Introduction

The core idea of the DHO was to solve a fundamental issue that complex oscillators usually have. Despite their amazing sonic potential, complex oscillators are very difficult to take full advantage of as they also require a large amount of modulation to give life to the sound they produce. To solve this issue, we designed an analog complex oscillator with internal modulation based on video game algorithms to facilitate this lively sound creation process.

The Dual Harmonic Oscillator, or DHO in short, is a complex oscillator where each of the two oscillators generates 4 octaves of triangle waves. The mix of these waves is used to modulate the other oscillator using through-zero FM. The two oscillators are then mixed into an analog wavefolder (designed by Befaco).

We then used simplex noise to modulate all the parameters, allowing for 10 parameters to be modulated using just one CV. Additionally, there are also 10 internal LFOs that one can use to control any of the parameters.

This module is, as far as we know, the first Eurorack module featuring haptic feedback. This allows the module to communicate information back to the user in a fashion that we are now all accustomed to through the wide usage of this technology in smartphones.

The inspiration

The DHO features internal modulation based on simplex noise — a parametric noise most commonly used for map generation in video games. When looking at a topological map in one dimension, the hills and valleys made us realise that it would be a perfect modulation source for synthesizers.

At ATOV, we are big fans of randomly generated patch systems found on synths like the Waldorf Blofeld. However, these have a major flaw: there isn't usually an undo button. Other instruments like the Intellijel Shapeshifter or the Melbourne Instruments NINA have a great feature that allows interpolation between the values on the front panel and values stored in a saved preset.

We decided to implement simplex noise as a source of modulation. Applied to one parameter, this noise will offset the set value up or down. Moving through the map causes this value to move up and down in a way that appears random to humans. As we have 10 instances of this noise, the offset value on each parameter will be different — meaning that if one turns the MOD encoder infinitely, eventually all possible combinations of parameters will be found.

The great thing about simplex noise is that even if it appears random to a human, it is in fact parametric. This means that if the same input values are given to the algorithm, the same output will come out. In the context of the DHO, this means that one can simply turn the MOD encoder the other way to retrieve previous results.

Furthermore, as this is a continuous parameter, we could add CV control that allows exploring around the coordinate set by the MOD encoder — enabling modulation of up to 10 parameters using just one CV.

We took the opportunity to make this complex instrument even more usable by implementing LFO modulations. One can apply LFOs instead of an offset, in which case the simplex noise controls the frequency of the LFO. The aim of these LFOs is to give life and movement to an otherwise static tone.

If more specific or controlled modulation is desired, there are CV inputs and attenuators for all but two parameters.

Interface

The DHO features two identical oscillators, and most of the controls are repeated for each oscillator. The controls are organised in rows — the first row corresponds to oscillator 1, and the second row to oscillator 2. The same applies to the CV attenuation section and the input/output jacks.

The modulation input CV range is +/-5V. All other CV inputs have a range of 0–5V and have an attenuator to reduce the modulation range.

Haptic feedback: The DHO is equipped with a vibration motor that provides haptic feedback for actions performed on the DHO. Please note that the performance of the haptic feedback is strongly dependent on the mass it has to move — the heavier the case, the less effect will be felt. This can be solved by slightly loosening the screws holding the DHO.

Powering the DHO

The DHO is powered using a Eurorack power supply and runs on +/-12V. The power connector is shrouded and the power lines are diode-protected.

Connect the provided 10-to-16-pin ribbon power cable from your Eurorack bus board to the module. If you are not using the provided cable, always make sure that the -12V aligns with the red line on the cable.

Here is a 30-minute video about how to plug your Eurorack module correctly.

The DHO consumes 215mA at +12V on average and can peak up to 340mA with haptic feedback. Haptic feedback can be deactivated — see the Config Menu section for details.

The DHO consumes 140mA at -12V.

Tuning

As the tuning is digitally controlled, we could implement a few tricks that most oscillators are not capable of.

Two encoders allow tuning up and down the frequency of each oscillator. The tuning resolution can be selected by pressing the tuning resolution buttons to the left of the encoders, and the current state is indicated by the LED to the right of the encoders.

