My recent build of a Befaco Rampage prompted me to revisit several variations of the Serge Dual Universal Slope Generator that I have in Eurorack: Befaco Rampage, Random*Source DUSG, Steady State Fate Mini Slew, After Later Audio Tilt, Make Noise Function, and Joranalogue Contour 1. I learned a few things, too. But first let’s briefly look at the original Serge. I built a CGS Serge DUSG board some years ago, in a banana jack format. It’s very much like the graphic above, with the addition of a Bipolar output.

There’s beauty in its simplicity. Patch from End to Trigger to make it cycle. Patch from Out to CV in for different curves. The Eurorack versions I’m looking at here all have added features to this basic design. And they are:

Random*Source Serge DUSG

The R*S DUSG is the closest in design to the original. It is also the largest, at 26hp width. I built mine from a kit.

Befaco Rampage

The Rampage implements the functionality with a completely different technical design. It’s 18hp wide. I built mine from a full kit. The Rampage circuit is open source, with schematic supplied.

Steady State Fate Mini Slew

The Mini Slew is a single unit; you need two for the equivalent of the DUSG, in 16hp total. I wrote a comparison of Mini Slew with the Make Noise Function.

After Later Audio Tilt

Tilt also comes as a single unit, so you need two for the Dual Slope functionality. But it’s only 6hp in width!

Make Noise Function

Make Noise Function has been in my kit for quite a while. Lately I decided to get another used one and found the perfect match to my existing Function, ca. 2012. It has the same revision PC board. I replaced the blue knobs with Davies clone black knobs, to which I’ve taken a fancy. The all black and silver scheme looks nice.

Joranalogue Audio Design Coutour 1

A relative newcomer to the universal slope space, the Contour 1 hits the ball out of the park. I bought a pair of them during a Spring sale. Same 8hp width as Mini Slew and Function and only slightly more expensive.

Feature Comparison

This is a simple list of features. We’ll get into functionality later.

All have the following common features.

• Input for slewing
• Main output
• Output indicator
• Rise pot
• Fall pot
• Rise CV input
• Fall CV input
• Trigger input
• Shape control
• Cycle switch

R*S DUSG adds:

• One square wave output
• One bipolar output (inverted)
• End pulse output
• 1V/Octave FM input
• Both CV input (summed to Rise and Fall CVs)
• Rise CV attenuverter
• Rise Exponential switch
• Fall CV attenuverter
• Fall Exponential switch

Rampage adds:

• Cycle gate input
• Trigger button
• Range switch
• Exponential FM input
• Rising gate output (10V)
• Falling gate output (10V)
• End pulse output (10V)
• A>B gate output (10V)
• Min output (trough)
• Max output (peak)
• Balance pot between A and B to comparator input

Mini Slew adds:

• Cycle gate input
• Variable level output (+/- 9.5V)
• Variable level CV input, i.e. 4-quadrant multiplication
• Exponential FM input
• CV Sum
• Time Compensation
• Shape CV input
• End of Rise pulse output
• End of Cycle pulse output

Tilt adds:

• Unipolar/bipolar cycling switch
• Both CV input
• Variable level output (+/- 9.5V)
• Bipolar output (+/- 4.5V)
• Rising gate output (9V)
• Falling gate output (9V)
• Separate shape knobs for rise and fall
• Gated ASR with manual sustain level

Function adds:

• End of Rise pulse output (11V)
• End of Cycle pulse output (11V)
• Exponential FM input
• Hang input
• Inverted output

Contour 1 adds:

• Trigger button
• Full/Audio range switch
• Shape controls (Bend) for both Rise and Fall
• CV inputs for both Rise and Fall Shapes
• Gate input
• Hold input
• Rising gate output (5V)
• Falling gate output (5V)
• Inverted output

Generating Envelopes

Envelope peak voltage

• R*S DUSG main output peaks at 5V when generating an envelope or cycling.
• Mini Slew main output is about 8.5V and the variable output can go up to +/- 9.5V.
• Rampage outputs peak at 10V.
• Tilt outputs up to +/- 9.5V unipolar, or +/- 4.5V bipolar.
• Function main output peaks at about 8.5V.
• Contour 1 output peaks at 10V.

