CHOMPI x0x

A TB-303 / x0xb0x-style bass line machine for the CHOMPI, with a TR-606-style drum machine alongside: a 303-style voice and seven 606 voices, each with a 16-step sequencer you program a step at a time or record into live. No menus: every control is a key, a knob or the toggle.

Download: get the latest .bin from the Releases page. With the Multi Firmware Launcher, add it to the FIRMWARE folder in the root of the SD card; with the standard bootloader, copy it to the root of the card as the only .bin there. See Flashing below for more.

Cheat sheet: every control on two pages, bass and drums (or both as a PDF to print).

Built from scratch on top of CHOMPI Club's TEMPO firmware, whose hardware layer, libraries and startup it keeps.

Status: in use on a CHOMPI. The filter and envelopes are fitted to recordings of authentic TB-303 circuitry (from an RE-303 I built a few months ago). The voice, sequencer and every panel behaviour are tested on the desktop (host/), and the firmware builds without warnings. Recently added features (A/B patterns, MIDI import and export, write protect, the delay's free time) are tested on the desktop but have had less time on the hardware. The drums' side (a TR-606: LOOP in step mode, or a CHOMPI double tap) is the newest part and has had the least time on the hardware.

The toggle switch: two modes

Up: STEP mode. Program the pattern step by step. Down: PITCH mode. Play live, record in real time while it runs, or enter steps one note at a time while it's stopped.

PLAY runs and stops the pattern in both.

Bass and drums: LOOP in step mode, or double-tap CHOMPI

In step mode, LOOP swaps the panel between the bass and the drums, and CHOMPI + LOOP mutes or unmutes the side you're on (the bass or the drums, effects and all). LOOP lights dim in that side's colour (purple for the bass, amber for the drums), and blinks slowly while it's muted. Both sides can be muted at once. The mutes are saved with your settings. Tap tempo is on the purple knob's click.

In either mode, tap CHOMPI twice quickly (each tap short, nothing else touched in between) to swap the panel between the bass and the drums. Both always play; the swap only picks which one the keys and knobs edit. The keybed flashes the new side's colour (purple for the bass, amber for the drums), and the CHOMPI light stays amber while you're on the drums. A shift combination never counts, however fast: a press only counts as a tap if no key, knob or button was used during it.

Patterns for each side, or both

Every pattern slot holds a bassline and a drum part, and each side plays its own slot: the bass might play 1A while the drums play 3B. Each side's pattern page shows and picks its own (on the drums' side, the slots with drum parts are lit); a change waits for the end of that side's loop (the drum part's own length for the drums), and the queued key again switches at once. To send both to a pattern, hold PATTERN (F#4) on the pattern page and press the pattern key (with CHOMPI too for a B): they switch together at the end of the bar, the drums from their top. Write protect (PATTERN held 2 s) only works while stopped, so holding PATTERN while playing never toggles it. The settings remember both sides' patterns (a settings file from before this starts the drums on the bass's).

The drums (TR-606)

Seven voices synthesised the way a TR-606 makes them (from Roland's block diagram), fitted to samples of real 606s: bass drum, snare, low and high tom, cymbal, open and closed hi-hat, plus accent. Every pattern holds a drum part alongside its bassline, so picking 3A changes both; the drum part has its own length, for polymeters. Accent is per step, as on the 606, and makes hits up to 3x as loud (the accent level sets how much). A closed hat chokes an open one.

Voices on the black keys. Step mode: C#3 BD · D#3 SD · F#3 LT / HT · G#3 CY · A#3 CH / OH; the toms' and hats' keys alternate press by press (the key again gives the other voice's page). Live mode: every voice its own key, C#3 BD · D#3 SD · F#3 LT · G#3 HT · A#3 CY · C#4 CH · D#4 OH. Each lights in its own colour (BD red, SD orange, LT yellow, HT lime, CY cyan, OH violet, CH magenta), the selected one bright, and flashes when it plays.

Mute and solo (both modes): CHOMPI + a voice key mutes that voice (it blinks slowly); again unmutes it. Holding a voice key 2 s solos it: every other voice is muted, so you can bring them back one at a time with CHOMPI + their keys. Holding a voice 2 s when it's already the only one playing brings the whole kit back, and so does holding the accent key (C#4) 2 s in step mode; every voice key flashes white. Mutes act on the drum part's playback: voices you play by hand always sound. They aren't saved.

Recording drums live. With quantize on (the default), each hit goes to the nearest point of the grid. With it off, each hit keeps its timing within the step, to 1/24 of a beat (a hit just before a step goes on that step, on time). Turning quantize on plays hits recorded off the grid on their steps instead; the timing is kept for when it's off again. A hit you record into the step about to play isn't played twice. Rolls are recorded too, one hit per voice per step (the first); a roll faster than a step (1/32 on the 1/16 scale) records its extra hits as that hit's ratchet. The pattern, copy and clear keys are in step mode only.

The drums keep to the grid: a bass note recorded late doesn't move the drums on its step.

