DIY Pitch Shifting Pedal

[kevinng]

I'm looking to build an analog pitch shifting pedal.

I want to avoid digital because I'd like to explore low power battery powered options.

Does anyone know if there are existing circuits that lets us shift pitch, say, with a potentiometer?

[FiveseveN]

You can do frequency shifting: https://www.diystompboxes.com/smfforum/index.php?topic=77195.0
But it will be much more complex and potentially more power-hungry than a contemporary DSP.

[Rob Strand]

Check out this thread,  in particular Reply #14 from duck,
https://www.diystompboxes.com/smfforum/index.php?topic=106418.0

Something to think about.

[ElectricDruid]

avoid digital
low power battery powered

These two might be pulling in opposing directions. Any analog circuit that can do pitch shifting (or frequency shifting as has been suggested) is probably going to be much more complicated and many more parts than the equivalent digital circuit.

[Mark Hammer]

Do you mean fixed pitch-shift, like a harmonizer, or variable pitch-shift, like a Digitech Whammy pedal?

[EBK]

Note, there is a big difference between frequency shifting and pitch shifting. 

If you just want to warp sounds in a not-necessarily-musical way, you can use a modulator (with some careful filtering) to create shifted frequencies. This would basically allow you to create a signal with additional frequency components that are all linearly shifted by the same amount. You will lose musical coherence between notes, i.e., a beautiful chord will become very, well, discordant.

Pitch shifting is trickier.  Instead of linearly shifting frequency components by a fixed amount, you are really multiplying the frequencies by a fixed number.  This can be accomplished by recording a window of sound and playing it back at a different speed.  If you want to do something like this as a pitch warping toy, modulated delays are an easy way to accomplish it.

If, however you are looking for something like a POG, then, honestly, just buy a POG. 

[amptramp]

If I was going to design a pitch shifter, I would probably use a Fourier transform to find the frequency content of the existing signal and stretch or shrink it to get the new frequencies then do an inverse Fourier transform to generate the new sound.  It all sounds like a digital project to me.  You could try optical computing where a lens is a Fourier transforming element and this was used in some of the original synthetic aperture radar units but I don't know how you would arrange the optics to do this.

If you are taking in a guitar signal, there will be six strings with the signals added together.  If you could get an output from each string, you might have a better chance of doing something but the harmonics may be higher in amplitude than the fundamental so you would need a lowpass filter to accentuate the fundamental and this just gives you a signal worth processing.  You could measure the period of these signals and introduce a signal with the period multiplied by a constant amount for all harmonics but you can see the difficulties right there - six strings, a period for each harmonic (because the harmonics are not exact multiples of the fundamental frequency) and a new signal with the multiplied periods.

There were some tape machines with rotating heads that added the peripheral speed of the head to the tape speed to get the signal and if you had a constant tone as the input and didn't mind the discontinuities where one head left and the other took over, this worked and actual machines were made that way.  Once you got into actual program material, there were artifacts.  You could simulate this by bucket brigade devices where one BBD took in the signal and read it out at a different rate with parallel devices where one was reading in the signal while the other was reading it out at a different speed.  You could also use PT2399 delays to do this but the need to run them in parallel and switch between them when they are running at different speeds is more of a project than just a simple low-power device.

[anotherjim]

As I understand it, BBD's were intended for multiplexed communications or scrambling where a BDD records at one speed and then speeds up to send a compressed (in time) package that gets the reverse treatment at the other end. The BBDs with their 256, 512 or 1024 stages happen to be easy to time with binary counters operating to the BBD clock. It ought to be possible to run several taking turns to keep the shifted audio going but there will be glitches, you'd have to play a doubled package twice to keep time or drop alternate packages for a half-speed effect.

[Mark Hammer]

In the late '70s, EHX produced a bunch of interesting devices.  One was the Memory Man, and another was the Hot Foot controller, a mechanical expression pedal that could be attached to the shaft of pots to physical rotate them with your foot.  One of the "killer apps" was to work the delay-time control of the Memory Man to achieve pitch manipulation.  Set it for long delay time and quickly shorten delay (faster sample-out than sample-in) to achieve higher pitch, or set for short delay time and quickly lengthen it (faster sample-in than sample-out) to drop pitch.  Limited in many ways, but it's what we had/used, decades before the Digitech Whammy was a gleam in anyone's eyes...and it was all analog.