Rob Hordijk Dual Envelope Generator

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Panel design for an early Rob Hordijk DUAL ENV

The DUAL ENV is a module by Rob Hordijk. It features two paired envelopes with different shapes, ENV A is a four-stage (attack, decay 1, break level, decay 2, release) and ENV B a two-stage (attack, decay). It also features a Sample and Hold function. There have been a few versions with different envelope stages, toggle switches, and modulation possibilities.

Both envelope generators share the same gate input, meaning that they can not be triggered separately, though in later versions ENV B featured a toggle switch to allow self-cycling. This also features two LEDs to indicate the gate status of each envelope.

Triggering threshold is roughly 100mV above ground, and can be triggered by wave forms other than pulses, e.g. triangle waves.[1]


A flow chart explaining the functionality of the DUAL ENV

The ADBDR envelope is primarily intended to be used for volume envelopes. When the decay2 knob on the first envelope generator is fully open the decay2 acts like the sustain that you find on most of the traditional envelope generators. In this case the break control will act like the sustain level. There are CV inputs for the attack, decay1, decay2 and release rates.[1]

The CV input for attack is inverted, so increasing the CV level will shorten the attack time while increasing the decay times for decay 1, decay 2, and release. This means that when MIDI velocity is used as a CV a higher velocity will shorten the attack and increase the other decay times.[1]

Rate settings can be from really snappy to pretty slow. Care was taken that you still have good control over the rates when in the snappy range.[1]

The ADBDR envelope is designed to finish its attack phase before it can be re-triggered. When used for sequencing this allows for complex envelope shapes producing interesting rhythmic effects, but when used for fast keyboard play it might feel a bit strange to play the module with long attack times.[1]

Note that there are no attenuator knobs to set the amount of modulation for the ADBDR envelope CV inputs allowing for direct connection to velocity CV or CC#. However when modulated from other sources they might need extra CV processing to set the modulation levels properly.[1]

Gate or Trig Toggle

New versions of the module feature a toggle switch to select between GATE or TRIG modes. When using long pulses or 'gates' as inputs the envelope will pause at the break level while the gate is high, and continue when the gate is low. Toggle mode will simply run through all stages of the function without interruption.


The second envelope generator is intended as a modulation envelope generator to e.g. sweep a filter or control the harmonic wave shaping of an OSC HRM. The range is from several milliseconds to about a minute.[1]

Earlier versions featured an extra output that is controlled by a bipolar mix knob that can invert the envelope shape and also gives some extra overall ‘sink’ or ‘lift’ when the output level is increased. Later versions traded this for a plain inverted output, and instead gave the envelope a second stage (attack), with a pot for control.

Sample & Hold

When modulating decay times with control voltages it is good advise to keep the voltage fixed while the envelope is developing. For example trying to modulate the decay time with an audio rate signal does in general not produce sensible results. A S&H is integrated into the module to sample the decay time modulation input signal (S&H IN) on every new gate trigger. This way the modulation amount will stay fixed until the module is triggered again by a new gate pulse. The full sampled signal is sent straight to the S&H OUT, so it can be routed to an input of choice on ENV A or use elsewhere, but is attenuated using the ENV B MOD knob before being sent to the ENV B DECAY function.[1]

A Rob Hordijk DUAL ENV filter (centre) in triple-module format with a TWIN PEAK (right) and OSC SYNC (left) - this version features a toggle switch for GATE or TRIG modes, and another for ENV B self-cycling mode.


Many of Rob's designs feature internal connections or 'normalisations' between inputs and outputs. These can be broken by inserting jacks into the inputs. When found together in triple-module formation the following normalised connections were made:



This page uses Creative Commons Licensed content from Mod Wiggler Wiki:Rob Hordijk Designs (View authors).

  1. ^ a b c d e f g h Rob Hordijk

External links