Max 261 filter with axoloti?

#1

would it be possible to control one (or in my case up to 5) of these filter chips from the axoloti?

https://www.maximintegrated.com/en/datasheet/index.mvp/id/1488

these are really nice (special and unique sounding) analog filters, cheap and can be programmed from an arduino, see here for an implementation: http://devsound.se/2012/sjs-one
i guess i would need to adapt the relevant arduino code and implement a custom object to control the filter, or would that be a script? i have not touched this area yet. it would be great if somebody could tell me if this could be working or if it is totally out of reach.

a quick look at the source of the arduino library at the link suggests that i need two timers to set the filter clock. and 8 digital outs to control the filter. also spi (soft or hardware) to control some 4053 switches to control the various output modes (the filter outputs them all in parallel)

i actually built and modded the sjs-one and know it quite well. (hardware and software wise) i think those filter chips could add a lot of color to the axoloti palette. after all the marriage of digital signal generation and analog filter is very sucessfull :slight_smile: waldorf microwave...

#2

Extremly dope Idea! I cant practically help you with that but you have my full mental support here :slight_smile:

#3

These look really interesting. I'm curious, however, how the 64-step F value would sound for filter sweep type gestures. Is there any stepping in real use? Or is it just the programmatic setting of F that has such coarse granularity?

#4

I think Floats in Axoloti are not 64 steps but 14bit, so filter sweeps should sound great (just as they would sound in Axoloti :wink:)

#5

Absolutely.

But the MAX261 tech specs state it features "64-Step Center Frequency Control", so I was wondering if there are control limitations inherent to the circuit itself.

#6

yes it will be somewhat steppy for sure. but it is still very useable i think. i will try to hack a version together with axolotl and an arduino that controls the filter via midi (since i have an arduino implementation already) and post some samples. but currently my days are too short...

#7

The MAX621 can also be frequency-swept by adjusting the input clock frequency, generated for instance by a gpio PWM/timer on Axoloti. Current PWM output objects can only adjust the PWM duty cycle but a variation of the PWM objects could be made for this.

#8

you have my attention @johannes :slight_smile:

#9

Wow! Mine as well! Really curious how these filters could round out an axo build.

@johannes, would that render the arduino part redundant?

#10

Yes, as long as you don't want to generate independent clocks for more than three of those filter chips.
I don't plan to develop this, the critical part is getting the power supply right (single ended +5V, or symmetrical +/- 5V supply?), and biasing the audio in- and output (DC coupled or AC coupled?)

#11

the powersupply (single ended) and dc biased audio in, is straigthforward, at least it works very well on the sjs-one. i'm more worried about the coding in axoloti, because i never did it before (scripting etc.)

i will attempt to hack a filter input into the sjs-one and connect the axoloti to it, just to see how it sounds...

#12

As the Axo doesn't have audio outputs per voice you wouldn't be able to have a filter per voice like a poly synth, just one filter on the output (or 2 - one L and one R). If you're going this route anyway you might want to have a look at other analogue filter designs which can be powered by 5v too as you will be able to control them with cv from the analogue out gpio. This will obviously depend on how much additional circuitry you want to add. The best suited to this might be the Wast filter - sounds cool and operates with on single 5v supply. See here:

http://modular.fonik.de/pdf/Wasp%20Filter%20clone.pdf
http://jhaible.com/legacy/tonline_stuff/hj_wasp.html
http://electro-music.com/forum/topic-30946.html

You can replace the ca3080s with an lm13700 and scale and sum the cvs from the analogue out gpios (can't remember which ones they are) for cv control. You could leave the distortion off or add it on too.

#13

thanks for your post.

i saw the wasp filter many times on my internet searches, hehe :slight_smile: looks nice!

i know that the axo only has stereo out, but on a monophonic synth (or paraphonic for that matter) it still works very nice with only one (or two) filters. what is so intruigin about the max261 filter is that it uses almost no extra parts.

#14

It might be easier to use the wasp filter as it already runs on 5v and doesn't need any funny business to control it with the pwm :wink: that's what I'd do anyway.