The DiSH
A Digitally-Controlled Sample/Hold Effect
Overview
I have to admit, the blurping, blooping sound of a filter sample/hold effect has always been something I've wanted to make. However, the many Maestro FSH-1 clones out there have their share of issues. Many have ended up in a junk heap and others suffer from noise. Then I came across the Subdecay Proteus. I thought it was quite innovative with the tap tempo and multiplier settings and I thought it would be so cool to build one. Well, I didn't have the skills for a good while, but several months ago I started getting in to microcontrollers and studying the FSH-1 schematics.
At its core, the FSH-1 is a voltage controlled filter (VCF), where the filter characteristics are controlled by a control voltage (CV) supplied to two pins of the main chip. Turns out a little microcontroller can easily supply this level of voltage, so I started writing code and breadboarding. Several weeks later, I finally emerged victorious.
The DiSH (Digitally-controlled Sample/Hold) has a few cool features:
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Tap Tempo
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Manual speed adjustment for fine tuning
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Multiplier selection of 1, 2, or 4
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A pattern mode that uses a full length pulse followed by two half-length pulses
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No noise transistor selection or complicated biasing
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Minimum voltage level to fine tune the filter range
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A minimum voltage level difference between old and new value so that you don't get multiple samples of a very similar filter setting
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Provided code so you can tweak it to taste!
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In addition to the above, I removed the envelope filter portion of the circuit, as it simply is not an effect that I would use. Plus, I already have that effect elsewhere.
How It Works
The heart of the DiSH is a voltage-controlled filter (VCF) that is created by two operational transconductance amplifiers (OTA). OTA's differ from a standard op amp in that an OTA converts a voltage in to a current out. The VCF is created using the LM13700 dual OTA. This topology is fairly standard and is extremely similar to the VCF in the FSH-1.
DiSH Signal Path
The control voltage for the VCF is provided at the point labeled "TAP" in the schematic above. The control voltage generation, tap tempo control, and tempo indicator LED signals are all produced using an ATTiny85 microcontroller. The configuration is shown in the schematic snipped below. The switch on the encoder is used for tempo division and the control voltage output is labeled "TAP_RAW". This signal is then low pass filtered to remove the PWM high frequencies and goes through a trim pot that is set up as a voltage divider. The setting of this trimmer governs how strong the maximum effect is with the sensitivity control set to maximum. This is really subjective, so play with it, but I found that somewhere around the 30-50% region was what I preferred.
DiSH Digital Section
The VCF runs off of +/-9V bipolar power, so a MAX1044 (or similar) is used for the negative voltage conversion. There are also pads for the bypass indicator LED. It's pretty straightforward.
DiSH Power Section
All of this fits easily in a 125B enclosure. All of the code, board layout, and build documents can be downloaded here.