Effects documentation

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Designed to avoid an ultra-smooth fade-to-noise decay.


This effect was designed to satisfy my need for a more "natural" reverb effect that sounds more like a collection of gradually diffusing distinct echos rather than the excessively smooth "to noise" diffusion some other effects are designed to produce.

This is the fourth generation of my reverb effect. The first two used prime lengths for delay lines to get much smoother and less metallic timbres using a typical old-school "parallel combs" configuration. The second generation improved this slightly with some extra parameters. The third generation switched to using roots to produce irrational numbers for the delay lengths.

In this version I've implemented both configurations; You can get sounds similar to the first and second generation using algorithm A and the third generation using algorithm B.


Reverb screenshot



The diffusion stage is often called "early reflections". The purpose of this stage is to create a complex multi-tap comb filter which feeds into the tail stage thereby coloring the timbre of the entire reverb.

Pre-delay time before first reflection.
Whether the early reflections should fade in or out.
Number of reflections. A zero setting here disables the diffusion stage.
Spread decides how much the times for different taps "fan out" toward zero.

Maximum will give you the largest variation in delay lengths and will sound more metallic. Around 50% is usually ideal. This parameter can be used to adjust overall timbre. You may want metallic (high = plate), a balanced setting (medium = room, hall) or a very phasey/delayed timbre (low = weird).

Decides if the taps should be concentrated on one side or the other.

In other words many short times with a few long ones or a few short times with many long ones.


The tap positions are modulated by an LFO to help smooth any static peaks. It can also be used for pitch effects and chorus.


The tail stage diffuses the phase of the early-reflections input over time from distinct clicks to entirely diffuse noise.

Over-all "size" of the reverberation.

This has a direct influence on the average length of all delay taps. Very short for metal bucket timbres, first third for rooms, middle third for halls, last third for canyons and extremely long complex echo patterns. Please do not set the size to 100% and expect it to sound like a reverb as this is essentially setting the space between walls to several kilometers!

Amount of feedback in individual elements; This influences the decay time.
Amount of diffusion between elements.

Low settings will produce more direct, delayed sounds. High settings will produce over time a very high impulse density; Over time the signal will move in more and more directions eventually becoming a hiss or white noise in terms of phase.

How many individual delays/diffusors/etc are set up.

More elements will give you less "grainy" or "echoy" sounds as more filters will be applied in series with the consequence of proportionately increasing processing cost.

Algorithm A is a standard parallel configuration, B is a unique series configuration allowing for infinite sustain/decay.
Whether the main reverberation should fade in or fade out.
How echoy or metallic the timbre is; how far apart delay taps are.
How many delay taps are allocated to near or far times.

This tone control is applied to the tail delay lines.

Amount of high frequency content (like high-cut).
Amount of low frequency content (like low-cut).

Global delay + feedback.

Delay time applied to the whole reverberation.

Algorithm A applies this delay before the output. Algorithm B applies it after the output and before global feedback.

How much feedback is applied.

For algorithm A this is a plain old delay line with feedback. For algorithm B you can use 100% global feedback to produce infinite sustain. The filters (including interpolation) will remove content so you'll still hear a decay of the frequency content. For true infinite sustain you must disable all filters and modulation and set both feedback (decay) parameters to 100%.

A cross-blend between wet and dry.
Offset added between left and right channel tap positions.
Gain applied to the wet signal output if needed to make up for any losses due to filters.

This tone control is a band-pass filter applied to the wet signal immediately before it is mixed to the output.

Center frequency of band-pass tone control filter.
Width of band for the tone control filter.


This reverb has a somewhat unique implementation for algorithm B so don't be surprised if you don't get the effect you would expect. Algorithm A is a typical old-school parallel configuration and may behave in a more familiar way.

A common complaint about older parallel delay lines reverb effects is with regard to the resulting heavily comb-filtered metallic timbre and fluttering decay. These effects tend to sound far less than ideal when unnatural sounds such as those from a subtractive synthesizer are processed. Those old effects also tend to sound odd on vocals and mixed content and are generally very easy to identify as an effect which makes them draw the attention of the listener. This is often frustrating and counter-productive to the aim of creating natural ambience.

A natural room or hall does show these effects only to a lesser degree for a number of reasons. Digital delay lines act as almost perfect mirrors lacking any diffusion of the signal and while the signal is "in transit" via the delay lines it does not interfere with other reflections. Real air or other mediums do not transmit an impulse in this "perfect, 1d" way but are more like an all-pass filter in multiple dimensions including not just simple reflection but refraction, diffusion, absorption and other effects.

In order to reduce the negative effects produced by isolated delay lines both algorithm A and B utilize only a few simple techniques:

Multiple feedback path network
The first technique is to increase the diffusion slightly by shifting the phase with all-pass filters as well as running the delay lines both in series and parallel.
The second technique is to choose delay lengths which are irrational to minimize cumulative comb-filtering effects.
Modulation is also applied both to delay length and filter coefficients to very roughly (very poorly ... but usefully) approximate the interference which would occur during transit through the air.

The result of these simple techniques is not perfect but does satisfy my own desire for a more realistic reverb effect with minimal complexity. A combination of these and other techniques are used in more expensive and complicated algorithmic reverb effects.