[sdiy] digitally controlled potentiometers

[Tom Wiltshire]

On 6 May 2007, at 19:59, Heiko van der Linden wrote:

>
> Thanks Paul, Andre and Ingo,
>
> Your input is very helpful to help me assess what I really need  
> from my
> circuit.
> Concerning the resolution, I definitely think that a high  
> resolution is needed
> for the potentiometers that control an audio signal such as filter  
> cutoffs,
> resonance and ADSR potentiometers. I wonder if a nice filter sweep  
> can be
> done with a low resolution potentiometer without zipper noise.

Hi Heiko,

I've thought about some of these issues too, but I was trying to  
produce control voltages from a control position.

Paul is absolutely right that you can't really use high resolution  
from a knob. Where exactly the limit is a bit of an open question  
(Andre and his MS20 might need 350+ values, but no-one really  
criticises the Prophet 5 with only 127) but it is certainly less than  
10-bit, so 14 or 16-bit control resolution is unnecessary and will  
give you other headaches to sort out.

However, you're also dead right to point out that a filter sweep with  
only a 8-bit value would give noticeable zipper noise.

The point here is that these two things (input and output) don't have  
to be the same. Although the Prophet 5 might have output used 7-bit  
control resolution (input) it used a 14-bit pitch CV to eliminate  
zipper noise (output).

When I was thinking about these issues, I'd more or less decided that  
I was going to use a limited input resolution and then use  
interpolation where necessary to produce a much larger output  
resolution. Perhaps this would help you.

Regards,
Tom