Fun With Filters (Pt.4) - Of Envelopes and Filters

If I'm going to use a Yamaha CS style filter with the SS-30M then I need to think about envelopes. The CS synths had some different ideas about envelopes!

Pushing The Envelopes

A typical envelope is ADSR. Attack, Decay, Sustain, Release.

For a filter, the idea is to increase the Attack portion from 0 to the maximum desired level of cut-off. If the filter control is set to Fc = 0 that is. If the filter is already partway open the envelop picks up from there and returns to that level.  Inside the filter module, a summing amplifier is adding the envelope level to the Fc level set by the module's own control.

Negative voltages will shift the cutoff back down, but for an envelope this is just a releative shift to the Fc control setting. You can shift and scale the envelope input, but it will always start from and return to the same voltage, which is the CVin + Fc. This is generally fine and means you can easily invert the filter start from open and close before reopening, using an inverted envelope (useful for bandpass and bandstop more than high and low pass, perhaps), but does limit the control of the filter envelope a bit. 

CS Filter Envelope

The CS-80 VCF has an ADR envelope with two additional Attack settings. The missing Sustain setting is actually determined by the combination of VCF cutoff frequency (VCFc) and Brilliance. The two additional settings are Initial Level (IL) and Attack Level (AL). IL is a setting below the Sustain level(Sl) and AL is a setting above it. When an envelope starts the Attack portion increases from a level determined by Sl minus IL. It then rises past Sl up to AL over the Attack time (At) period. As normal, it then decreases over the Decay time (Dt) to Sl, where it is held until the key is released and decreases down to 0 over the period of the Release time setting (Rt).

Curve Swerve

What type of curve is the CS-80 using for this envelope though?

This is the block diagram reprentation from the service manual:

That look like attack is linear, decay might be exponenetial and the same for release.

But there are notes on the special Yamaha IC that generates this envelope . What does that say?

Those all look like inverse log curves! Hmm! Well, then what about the Service Notes on tuning etc?

OK. that's different again. Someone must know!

User manual? No, unless they are all linear.

The Arturia CS-80V manual?

Well, that could be righ, but I need corroboration.

Gordon Reid? Umm, just a comment about the GX-1. And that envelope?

Now come on!

Let's try another clone of the CS-80 - Deckard's Dream

I can't find anything. It's created digitally though.

It's getting annoying now.

Eventually, checking the output of the CS-80V convinced me that the curves are log.  Moving on!

CS Levels

Here's a diagram of a typical ADSR with the IL picking up just above 0.

 This combination can be rather confusing to start programming from. In general, setting Brilliance to Full and then adjust VCFc to Low is a good midpoint, but if you want to start adjusting from there it's easy to lose your place.

One way to get a normal ADSR envelope is to set Brilliance and, VCFc to half and AL to +5 IL to -5. The combination of middling Sl with maximum IL and AL gives the classic shape, but finding a good balance can still be difficult.

Some sort of responsive diagram, showing the levels as in the above diagram would be nice.

If both IL and AL are set to 0 then there is no AD VCF envelope. Not just no Attack portion, because there is nothing to increase from or to, but no Decay either, because there is no AL to decay from. In this case, the envelope starts at the setting of VCFc + Brilliance, i.e. Sl, and if there is a Release setting it will fade away in Rt to 0. The Sustain level is the whole envelope until release.

This arrangement is a nice feature on the CS synths, but it relies on the envelope being told what the Fc level is. This then sets the Sl and give a reference from which to set the IL and AL.

To do this in Eurorack is not impossible but it's not straightforward and it would use up a few cables and probably more than a few modules to get it to work in exactly the same way.

It does not need to work in exactly the same way though. The main advantage of the IL/AL is not the AL. If a lower level of attack is required then this could be achieved easily with attenuation. The real problem is the IL. Nearly all envelopes start and end at the same level, but what are the other options?

Envelope Options

The other options fall in to three catagories

1) Very expensive modules that do anything you want with voltages, like the Rossum Electro-Music Control Forge >£400

2) Mid-priced multi-stage generators like ALM Busy Circuits Quaid Megaslope or  Mutable Instruments Stages   > £200

3) Ladik C-216. £55

Control Forge is complete overkill, so let's look at the others.

Ladik C-216

 Only the Ladik C-216 delivers a dedicated envelope generator with IL setting. Is the Ladik C216 everything I need to replicate the CS filter envelope?

It certianly appears to be! With a bit of attenuation and/or some decisions about how to set the filter's cut-off control even the AL type behaviour can be achieved.  Other notes in it's favour is that it is cheap (at that price two or three would not be extravagant!), low on HP count (4HP) and power consumption.

However, I was reading the excellent Patch & Tweak, in particular the section on envelope curves, and apparently all the Ladik C series envelopes are linear.Which as we saw above is not what the CS uses. Shame that.

Up Stages

Stages is more reasonably priced than Control Forge, but might still be too much for my needs. If I did get Stages I'd want to do more with it than just envelope duty. My thinking is to have modules dedicated to the SS-30M, more or less, and Stages would be tied up completely when running duty as an IADSR.

Setting up Stages for IADSR is even in the manual.

Stages has everthing and more that I need. It has just one competitor.

Megaslope

ALM Busy Circuits' Quaid Megaslope would also fall into this trap of being too much for my needs, but it appears to get closer to CS-80 functionality because it has control over the destination level, as well as time, of every stage. It also has CV control over both parameters and that would allow for inputting a Sl CV and offsetting the same voltage for IL and AL inputs.

It would go like this.

	Stage 1	Stage 2	Stage 3	Stage 4	Stage 5
Env		Attack	Decay/Sustain	Release	-
Level	Sl - IL	Sl + AL	Sl	0	0
Time	0	At	Dt + Sl	Rl	0
Slope	N/A	Log	Log	Log	N/A

So?

I just decided to buy the Megslope. It's an expensive sledge hammer to crack the Yamaha CS envelope nut, but of course I'm going to use this for more that just that.The full CV control sold it to me over Stages.