An Introduction to Envelope Filters


An Introduction to Envelope Filters: Controlling the Funk

My envelope filter journey began when I purchased an Electro Harmonix Mini Q-Tron on my 17th birthday. To this day I can’t remember exactly what caused my craving for one, other than a healthy diet of 70’s Herbie Hancock and Grateful Dead.

Whatever the reason, it quickly became one of those pedals I could never turn off. It still remains one of those effects that leads me into all sorts of quacky territory as soon as I step on the switch. What’s most compelling is that it’s a pedal where a minor change in playing style can drastically alter the sound from a low, warbly growl to something similar to an agitated mallard.  

Most guitar effects can be utilized as set-and-forget devices where you find a sound you like and simply switch it on and off when you need it. This is not to say you can’t sacrifice a moment of guitar playing to bend down and tweak repeatedly, but such fiddling isn’t essential for operation. While something like an overdrive will respond to how hard you play in that the clipping may increase or decrease, the parameters inside the box remain constant.

Some pedals—most famously the wah—but also volume pedals and some modulation effects will have an expression pedal built in and you change a certain parameter by rocking your foot, allowing for hands-free tonal shaping while playing your guitar. This makes for a more interactive sonic experience, but can be a little challenging in a rub-your-head-and-pat-your-belly kind of way. Also, depending on your footwear, an expression pedal can be a hazard to one’s balance, especially in those popular high wind jam situations.

Envelope filters are a rare combination of these two different control types. You set the parameters to work with your rig, playing style, guitar, pick thickness, etc, yet the dynamics of your playing will elicit sonic changes; in the case of an envelope filter this involves altering the cutoff frequency of an audio filter. By playing louder or softer or switching guitars, the pedal will behave in dramatically different ways.

What is an envelope filter?

An envelope filter is a tone altering effect that is controlled by the dynamics of your playing. As you play louder, that change in tone gets more intense. Where other effects are manipulated through changes in control settings or the use of an expression pedal, envelope filters are controlled by the level of the signal going into them.

To create a filter we must first accept the concept of complex sound. Simply put, all sounds are comprised of multiple sine waves of various frequencies and amplitudes, which change over time. Any sound wave can be broken down into a multitude of these smaller component sine waves which add and subtract from one another to create the final sound.

Most often the loudest and lowest harmonic that we hear in a sound is what we call the fundamental frequency, though it is possible to create a phantom fundamental from other notes - check out the “Smoke on the Water” opening riff. That being said, if a trumpet, a piano, a violin, and a vibraphone walked into a bar and played concert middle C, each instrument would be producing the same fundamental, but with wildly different overtones.

An open low E string oscillates with a fundamental frequency of about 82.4 Hz, but it also has various harmonics oscillating at frequencies above that, the makeup of which tell us that it’s a guitar and not an oboe, a Telecaster instead of a Les Paul, and a Deluxe Reverb instead of a 5150.

The specific overtones heard, the relative levels of them, and how they change over time will be affected by the guitar, amp, pedals, pick type, string gauge, room size, etc. This is why we all need at least 12 guitars, 8 vintage tube amps, and several dozen pedals. Any discussion about “shaping tone” is, at its core, a study in manipulating these overtones in some manner, whether making them more pronounced (as a treble booster pedal might do) or taming those that feel offensive (the primary function of an EQ pedal).

An audio filter is a device that removes or reduces the volume of a certain frequency or set of frequencies from the signal going through it. Make the “eee” sound with your mouth, hum a note of your choosing, and then slowly shift your mouth and tongue into the “ooo” sound while continuing to hum. Notice how the airy high frequencies go away, but the lower frequencies remain? The note doesn’t change, but its sonic character does. You’ve just made and modulated a simple filter and possibly weirded out your coworkers.

A low pass filter removes or attenuates frequencies above a specific cutoff point, allowing the content below that point to pass; a high pass filter removes or attenuates frequencies below a specific cutoff point. A bandpass filter can be thought of as a low-high pass sandwich as it reduces or cuts spectral content on both sides of a specific center frequency. Most of the classic wah pedals are bandpass filters wherein the pedal sweeps the center frequency.

In pro audio, DAW plug-ins, and stereo equipment we sometimes find other types of filters such as notch and shelving filters, but these don’t typically come into play with the envelope filters on our pedalboards.

On something like an EQ, the cutoff point or center frequency for a filter is typically set via its own control and on a wah pedal it’s determined by the angle of the expression pedal. However, on an envelope filter, the amplitude envelope (or average signal level over time) of the incoming sound determines and changes this point.

By using a sub-circuit called an “envelope detector,” the size and shape of the input signal gets translated into an average change in voltage over time, this resulting signal is then used to control the audio part of the circuit. Under normal circumstances, as the guitar sound gets louder, the filter’s cutoff frequency goes higher. As the sound gets softer the cutoff frequency goes lower, often so low that the entire signal is cut out.

In envelope filter pedals with 2 directional modes, this scenario would be considered “Up” and the inverse of this would the “down” mode. For pedals with a down mode, the cutoff frequency starts high and then is lowered as the guitar gets louder and then goes back up as it gets softer; an “ow-eee” instead of an “ooo-wow” effect.

The trajectory of an envelope filter’s behavior throughout the course of a single “wow” sound.

