Tuesday, July 17, 2018

Hitchhiker Laboratories susSub

A few years ago, I made a guitar pedal to emulate the sustain substitute function from the Echoplex Digital Pro. Should you need a demonstration of this and other EDP functions, Andre Lafosse has been documenting the EDP through excellent and in-depth videos on his YouTube channel. While I've never played with the EDP, I've gotten hooked on sustain substitute by playing with the Mobius looper plugin that implements many of the EDP's features.

My susSub pedal implements sustain substitute with the Pigtronix Infinity looper. While it could, in principle, work with other loopers that allow you to change the loop feedback level while overdubbing, much here depends on the particularities of the looper's implementation. For instance, any attempt at changing the feedback level while overdubbing on the Boomerang looper produces undesired artifacts that cannot be avoided.

To work its magic, the pedal does two things at once:
  1. The audio signal is gated so that sound is allowed through only when the button is pressed.
  2. Pressing the button switches between two potentiometers that control the Inifity's loop feedback level through the expression pedal input. 
In overdub mode, while the button is pressed, sound is allowed to flow to the Infinity to be recorded into the loop and the loop feedback level changes during the recording. If the "button down" potentiometer is set to 0%, the loop content will be completely replaced with the newly recorded sound. Varying the level of the potentiometer lets the performer retain the previously recorded sounds at a lower level.

While this is pretty close, it is not a completely accurate emulation of the EDP's sustain substitute function as it mutes the dry signal, but this is quite acceptable for my use. A more faithful recreation of the EDP's function would require to split the audio signal in two and blend the dry signal with the loop audio. Something like the Xotic X-Blender would be a good solution, but DIY options abound (for instance the Splitter-Blend pedal could be easily adapted).

The hardware implementation is rather simple. The audio is attenuated through two vactrols (one in series and one in parallels going to ground). These are controlled by the arcade button, although one of them is always receiving the logical inversion of the signal. For instance, when the button is up, the vactrol in series receives 0V and presents a resistance of approximately 10 Mega-ohms. The vactrol in parallels receives 5V and presents resistance between 1.5 and 5 ohms, effectively shunting to ground whatever signal passes through the first resistor. What makes vactrols interesting for this application is the short delay (2.5 ms to turn on and 35 ms to turn off) which create a smooth and fast fade in and fade out, thus removing the clicks usually associated with rapidly switching an audio signal.

The same logic signal is also used to switch between the two potentiometer wipers using a CD4066B analog swtich integrated circuit.

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