Digital Filter Module


I hope this is the right forum to ask. :slight_smile:

There are a ton of digital oscillators all sounding great and having many more possibilities than it’s analog counterparts.

But why aren’t there many digital filters? Be it Shruthi, Ambika, Prophet 12 or Peak. They all use digital OSC, but analog filters for making the synth sounding more analog/fatter, etc.

Is it so hard to emulate filters digital? Why just oscillators, but not filters?

Isn’t there a big possibility for a Braids/Plaits type of module with many different filter types via 3 knobs (multi-filter, ladder, morphing, etc.)

I know… Not everything needs to be cloned digitally, but I would love to see a module with many different filter flavours.


Actually, there was a digital filter option for the Shruthi.

Im no coder, but im sure I read that good filters are one of the more difficult things to do well in software… But there are cleverer folk than me around here that could tell you more about that.


I think it might have quite a bit to do with cost and complexity. A stand-alone digital filter module needs to be able to acquire analogue audio signals and convert them to digital form with high fidelity - so that means at least 48Khz sample rate and at least 16 bits, but probably more using an audio codec - and then apply fairly heavy computation to it - so that means a fast MPU with DSP or at least floating point operations done in hardware - such MPUs are now available at reasonable cost - and then the DC-coupled control voltages need to be acquired and converted to digital form at audio sampling rates too, to allow FM etc, but in DC-coupled form. And then you need at least 48kHz and at least 16 bit of DAC output conversion.

Those sorts of specs can be found in the now discontinued Roland ARIA modules (and there are some nice but fairly vanilla digital filters available for them in the customisation tool). But it was clear that Roland was making a large loss on every one it sold, and they were bulky and power-hungry. Some of that was due to some curious design and engineering decisions by Roland (those ARIA modules were clearly a “skunk works” product), but also due to the intrinsic nature of the digital filter beast.

Nevertheless, I bet that we will see some innovative digital filter modules come to market in the near future.



Well, if the Shruthi didn’t have analog filters, people would have thought “meh it’s just a VST in a box”.


Yes, and although modular synths can be defended as a form or creative anachronism, given that they have “somewhat fallen out of time” as Max Stadler observed, it starts to become faintly ridiculous to interface a digital oscillator module with a digital filter module, all modulated by digital LFOs and digital envelope generators, fed through digital delays, and all driven by digital trigger sources and digital sequencers, when all the interfaces between those digital modules are 19th Century technology patch cords carrying analogue voltages… and then the signals are mixed in an analogue mixer before being converted back into digital form by your audio interface, stored and processed digitally in your DAW, and then uploaded and published in various digital audio formats to YouTube, SoundCloud, band camp etc. At some stage, the question needs to be asked: “Do you really need all those ADCs and DACs and audio codecs?” Sacrilege, I know, but may I point out the Mungo State Zero, which was probably somewhat before-its-time, as one way forward that keeps the patch cords but ditches the ADCs and DACs and analogue voltages and signals. But even then…


I wonder, if at some point, we will see the introduction of a digital connection between digital modules so that digital modules (from different manufacturers) can communicate audio and control signals digitally to each other without the need for multiple DA to AD conversions, at least until the need to interface with a analog module is needed.


just my two cents, but although you lose the tactile control, at that point my thinking becomes why not just use Max/MSP or VCV Rack or some other software patching environment.


Digital oscillators + Analog filter will sound more natural than Analog oscillator + Digital filter. I’ve been able to pick out the harshness of a digital filter very easily with all my digital synths. They always sound washier in a sterile way, and usually, during a filter sweep, at least one or two spots in the motion would sound unnatural.

I think digital EQs are the same way, and I use them to be surgical and cut specific frequencies in mixing, but never to completely shape, because of the washy harshness. You can be a lot more extreme with analog filters before they start to sound bad.

Especially with the analog resurgence, I don’t believe a digital hardware filter would be desirable to most. The only time I’d use a digital filter is when it is tied to a digital synth that can only make a unique sound I’m after.


Yes I realised that too. But I’m wondering why digital filters don’ really manage to sound analog at all while there are many digital oscillators with analog sound.
That’s why I’m wondering if it’s so much harder or requires much more processing power to do good analog-like digital filters. Maybe current micro controller are too weak?




Part of the appeal of analog filters and VCAs to me is that they have imperfections all over the place. Be it distortion, noise, CV bleedthrough or weird behaviour when you FM or AM. A little compression can really bring these artifacts to the surface and create a ton of character and vibe.
Of course you can emulate all that. But recreating all those side effects perfectly would require so much effort that in many cases it’s probably simpler to just stay analog in the first place. At least in eurorack-land.

I think the biggest point is that people like to read the word “analog” (whatever that means)


The idea of configurable characteristics and quirks in a filter appeals to me immensely. Or even better, configurable voltage-controllable quirks and character.

There was a line of eurorack modules shown off at, I think, NAMM, maybe last year or the year before, which seemed to use a behind-the-scenes digital bus as well as analogue patch cords. The bus protocol was CANbus, iirc, as used in cars and trucks and trains. However I can’t find any links to it. I’m pretty sure I didn’t just dream the whole thing.


I remember that too. Each module had a small display, and was configurable to multiple functions.
The Teensy has nice CAN implementation.


The Audiothingies Micromonsta is a great example of a fully fledged VA in a box that sounds awesome if not a little vanilla.

I would love to see a fully digital Super Ambika desktop synth from Oliver/@pichenettes with 2 plaits(+ FM with 4 operator mode)-per-voice -> vcas(or whatever) -> ripples/rings/ (and something like clouds) with highend DACs

this would make my heart explode.


you can make it in a skiff… just might be a bit pricey :stuck_out_tongue:

or really anyone could use the open source code to assemble something more economical with less parts.


I’m guessing you’ve mainly dealt with older digital synths? I agree that sometimes you can hear stepping when a full filter sweep is 128 steps. However, those kinds of limitations where largely abandoned quite some time ago. My Alesis Ion, released in 2003, has 8000-someting steps for every filter sweep. You’ll never be able to hear any of those steps, i can tell you that. :grinning:

Also, the very idea that digital = harsh or any other unavoidable sound character is absolute nonsense. A digital filter will sound the way it’s made to sound. You’d never be able to pick out the digital filters in a double blind test of high quality, modern filters.


Oh yes. I’m totally not saying it’s impossible. In fact, a while ago I started a thread at the Axoloti forums where I try to deepen my knowledge in modelling these quirks by working my way from a diode waveshape to a full filter model. I’m not yet there but that is mostly due to lack of time.

But my feeling is that in digital models, you only really can have what the designer specifically implemented. There is less to be discovered by abusing the system. Maybe the possibilities for exploration are what people are missing in the virtual analog domain?

I’m basically trying to understand why most musicians prefer analog over virtual analog. And why I find myself having so much fun with the quirks of my analog synths while totally not enjoying the quirks of digital synths (Blofeld for example. That thing has many, many quirks). It goes against my engineering brain that says “there’s nothing clever math can’t replicate”.


Feel the need to chime in here and agree with lisa: if you get to the highest levels of hardware and software you will never be able to pick out a digital filter. In my ‘professional life’ I am a re-recording mixer for film, and the EQs, filters and dynamics processing modules of the Neve DFC console are superb beyond belief, and this is a desk that was released many years ago.

I understand the question is geared towards synth filters and “affordable” implementations, but I have to call foul on the very idea that one can “pick out” a digital filter.


What about the 4MS SMR? I don’t have one, but really want one. Sounds good to me.


It sounds like you are describing a modular :grinning: