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This talks a bit more about the use of the HiRose connectors for the microcontroller and codec boards and the trade-offs involved there. They would have been a horrible choice in any situation where you knew what you wanted from the first get-go. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
276 lines
11 KiB
Markdown
276 lines
11 KiB
Markdown
## Resurrected random guitar pedal project
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This is a resurrected version of my old guitar pedal project, except
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this time with a screen and a few rotary encoders instead of the old
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horrid analog potentiometers.
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There's a 'Hardware' directory with the kicad files.
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There's a 'Software' directory that contains the firmware to make it do
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something.
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And there's a 'Documentation' directory, which is a very optimistic
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thing for this project.
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Anyway, with the update to have a screen and proper rotary encores, the
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thing can now have multiple effects and a sane-ish UI to them. Except
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I'm not exactly known for my mad UI designing skillz. So...
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## Firmware
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I've only ever built the firmware on Linux, but it *should* be perfectly
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possible to build on MacOS or Windows too if you just figure out the
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platform requirements. The project depends on the `pico-sdk` and
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`tinyusb` libraries, and has submodules for both, so they get built
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automatically, but the build tools your platform has to provide.
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Regardless of platform, you'll need the basics:
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- git
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- make
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- python3
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- cmake
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and a 32-bit arm cross-build environment. On Linux, that would be
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something like
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- arm-none-eabi-binutils-cs
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- arm-none-eabi-gcc-cs
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- arm-none-eabi-newlib
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and if you have all the requirements, doing
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```
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git clone https://github.com/torvalds/GuitarPedal.git
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cd GuitarPedal
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cd Software
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make prep
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make
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```
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should get the build going, and you should find the resulting
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`blink.uf2` file in the `build/` subdirectory. You can just write that
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file to the USB filesystem after you've set the pedal into
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programming mode (see below).
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If you have installed picotool with USB support (the pico-sdk build only
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builds a cut-down version without it), you can also just do ``make
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flash`` to flash the image that way.
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## Hardware
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The kicad design files (and some supporting infrastructure, like the 3D
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printed insert and the enclosure drill rules) are in the ``Hardware``
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subdirectory.
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The board files are perhaps somewhat strange, in that there are two
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modular boards for the "core" hardware: the RP2354 microcontroller
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(`Hardware/rp2354`) and the TI TAC5112 codec (`Hardware/codec`)
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respectively.
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Then there are boards for the audio and 9V DC power jacks
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(`Hardware/audio-jacks`) with a connector for the codec board, and a
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main board (`Hardware/pedal-board`) for the pedal IO (i2c connector for
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the screen, USB-C programming port, rotary encoders, pin header for
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stomp switches) which then has the connector for the rp2354
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microcontroller board.
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I'm using the nice HiRose BM28 series connectors on the modular boards.
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They are absolutely tiny, which makes for a great board footprint but
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admittedly also makes for a slightly more complicated board due to the
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tiny 0.35mm pitch. I'm not a fan of the traditional pin headers simply
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because they make it so hard to do compact form factors.
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The inter-board connector is a 12P 0.5mm FFC cable that carries power
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and data lines (i2c for control, i2s for audio).
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This modular design is purely so that I could try out different form
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factors, and if you know what you want you should just put the TAC5112
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directly on the audio jack board and the rp2354 on the IO board. The
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modular setup makes for more complicated boards (the core boards have
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components on both sides due to the connector, for example), but allowed
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me to separate out the more complex and slightly more expensive boards
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from the "let's try this layout" boards.
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### Images
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## Basic UI
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The pedal has a 128x128 monochrome OLED screen and two rotary encoders
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you can turn, and both of them also have switches so you can press down
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on them to do things. There are also two stomp-switches.
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The top rotary is the "value" rotary, which changes the values when you
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rotate it, and switches to the next value in the list when you press it
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(you can also *hold* the rotary and rotate it at the same time, which
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allows for moving back and forth in the effect value list, but most of
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the time it's easier to just click forward).
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The rotary below it is the "effect" rotary, which walks through the
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effects in order when you rotate it. You can also enable/disable each
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effect by pressing it.
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The left stomp switch is _also_ a "enable/disable current effect"
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switch, but for your feet. You do not want to stomp on the rotary
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switches.
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The right stomp switch is a "disable/enable the whole pedal" switch.
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There are also two status LED's associated with the stomp switches: the
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left one shows the "currently selected effect status", and the right one
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shows "global status".
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*Mostly* those status LEDs are just about on/off, but some effects will
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also indicate whether they are in an active state by making the LED glow
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more brightly. For example, the noise gate will glow more brightly when
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the signal is gated, and the compressor effect will glow more brightly
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when it's compressing.
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The "global status" LED can also glow more brightly, but it will do so
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when things are bad: if the signal is hard-clipping past the range of
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the output. You typically wouldn't want that, but hey, maybe you really
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want an insane boost with hard clipping that drives the amplifier to do
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nasty things.
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Finally, there is also a special 'reset sequence" - if you press and
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hold *both* rotary switches, that is a reset signal, and if you are
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connected to a computer over USB, the pedal will go into programming
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mode.