A common use for slope generators is to make an Attack-Decay envelope. Attack is started by a rising edge on the Trigger input. The output voltage rises to maximum at the rate set by the mix of Rise pot, Rise CV input, and FM input. The curve of the rise can be linear, logarithmic (log) or exponential (exp). Upon reaching the max voltage, the decay phase starts heading back to zero at a rate set by all the fall controls and with a linear, log, or exp shape. Of special interest is what happens if a new trigger is received before the cycle is completed. For all of these modules except for Contour 1, a new trigger (positive transition) arriving during Rise is ignored (contrary to what the Rampage manual states, that it will be a hard sync, i.e. a reset to zero). Contour 1 resets to zero when a new trigger is received at any time during a cycle.

Contour 1 is the only one here besides Rampage to offer a trigger button (and it’s big). Contour 1 also provide a Gate input, which enables a 10V peak Attack-Release (AR) envelope to be generated from any gate signal above a threshold, whereas the other USGs rely on using the input as a gate input for AR envelopes. Those others rely on the height of the input to determine the peak output. Contour 1 can also be used this way.

Behaviors differ when a new trigger is received during Fall.

When a trigger is received during Fall

R*S DUSG just ignores triggers throughout the whole cycle. This makes it ideal for use as a pulse frequency divider. The End pulse will be a division of the number of trigger inputs that happen during the cycle.

Mini Slew, touted also as a pulse divider, exhibits this behavior as well, but very oddly only if the trigger pulse height is around +5V. It was very confusing when triggering Mini Slew from a Rampage 10V pulse output. With a 10V trigger Mini Slew will re-trigger during Fall! I view this as a design flaw, since the level of a trigger should never make a difference to anything.

Rampage behaves as stated in the manual and re-triggers upon receiving a new trigger during Fall. Re-triggering means that the voltage begins rising again.

Tilt also re-triggers when getting a trigger during Fall.

Function also re-triggers when getting a trigger during Fall.

Contour 1 always resets to zero when getting a trigger at any time during the cycle.

Envelope shapes

Each of these modules has a different idea about shape. The traditional curves for an envelope uses a logarithmic rise and an exponential fall, or a linear curve for both.

R*S DUSG has one of the more flexible shape controls. Rise and Fall shapes can be set independently, making any combination of lin/log/exp Rise and lin/log/exp Fall possible. It’s accomplished by a reversible attenuator (attenuverter) plus an Expo switch. It works by feeding back some of the output into the Rise or Fall CV input. Thus, it mixes with whatever CVs are patched into the Rise, Fall, and Both CV inputs. And there are two positions of the Expo switch, which differ regarding the target voltage of the curves. In one position the target voltage is higher than +5V, resulting in reaching this max in a shorter period of time.

Mini Slew has a Shape knob that goes from log through linear to exp. There’s also a Shape CV input which sums with the Shape knob setting. Given that changing the shape impacts rate, using this CV input might have unexpected results. With Mini Slew both Rise and Fall are the same shape, log, lin, or exponential.

Rampage has yet another way of adjusting shape. A shape knob allows to choose between log attack and exp decay, or linear, or exp attack and log decay.

Tilt features separate pots for rise and fall shapes, which facilitates the traditional log attack and exponential decay, if desired.

Function has a single shape knob, like Mini Slew, with both Rise and Fall being affected in the same way. Function has no CV over shape.

Contour 1 has the most control over shape (which it calls Bend). It features separate knobs for rise and fall shape, plus CV inputs to go with each of those. It’s easier to understand than how R*S DUSG does it.

Here’s a nice instructional video about envelope shapes by someone else.

Relation of shape change to frequency

Altering shape always has some impact on the rate (frequency). Each module behaves differently.

R*S DUSG sees the rate slowed by increasing log shape and made faster by an exponential shape.

Rampage sees the rate slowed by turning the shape knob either direction, away from center (linear).

Mini Slew has two different behaviors. With T-Comp off it’s like the DUSG: log shapes make it slower and exp shapes faster. With T-Comp on it behaves like Rampage, with rate slowed by by either log or exp shapes. The T-Comp feature doesn’t work very well to keep the rate the same as the shape changes.