The following resolutions are available:

• Octave
• Semitone
• Fine-tuning
• Off (in this mode the encoder is deactivated)

Tuning modes: To switch between modes, hold the first tuning resolution button and tap the second one.

• Independent tuning: The two oscillators are tuned independently. In this mode, the LEDs of both tuning resolution buttons are solid.
• Linked tuning: The encoder of oscillator 1 controls the tuning of both oscillators, and the encoder of oscillator 2 tunes only the second oscillator. In this mode, the LED of the first tuning resolution button is solid and the second is breathing.

VCO/LFO mode: The tuning range of the oscillators can be changed by pressing and holding the tuning resolution button and turning the encoder left for LFO range or right for VCO range. The haptic feedback will send a pulse to confirm the operation.

• VCO mode: Frequency tunable between 20Hz and 1.2kHz. The LED indicating the tuning resolution is solid.
• LFO mode: Frequency tunable between 27mHz (37 seconds) and 2Hz (0.5s). The LED indicating the tuning resolution is breathing.

External control: To control the tuning of the oscillators externally, use the V/Oct inputs. The V/Oct input for the first oscillator is normalised to the second one — this connection can be broken by plugging a cable into the V/Oct input of the second oscillator. These inputs are calibrated to follow the volt-per-octave Eurorack standard.

Sound shaping

Harmonics: The DHO is a harmonic oscillator — each oscillator generates 4 octaves of triangle waves. The frequency of the second harmonic is twice the first, the third is four times, and the fourth is eight times the first.

The volume of each harmonic is controlled by the H1-4 potentiometer, with the internally set value represented by the LED to the bottom right of the control. This can also be modulated with external CV via the Harmonics CV (H1-4) inputs, attenuated by the FM CV potentiometers before being mixed with the internal setting. The resulting mix is outputted to the corresponding VCO out jacks.

Harmonic outputs: Each harmonic has a direct output. These outputs are pre-VCA and are not affected by the harmonic volume.

Frequency Modulation: The DHO features through-zero frequency modulation — oscillator 1 modulates oscillator 2, and oscillator 2 modulates oscillator 1. For more on the differences between FM types, see this article from Learning Modular.

The output of VCO 1 is normalised to FM input 2, and VCO 2 is connected to FM input 1. These connections can be broken by plugging a cable into the corresponding FM input. The amount of FM is controlled by the FM1/FM2 potentiometer, with the LED indicating the internally set value. External CV via the FM CV inputs is attenuated before being mixed with the internal setting.

Internal modulation

One of the very special features of the DHO is its internal modulation system based on simplex noise — an algorithm commonly used in video games for procedurally generated terrains and textures.

This noise applies an offset to any of the harmonic volume or FM amount parameters. Turning the MOD encoder travels through the map, and the offset value will differ for all 10 independent instances of noise running in the DHO, allowing all possible combinations of parameters. As this is algorithmic and not random, the same input always results in the same output — meaning you can undo changes by turning the MOD encoder in the other direction.

This can also be controlled via CV using the MOD input and its attenuator (range: -5V to +5V), allowing scanning through the simplex noise map around the coordinate set by the MOD encoder.

Assigning modulation: Press the MOD button to enter the modulation page (MOD button LED lights up). On this page, the FM and H1-4 potentiometers set the amount of modulation for each parameter, and the LED next to each parameter indicates the amount assigned.

Modulation type: While holding the MOD button, turn the destination control potentiometer (FM1-2 or H1-4): left for offset modulation (LED off), right for LFO modulation (LED breathing). Haptic feedback triggers when the modulation type changes.

Clear all modulations: Press and hold both tuning resolution buttons for 4 seconds. All modulation will be cleared and the current potentiometer settings will be applied.

Mixing and Folder section

The DHO features a small mixer section and a wavefolder. The VCO1 and VCO2 volume potentiometers control the volume of each oscillator sent to the wavefolder. Note that the mixer is internally DC coupled — if one oscillator is in LFO mode, it will control the distortion symmetry of the wavefolder.

External Signal: The MIX IN input adds a signal into the mixer. This input is DC coupled, so any low-frequency or DC signal can be used to affect the symmetry of the wavefolder.

