has a modulation or control voltage input. When the CV input gets a changing voltage, the filter alters the center frequency of the pass band. Essentially, the changing voltage "sweeps" the parametric band that the filter responds to up or down the frequency spectrum.

Many synths also route the keyboard pitch CV to the filter. This is called "key tracking" or "key follow". The reason is simple if you consider a typical lowpass filter. If the cutoff is set low, the filter will only allow bass notes to pass, while high pitched sounds are blocked. If you were to play a high note, you wouldn't hear it. Making the cutoff follow the keyboard pitch means the filter will "open" more when playing higher notes, allowing you to hear them.

How it all Ties Together:

When you press a key on the keyboard, it creates a control voltage from the key you pressed, and a trigger or gate signal. The control voltage is sent to the VCO, and determines the pitch of the note played. The output of the VCO is then sent through the VCF's audio input and on to the VCA.

Simultaneously, the trigger or gate fires the envelope generators for controlling the VCA and VCF. Both EG's send out their envelope. One envelope generator (typically an ADSR) makes the VCA open and swell in volume to make the appropriate sound. The other envelope generator (typically AD) adjusts the VCF to gve it it's charachteristic timbre.

The output of the VCA is then connected to your mixer, amplifier, etc.