Resurrected random guitar pedal project

This is a resurrected version of my old guitar pedal project, except this time with a screen and a few rotary encoders instead of the old horrid analog potentiometers.

There's a 'Hardware' directory with the kicad files.

There's a 'Software' directory that contains the software to make it do something.

And there's a 'Documentation' directory, which is a very optimistic thing for this project, and it is currently entirely empty. Some day...

Anyway, with the update to have a screen and proper rotary encores, the thing can now have multiple effects and a sanish UI to them. Except I'm not exactly known for my mad UI designing skillz. So...

Images

Basic UI

The pedal has a 128x128 monochrome OLED screen and two rotary encoders you can turn, and both of them also have switches so you can press down on them to do things. There are also two stomp-switches.

The top rotary is the "value" rotary, which changes the values when you rotate it, and switches to the next value in the list when you press it (you can also hold the rotary and rotate it at the same time, which allows for moving back and forth in the effect value list, but most of the time it's easier to just click forward).

The rotary below it is the "effect" rotary, which walks through the effects in order when you rotate it. You can also enable/disable each effect by pressing it.

The left stomp switch is also a "enable/disable current effect" switch, but for your feet. You do not want to stomp on the rotary switches.

The right stomp switch is a "disable/enable the whole pedal" switch.

There are also two status LED's associated with the stomp switches: the left one shows the "currently selected effect status", and the right one shows "global status".

Mostly those status LEDs are just about on/off, but some effects will also indicate whether they are in an active state by making the LED glow more brightly. For example, the noise gate will glow more brightly when the signal is gated, and the compressor effect will glow more brightly when it's compressing.

The "global status" LED can also glow more brightly, but it will do so when things are bad: if the signal is hard-clipping past the range of the output. You typically wouldn't want that, but hey, maybe you really want an insane boost with hard clipping that drives the amplifier to do nasty things.

Finally, there is also a special 'reset sequence" - if you press and hold both rotary switches, that is a reset signal, and if you are connected to a computer over USB, the pedal will go into programming mode.

If the pedal is powered on, but not connected over USB (so either using the 9V guitar pedal power, or using USB from just a charger), the reset sequence will reset all the effects - turn them off, and reset them to default values.

Audio effects

The current effects are:

• Noise gate

This one is fairly simple. Depending on how noisy your guitar environment is, you may or may not need this one. But particularly if you use the boost effect very aggressively, you probably want it even if you don't have a lot of 50Hz / 60Hz hum.

The default level is -70dBV, which is pretty quiet.

Anyway, 0dBV is very loud - most guitar levels are roughly in the -20dB range (0.14V peak, aka 280mV peak-to-peak voltage).

-40dB is a "quiet sound" (14mV peak voltage), and -60dB is pretty much silence. So a -70dB noise gate should be a good starting point for a good low-noise pickup.

That noise gate allows going down all the way to a -100dB noise floor, which is ridiculously border-line for what the hardware can actually do. But my environment and guitar is actually quiet enough that I can go down to -85dB, and it will glow brightly to show that the gate is on and the signal is smaller than that.

I'm actually pretty happy with that, in that it's about a 0.1mV peak-to-peak signal. It's not just that my guitar isn't picking up a lot of noise from the environment, it also means that the pedal itself is not noisy.

Alternatively, it just means that I got all the math wrong, and it's lying to me.

There's also attack/release values that can tune just how the size of the envelope is calculated, and how quickly it reacts to noise (and how quickly it goes back to gating).

• Compressor

This does what a compressor does. Like a noise gate, there's a attack/release to tune how the signal envelope is tracked. It has a "boost" setting to allow it to just boost the signal in general, but the "level" is then the level at which it starts compressing.

The "ratio" is how aggressively it compresses signals that go over the level (but the attack is also very relevant: the attack is about hoq quickly - or slowly - it reacts to signals that go over the level). So the "attack" basically says how quickly it starts reacting to a signal that goes over, and then the ratio is how aggressive it is once it starts reacting to it.

• Boost (w/ distortion)

I like this one. Others may not.

It can be used as just a clean boost - but so can the compressor. But what I like doing with it is to set it to some ridiculously high boost value (like +20dB), and then set the level down to something fairly low (like -20dB).

A +20dB signal boost is basically increasing the voltage level by 10x, but then the "-20dB level" means that the "level" is set to 0.14V.

And what that boost effect does is that when the signal hits the voltage level, it "folds" it down (or up, if it hit the negative level). So the +20dB boost will first make the signal much bigger, but then the level folding will limit the end result to sane levels, but instead of just clipping at that level, the signal folds down and you get higher harmonics.

I think it sounds more interesting than the typical soft- or hard-clipping effects.

• phaser
• flanger
• echo

Nothing particular about these. They are very simple effects. The echo in particular is not great - it's too simple. In a better echo, there should be more delay taps, and do more of a chorus/reverb thing. I got bored with it.

• pitch shifter

This one is almost certainly not useful, but it's fun. It's a pitch shifter, but it's not the smart kind of "do an FFT, shift frequencies up or down".

Instead it's based on a delay loop, and walking the delay either faster than realtime (shifting the pitch up) or slower than real-time (shifting it down). And then to avoid the sudden discontinuities when you have to jump backwards (or forwards), it actually walks the delays in two phases, and multiplies by a function that goes down to zero at the discontinuity point (the function happens to be sine/cosine for the two phases, but it could be something else).

End result: it does shift the pitch, but it also has a delay due to how it's done.

And to make it sound even more complex, it has a feedback thing, so it can feed back its own pitch-shifted signal into the delay loop, and you get another pitch shifting (with an extra delay). So you can kind of think of it as a short echo with a pitch shift.

It tends to sound most natural - which isn't saying much - with a +1 octave shift, but it isn't limited to whole octaves. You can shift the pitch up by random fractions. Play around with it.

• 10-band EQ

This is the most complicated from an actual algorithmic standpoint, and also has the fanciest display.

Each band goes +-20dB (so 0.1 .. 10x). At the extremes, it will tend to distort the signal - all the math is done in 32-bit single-precision float, I won't guarantee it's entirely stable or smooth.

• "USB"

This doesn't affect the sound, but it turns the USB audio interface logic on and off, and you can pick whether you want the stereo signal to be either all dry, all wet, or "left channel wet, right channel dry".

It's a work-in-progress. It works, but not entirely reliably.