Analog synthesizers (and software synthesizers that emulate their design) have a jargon. Before looking at the synthesizers themselves, here’s a brief glossary of terms commonly found in Serge, ARP, and other modular synthesizer documentation:
Control Voltage / Gate (CV/Gate) - a common schema for transmitting signals around a synthesizer that are not meant to be heard directly, but are meant to control and trigger sound-producing modules. When the control voltage in question represents musical pitch, both Serge and ARP systems support a standard scaling of 1 volt-per-octave (over a range of 0-5 volts on Serge systems, 0-10 on ARP systems). In addition, Serge synthesizers feature a linear voltage system that is usually paired with a scaling knob. When the control voltage represents a pulse, trigger, or “gate”, then it is treated similar to digital signals, with 5 volts (Serge) / 10 volts (ARP) being HIGH and 0 volts LOW. The ARP’s higher voltage range means a pulse from a Serge synthesizer won’t fire the trigger/gate inputs on an ARP 2600 without amplification.
Voltage Controlled Oscillator (VCO) - an oscillator with a variable frequency controllable by an external voltage source. This is a core component of almost any analog synthesizer. Analog oscillators can often generate a variety of waveforms depending on their design. Serge systems have only a few true oscillator modules, preferring instead to leverage “patch programmability” to repurpose slope and timing generators to create additional oscillators; the ARP 2600 has three VCOs, all with slightly different capabilities. Analog oscillators can generate a variety of waveforms depending on their design, and all ARP VCOs can generate more than one kind of sound. Typically, a VCO’s circuit is designed such that its “oscillator core” generates a sawtooth or triangle wave, with the other waveform outputs constructed via signal processing.
Voltage Controlled Filter (VCF) - a filter with one or more parameters that can be adjusted dynamically by an external voltage source. Filter modules are a core element in a subtractive synthesizer, allowing you to carve away parts of the frequency range of any input signal, and as such are considered hallmark components of “East Coast” synthesizers. The Serge system has a number of different kinds of VCFs, most of which function as state-variable filters, with the parameters adjustable through voltage control; the ARP 2600 has one VCF which functions as a resonant low-pass filter, with a VC-controllable cutoff frequency.
Voltage Controlled Amplifier (VCA) - an amplifier circuit where the amount of gain or attenuation applied to the incoming signal can be varied by an external voltage source, similar to a transistor. Serge systems have a number of VCA modules, both to serve as classic “note” generators but also to allow voltage-controlled mixing; the ARP 2600 has a single VCA.
Low Frequency Oscillator (LFO) - an oscillator used to generate low-frequency control voltage rather than an audible audio signal. Serge systems avoid making the distinction beteween audio- and control-rate oscillators and signal-generators, so most Serge modules that generate repeatable signals can serve as LFOs; all three VCOs on the ARP 2600 can be used as an LFO.
Envelope Generator - a circuit that generates time-varying control voltage intended to have a start and an end (as opposed to an oscillator). These circuits often output these voltage “envelopes” according to a common design, such as an ADSR (Attack/Decay/Sustain/Release) envelope. Most Serge systems have generic “slope generators” and slew limiting circuits instead of complex envelope generators; the ARP 2600 has two envelope generators (an ADSR and a simpler AR) that have a common trigger circuit.
Envelope Follower - a circuit that takes an audio signal and rectifies and low-passes it to generate a control voltage slope that follows the contours of the original signal. These circuits are often used to allow an acoustic instrument (via a microphone) or an electric instrument (via a pickup) to control a parameter on a synthesizer. They are also used in audio processing to generate the “key” signal for a dynamics processor, such as a compressor, limiter, or noise gate. The Serge Dual Universal Slope Generator can be used for a wide variety of envelope following tasks; the ARP 2600 has an envelope follower, designed to use with an external audio source.
Ring Modulation - a simple signal processing technique where two audio signals are multiplied - the result is a signal with additional sideband frequencies resulting from the sum and difference of the harmonic content of the two signals. If one of the input signals is rectified, it is referred to as Amplitude Modulation. Serge systems have both dedicated ring modulator modules as well as VCA circuits that support amplitude modulation (to simulate musical “tremolo”); the ARP 2600 has a ring modulation circuit that can do both ring and CV-based amplitude modulation.
Frequency Modulation - a signal processing technique where an oscillator’s frequency is modified by a second source, such as an oscillator. If the modulating source is an LFO, the result is a musical “vibrato” effect. When high frequency oscillators provide the modulation, sidebands occur, as they do with ring modulation, but in a richer and more complex manner. As a result, frequency modulation can be used to generate rich spectra from comparatively simple input sources. The voltage control circuitry on both Serge and ARP systems support stable tracking of audio-rate frequency modulation.
Sample and Hold (S/H) - a circuit that samples one voltage based on a secondary voltage fulfilling a triggering condition (e.g. rising in value above a certain value), and then holds that voltage until the trigger fires again. In analog synthesizers, the two voltages are often oscillators run at different frequencies (or a noise source sampled and held by an LFO). S/H circuits can be used to create arpeggio, sequencer, and rhythmic effects by generating control voltages for sound-producing modules in a synthesizer. The Serge Smooth / Stepped Generator has a sample-and-hold circuit with a variety of inputs, outputs, and controls, as does the ARP 2600.
Slew Limiter - a circuit that limits the rate of voltage change in a system. This can be used to glide between values to create, e.g. musical “portamento”, or as a type of filter or waveshaper. West Coast synthesizers such as the Serge often used slew limiting in lieu of filters, and as a building block of slope generators - the Positive and Negative Slew modules on the 73-75 Serge are slew limiters; these circuits are also built into the Dual Universal Slope Generator. The ARP 2600 has a slew limiter in the “lag” circuit on the Voltage Processors module.
Waveshaper - a circuit that distorts the incoming signal to change its spectrum, usually by clipping, folding, inverting, or otherwise transforming (“shaping”) part of the signal’s waveform. Unlike filters, waveshapers are amplitude-dependent, meaning that the strength or character of the transformation will change depending on the amplitude of the incoming signal. Waveshapers are important components in “West Coast” synthesis - the Serge Wave Multipliers and Triple Waveshaper are examples of such circuits.