[sdiy] MIDI Velocity on analog synths

[Tom Wiltshire]

H Grant,

On 9 Feb 2014, at 15:41, MTG <grant at musictechnologiesgroup.com> wrote:

> I have a couple of questions based on the helpful input from last week.
> 
> On 2/5/2014 7:05 PM, rsdio at sounds.wa.com wrote:
>> 
>> I agree that it would be rather difficult to make the Pro-One ADSR
>> respond to CV directly. You could implement a parallel digital ADSR in
>> the processor, and sum its output on the VCA CV input, but that might
>> require alterations of the ADSR settings when the user makes a patch.
>> Also, I'm not sure whether digital ADSR would perform very well with a
>> small 8-bit.
> 
> I'm not seeing a VCA CV input on the Pro One. It looks to me like each ADSR is set by 4 different pots (rather than the summing amplifier scenario of the VCF, VC0). Unless I'm missing something…

Each one of those pots is just providing a CV to the CEM3310, so in a sense, they're all CV inputs. You could perhaps try feeding some velocity CV into the Attack input to shorten the attack with increasing velocity, but it's likely to involve hacking a mixer in between the pot and the CEM3310.


> On 2/6/2014 2:05 AM, Tom Wiltshire wrote:
> 
> > What the CEM3310 doesn't have that the CEM3312 added is the Level CV > to avoid a separate VCA for amount control. On the Pro-One this isn't > a problem, since it's not a programmable synth and the filter
> > envelope simply goes via a "Filter Envelope Amount" potentiometer. As > Florian says, that would be the ideal destination for a Velocity CV.
> 
> To me that looks like an input to the filter and so how is that different than what I'm doing? I give the user CV control for the cutoff and resonance. I already planned to inject the velocity at those two places (under user selection of course).

There's two different things you could do with the velocity and the filter:

a) Use velocity to control filter cutoff - essentially, turn up the cutoff knob with increasing velocity

b) Use velocity to control the filter envelope amount - turn up the envelope amount knob with increasing velocity.

On programmable synths where the envelope amount is controlled by a VCA, you've got a simple way to do (b). On the Pro-One, where a pot does that job, you haven't. More's the pity.

Incidentally, I was wrong about the CEM3312. I'd been mislead by the marketing speak on the front page. The Peak CV isn't a Level CV, and wouldn't have let you do want you wanted without modifying the Sustain CV too.

> 
> Maybe what Brian is implying is that rather than impress a simple velocity value on the VCF, make my own ADSR that's influenced by the velocity and apply that to the VCF?

Yep. Since you can't easily control the amount of the VCF envelope that's there, make your own. It's one way to solve the problem.
Hack the 3310 the same way that Oberheim did in the OB8 / ECO405 is another way.

> So I guess there is no easy way to control the ADSR's themselves or the volume without a fair bit of construction.

Yeah, that's basically it. None of this is "easy", but all of it is "possible".

> So far the project requires only "wires and resistors" for the CV aspect (as far as carving up the Pro One is concerned no cuts on the PCB, etc).

I think for what you're doing, you've taken the right course. Most people don't want to hack the traces on their beloved old Pro-One, but they'll be very happy with something that just plugs in and gives them a load of new features. I'd try and go as far as you can like that (I loved your extra LFO with MIDI sync - add some more wave shapes!). If you could somehow get another envelope in there, that'd be cool too.

On my own Pro-One, I added two external inputs which patch into the "Direct Source" and "Wheel Mod Source" modulation buses (U101). This lets me stick whatever I want into the synth and then route it wherever I want too. It involves the absolute minimum of track cutting, since all you're really doing is adding 100K resistors to the standard inverting mixer. But my Pro-One is getting a bit semi-modular these days...

Regards,
Tom