What is the Entropic Doom?

The Entropic Doom is a very unique module, we like to think of it as an oscillator for noise - a noisillator! It has a unique and unusual topology, featuring a cmos logic gate that is biased for use in the analogue region between logic zero and one. We use this for controlled noise generation, and it is in a closed loop circuit along with a variable pole low pass filter and VCA. The output of the filter is fed back into the input of the cmos logic gate, which generates noise that can be tuned depending upon the resonant loop gain, filter frequency and filter pole selection. We can take the output from two different points in the noise loop, either pre-filter (Bright Out) or Post filter (Dark Out). We can also mix an external audio signal into the noise loop with the Audio In jack and level control, and also use an audio sync signal (Sync IN) to reset the cmos gate and / or mix in adio to a different part of the noise loop. We also have included an XOR Ring Modulator and XOR VCA too, each of which is rather different in function from its regular analogue counterpart.

 

Here's a demo video from Wayne Taylor which gives an introduction to the Entropic Doom

Usually in analogue synthesis we use a reverse biased transistor or Zener diode in avalanche mode as a noise source, however the Entropic Doom takes a different approach and uses a CMOS logic gate in a biased, analogue configuration as a noise generator - this is useful as we can also apply as Sync signal to the other input of this gate so that the noise generation is reset, in a similar way that Oscillator Sync works on an analogue VCO.

The Noise core itself is contained within a loop that also includes a low pass filter, which can be switched between 2 pole, 3 pole and 4 pole configurations, and the feedback from this filter is fed back into the noise core itself to create a resonant feedback loop. We have two noise outputs, the Bright output is taken directly from the Noise source itself, and the Dark Output is taken from the output of the filter.

External audio can be mixed into the pre-filter section of the noise loop using the Audio In jack and its associated Audio In level control.

There is also a Sync Input, to allow a signal to be fed into the other channel of the Cmos logic gate that is used as a noise generator - if a square or pulse waveform is used at a suitable frequency then this gates the noise source to give a similar effect to Oscillator sync in a VCO - the effect is subtle, and most obvious when using the Bright output. The sync signal is also mixed into the noise loop, so it serves as a second Audio input too.

 

Available with either Black (Dark Edition) or Silver front panels.

 

Control functions

FREQ-C control: This allows a course manual adjustment of the Noise frequency

FREQ-F control: This allows a fine manual adjustment of the Noise frequency

1v/OCT INPUT: An external control voltage can be patched to this input to allow 1V/Octave pitch control of the Noisillator Output

FREQ-F control: This gives a fine manual adjustment of the Noise frequency

RESONANCE control: This control adds a passband resonance into to the noise loop, so that the noise becomes more bandpass filtered and pitched

RES-CV Input: An external control voltage can be patched to this input to allow external voltage control of passband resonance, and in this case the Resonance control becomes an attenuator for any voltage fed into the RES-CV input jack.

POLE Switch: This three way switch selects 2 Pole, 3 Pole or 4 Pole response from the filter in the noise loop

AUDIO IN control: This controls the level of any audio signal that is fed into the AUD IN jack, and this audio is mixed into the Noise loop upstream of the filter.

SYNC control: This controls the level of any audio signal fed to the SYNC Input

 

Noise Generation

The Entropic Doom uses a novel and quite different method of "analogue" noise generation, rather than the reversed bias transistor or zener diode commonly used in analogue synthesiser noise generators we generate noise by operating a Cmos Logic chip within a varying analogue region, rather than its usual on or off logic state. This generates noise with a unique texture compared to conventional sources, and as we use a two input gate we have configured the second input to sync the noise generation - so that under certain conditions of level and frequency a SYNC waveform can reset the individual noise bursts from the gate. This effect is most obvious at the Bright output, as the filtering tends to mask the effect at the Dark output.

We also have a feedback loop in the circuit, which passes the output of the noise chip through a two, three or four pole filter and then mixes this feedback to the input of the Cmos Logic gate. In addition we have a resonance circuit which can apply a varying level of passband resonace to the noise loop itself. The combination of these two functions allow us to "tune" the noise so that we can change the frequency with either the front panel Frequency controls or an external 1V/Oct pitch control voltage.

With maximum resonance applied and using four pole filtering the output waveform will approximate a sine wave, however its frequency will "warble" a little because of the random generation of the noise - this is actually a very accurate electronic equivalent of a steam whistle kettle, which warbles in a very similar manner due to the randomness of the steam passing through it.