Tilt acts still differently. In bipolar mode, the rate is slowed by deviation from linear shape, like Rampage. But in unipolar mode, log shapes make it slower and exp shapes faster.

Function sees the rate slowed by increasing log shape and made faster by an exponential shape.

Contour 1 tries to keep the rate the same with changes in shape, but there is still enough deviation to hear a pitch change when changing shapes.

Let’s get cycling!

On four of the modules, patching from End to Trigger will cause cycling. But each module has a switch to turn on cycling. R*S DUSG has a toggle switch; Mini Slew has a button (with status LED) that turns cycling on and off (remembered across power off and on), plus a Cycle gate input that toggles the state set by the button. Rampage has a toggle switch plus a Cycle gate input, but when toggling into cycling it’s also necessary to press the trigger button to get it started. (Patching to both Trigg in and Cycle gate will turn on cycling and start it.) Function has a button with status LED to toggle cycling. The version I have of Function powers up with cycling mode on.

Tilt has no End pulse, so to get cycling one must toggle the three-position switch to either bipolar or unipolar. (Center is for gating envelopes or following an input.)

Contour 1 has a toggle switch for Slew/Loop to enable looping. When looping, the Gate input takes on a new ability: when Gate is patched during cycling on, a high gate enables cycling, while a low gate disables it.

Frequency control

On five of the modules, (DUSG, Rampage, Mini Slew, Tilt, Function) the 1V/Octave, FM, or Exp CV input (same function, just differently named) causes an exponential increase in the frequency with a positive voltage and a decrease with a negative voltage. R*S DUSG tracks closely 1V/octave and has a trimmer for that.

On R*S DUSG turning knobs clockwise and adding positive CV to the Rise, Fall, or Both CV inputs will increase the frequency.

On Tilt, the Both CV input acts exponentially, at a measured 1.1V/octave. Patching to both the Rise and Fall CV inputs gets the same response as patching to the Both input. Turning knobs clockwise decreases the frequency, but adding positive CV increases the frequency.

On the Mini Slew, turning knobs clockwise or adding positive CV to the Rise and Fall CV inputs will decrease the frequency.

On the Rampage, moving sliders upwards decreases the frequency. Adding positive voltage to the Rise or Fall CV inputs also decreases the frequency.

On the Function, turning knobs clockwise or adding positive CV to the Rise and Fall CV inputs will decrease the frequency.

On the Contour 1, like Rampage, moving sliders upwards decreases the frequency. Contour 1 has only two frequency control inputs: for Rise and for Fall. Patching to Rise normally connects to Fall. These inputs respond 1V/octave (with a trimmer right on the front panel), in both ranges, Full and Audio.

Audio Oscillating

Contour 1 and the R*S DUSG are the best audio oscillators here. Both have 1V/Octave CV inputs, as well as trim pots to tweak this response.

Contour 1 in Full range or Audio mode maxes out at about 1 KHz manually, but reaches 25 KHz with +5V on the CV inputs and even 35 KHz with +7.5V on the CV ins! The difference with Audio mode is that the lowest manual frequency is about 14 Hz, whereas Full range mode will manually go lower. Negative CV will lower it even more.

R*S DUSG has a maximum manually set frequency of 10.5 KHz, going up to 15.5 KHz with +5V on either the 1V/octave or the both CV input with attenuators maxed.

Mini Slew tops out at 300 Hz with both knobs full CCW and linear shape, going up to 560Hz with +5V into the FM CV input.

Function tops out at 590 Hz with both knobs fully CCW and linear shape, going up to 1.0 KHz with +5V into the FM CV input.

Rampage top in Lo, Mid, and Hi range is respectively 2.5 Hz, 24 Hz, and 500 Hz, going to 1.2 KHz with +5V on the Exp CV input.

Tilt max frequency with linear shapes is 1.2 Khz with both knobs fully CCW, and will go to 2.3 KHz with +5V on the Both CV input.

Hold it!

Function and Contour 1 have hold capability. A high gate on the Hold input (Function’s Hang) stops movement at that moment, until the gate goes low again. This affords some interesting modulation effects when gating Hold by another oscillator or function generator.

Conclusions

What a fascinating range of design choices. Different voltage and frequency ranges; different frequency controls; different output options; different ways of visually indicating activity. And all of these stem from the same basic idea.