In shopping for an envelope filter you may come across a few different notes on design; some may mention optical control and others may mention VCFs, OTAs, or FETs. These are all different ways of achieving the same goal. Each design will have its own personality, but ultimately each one is carrying out the same function in terms of control and filtering.

In short, an envelope filter can be thought of as a very intense tone control that is controlled by the intensity of your guitar playing. Louder notes make for greater movements in that tonal change.  

Envelope filters consist of two smaller components; the first being the audio filter itself and the second being the envelope detection and control circuit that controls the audio filter.

What do all these knobs do?

Envelope filters typically have at least 2 knobs. The first is usually “gain” or “sensitivity”. The other control will be labeled “peak,” “Q,” “resonance,” or “feedback.”  

The Sensitivity Control

The “Sensitivity” knob affects the strength with which the input signal is able to control the filter cutoff. By dialing this back the effect tends to be more subdued and requires much heavier picking (or a louder guitar) to really feel like it’s responding. By cranking it up, the effect becomes more touchy so that even the quietest notes trigger the envelope to go high.

In many envelope filter designs this control is labeled as “Gain” and pulls double duty by changing the response of the envelope while also changing the amount of audio going into the filter. This way, a setting that yields a more sensitive touch would also lead to a slightly distorted signal in the filter itself.

Depending on the pedal you’ve got, you may find separate controls for gain, level, or boost. These will contribute to the relative loudness of the audio signal either pre or post filter. In our Disco Terrapin, for example, the “Gain” control changes the amount of audio gain going into the filter and “Output” adjusts the level of the output amplifier, which also offers a little bit of a boost.

The Peak, Q, Resonance, or Feedback Control

The second knob found on all envelope filters refers to the amount of positive feedback in the circuit, which makes the effect more pronounced. Higher feedback settings begin to create a small bump in the region of frequencies at the edge of those that are being rolled off. Higher peak settings will yield a louder bump, with a smaller bandwidth. If the filter effect were a minivan, feedback is like the flags your parents taped to the front bumper when you were learning to drive so that the edge of the vehicle would be more noticeable.   

By increasing the feedback, the effect becomes more intense and in some cases will approach a ringing or self-oscillation. This control also makes a filter sound more vocal.

There’s no right or wrong setting for this (or any other) parameter, but a delicate hand is recommended when exploring because it can easily wind up hurting your ears if cranked, especially with chunkier sounding guitars.

Resonance emphasizes the region of spectral content at the edge of the area that will be rolled off.

Filter Type

Many envelope filters will also have some sort of selector to change between low pass, band pass, or high pass. As described earlier, this will affect which frequencies are cut or heard. The conventional envelope filter sound (at least for Grateful Dead fans) utilizes the low pass mode. This is the default setting for designs which do not have switchable filter types.

This is not to say that the other filters do not have their place, as evidenced by what sounds like a bandpass filter in the guitar solo on Frank Zappa’s “Stinkfoot.”

The 3 primary filter types in a state-variable filter are low pass, band pass, and high pass.

Range or Sweep Controls

A less common set of controls found in these pedals are labeled as “range” or “sweep.” These affect the limits of the filter cutoff. While sensitivity will adjust how easily the pedal responds to your volume and attack, “Sweep” will push back on how high the cutoff can go. This can be useful for keeping the pedal from reaching harsh, icepick like sounds.

It can also be fun to explore lower settings where the effect may behave less like a quacky funk machine and more like a mildly evolving vowel-generator in a box. A sweep control will work in conjunction with the sensitivity control, so if your pedal has one, it’s important to explore them at the same time.

If we were to think of this effect as a bicycle, “sense” would be your gear where higher sensitivity is an easier gear and lower sensitivity a more difficult gear. “Sweep” sets the grade of the hill you’re climbing with lower sweep settings being a steeper hill and higher sweep settings a more gradual hill. A very sensitive gear and a very easy hill and we wind up pedaling too fast; the filter stays wide open and potentially sounds harsh. If we’re in too challenging a gear and with too steep a hill we get nowhere despite putting in significant effort; the filter remains closed and we barely hear the guitar coming through.

We’re usually looking for that happy medium where we have room to push it harder or pullback a little for the greatest range of control.

Attack, Decay, or Response Controls

The final set of controls you may find in an envelope filter will relate to time constants. These are usually labeled things such as “attack,” “decay,” or “response.” These controls will affect either how quickly the envelope charges up, how slowly it drops back down, or some combination of the two.

In the Disco Terrapin, we include a “Decay” control. This adjusts how quickly the envelope will clamp back down as the note fades out. When set very low this offers a quick plucky sound. When turned up, it allows things to ring out a little longer which can be quite helpful for chords or slower passages.

Envelope filters are a lot of fun, a little weird, and perhaps not everyone’s cup of tea. However, few other effect types respond so drastically to the slightest change in your playing.

Few other effects bring down the funk in such healthy portions as well.

While it can be a challenge to get the timing, sensitivity, and response dialed in to your taste and playing style, once you get it right, it can prove to be a very expressive tool.

If you’ve not played through one before, I highly recommend checking one out. If you’re not a guitar player that’s ok too; envelope filters sound great on electric pianos and clavinets, and are rooted very heavily in concepts that control many synthesizers.

Most importantly, envelope filters offer yet another unique opportunity to craft and explore the sounds you use to make music and we think that’s cause for a funky celebration.

If you’re looking to get your hands on an envelope filter ASAP, may we suggest our Disco Terrapin?