Controlling Potentiometers from a PC

martin

On a sunny day (Tue, 4 Nov 2008 01:05:35 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

How about using C? It is better then C++ :-)

You *could* build an interface for the PC that outputs a voltage that can drive a: *Motorized Potentiometer* See for an example:

mm ;-)

OH YES, digital is BAD it is all or nothing ... big steps..... BTW did you not write in C++? did you use type double, or float? Or unsigned 8 bits char?

A DAC HAS STEPS TOO!!!!!!!

Use type double, provide step and direction pulse on your PC interface, and selector number to select one pot, stepper motor -> 1000 : 1 gearbox ->

potmeter shaft.

It will be slow but a lot smoother.....

Rabbit traps.

Are you sure?

Dark Fader.

Well there's half your problem right there. C ick poo ick poo!

Does the adjustment need to be able to hit exact values or can we be only roughly right?

If the steps are small enough, a step is just fine. Digital pots usually have too few steps. A DAC may be a better option.

A Gilbert cell or just a single JFET or BJT can be used to control the gain by multiplying two analog signals. You can buy volume control chips that are basically Gilbert cells. For all of these you need a DAC to make an analog voltage to control them. The signal from the DAC needs to be filtered to clean it up.

Since you are into writing software, why have the circuit at all. Use the sound card in your PC and software to do all of the work.

The big problem with the digital POTs is that they are actually RC-RC-RC-RC... This RC of the high and the variable order comes into play. Another problem is that the THD of the digital pots is substantial. For those two reasons, I wouldn't recommend using digital pots for audio.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Your comments are appropriate only when the digital pot is improperly loaded. I've rolled my own for use as RF attenuators up to several hundred MHz.

I've even made them with dB steps ;-)

...Jim Thompson

One dB is a large step. I did tunable narrowband filters with the off-the-shelf digital pots; it was quite tricky to compensate for the variable RC while maintaining the reasonable SNR and THD.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

I have no significant experience with off-the-shelf digital pots, since I roll my own in Silicon. But it sounds like you might have had significant current flow thru the analog switches. (My typical venue is BiCMOS)

...Jim Thompson

We're all assuming that these pots are plain old gain (volume) controls. As others have suggested, amplifiers with voltage controlled gain might be the way to go. You can select from a wider range of DACs to give you the step size you require.

If these pots are for tone controls, or other effects, the proper analog circuitry will have to be developed to do voltage to whatever. Not knowing what kind of 'audio circuit' you are starting with, consider the fact that it might be simpler to throw the whole thing out and design something optimized for digital control.

Motorized pots are a nice option in that the operator can see (and in some cases manually tweak) familiar looking controls. Some sound technicians like the old analog controls because the knob (or slider) is both the control plus a quick visual indication of the status of their sound board.

As you switch a digital pot, there is also a step because of the gain change not happening at zero voltage. These tend to jump out at you.