Absalon — a tunable, dynamic feedback reverb

**This patch introduces audio feedback. Please, be careful in operation. I am not responsible for any blown speakers or eardrums. If used with care, the chances of that occurring are minimal. But still, when first learning the patch, adjust parameters with a light touch.**

Named after Soren Absalon Larsen, who discovered audio feedback (sometimes called the Larsen effect), and inspired by my recent purchase of an EOB Strat, Absalon is a tunable feedback reverb. Pitch is detected, and then applied to a peaking filter, which sits in the feedback loop of a plate reverb. Drive is added to the signal to give it more character and provide an ‘amp-like’ feedback sound. The patch can be used for almost violin-like swells, swirling cacophony, and all sorts of cool sounds. (I made the demo video with my digital piano because it’s not an instrument you commonly associate feedback with — one of the perks of the patch.)

Audio enters through a stereo pre-delay, after which it is analyzed (right channel only) to extract pitch and envelope. Then the audio passes through a stereo plate reverb, and the audio path splits, with one signal, stereo, going to the audio output, while the other signal is summed to mono and sent to the feedback loop.

In the feedback loop, the audio passes through a peaking, or bell, filter. The Q and gain of the filter (see control page) determine the clarity and emphasis of the tuned feedback. From the peaking filter, audio passes through an overdrive module (germ; you can try changing the drive type — different overdrives have different character, I found the germ was the most complex, but other types have strengths and weaknesses — the ‘pushed’ overdrive has a mellow, violin-like quality). Then, audio passes to a VCA/compressor combination (the VCA mostly exists to cut off audio entirely when the envelope is closed); the compressor is set to 1:inf ratio, to limit the feedback loop. After the compressor, the audio continues to low- and high-pass filters, which have a two-fold effect of characterizing the high- and low-end of the feedback while also keeping subharmonics and high pitched squeals (relatively) at bay. Finally, the signal passes through a stereo spread before returning to the reverb input.

Along with the pitch-tracking, an envelope follower allows you to dynamically control feedback. Since the envelope follower responds to amplitude, notes played harder will have more feedback (or feedback suppression, more on the envelope follower in the control section). The envelope controls allow you to dictate how the feedback swells and decays.

Since the pitch-tracker is monophonic, chords will not be translated into responded feedback frequencies. However, when using the reverb to smear out the feedback, you can definitely play chords and get cool results.



Left — turns the envelope follower on and off (this is also replicated by a UI button on the control page; when active, the envelope follower UI button will get brighter)

Front page:

Across the top are controls for the mix and reverb. WET and DRY LEVELS can be used to create a mix. There is PRE-DELAY (up to 1000 milliseconds), which can be used to create a sense of ‘space’ and add interesting dynamics to the play between instrument and feedback. When the reverb is delayed, so to are the pitch-tracking and envelope following. REVERB DECAY sets the decay of the reverb; you can remove the reverb decay entirely, if you want to just use the tuned feedback. Although it’s my inclination to push reverb decay to… the maximum most of the time, here I advise restraint: since the reverb is fed back into itself, longer decays can produce more muddled feedback.

The next row has controls for the drive and compressor. DRIVE controls both the input and output gain of the overdrive module. While the overdrive contributes to the feedback amount of the circuit, it also changes its character and adds complexity to the signal. The COMPRESSOR THRESHOLD determines how much feedback is allowed through the circuit. (I generally keep it at 0 and allow the envelope to control the feedback. Unless I’m using negative gain from the envelope to suppress the feedback while I play. But you can set it to allow a little or a lot of the feedback to recycle regardless of the envelope.) COMP ENV has really minimal affect, but it does somewhat shape the feedback sound, so I threw it in.

The middle row has the controls for the envelope follower. First, there is a UI button that indicates if the envelope follower is ACTIVE or not. You can push it to change the state (also available via the left stompswitch). Then there is a control for ENV GAIN — this determines whether the envelope follower lowers (negative values) or increases (positive values) the compressor threshold. ENV ATTACK and ENV DECAY shape the envelope follower’s respond; in particular, when using the envelope follower, decay can be thought of as complementing the reverb decay, with long decay times extending the feedback loop.

The next row has controls to select how the mid frequency is controlled: PITCH-TRACKED, MANUAL (set by the FREQUENCY control next to it), or EXPRESSION (or CV, using the expression/CV input). When pitch-tracked, there are some additional controls: HARMONIC allows you to offset the tracked frequency, for instance, setting it to E1 would produce a harmonic one fifth above the played note. DRAG controls the rate of a CV filter — when the feedback jumps to new pitches immediately it sounds a bit unnatural. I tried a slew limiter, but it was too… synth portamento-y. Finally, you can control whether or not the pitch-tracked audio passes through INPUT FILTERS. This can improve tracking, but it also makes the feedback a little less lively, in my opinion.

The final row is devoted to filtering. LO CUT and HIGH CUT control the frequencies of the high- and low-pass filters in the feedback path, respectively. You can use them to shape the tone of the feedback and eliminate rumbles and squeals. (Or at least mitigate them — this is feedback. Without rumbles and squeals, what are we even doing?) MID Q and MID GAIN control the peaking filter. MID Q will set the clarity of the tuning by adjusting the bandwidth of the filter; while you may be inclined to set this high, allowing in some surrounding frequencies can produce a more natural sounding feedback. MID GAIN determines how much emphasis is placed on the peaking band; you can also set it to negative gain, which allows you to sort of create a ‘negative space’ around the played note’s frequency.

And… that’s it. This is a fun, wild one. Maybe not for everyone, but I tried to make it as flexible as possible.

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