Wavefolder: The wavefolder is a four-stage diode circuit based on circuit B of the Befaco Chopping Kinky, with the voltage control section completely modified. The wavefolder potentiometer controls the intensity of the effect, with a corresponding CV input and attenuator. At its minimum setting, the wavefolder acts as a limiter, folding all signals outside +/-5V to stay within the Eurorack standard range. For a purer signal, decrease the VCO volumes or use the individual VCO outputs and bypass the wavefolder entirely.

Calibration

If the tracking of the DHO is off, the V/Oct input tracking can be recalibrated using the calibration trimmers on the back of the module. We have published an online tool to perform the calibration in one pass:

Single Pass Oscillator Calibration Tool

After performing this calibration, the internal calibration should be performed again — see the Config menu — Internal calibration section below.

Config menu

To enter the configuration menu, press and hold the MOD button while turning your system on. The MOD button LED will flash to indicate you are in the configuration menu.

Haptic On/Off: Press the oscillator 1 tuning resolution button to switch haptic feedback on (LED on) or off (LED off).

Version check: Press the oscillator 2 tuning resolution button to display the firmware version. The current version is V1.0. The LEDs should appear as follows:

Press the MOD button to exit the version check page.

Internal calibration: The tuning of the DHO is digitally controlled via two DACs that send voltages to each oscillator to offset their frequency, requiring precise calibration. If octave shifts are not accurate when turning the encoders, recalibration may be needed.

While in the configuration menu, press and hold both tuning resolution buttons. The LEDs in these buttons will turn on, and the LEDs next to the FM and harmonic volume knobs will light up one after the other to indicate progress. This process takes about 1 minute, after which the module restarts normal operation.

Exit configuration menu: Press the MOD button to resume normal operations.

Firmware update

WARNING: the micro-USB connector is fragile — please handle it with care.

Start by checking the firmware version of your unit using the procedure described in the Config Menu section above. The current version is V1.0.

Procedure:

1. Turn off your Eurorack system.
2. Connect the DHO to a computer using a micro-USB cable.
3. Power the module while holding the USB BOOT button situated on the edge of the second PCB.
4. The DHO will appear as a storage device named RPI-RP2.
5. Drag and drop the firmware file into this storage device.
6. The unit will automatically restart.

The Expander — DHOex

The DHO ships with its expander (DHOex), designed to bring more traditional oscillator features to the DHO. As with the main module, the DHOex is organised by rows of controls corresponding to each oscillator.

Note: All outputs are calibrated to +/-5V.

Connecting the DHOex: Use the provided 12-pin ribbon connector. Mind the orientation — connect it to the connector on the back of the DHOex and to the expander connector on the main module.

SYNC Inputs: These are the sync inputs for the corresponding oscillator. The DHOex features inputs for both hard (HRD) and soft (SFT) sync, with comparators to allow non-square waves as sync inputs — enabling the use of individual harmonic outputs or VCO outputs.

PWM: The pulse width of the square wave can be modulated using the PWM CV input. The PW1-2 potentiometers function is set by the switch on the back: in position A, CV is mixed with the potentiometer value; in position B, the CV input is attenuated by the potentiometer. Two switches are available, one per pulse width control. Note: the PWM on this module features hysteresis and is derived from the sine wave (rather than triangle or sawtooth as in most synthesizers), giving a different flavour to this classic sound.

Waveform Outputs: Calibrated to +/-5V, following the frequency of the base harmonic of each oscillator. Available waveforms: sine, square with PWM, and sawtooth.

Acknowledgements

We would like to express our deepest gratitude to all the testers who dedicated their time and expertise to help us refine and improve the DHO. Your commitment and valuable insights have been hugely important in making the DHO what it is now.

A huge thank you to:

Marius Kramer aka Bassinfected, owner of the label Filter Movement and an incredibly skilled mixing and mastering engineer.

Steven Williams aka Drusnoise, sustainability scientist and live performer. Steve also runs great workshops on how to perform live with music gear and is a great supporter of the live-performing electronic musician community in Berlin through the Berlin Modular Society collective.