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Using a filter


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I want to put a filter on my FM, and have been brainstorming about how to build a filter box that the SID and FM can share.. BUT, I've just realized I need to figure out something even more basic, first.

There's no way to set the initial frequency and resonance. Unlike the SID, I'd have to put knobs on the filter box for the FM, right?

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From the docs:

The AOUT page allows to assign the modulation sources of an instrument to an AOUT channel. Note that analog (CV) outputs are an optional feature which require a MBHP_AOUT, MBHP_AOUT_LC or MBHP_AOUT_NG module.


AOUT: selects the AOUT channel which should be configured (1-8)

Inst: assigns an instrument to the AOUT channel (1-4)

Assg: assigns one or more modulation sources to the AOUT channel (L: first LFO, L: second LFO, E: envelope generator EG5) -- the modulation depth has to be adjusted separately in the LFO/EG5 menu page and is saved with the instrument patch

Offs: adds an offset to the modulation sources (-64..63)

I can point you to integration details (schematic) once you decided which AOUT* variant you would like to use.

I'm using four AOUT_LC modules by myself to control 4 CEM3378 based filters, because it was the cheapest solution some years ago:

See also:

Meanwhile I would prefer the AOUT_NG module, because it's less expensive than AOUT, and less soldering effort than AOUT_LC

Best Regards, Thorsten.

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I get all that. I've got the aout ng built, I've got filters. I've got them working together. (both w/ the SID and the FM. I do have one issue there... SID needs 2 aouts for the four filters, fm needs one.. But that's for another topic)

But, there are obviously no filter control knobs on the FM. So, say I want patch 1 to have the filter frequency at 50%, and patch two to not use the filter at all (so, frequency at 100%). Where do I set that? And, the resonance as well...

I get that I can assign LFO's and EG's to them, and set the level those will affect the filter... But just to leave the filter on steady at 50% vs 100% frequency? Or, doing a filter sweep while I fade in a part....

My guess is I'd have to go in to the menu and use the offset value, right? But, I like to tweak filters a lot, so I'd rather have knobs for it.

(which by the end of the day yesterday, I started thinking about building a dedicated filter box w/ the cv2 or something, to give me full recordable filter control for the FM... But, thats not realistic, as I'd like to save my money and share the filters between the FM and sid.)

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Having built a polivoks filter and being confronted with the same problem, I realized that there is a tradeoff between tight instrument (patch engine) integration vs reusability for other synths, if you don´t want to patch many cables around...

Fortunately, the polivoks filter has dedicated pots for manual cut/res control (which serve as an offset when using control voltage inputs), so i´ve decided to connect it to the MBSEQ AOUT later on, which allows for recording and sequencer automation, and using the filter via the audio mixer fx send/return channels to enable it for all synths... but obviously this disables the benefits of filter control by the synth engine, which is very powerful in both MBFM and MBSID...

It was a hard choice, because I´ve really enjoyed TKs fine demos of the FM engine in conjunction with directly coupled filters!

So, you have to decide for yourself what you need, a possibility would be to use an external MIDI controller/sequencer to manually change cutoff and resonance values, send these changes via MIDI to the MBFM, which will control the "potentiometerless" filter for you, but I don´t know how these CCs are internally "mixed together" with the filter changes generated by the internal patch engine.

Many greets,


Edited by Hawkeye
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Or, just make them analog for real time control (in addition to the CV from the FM), and accept I can't record their values......... (for live stuff and having fun).

The dedicated filter box seems better and better all the time... I have an electrix filter factory, which is a midi, time synchronized filter and lfo. It's very useful. Can use it on drums, turntables, whatever synth, etc.

Building one w/ the cv2 could be very interesting. Not only midi/time synch LFO, but EG's as well. Could make a control surface w/ the LFO and envelope controls. Assign other CV's to the pitch and res, and I think all of that is saveable in a patch.. I'll have to read up on the cv2 again.

But my idea for that is I think (correct me if I'm wrong) I could put it on the same channel as the FM, and when a key is pressed, it would trigger the envelope on the filter box, too... So then the FM's built in EG5 and LFO's are redundant.

Edited by wicked1
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Isn't it possible to addd a couple of rotary encoders and a pushbutton with 4 leds (for instrument selection) and customize the firmware to assign it to the corresponding DIN/DOUT pins connected, then make the knobs control the AOUT overall fixed voltage offset per patch?

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