Temperature-compensated resonance ( was Re: [sdiy] RE: [AH] Parametric EQ)

[Tom Wiltshire]

On 22 Feb 2010, at 09:49, cheater cheater wrote:

> Tom,
>> Anyway, if there was a DAC controlling it, I'd still save the  
>> hardware and
>> use a lookup table to linearize the resonance response. But that's  
>> just me.
>
> But a LUT is not temperature dependant, and will probably not work
> that well with resonance settings which can be really delicate around
> the generative point.

Aww, c'mon! Who ever bothered having a temperature-compensated  
resonance control anyway? Certainly no synth I own has one. You're  
lucky if you get a tempco in the filter *cutoff*, let alone the  
resonance.

The nearest I can think of is something like the Oberheim Xpander,  
which calibrated the resonance control as part of its full auto-tune.  
But even that doesn't avoid the temperature dependence  - it just  
ensures that you get the same response as you got at whatever  
temperature you were at last time you tuned it. If you take it  
outside from a warm room, it'll still drift out, but you're only an  
auto-tune away from having it right again.

It's true that you're going to need a fair bit of DAC resolution,  
since you're trying to linearize an exponential curve, but my recent  
experience with the pan law suggests that 12-bit would do it.
A practical example is the Korg Polysix which uses SSM2044 filters  
( which need an inverse-log pot to linearize the resonance control ),  
but seems to manage to have a reasonable resonance response. It only  
uses an 8-bit DAC, and I suspect there's a LUT involved there.

One thing I'm really learning about synth building is that you can  
often get away with a lot less than you think you need. It's amazing,  
but the human ear really isn't "hi-fi" at all. I thought I needed 16- 
bit parameter values for everything, but 12 will do, and often 8 is  
enough.

Regards,
Tom