[sdiy] Filter frequency cutoff with linear response?

[Tom Wiltshire]

On 20 Feb 2012, at 13:19, Florian Anwander wrote:

> Am 19.02.2012 21:32, schrieb Olivier Gillet:
>>> But is there any
>>> of these synths that does this VCF modulation linearly?
>> 
>> The Doepfer Dark Energy does it.
> ???
> The DE is based on a CEM 3394, so most of the 3394 based synths should do that: AX60, SixTrak, MAX, M1000...

I was just having a look at the CEM3394 datasheet, and they did it that way for exactly the reason predicted - linear FM of the filter. The cutoff CV is exponential (3/8V per octave) but the chip also includes a filter modulation VCA from the VCO triangle output, and that input is linear.
Presumably Florian is right; since this is a feature of the chip, you'd expect the other synths based on it to have the same characteristic.
Interestingly, none of the CEM3396, CEM3378, CEM3379, or the CEM3372 include this feature - the only filter cutoff CV input is fed through an exponential convertor block in all cases.

T.