Digital components in analog circuits?

[AndrewBischoff]

Hey all,
I'm brand new to this forum so I hope this question isn't out of place... I didn't see a more appropriate place


I had this weird thought while I was in the shower the other day. I want to make a multi-effects pedal that has a digital interface, but analog effects. So essentially, it would be multiple analog effects circuits built into one enclosure that all can be controlled digitally. I'd have a microcontroller using digital potentiometers to control all of the tone/volume/etc controls, and then relays to switch individual effects on and off. Then I'd have features letting you save/recall presets for tones w/ the microcontroller, and other cool stuff you can only do digitally.
I dunno, probably not the most practical idea but it sounds like a fun project haha.

So the only thing I'm not really sure about with this project that I'm hoping some of you more savvy folks will know: Will using digital potentiometers and/or relays have any impact on tone? I'll be honest, analog circuits aren't exactly my forte. I just have a fear that the circuitry in a digital pot could in some way disturb the signal? Audio signals are pretty delicate and any impact on tone (or grounding buzz) can be a pretty big deal, so I wasn't sure if digital pots would be designed for an application like this.

I'm pretty sure relays for switches would be fine, right? My first thought was to use FET's controlled by the microcontroller to switch effects on/off, but I'm almost certain that would have a pretty big impact on the tone.



Any thoughts?

[edy_wheazel]

I usually have breakthrough ideeas while I'm smoking (cigarettes), not so much in the shower.
The ideea is not bad, I was thinking at something similar (while I was smoking, of course), except the digital pots and presets. For example, in a box I want to put a multichannel preamp (tube or solid state) with a cab-sim (ADA Varicab). I will have a 4x16 display on wich I will display the channel (clean, drive, satanic), the cab-sim mode (2x10, 1x12, 2x12, 4x12, bright or dark) and other stuff. It's not a big deal (engineerly speaking) and it will be my next project after I'm done with all the things I'm working at.

[notnews32]

check out Chase Bliss' lineup of effects.. they've created a line of pedals with all-analog circuit paths that are controlled by a digital brain type thing. The audio signal doesn't touch any digital components, but it's directed and orchestrated by the digital components. They have amazing effects.

[Dirk_Hendrik]

I had this weird thought....

Any thoughts?

First of all, welcome!

Regarding your weird thought, you're not the first one. The thought you had is about using the most suitable solution for the job.

A while ago I had a discussion with a rather experienced programmer who was building a full hardware based switcher because he did not have any microcontroller experience. He was rather hard to convince getting a simple arduino.... and was off in no-time when he did.

[teemuk]

You may not always need a "digital potentiometer" if you can substitute it with OTA, LDR, FET or multiplexer IC. All can be used for various purposes of "gain controlling", whether frequency-selectively or at full bandwidth, and you can generate an analog control signal to them with digital solutions.

I suggest you study relevant product schematics of ADA, Digitech/DOD, Hughes&Kettner, A.R.T., or Dynacord, who all have extensive history of making devices you describe: Analog signal path with digital control and interface. Take a look at circuit diagrams of units like ADA MP-1, Marshall JMP-1 (preamp), or perhaps Digitech's RP-1, GSP2101, or Johnson's JM-150. Schematics to all should be found from Internet with no particular trouble. Also, many of the modern "digital modeling" amplifiers incorporate some elements of analog signal processing with digital control. Peavey Vyper and Vox Valvetronix series are good examples.

Anyway, guitar processing concerned, first signal processing units (preamps, amps, effects, etc.) that had an analog signal path but digital control and user interface appeared already in the mid 1970's so you should have no problem finding reference material for your project.

Digital circuitry will always be noisy. Just follow recommended rules of design and layout and digital noise should pose no relevant issues: Keep conductors carrying digital pulses as far away you can from delicate parts of the analog circuitry, avoid running parallel conductor paths with analog and digital circuiry, keep analog and digital grounds separated, inputs and outputs may need additional RF filtering, etc. Analog and digital circuits have co-existed for decades so digital noise creeping into analog signal paths should really be no issue if you just put some effort into avoiding it. Reference material should again be found a plenty.

I'm pretty sure relays for switches would be fine, right?

Yes. You don't always need them either, since you can often substitute them with solid-state solutions of switching. ...But solid-state solutions tend to have input voltage range and current limits, which may prevent using them in high voltage circuitry associated with tube amps and alike. Then relay is pretty much only option. It can degrade signal much more than FET if the contacts are corroded, BTW.

My first thought was to use FET's controlled by the microcontroller to switch effects on/off, but I'm almost certain that would have a pretty big impact on the tone.

Huge percentage of effect pedals, effect units and amplifiers use FETs for that very purpose and people don't seem to mind. In any case, you'd be sending an analog pulse to FET gate and you need to intrdoduce some gradul sloping to on and off characteristics of the device to prevent "pops" during switching. Just study schematics and you quickly notice some common schemes how that is achieved.

[teemuk]

Concerning "digital potentiometers"...

They are actually somewhat broad product category with different types of arrangements for control and different types of arrangements for attenuation.

Most complex "digital potentiometers" actually run the signal through Analog-Digital-Analog -conversion process, during which the signal is in digital realm attenuated to proper magnitude. Naturally quality of the output signal is hugely dependent on these processes and I think performance of each digital potentiometer in this category should be evaluated separately. Generalisation proably serves no one.

Most usual "digital potentiometers", on the other hand, are just stepped resistive networks fabricated on an IC with digital/analog switching scheme. With enough resistors in the chain good resolution and seemingly stepless adjustment is achieved. At worst, the fabricated resistors will be noisier than discrete ones, the switching will generate noise, and resolution is no better than that offered by other analog options. At best, the designers have focused on noise performance and alike and the chip is the easiest scheme to implement.