Pedalsync midi control over analog circuits and pots

[rhdwave]

Hey all,

I posted this in the Build Your Own Stompbox forum section but then thought it might be more appropriate here

Has anyone had any experience working with any of the pedalsync stuff? Specifically, I was looking at the four pots module and the Hi-V Pots.  Here's a link to the site:

http://www.moltenvoltage.com/pedalsync/PedalSync_audio_control_MIDI_chips_modules_index.html

I want to be able to save presets four pot positions and then recall them using a switch.  It seems like all the available gear from pedalsync would achieve this end and I've studied the data sheets, just looking for some confirmation or someone who has used the stuff before jumping in.

Much thanks for any help!

[potul]

Have you tried to contact Molten Voltage for help?

Bill from Molten Voltage used to hang around this forum, although I haven't seen much activity from him lately.

Mat

[rhdwave]

I did, he got back to me with a brief response, basically saying that all the info is in the datasheet but I had more questions which he hasn't responded to.  I'm wondering if they've chosen to concentrate more on their other designs and time is an issue? I figured i'd ask around here rather than bothering him with newbie esq questions.

[rhdwave]

So, one of the basic things I was confused about is regarding the actual mechanical pot value vs the digital pot value.  In the literature I read both on the pedalsync website as well as the datasheet, for the fourpots module it says to use 5k mechanical pots, which as I understand it are connected to the digital pots which are of whatever value you need.  In the response which I got from Bill, unless I misinterpreted it he stated that the resistances needed to match up. 

I wasn't sure why the 5k pot value was critical or if it is.  In my thinking, the digital pot would somehow attach a value to the resistance of the mechanical pot and than convert that to a corresponding resistance in its own value, so say the mechanical pot wiper was set at 50%, and I was using a 100k digital pot, it would assign a 50k resistance value.  I'm not sure if the circuit board for the four pots module is designed only to interpret signals from a 5k pot for its conversion...?

Sorry, i'm sure this is pretty basic stuff digitally, I just have no experience with this though it seems like a pretty cool rabbit hole to go down and get lost for who knows how long...

Rick

[ElectricDruid]

The datasheet is pretty clear that 5K pots are required. That much is true.

In practice, I doubt it's critical. There's one reason it might be, which is that the PedalSync chip's internal ADC likes to see a low impedance. If it's scanning the pots fast, there won't be much time for it to charge up the internal sample-and-hold capacitor, and a large pot value would mean that the capacitor would only charge slowly - potentially too slowly, meaning the value would be read incorrectly. Consequently, I might be tempted to try it with 10K, but I'd think twice about using 47K or 100K pots with it, unless they were buffered somehow.

I don't see any reason why the control pots should match the digipots. They're doing totally different things and they're not even related, except by software.

Your thinking about it looks spot on to me; the chip reads the 5K pot, sees it's at 50%, and then sets the appropriate digipot to 50% resistance. What actual value that is depends on which digipot chip you use. They all have exactly the same interface, so the PedalSync chip won't even know what value the digipot is. It doesn't need to. It just sets it to 10%, 50%, whatever.

Tom

[rhdwave]

Thanks Tom for looking that over!
In rereading Bill's email response to me:

"Yes but the digipots need to be able to handle the voltage and current of the existing ones and should have the same resistance.
The datasheet should give you all the info you need but it is not a project for a beginner."

I think I was misinterpreting what he was saying.  I think he's simply saying that the digipot resistance should match the resistance I had in the original analog pot (if I for instance was modifying an existing circuit that already had pots in them).  I originally read it as saying that both the analog pot and the digital should have the same resistance value.  But, as you said the datasheet is pretty clear on this that it should be 5k for the analog pot.  Pedalsync sells two versions of the digipots...one which I believe can only handle 5V and another hi voltage one that can go as high as 18V and 5ma.  I'm waiting to hear back from Bill about the values which are available in the hi voltage pots.  On the website they say they are only currently available as 100k, with other values coming, but I don't think it has been updated in quite a while.

Ok, getting there...Thanks again Tom, I really appreciate it.

As a related question, would you say this would be the easiest way to implement what I want to do or is there another, perhaps better method to achieve the same aims?

Rick

[ElectricDruid]

As a related question, would you say this would be the easiest way to implement what I want to do or is there another, perhaps better method to achieve the same aims?

That's an interesting question. It probably depends on the circuit. In general terms, yes, I'd say the PedalSync stuff is going to make adding programmability to a pedal about as simple as you can, because you just pull out the pots in the circuit, replace them with digipots instead, and then hook up the PedalSync chip. That's pretty straightforward, so it's the "easiest way" in that sense.

For some circuits, there might be other ways. Using multiplying DACs instead of Digipots to control levels for example. Some circuits could be redesigned with VCAs to give variable gain stages or variable filters, or sweepable phase shifts, or whatever. In many cases those could be controlled directly by a processor. For example, my Flangelicious design is based on a PIC processor already, and could have programmability added by (a) selecting a slightly bigger PIC to give yourself more room for code, and (b) adding a MIDI input. The only analog control is the resonance, which could be done either with a digipot, a VCA, or a multiplying DAC. In short, you'd only have to add one part - that's "the easiest way" in that sense. But it'd involve modifying the circuit design, and recoding the processor chip, all stuff which is definitely not straightforward.

Overall, the advantages of being able to just paste the programmability into an existing design make the PedalSync chip an appealing solution. But if you were starting from scratch designing a programmable pedal, you would be highly unlikely to come up with a design that looked anything like a PedalSync circuit with four digipots and processor bolted onto a analog circuit. You'd design the programmability in at a much deeper level and save a lot of parts.

Tom

[rhdwave]

These are all great points and ideas.  Thank you again.  I think you're right though in terms of ease of use for someone like me who is completely new to the digital world as far as effects goes to perhaps give the pedalsync stuff a shot.  I have no experience with programming in terms of pic or arduino or anything along those lines though i'd like to get into that.