Developing your own analog signal processing board

Thanks to the well-defined border between the analog and digital subsystems of the Shruthi-1, it is easy to design an alternative analog signal processing board. Such a board will include a power supply, probably a VCF and a VCA, but you are in no way restricted to this configuration. For example, the analog signal processing board may also contain:

  • Additional signal processing elements such as distortions, waveshappers (VC or not) throughout the signal chain
  • Additional connectors breaking out the CV inputs and outputs
  • Sensors, pots, envelope followers… generating control signals
  • Delay, chorus elements
  • An additional sound generator tracking the frequency of the digital oscillators’ frequency…

There are so many ways in which the DCO->VCF->VCA paradigm can be extended! But independently of what the board actually implements, we will use the term “analog signal processing board” (ASPB) to refer to it.

Requirements

Inputs

The Shruthi-1’s digital board expects to receive the following from the ASPB:

  • Ground, regulated +5V rail. The control board will not draw more than 100mA.
  • Four control voltages in the 0-5V range. These are directly connected to the ADC pins of the ATMega644p. Thus, those voltage should not go outside of the -0.5V..5.5V range. If the CVs you send to the Shruthi-1 are expected to be in a wider range, a resistor divider and a 5V zener to ground can act as a minimalistic voltage scaling and protection circuit. It is also possible to leave these pins floating, or to ground them.

Outputs

The Shruthi-1 outputs 6 PWM signals with a carrier frequency of 39062Hz:

  • OSC: oscillators mix
  • VCA: VCA gain
  • FREQ: Filter cutoff CV
  • Q: Filter resonance CV
  • CV1: Routable CV1
  • CV2: Routable CV2

For hardcore hacking, there’s an extra digital line, labelled TX< connected to the second UART of the ATMega644p. This might be used in case the ASPB contains a digital section that needs a more sophisticated level of communication with the main MCU. The firmware will have to be modified to support this, though!

Recommendations

The recommendations in this section are just… recommendations. They can be ignored, but an ASPB will benefit from a better integration with the system if these rules are followed:

Mechanical constraints

The ASPB should have the same size as the filter board, with holes in the same locations, and with the audio connectors placed in such a way that the two boards can be stacked together. In case more board space is required, increasing the width of the board is a recommended option.

Power supply

A nice power supply section:

  • allows the user to power the ASPB from a wall-wart with a commonly found voltage (7.5V to 12V) or from a 9V battery.
  • Is robust to a wide range of input voltages up to +15V.
  • Is robust to reversed polarity.

Working with a single-rail supply, or generating a negative rail with a DC-DC converter are two equally respectable approaches. The supply used in the SMR-4 and CEM3379 board does its job quite well… Its main advantage is that it handles up to +15V, doesn’t require fancy LDOs, and has a switching frequency well above the audio range.

A main power switch is definitely not a requirement.

CV conditioning

The CV signals are internally generated at 976 Hz. Thus, filtering everything above this frequency is helpful, especially because this kills the PWM carrier at 39kHz. Many circuits have a CV summer / scaler: it’s only a matter of adding a cap there with the right value.

The cutoff CV is expected to be translated into an exponential cutoff frequency, with a 0.4675V/Hz scale, and a minimal cutoff in the 10-20 Hz range (so that the maximum cutoff is above the limit of the human hearing range).

The resonance circuit should be designed in such a way that when the CV is equal to +5V, the filter self-oscillates.

CV routing

If the ASPB doesn’t include a VCA, it is not a very good thing to reuse the VCA CV for a totally unrelated purpose. If the board doesn’t include a VCF, reusing the resonance and cutoff CVs for other purposes is OK, as long as these purposes somehow relate to the signal’s frequency content.

As a rule of thumb, patches from the preset library should not sound like bursts of white noise when played through the board. They don’t have to sound the same either (what would be the point?)!

Audio signal path

While this is not done on the SMR-4 board, it is advised to use a dual op-amp chip for the input mixing and final output stage, so that users will be able to replace the op-amp by a higher quality one if necessary.

Gain/volume pots are not a requirement.

Resources

Board template

In the shruthi/hardware_design/pcb directory of the source code, you’ll find the board and schematics files Shruthi-Analog-template-v01. They contain an empty ASPB with the connectors and holes already in place.

Playground board

This board is useful for quickly getting started on a filter design project: it already contains a +/-5V supply, breaks out all the connectors to the digital board, provides an input and output amplification stage, and, last but not least, a proto area. It is described in the Shruthi-Analog-dev board and schematics file available from the source code distribution.

A question…

Q: Can I sell the Shruthi-1 filter boards I have designed?
A: Of course! The cc-by-nc-sa licence applies to the original filter boards from Mutable Instruments, not to the template and playground board upon which you might base your design.