Shruthi as a digital effects processor?

My shruthi works great, but I was wondering if I could use the audio in with my guitar.

I’m not sure how to do it, I think I need midi signals to trigger the output, or can I just play my guitar through shruthi, and choose a patch.

Very curious, but I don’t want to fry anything either.

yeah, you can put a signal into the “audio in” and process it. If you didn’t install a pot at the input jack then you don’t have control over the gain. Any patch will work, and the signal will get mixed with the oscillator, go through the VCF and VCA.

If you want to just process guitar without any other signals, you should set osc. 2 to “none”, and set the mix to 100% osc. 2 to disable the oscillator. Then I think if you put a VCA offset in the modulation matrix, with a positive value, it should open up the VCA so you don’t have to play a note in order to hear the audio through. Then it’s just a matter of modulating the VCF and VCA with LFOs, envelopes, seqs!

First of all, the Shruthi-1 with the “traditional” filter boards can only be used as an analog effects processor. The signal only goes through the VCF and VCA sections, it is not digitized. This limits you to volume and filter changes, but with the LFO and sequencer and modulation routing and all there are a couple of nice effects that can be achieved. If you have the firmware v0.92 and the latest preset banks, there are indeed a few presets prefixed by “in:” which are showing how to process the external input (all oscillators off, VCA opened, crazy modulations on the filter, VCA, and noise generator).

There’s a demand for digital FX too, so this is here that this:

will come into action. Digital filter board+FX for the Shruthi. Neat! Digitizes the signal, runs it through another AVR with filter+FX code, spits it to a DAC. Has an external input too! Will be released as soon as I build one and write the code!

I’m guessing there will be new screen gui in future shruthi updates for parameter tweekage?

cool! unipolar supply! curious what the FX will be! oh TLV rail to rail opamps, any chance you’ll provide these to order? they’re a pain to source (had to import them for wtpa from the US)

btw one question i’ve been wondering about (as i’m mounting all control etc on the panel, offboard), is there a particular reason why you’re using the specific spacing for the volume pots soldering pads?
the way they are i can’t use a 3 header row like for the other pots, so i either have to use 3 individual header (which is not a very steady connection) or solder the cables directly.

anyway, count me in!


Yes, I’ll post a firmware update with a new screen to tweak the FX parameters. You’ll select “dsp” in the filter board selection menu and a new menu will appear on the same page at the filter.


I’ll be doing more stuff in the unipolar department (including a stripped-down version of the SMR-4 with a much lower part count), so get ready to see rail to rail op-amps in more places! They will come with this board, along with a programmed 328p (the silkscreen says 168p because 328p were hard to find for a while… now it seems easier to get them). TLV274 should work too.

I have a “pot” part in my Eagle library with the right spacing for soldering a metal/front panel pot directly to the PCB and that’s what I’m using here. Obviously it doesn’t make sense to solder it there. I haven’t though that people would use header there instead of cables. I’ll change this in future board revisions!

I love this… I’m already trying to figure out how to fit it in to a stomp box, actually I think a nice pedal board would work… maybe something like this.

Great news!
Perhaps you might consider rotating / repositioning the pot pads so they could be used as such and maybe designed into a compact enclosure form factor. And duplicate the connections with header spacing?

Also it looks like you’re summing the signals from the ext. in and the osc. at IC2 before the adc? I’m wondering if it might make more sense to digitize both signals independently so they could be addressed independently by the code? Looks like you’re using all of your analog ins. The SMD 328 has an extra 2 adc’s… :wink:

@smrl: not sure I could have got some wait-free code to sample two inputs at 39kHz and the CVs at more than 1kHz.

Anyway this is not intended to be a stand-alone FX processor - something I might give a shot later, but probably not on AVR.

I was just waiting for another board to come along for my green Shruthi- this could be it!
One thing that might be cool- stereo outputs. That way you could have stereo delay/reverb FX, and panning modulation. Don’t know if there’s physically room on the board for two jacks, but you could use a single stereo socket. I mention it really because I have a Waldorf Miniworks filter, which has mono input and stereo out, and can do nice panning effects.
Just a thought…


Incidentally, just to echo rosch (pun semi-intended): I’d also love to know what the FX on the board will be. Also, I don’t know if this is feasible, but I’m wondering if digitising the filter means multi-mode high/low/band/notch 2/4-pole filters might be possible. If so, that would be very cool! I love the analogue low-pass filters, but also really like high and band-pass variants, and think they’d complement the more harmonically-rich waveforms the Shruthi is capable of producing very nicely.


What is planned for the digital board:

  • 2 poles SVF (so lp, bp, hp), since this looks like the simplest to code and keep stable.
  • Distortion/Fuzz/shaping, pre or post filter.
  • Bitcrushing/Bit reduction, pre or post filter.
  • Flanger, post filter.


  • 4 poles LP.


  • Long delays, unless the signal is downsamples to 10kHz (there’s only 2kb of RAM on board, 512 bytes of which being taken by buffers and book-keeping data).

Note that I will receive soon the prototype of the dual SVF board which will be the way to go for HP, BP, LP, and combinations of.

Cool, cool. The Dual SVF board isn’t the same as the Shruthi-DSP board above, then?


Sorry, scrub that question- just did a search of the forum and found it. The dual SVF looks very cool!