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A voltage-controlled filter (VCF) is an electronic filter whose operating characteristics (primarily cutoff frequency) can be set by an input control voltage. Voltage controlled filters are widely used in synthesizers.



A music synthesizer VCF allows its cutoff frequency, and sometimes its Q factor (resonance at the cutoff frequency), to be continuously varied. The filter outputs often include a lowpass response, and sometimes highpass, bandpass or notch responses. Some musical VCFs offer a variable slope which determines the rate of attenuation outside the bandpass, often at 6 dB/octave, 12 dB/octave, 18 dB/octave or 24 dB/octave (one-, two-, three- and four-pole filters, respectively). In modular analog synthesizers, VCFs receive signal input from signal sources, including oscillators and noise, or the output of other processors. By varying the cutoff frequency, the filter passes or attenuates partials of the input signal.

In some popular electronic music styles, "filter sweeps" have become a common effect. These sweeps are created by varying the cutoff frequency of the VCF (sometimes very slowly). Controlling the cutoff by means of a transient voltage control, such as an envelope generator, especially with relatively fast attack settings, may simulate the attack transients of natural or acoustic instruments.

Historically, musical VCFs have included variable feedback which creates a response peak (Q) at the cutoff frequency. This peak can be quite prominent, and when the filter's frequency is swept by a control, partials present in the input signal resonate. Some filters are designed to provide enough feedback to go into self-oscillation, and it can serve as a sine-wave source.

ARP Instruments made a multifunction voltage-controlled filter module capable of stable operation at a Q over 100; it could be shock-excited to ring like a vibraphone bar. Q was voltage-controllable, in part by a panel-mounted control. Its internal circuit was a classic analog computer state variable "loop", which provided outputs in quadrature.

A VCF is an example of an active non-linear filter. The characteristic musical sound of a particular VCF depends on both its linear (small-signal) frequency response and its non-linear response to larger amplitude inputs.

Resistive opto-isolator filters
Resistive opto-isolator filters, also known as vactrol filters, are an early type of VCF design that remains popular to this day because of its low production cost and inherent linearity. It is commonly encountered as so-called "low-pass gates" and in auto-wah pedals.

The core of such a filter is a simple RC low-pass where the resistor is swapped for a photoresistor. A LED is powered by a control voltage to dynamically change the resistance of the photoresistor, thereby changing the cutoff frequency of the RC circuit. Adding a resistor parallel to the capacitor turns this circuit into a low-pass gate. Some designs use two photoresistors per LED to create a sallen-key topology, making the filter capable of resonance and non-linearity.

Photoresistors have an inherently slow, "natural" response to light, giving resistive opto-isolator filters a poor capability for audio-rate modulation. In some cases, trying to audio-rate modulate a resistive opto-isolator can produce modulations typical of an envelope follower, which may be desirable. The slower response also allows for the use of binary voltages (e.g. gates or triggers) directly, reducing the need for envelope generators and therefore further decreasing the cost per voice.

Diode filters
Passing a bias current through a diode (or the PN junction of a BJT) decreases its small signal equivalent resistance. Adding capacitors to a network of forward-biased diodes creates an electronic filter. The aforementioned bias current can be modulated to dynamically change the filter's cutoff frequency ; this property is used to create voltage-controlled filters.

The aforementioned bias current passes through the diode-capacitor network, therefore it is inherently band-limited, low-pass filtered to the filter's cutoff frequency. This means that the filter's time constant, when modulated, won't decrease in less time than itself. This causes diode filters to have a specific response to audio-rate modulation. In some implementations, a large enough feedback or input signal will create said audio-rate modulation. This non-ideal characteristic is known as "self-FM" and plays a small, but significant role in the characteristic sound of diode filters.

Notable synthesizers that use diodes to implement a voltage-controlled filter include:
 * Korg MS-50
 * Steiner-Parker Synthacon
 * EMS Synthi and EMS VCS3 (diode ladder)
 * Roland TB-303 (diode-wired transistors - diode ladder)

Transistor ladder filters
Similarly to a diode filter, a transistor ladder filter can dynamically change its cutoff frequency by increasing or decreasing the bias current going through the PN junctions of bipolar transistors. The major difference with a diode filter is that said bipolar transistors work double duty by being current buffers. When each filtering stage is buffered, a lower feedback gain is required to produce a high resonance, and the poles and zeros on a

Up until october 1986, transistor ladder filters where patented by Moog, making their availability very low. They were also notoriously expensive, even though this filter topology was specifically designed to reduce production costs and the number of components.

Synthesizers that use a transistor ladder filter include :
 * Moog synthesizers
 * Paula Maddox's Monowave

OTA filters
OTA filters often use general-purpose OTA ICs such as CA3080, LM13600, and LM13700. There are also specialized OTA ICs that have optimal characteristics for the implementation of musical filters, such as IR3109 and CEM3320.

One the most well-known exemple of an OTA filter is the VCF from the late revisions of the Korg MS-20, which uses the aforementioned LM13600. This filter has a characteristic, unapologetically harsh sound when it's set to a high resonance, which is responsible for a lot of the commercial success of the synthesizer.

The popularity of this specific OTA filter unwillingly created an expectation that all OTA filters sound -or should sound- like an MS-20 VCF, while in actuality the vast majority of OTA filters have exceptionally ideal characteristics from a technical standpoint.

Note that said ideal characteristics do not necessarily translate to musically useful characteristics, to the contrary more often than not. This is why many OTA filter designs implement non-ideal characteristics voluntarily. The advantage over previous filter designs is that these non-ideal characteristics are customizable to an unprecedented extent.

Notable synthesizers that use OTA filters include :
 * Korg MS-20
 * Oberheim SEM, OB-X
 * Roland SH-101, Jupiter-8, Juno-60
 * Sequential Prophet-5

Special cases

 * The PoliVoks VCF uses a specific programmable op-amp IC (K140UD12) whose bandwidth can be dynamically controled by misusing its compensation pin.
 * The Wasp VCF is a type of OTA filter, but uses op-amp integrators typical of state variable filters, its OTAs in an open-loop configuration, and a binary inverter IC (CD4069) to approximate the necessary negative feedback.
 * Some VCFs use principles from mixed-signal circuits. A dynamic resistance can be approximated with a PWM-controlled electronic switch to form an RC filter. Clocked electronic switches are also used in switched capacitor filters. The cutoff frequency of the former is controlled by the clock's pulse width, while the cutoff frequency of the latter is controlled by the clock's frequency.