Audio Combine (2025)

 Audio Combine is a simple audio processor for the [Nonlinear] Dynamics system. It can accommodate gating, filtering, panning, and crossfading.

Audio Combine is named after a piece by composer John Bischoff, whose work I admire; I am hopeful the he can forgive my playful appropriation of his (very good) title.

Functional Explanation

Audio Combine has two monaural input channels, labeled A and B. Each of these channels may operate in either “gate” or “filter” mode, selected via the associated yellow tactile switch. Each channel features a Level control and an associated CV input; however, the minimal nature of the graphics does not fully explain the function of the level controls. Why? Well, when designing the PCB and panel graphics, I worried that the device’s original intended functionality would be too unusual, and I settled on a simpler, more straightforward compromise. Eventually, I rolled back this decision and updated the code but, alas, already had ten panels on hand. So, for the time being, the graphics will remain somewhat misleading.

In Filter mode, the level controls alter the cutoff frequencies of a lowpass and highpass filter, each routed in series. Each filter is non-resonant, and has a slope of -12dB/octave. In the lower half of the travel of the Level slider, the lowpass filter gradually opens; in the upper half of the travel of the slider, the highpass filter’s cutoff frequency gradually increases. The effect is somewhat like a DJ-style “tilt” filter—allowing one to “darken” the sound on the lower half of the control, or “thin out” the sound in the upper half. The associated Continuous CV input and attenuator slider performs unipolar, positive modulation of the Level parameter. This functionality is similar to the multiple “tilt” filtering stages found in my prior map01 and map02 Delta Scan Mapping Interfaces. In Gate mode, the lower half of the Level sliders act as a simple amplitude scaling factor (like a VCA). The upper half of the Level sliders still behave like highpass filters.

The Momentary!! inputs convert incoming control voltages into momentary triggers that briefly apply positive modulation to the filters’ cutoff frequencies. One can think of this as being similar to a “strike” control on a modern low pass gate module. It is useful for building more complex articulated structures than most CV sources will provide; it can be useful to use in tandem with the Continuous CV input for especially interesting/dynamic acoustic envelope shapes.

Each channel features a Location control and associated CV input, which allows for dynamic panning of sounds in the stereo field. Note that the CV input and Location potentiometer controls are summed and the folded into the range of 0–1, where 0 = hard panned left and 1 = hard panned right. So, for instance, a repeating ascending ramp patched to the Location CV input while the Location knob is turned fully counterclockwise will result in a sound that pans from left to right, and then suddenly jumps back to the left position. Keeping the same ascending ramp patched but turning the Location knob fully clockwise will result in the opposite pattern, panning gradually from the right toward the left and then jumping back to the right suddenly at the end of the cycle. This can be useful for scenarios in which inverse/mirrored panning is desired between both channels, or in situations in which it is desirable to have indirect overlap of panning regions for each channel. This feature was, most importantly of all, a more interesting option than any more typical alternative I considered.

The stereo-panned outputs from each of the two channels are then passed to a stereo crossfader labeled Source Blend. The crossfade amount is determined via the Source Blend potentiometer and associated CV input + attenuverter. The crossfader uses a special curve in which the amplitude of either source is only altered when the crossfader enters the opposite source’s “side.” This maintains a greater degree of amplitude independence between channels, and is still convincingly smooth/continuous. The A and B LEDs illuminate to indicate the current balance between channels. Naturally, voltage control of the crossfader position can be used to impart dynamic articulation onto sounds. Likewise, depending on the sound passed into each channel, a number of other panning and filtering-related effects can be realized by modulating the crossfader.

There are many interesting use cases, for instance, in which sending the same sound into each of Audio Combine’s input channels can yield interesting and fruitful results. Notably, the two Nonlinearity outputs from the Compact Oscillator Network are inverse in amplitude; when selecting the same transfer function for both outputs, it becomes possible to apply Audio Combine to produce fairly predictable variable-width bandpass filtering effects—a trick inspired by the Hordijk twin peak/inverse summing methodology.

Similarly, sending the same or similar audio into each of Audio Combine’s audio inputs and applying different styles of modulation to each channel can be an interesting way of crafting more complex articulative structures. Combining this with automated control of the crossfader and/or panning can be quite fruitful.

Audio Combine is a concept/prototype developed primarily for personal use. It is not available for sale.