Digitally controlled analog

[Reinerterig]

Hi,

I have no experience in the digital domain but I am quite comfortable with analog circuits. I realize adding a digital element can only enhance my design.

I know some companies digitally control their analog circuits like chase bliss audio. My approach would be digital potentiometers. This would allow me to manipulate level, eq or anything a regular pot would do --witch is everything--.

Is this an extremely difficult thing to accomplish, what would my limitations be, any pitfalls I should look out for, what is the alternative?

Thanks!

[Digital Larry]

You have to learn how to program a microcontroller.  With all the resources around Arduinos for example, at least there will be lots of things to study and maybe even one of them is close to what you want to do.

I looked at some digipots at one point and there are a few things to note:
a) If they are powered by 5 volts, then all signals going in must be between 0 and 5 volts.  There may even be a little margin at the edges required.  This would require you to do level shifting and possible scaling of analog signals.

b) Depending on whether you are trying to get a linear or log taper, you will have to choose an appropriate part.  I believe linears are much easier to find.

c) You may have to deal with "zipper noise" which is the result of the steps not being small enough to avoid an audible jump.

[ElectricDruid]

You have some other options to consider beyond just replacing the pots with digipots. That's initially appealing because it sounds so simple, but in practice it may not be the best solution. Other possibilities are:

1) DAC controlling VCA gain - either linear OTA like LM13700 or exponential VCA like V2164 or THAT 2181. There are now CEM chip clones available too - AS3360 and AS3330.
2) Multiplying DAC controlling levels or gains (MDAC). This is great option, since it's basically a one-chip solution. You can fairly easily find 10 or 12-bit MDACs, which you won't mange with digital pots. Bonus points if you've got an easy solution for bipolar Vref inputs (some MDACs duo this, but by no means all).
3) Replace non-signal path parts with digital elements. Instead of controlling an LFO's rate and depth with digipots (for example), you can generate the LFO digitally and output it at the required rate and depth. Since you're dealing with LFO frequencies, the sample rate demands are low and you can easily provide vastly excessive (in theory terms) sample rates and use simple filters to provide an indistinguishable-from-analog output.

I think there are others but it's getting late and I can't recall them right now. Anyone else care to step in?

Tom

[Reinerterig]

You have to learn how to program a microcontroller.  With all the resources around Arduinos for example, at least there will be lots of things to study and maybe even one of them is close to what you want to do.

I looked at some digipots at one point and there are a few things to note:
a) If they are powered by 5 volts, then all signals going in must be between 0 and 5 volts.  There may even be a little margin at the edges required.  This would require you to do level shifting and possible scaling of analog signals.

b) Depending on whether you are trying to get a linear or log taper, you will have to choose an appropriate part.  I believe linears are much easier to find.

c) You may have to deal with "zipper noise" which is the result of the steps not being small enough to avoid an audible jump.

Thanks Digital Larry,

I was concerned that digipots might have a very low voltage limit.

I could use low voltage low noise opamps but low noise wouldnt help my SNR at all if digital clock noise etc. is leaking through.

You have some other options to consider beyond just replacing the pots with digipots. That's initially appealing because it sounds so simple, but in practice it may not be the best solution. Other possibilities are:

1) DAC controlling VCA gain - either linear OTA like LM13700 or exponential VCA like V2164 or THAT 2181. There are now CEM chip clones available too - AS3360 and AS3330.
2) Multiplying DAC controlling levels or gains (MDAC). This is great option, since it's basically a one-chip solution. You can fairly easily find 10 or 12-bit MDACs, which you won't mange with digital pots. Bonus points if you've got an easy solution for bipolar Vref inputs (some MDACs duo this, but by no means all).
3) Replace non-signal path parts with digital elements. Instead of controlling an LFO's rate and depth with digipots (for example), you can generate the LFO digitally and output it at the required rate and depth. Since you're dealing with LFO frequencies, the sample rate demands are low and you can easily provide vastly excessive (in theory terms) sample rates and use simple filters to provide an indistinguishable-from-analog output.

I think there are others but it's getting late and I can't recall them right now. Anyone else care to step in?

Tom

Hi Tom,

First of all can I say I have a lot of respect for ElectricDruid. I ordered your Tap tremolo recently and before I got started building I already had replacements on their way because of a bug. You guys are great!

Digital to analog converters could be a good solution used in conjunction with LDRs, optocouplers, and transistors to either control volume or frequency.
I am not sure what an MDAC is but I'll look it up.

Thanks!

[dschwartz]

Has anyone tried using PWM outputs from microcontroller into analog switches (like 4066) ?
Using 2 switches, one with inverted pwm signal should make a virtual linear pot...
My only concern is if i use more than one "switching pots" there could be aliasing noise if the pwm are not exactly at the same frequency....