3372 VCF

[gavgomad]

Hey all,

Well, as it looks like there are a couple group buys for the 3372 which look like they may pan out, I thought I might start a thread on the subject. It was right to the wire whether I was jumping SSM2044 or CEM3372, but the multimode filter possibilities made the decision for me....

Initially, I think I may keep to the basic 4-Pole Low Pass design so that I can get things going sooner than later. I would, however, like to implement the Xpander design further down the road.... I think that some discussion and collective effort in both layers of the project would be helpful?

As my 3372s will be going into my full version of the "Wilba Beast", I'm thinking in stereo pairs.... The basic CV controls would be Frequency, Resonance, and VCA for each channel (6/8 CV channels spoken for). I am including a separate pair of VCAs per voice (via an SSM2164; 1/2 2164 per voice) running off of the same CV signal to control feedback. The last CV channel would drive a single ADC (see this link for the general idea: http://m.bareille.free.fr/modular1/vcf_multimode/extrapole.htm) which will control two channels of CMOS switches to derive the filter functions.

I figure that with some creativity, the ADC circuitry could be combined with two channels of switched 3372s on a single sided pcb.... My 6582 is going into a three space rack anyway, so I have lots of acreage to play with for stacking purposes. As with any filter design, as we're going for CPU control, the inputs can be drastically pared down....

Anyone else toying around with the 3372 out there?

Gavin.

[SounDuke]

Ahoyhoy Gav!

I have some 3372, not had already the chance to test them to my sid (waiting for aoutng). Thanks for the link tho!

[nebula]

I wish I could hear that filter ... looks awesome.

But since it's not going in a modular, I sorta question the merits of making the mode selection voltage controlled if you intend to use this as part of a MB-SID.  Wouldn't it be easier to just add some DOUTs?

[Ashiman]

i would like to see the phase shift of diffderent types

[gavgomad]

Hey all!

Been off the boards a couple of days, so I didn't see the recent activity....

My thought on the voltage controlled selection of the filter mode was that as the 16 modes are divided (theoretically evenly) across the 10v CV range, there will be a relatively stable voltage range which correlates to a particular mode. Therefore, if I want the 1 pole lowpass option for a particular patch, VOILA! It's storeable as an AOUT setting....

Also, the "phase shift" filters in this design would be quite unique!

Basically, in this instance, the 3372 would far outdo the range of options from the SID filter, and still be storable in each patch. Still haven't had the time to bang around with the PCB for such a beast, though... maybe over upcoming hols I can take a stab - my brain needs lots of time, as frankly PCBs aren't my strength!

Gav

[nebula]

True, but ... you would need a D/A plus an A/D when the circuits are right there beside each other.  Voltage control would work, but would add extra expense and more to go wrong.  If, down the road, either of those components goes out of calibration, most of your patches would end up using the wrong filter mode!

A standard DOUTX4 module would be usable to select 16 modes from 4 filters.

Looking at this schematic you could lose the A/D and everything to the left of it, and connect the 4 digital lines directly to half of a 74HC595 on a DOUT.

I think if external filter mode switching were ever to be added to the MIDIbox app, it would use this method instead of the voltage-control method.  As it is, Seppo's SSM2044 board in development provides for a simple digital on/off control.  If we think of "on" and "off" as two possible filter "modes", then this scheme becomes scalable (interfacing with a hypothetical 256-mode filter would not be a problem, but would you want to use a CV to switch between 256 modes?)

The filter "modes" could then be named by the builder as part of a table when compiling the SID app.

[gavgomad]

I agree with you there, Nebula.... My thought was simply that I had a free CV out anyway (once I build the hardware, of course! ;-P), and it would be easier to cycle through the modes with a cv than actually going into the external switch window in the SID architecture and turning on the required x-number of the four switches.... +1 switch if you add a true bypass....

In some ways I regret not sticking with the 2044 given that Seppo's already "on the job" on a board (sounds nice too.... some cmos bypassing it sounds like from your post?), but I couldn't pass up the possibility of multiple modes of the 3372....

My kludgy implementation also falls short in that you still need some sort of hard-wired display for the mode for each voice. If such an implementation made it into the SID architecture, that would be ideal, but my skills are lowly on the programming side. Maybe once I come up with a board for this beast (I can try it without the A/D architecture - it'll be awhile anyway!), some kind soul with a similar interest might meet me halfway and contribute a bit of a code insertion for the SID app.... Basically would require naming of the modes, and linkage of the four switches to act as one total binary output as you step through the modes.... Dunno how many people are looking at 3372s, though, given that absent a group buy (which I was lucky enough to get in on) they're rare and pricey....

I may try a simple 4-Pole LP implementation with bypass switch to start with on the 3372 anyway, so I can get some sound going! ;-)

Gav

[drsyncenstein]

I thought about this too, have the 3372's waiting on the shelf until the wilbabox is ready. But as you know, the problem is adapting the software.

[nebula]

In some ways I regret not sticking with the 2044 given that Seppo's already "on the job" on a board

Without innovation there is only stagnation!  I find the 3372 concept more intriguing, but probably out of my league, unless the CEM fairy sprinkles my house with filter chips.

(sounds nice too.... some cmos bypassing it sounds like from your post?)

I was surfing the Wiki (yes I do have a life thank-you) and found this:  http://www.midibox.org/dokuwiki/ssm2044_pcb

My thought was simply that I had a free CV out anyway (once I build the hardware, of course! ;-P), and it would be easier to cycle through the modes with a cv than actually going into the external switch window in the SID architecture and turning on the required x-number of the four switches.... +1 switch if you add a true bypass....