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If the pedal is powered on, but not connected over USB (so either using
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the 9V guitar pedal power, or using USB from just a charger), the reset
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sequence will reset all the effects - turn them off, and reset them to
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default values.
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## Audio effects
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The current effects are:
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- Noise gate
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This one is fairly simple. Depending on how noisy your guitar
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environment is, you may or may not need this one. But particularly if
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you use the boost effect very aggressively, you probably want it even if
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you don't have a lot of 50Hz / 60Hz hum.
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The default level is -70dBV, which is pretty quiet.
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Anyway, 0dBV is very loud - most guitar levels are roughly in the -20dB
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range (0.14V peak, aka 280mV peak-to-peak voltage).
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-40dB is a "quiet sound" (14mV peak voltage), and -60dB is pretty much
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silence. So a -70dB noise gate *should* be a good starting point for a
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good low-noise pickup.
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That noise gate allows going down all the way to a -100dB noise floor,
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which is ridiculously border-line for what the hardware can actually do.
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But my environment and guitar is actually quiet enough that I
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*can* go down to -85dB, and it will glow brightly to show that the gate
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is on and the signal is smaller than that.
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I'm actually pretty happy with that, in that it's about a 0.1mV
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peak-to-peak signal. It's not just that my guitar isn't picking up a
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lot of noise from the environment, it also means that the pedal itself
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is not noisy.
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Alternatively, it just means that I got all the math wrong, and it's
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lying to me.
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There's also attack/release values that can tune just how the size of
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the envelope is calculated, and how quickly it reacts to noise (and
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how quickly it goes back to gating).
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- Compressor
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This does what a compressor does. Like a noise gate, there's a
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attack/release to tune how the signal envelope is tracked. It has a
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"boost" setting to allow it to just boost the signal in general, but the
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"level" is then the level at which it starts compressing.
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The "ratio" is how aggressively it compresses signals that go over the
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level (but the attack is also very relevant: the attack is about hoq
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*quickly* - or slowly - it reacts to signals that go over the level).
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So the "attack" basically says how quickly it starts reacting to a
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signal that goes over, and then the ratio is how aggressive it is once
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it starts reacting to it.
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- Boost (w/ distortion)
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I like this one. Others may not.
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It can be used as just a clean boost - but so can the compressor. But
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what I like doing with it is to set it to some ridiculously high boost
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value (like +20dB), and then set the *level* down to something fairly
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low (like -20dB).
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A +20dB signal boost is basically increasing the voltage level by 10x,
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but then the "-20dB level" means that the "level" is set to 0.14V.
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And what that boost effect does is that when the signal hits the voltage
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level, it "folds" it down (or up, if it hit the negative level). So the
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+20dB boost will first make the signal much bigger, but then the level
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folding will limit the end result to sane levels, but instead of just
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clipping at that level, the signal folds down and you get higher
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harmonics.
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I think it sounds more interesting than the typical soft- or
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hard-clipping effects.
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- phaser
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- flanger
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Nothing particular about these. They are very simple effects.
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- echo
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This is the Echo King effect from Cleveland Music Co, converted from the
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Hothouse example effects to this pedal. All credit for it goes to Ricky
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Sheaves, except if I screwed up in the conversion, in which case you get
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to blame me.
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It's a DSP model of the Maestro Echoplex family of tape delay.
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- pitch shifter
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This one is almost certainly not useful, but it's fun. It's a pitch
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shifter, but it's *not* the smart kind of "do an FFT, shift frequencies
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up or down".
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Instead it's based on a delay loop, and walking the delay either faster
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than realtime (shifting the pitch up) or slower than real-time (shifting
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it down). And then to avoid the sudden discontinuities when you have to
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jump backwards (or forwards), it actually walks the delays in two
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phases, and multiplies by a function that goes down to zero at the
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discontinuity point (the function happens to be sine/cosine for the two
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phases, but it could be something else).
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End result: it does shift the pitch, but it also has a delay due to how
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it's done.
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*And* to make it sound even more complex, it has a feedback thing, so
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it can feed back its own pitch-shifted signal into the delay loop, and
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you get another pitch shifting (with an extra delay). So you can kind
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of think of it as a short echo with a pitch shift.
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It tends to sound most natural - which isn't saying much - with a +1
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octave shift, but it isn't limited to whole octaves. You can shift the
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pitch up by random fractions. Play around with it.
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- 10-band EQ
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This is the most complicated from an actual algorithmic standpoint, and
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also has the fanciest display.
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Each band goes +-20dB (so 0.1 .. 10x). At the extremes, it will tend
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to distort the signal - all the math is done in 32-bit
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single-precision float, I won't guarantee it's entirely stable or
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smooth.
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- "USB"
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This doesn't affect the sound, but it turns the USB audio interface
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logic on and off, and you can pick whether you want the stereo signal
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to be either all dry, all wet, or "left channel wet, right channel
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dry".
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It's a work-in-progress. It works, but not entirely reliably.
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