Z-plane filter for shruthi?

Since I soldered my Shruthi 1 with SMR mk ll filterboard I started thinking about other options as well. One legendary filter is the EMU Z-plane filter. Reading through the available documentation, the filter core itself is created from one low pass with cutoff and Q parameter, and 6 parametric filters with frequency, Q and gain. Emu created a number of filter presets to create typical and unique filter responses. The user is not bothered with the 20 parameters directly, but picks just from the preconfigured settings.

What makes ist special, is the way the filters are handled in realtime: filters are inrerpolated for frequeny follow, time and some morph parameter that can be assigned to controllers like mod wheel, velocity or aftertouch.

What do you think, would it be feasible to build such a filter monster with 20 parameters for the shruthi? Are there any filter cores that would be prominent candidates for that?

I am curious about your opinions about that.

From what i know the Z-plane is a digital filter. You could probably recreate it in analog but i think it would be far to big and complex for a shruthi.
If you want to design a filter there probably are still some strange analog filter designs left to try out first.
What about a shruthi with Low Pass Gate? And the wasp filter is also still on the list.

You may be best off with the Dual SVF Shruthi filter.

The other option is to add a “separation” parameter to the 4PM filter. This way you could space the relative cutoff value of the filter poles. That would act a bit like 4 separate EQs. No idea how good it would sound though.

Before I forget, here is a discussion where I describe the sound of using the 4PM filter in the wrong mode. I guess what I am trying to say is that the other filters are very flexible compared to other standard filters.

If you are interested you can even do the SDE mods to your SMR4MKII.

@ shiftr I jjust read what is a low pas gate

but tell me if im wrong, it is something possible to simulate with the matrixmods of the Shruthi…?

@Denis … I just mentioned the Low Pass gate as an exotic example. You could get something LPG like maybe with setting the VCA open and using the filter to cut of the notes.

I think there are maybe also some nice filters to think of by combining the idea’s of the 4PM the SMR4 SDE and some of tubeohms designs.

@Denis: A Low Pass Gate (LPG) is basically a VCF and a VCA in one module. The major difference comes from the use of vactrols.
While a vactrol turns on very fast, it takes a while to turn off lending itself very well to percussive patches. One only has to ping a LPG with a gate trigger to get a short percussive blip.

To recreate this in the mod matrix, you can assign the same envelope to the VCA and VCF cutoff. Make sure to use a fast attack and a slower decay. You can even trigger the envelope with an LFO to simulate the exact ping example above.

If you want to get extra fancy, add a lag generator to the envelope with one of the operators to get “smoother” transitions.

it’s interesting, I will try this soon, and also with the Oberheim matrix 6r I am about to have :slight_smile:

So a vactrol based shruthi filter board with a wave folder and a LPG?

@Denis: Call me a kill joy, but I wouldn’t recommend modifying the Oberheim filter IC. You could remove it and add your own filter, but then you would still be stuck with only the variables the firmware has set.

The irony of a z-plane filter Shruthi would be that the MCU (something that would require at least a 32-bit MCU running at 100 MHz) could be powerful enough to run the oscillators too. So the overall design would be quite kludgy and inefficient - because things would be much easier to design and build if both the oscillators and filters ran on the same chip.

A wavefolder+LPG Shruthi would be a cool idea… in theory. In practice, it wouldn’t be that great because all the waveforms / oscillator algorithms in the Shruthi are focused on subtractive synthesis, not on the kind of mildly complex tri/sine FM timbres typical of west-coast systems. Also, aliasing/quantization noise doesn’t sound that great through wavefolders/LPGs.

Incidentally, pichenettes, did you consider using the extra PCB real-estate on the XT version of the Shruthi to add a 12 or 16-bit DAC?


I imagine he didn’t want to create a two tier system. Those with XT getting new filter boards and those without not able to use them.

Plus designing two boards and having two types in stock.

Only the filter might need to be tweaked for each design?

I think there should be space also on the normal shruthi board for a dac… I guess it doesn’t make much sense with all the available filterboards which have filters for the PWM signal onboard…

> did you consider using the extra PCB real-estate on the XT version of the Shruthi to add a 12 or 16-bit DAC?

No because:

  • The two peripherals that would allow me to write to a DAC are already used for the MIDI output; and for buttons/LEDs I/O.
  • Even if they had been available, there wouldn’t have been enough CPU resources to write to it (writing to an external device always cause some overhead - except on more advanced MCU with DMA). As a comparison, the Ambika voicecards, which use a similar processor at 99% of its capabilities, do not have to handle MIDI, a sequencer, a user interface, etc.
  • Even then, you have to keep in mind that all computations on the Shruthi are done with an 8-bit resolution, so the only benefit of using a DAC would have been the lack of 39kHz PWM carrier on the output.