...If such an implementation made it into the SID architecture, that would be ideal...

The MB-SID application already allows envelopes and LFOs to be routed to CV for external filters.  I wish I had more experience than what the documentation suggests, but I hope to be getting sounds out of my MB-6582 by the end of next week (after which I may be able to speak with more nameity).

Since I assume the mode selection isn't there in the MB-SID app anyway, you could probably do as well for now with 4 toggle switches  ;D

Regardless, I don't think it would be unreasonable to expect a switched "filter on/off" parameter suitable for Seppo's board, which could be enhanced to be a list of choices other than on/off.  By making this control a user-selectable number of bits, you could theoretically tack on as many filters as you want (CEM, SSM, ladder, 303, whatever), pick from them within the SID UI, and store them with your patches.

By the way please don't think I'm telling you to do or not to do anything.  This is "Design Concepts", so I am just sharing my thoughts.

Cheers

[TK.]

From the MBSID V2 changelog:

  o 8 external switches can now be connected to 
     - DOUT shift register (-> DEFAULT_EXT_SWITCH_DOUT != 0)
     - port J5 of the core mode (-> DEFAULT_J5_FUNCTION == 3)
    The switch states are stored in the patch structure, and
    can be accessed via Control Surface (EXT menu) or via Parameter Change
    (NRPNs/Knobs/Wavetables)
[/code]
Note: if you want to sweep over different filter modes, just write the switch selections into a wavetable, and control the position from the modulation matrix. In difference to the CV solution this has the advantage, that you can easily select the interesting combinations, and change the order. 
Best Regards, Thorsten.

[gavgomad]

Nebula - thanks for contributing on this! I'm more than happy to have many viewpoints on this, as I'm still not clear on where I want this to go! Please, suggest away! My hope was to get some free discussion from those with an interest in the 3372! ;-)

TK - this addition in V2 is what originally drew me to the idea of the multimode vcf, actually.... I must admit, automating mode changes in a wavetable could get SCARY indeed! ;-P

The way I see it, TK gives us one POWERFUL synthesizer with an elegant interface, then builds in the possibility for even MORE power (ie. multimode through the switches), but it then becomes OUR responsibility if we want that added power to look more elegant and be more closely integrated into the interface. ;-)

I think the puzzle for those of us with 3372s is how to make the multimode function "elegant", as it were.... Right now, you can go into the external switches page, and if you want a 2-pole high pass, take the four switches and go: "on-off-on-on". But maybe its more of a 2-pole band pass you want: "on-on-on-off"....

Here's a quick thought.... Keep in mind I'm still at the planning and parts stage.... It should be easy to read the current filter mode from four sets of four LEDs (one for each pair to indicate mode - I think this is sufficient control, while leaving four switches for other effects, external vcf bypass etc.). Maybe a simple four-button solution on the CS that only affects the currently selected SID engine(s) that are dedicated to the four switches? It would be easy enough to do up a little legend on the panel to follow....

Not quite as elegant as what you propose, Nebula, (ie. twist a knob, the LCD reports the mode) but probably at least doable for a programming plebe like me! ;-)

Gav.

[gavgomad]

STOP THE PRESSES.... Just thinking more about this....

From the MBSID V2 changelog:

  o 8 external switches can now be connected to 
     - DOUT shift register (-> DEFAULT_EXT_SWITCH_DOUT != 0)
     - port J5 of the core mode (-> DEFAULT_J5_FUNCTION == 3)
    The switch states are stored in the patch structure, and
    can be accessed via Control Surface (EXT menu) or via Parameter Change
    (NRPNs/Knobs/Wavetables)

Note: if you want to sweep over different filter modes, just write the switch selections into a wavetable, and control the position from the modulation matrix. In difference to the CV solution this has the advantage, that you can easily select the interesting combinations, and change the order.

Best Regards, Thorsten.

I may have missed some additional genius here.... So rather than the simple binary 0-15 in order, we do a wavetable to set the switches, in the order we want, evenly spaced over a modulation range so that 0-7 or whatever = 4P LP etc.... Then  simply set up a modulation destination  to the wavetable position, and either modulate with an LFO/EG/Whatever (could get pretty sick!!), or simply assign a knob in the knobs layer as a source?

Correction. PURE genius TK! ;-)

Now I'd BETTER get to work on a draft of that 3372 board over the holidays! ;-)

Gav.

PS. Just noticed, by saving that CV, I can now voltage control inputs A and B into the 3372 mixers. Input A will always be the input from the SID belonging to that channel (1=SID 1 Left / 2= SID 1 Right), and Input B can be it's partner in crime. That way, by adjusting the Input B CV, essentially you can create a pure mono mix by combining the AB at equal levels?

Plus you get a VCF Pre Gain control for both A and B! ;-)

The 3372 is sounding better and better!

[nebula]

Hey, chipforbrains is selling 3372's at 12 for USD $72 + $14 shipping.  That's $86 for 12, i.e. under 8 bucks a pop, shipped.

http://cgi.ebay.com/12x-sequential-circuits-T8-Xpander-cem3372-vcf-chip-NEW_W0QQitemZ300177585039QQihZ020QQcategoryZ20080QQrdZ1QQssPageNameZWD2VQQcmdZViewItem

[drsyncenstein]

@ TK; that's pretty forward thinking! I did not know this, but it will open a lot of possibilities

@nebula; chipsforbrains has these regularly for reasonable prices. i got mine there too.