 

 

Ring Modulator

 

The Ring Modulator is a stand alone module within the Entropic Doom Module.

 

The XOR Ring Modulator uses a Cmos Logic chip rather than the regular analogue four quadrant multiplier arrangement as seen in our Ring SM module and other Ring Modulator circuits. We have two inputs, RM-X and RM-Y and a single output, RM-OUT. It performs as a Ring Modulator in that the output is the sum and difference of the input frequencies, however has an XOR based function, which means that unlike a four quadrant multiplier an output is available even if only a single input is used. Also, the output tends be be based on a pulse waveform, and it varies depending upon the type of waveforms fed to the inputs.

Because we get an output from the XOR Ring Modulator even when we just connect a single input it means that an audio signal can be patched into the second input via a VCA or mixer, which allows the Ring Modulation effect to be gradually added as the second audio signal is mixed in.

The RM-X and RM-Y inputs are not directly connected to either of the Noisillator outputs, however there is usually some interaction and cross talk between the two circuits inside of the X-OR chip itself, so a degree of ring modulation may also be heard on the DK-OUT and BR-OUT outputs. It is quite acceptable to patch either the DK-OUT or BR-OUT outputs to the RM-X or RM-Y Ring Modulator inputs.

 

XOR-VCA

 

The XOR VCA is a second stand alone Module within the Entropic Doom Module..

 

The XOR VCA has an input, and output and a CV Level Control. The input is RM-X, which it shares with the Ring Modulator, and the output is from the OUT jack. VCA Level is controlled by patching a unipolar control voltage to the CV IN jack. With the XOR-VCA control turned fully clockwise the VCA output varies from zero to full as the control voltage changes from zero to +5V. The XOR-VCA control acts as an attenuator to the incoming control voltage, so if set to 50% level then a zero to +10V input range is possible. The VCA does not resond to negative control voltages, it simply disregards them. If you wish to use a bipolar rather than a unipolar control voltage then you would need to level shift it with a module such as the AJHSynth CV Mixer-Offset-VCA module or similar.

The VCA acts in a very similar fashion to a regular Linear VCA, however an XOR circuit is present in the audio path, which creates distortion, particularly at higher gain levels. The nature of this dostortion is very dependant upon the waveform fed into the RM-X input.

It is a slight compromise that the Ring Modulator and VCA share a common input, this was simply due to the need to keep the module within a 14 HP module size, if we added more inputs and level controls then the module front panel would have become very much larger, and it is unlikely that you would need to use both the Ring Modulator and XOR VCA in the same patch as their distortion characteristics are quite similar.

 

 

DEMOS

 

Here's a demo video from Jason Barile that gives an idea of some of the sound possibilities of the Entropic Doom

Patch notes:

2 AJH Synth VCOs (pulse and triangle wave) are mixed together via Entropic Doom's ring modulator. The ringmod output is then routed through the MiniMod Ladder Filter and into Audio Input 1 of the Discrete Cascaded VCA. The DK-OUT of the Entropic Doom is routed to Audio Input 2 of the VCA. Output from the VCA goes into 4ms DLD for a little bit of echo and then into the AJH Finaliser R-EQ for some small church reverb. The kick drum part is a Crater sequenced on Metron. I'm playing the melody on a Keystep. You'll see me manually adjusting the filter cutoff on the ladder filter and on the Entropic Doom a bit.

 

 

The Entropic Doom module is being used here to generate a haunting steam whistle sound, introduced at 1:42 in the video below. It also features the Gemini 2412 filter and MiniMod system, in another expert performance from Wayne Taylor.

 

 

Entropic Doom module along with FFB914 and Sonic XV filter

 

 

The Entropic Doom takes on a percussive role in this composition from Wayne Taylor.

Warning - contains zombies!

 

 

 

A whip cracking percussive intro from the Entropic Doom leads us into a beautiful and hypnotic loop in this composition

 

 

 

Entropic Doom videos from our friends at Alex4 in Berlin

 

An introduction and overview of the Entropic Doom by Tom Koerting

 

 

Feeding two VCO's into the Entropic Doom

 

 

Using the Entropic Doom to mangle up drums

 

 

Parallel processing patch

 

 

Sine waves into the audio input

 

 

And last, let's just throw everything in!

 

 

 

 

 

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