Effects documentation

Reverb

A reverb effect designed to avoid an ultra-smooth fade-to-noise decay.

Description

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.

Screenshot

Reverb screenshot

Parameters

Diffuse

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.

Delay
Pre-delay time before first reflection.
Direction
Whether the early reflections should fade in or out.
Density
Number of reflections. A zero setting here disables the diffusion stage.
Spread
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).

Distribution
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.

Mod

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

Rate
...
Depth
...
Tail

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

Size
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!

Decay
Amount of feedback in individual elements; This influences the decay time.
Diffuse
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.

Density
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
Algorithm A is a standard parallel configuration, B is a unique configuration.
Direction
Whether the main reverberation should fade in, or fade out.
Spread
How echoy or metallic the timbre is, how far apart delay taps are.
Distribution
How many delay taps are allocated to near or far times.
Tone

This tone control is applied to the tail delay lines.

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

Global delay + feedback.

Delay
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.

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 parameters to 100%.

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

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

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

Comments

This reverb has a unique implementation for algorithm B so don't be surprised if you don't get the effect you would expect.

With algorithm A it takes on 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 less than ideal when unnatural sounds such as those from a subtractive synthesizer are processed. Those old effect also tend to sound awful on vocals and mixed content and are generally very easy to identify as an effect which makes them draw the attention of the listener.

A natural room or hall does have these effects present although not as much as many parallel reverb effects due to 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.

In order to reduce the effect of the perfect mirror-like reflections produced by isolated delay lines both algorithm A and B utilize only a few simple techniques.

The first technique is to increase the diffusion slightly by shifting the phase with all-pass filters as well as running the delay lines in series rather than 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 approximate the interference which would occur during transit through the air.

The result of these techniques is not perfect but does satisfy my own desire for a more realistic reverb effect.