Knob 2's and knob 3's clicks switch between the selected voice's attack / decay and the filter / its envelope; on the filter pages, CHOMPI + click resets the filter (and resonance) or the envelope (and decay); on the first pages, attack and tuning (knob 2) or decay and FM (knob 3). The filter and its envelope do nothing while the filter is in the middle (off). Knob 4's click steps through its pages; CHOMPI + click resets its effects (not the shared delay time). Hold knob 4 and press a voice key to take that voice out of the reverb and delay, or put it back in. While knob 4 is held, the voice keys show which voices are in (lit) and out (nearly off). A voice taken out still plays, and still goes through the filter, crush and distortion; only its reverb and delay sends are cut. Every voice starts in, and the choice is saved with your settings. In step mode the toms and hats keys toggle whichever of the pair they currently show (press the key again first to switch); in live mode every voice has its own key. Pushing knob 4 to do this doesn't change its page. The delay is shared: the bass reaches it through its own delay knob, the drums through their send, so their echoes always line up; its feedback and tone (the bass's knob 4, page 2) set it for both. The mix knob (volume, page 3) acts before the effects, so muting a side mutes its echoes too. MIDI out sends them on channel 10 (GM notes: BD 36, SD 38, LT 45, HT 50, CY 49, OH 46, CH 42), and MIDI files export and import the drum part on channel 10. CHOMPI + a knob pushed for 2 s resets every page of it at once (knobs 1-3): on knob 2, the voice's attack and tuning and the filter and resonance; on knob 3, its decay and FM and the filter envelope; on knob 1, its level and pan, the accent, the drum part's length, the LFO, and its LFO target and depth. The knob flashes white.

The 16 steps on 15 white keys

White keys 1-7 are steps 1-7 and white keys 9-15 are steps 10-16. Middle C (white key 8) is step 8 or step 9. The half in view (steps 1-8 or 9-16) is lit normally and the other half dimmed. The view follows the half you last pressed a key in, and while the pattern runs it follows the playhead once the second half has notes in it. D#4 flips it by hand (CHOMPI + D#4 in pitch mode's note entry).

Step lights: on = red, accent = bright red, tie = dim red; steps past the pattern's length dimmer still.

Step mode

Pages. On a parameter page the step keys toggle that setting for each step, lit in the page's colour (DOWN purple, UP cyan, ACCENT orange, SLIDE blue, TIE green). The page's key again goes back to the notes page; TIE's again opens the RATCHET page (its key pink), and once more goes back.

Ratchets. On the RATCHET page a step key cycles the step through 1-4 hits: the note plays that many times, evenly through the step, each gate half its share (pink, brighter for more; dimmer on a step that's off). A slide or tie out of the step holds its last hit. Ratchets follow the step scale and are saved and exported; on an external clock they land on its ticks, so 4 in a sixteenth comes out a little uneven.

Lights. The selected step is whitened (a selected empty step is dim white). The playing step flashes white.

Patterns. While the pattern runs, a new one waits for the end of the bar (it blinks); press it again to switch at once.

Pitch mode

The step lights only show while the pattern plays or you're recording; otherwise the keybed is just a keyboard.

Recording while running. A played note goes into the pattern; a note held across steps ties through them. With quantize on (CHOMPI + D#4) notes land on the grid: every step (1/16), every other (1/8) or every fourth (1/4). With quantize off they keep their timing within the step, to 1/24 of a beat. Turning quantize on also plays notes already recorded that way on the grid (each to its nearest grid point, so a note played just early lands on the beat it was aiming for); turn it off and their timing comes back, as nothing in the pattern changes (until you hold CHOMPI + D#4 for 2 s, which makes it permanent). Only notes with recorded timing move: notes right on their step, like everything entered in step mode, stay put. If two land on one step the nearer plays, and a note already there wins. CHOMPI + F#3 / G#3 / A#3 toggle accent, slide or tie on the step playing now. For octave jumps, shift the keyboard with CHOMPI + C#3 / D#3 and play.

Note entry (record on, stopped). Arming LOOP while stopped starts at step 1. Each note you play fills the next step (in the keyboard's octave), its white key lights up, and the pattern grows to it. CHOMPI + F#3 / G#3 put an accent / slide on the last step, CHOMPI + A#3 adds a tie step, CHOMPI + the blinking next step's white key a rest (rests you've entered show dim grey, the one just entered brighter). Press PLAY with record still on to carry on recording in real time.

Arpeggiator. With it on, the keys you hold are played one at a time in sixteenths at the tempo: up, down, up-down, random or as played, from up to two octaves below the chord to two above (CHOMPI + F#4 / A#4), in the pattern set with CHOMPI + G#4. While the pattern runs it locks to the pattern's steps (and swing), and the pattern is silent while it plays; with record on, the arpeggio is written into the steps. In note entry the keys go into the steps as usual.

Knobs

Turning the pattern length (knob 1, page 2), in either mode, shows it on the white keys for a moment: every step within the length dim white, the last one bright: white for steps 1-8, cyan from step 9 (middle C shows step 8 or 9, so the colour tells them apart).

Step scale (triplets). In step mode, hold D#4: the purple knob lights in that side's step scale, green 1/16 (the default), magenta 1/16 triplets or orange 1/8 triplets. Press the purple knob while holding D#4 to go to the next. The bassline and the drum part each have their own, so they can cross: the bass in sixteenths against the drums in eighth triplets is four against three, three drum steps to a quarter note. Twelve steps of 1/8 triplets make a bar (12/8), twelve of 1/16 triplets half a bar, so the length follows: a 16-step part becomes 12 steps when you switch it to a triplet scale, and a 12-step one 16 again when you switch back (other lengths you've set stay; steps past the length are kept, just not played). On a triplet scale that side's white keys are its steps one for one, keys 1-15 for steps 1-15, so a 12-step part sits on keys 1-12, its halves split between keys 6 and 7, with no middle C shared and no half to switch to (step 16, if a part is that long, isn't on a key until you go back to 1/16). The pattern page and COPY + a key still count pattern numbers 1-16 as usual. Both scales are saved with the pattern (patterns from v1.3.0, which had one for both, load with both sides on it). A new scale takes over at the top of that side's next loop, so it never lurches mid-bar; a pattern played on both sides (PATTERN held + a key) starts both together. Triplets aren't swung. Exported MIDI files keep each side's timing; importing reads every file as sixteenths.

Stepped controls (the wave, the lengths, the synced delay time, the LFO's shape and target) move a step every two clicks, so a nudge doesn't change them; turning back starts the count again. The knobs are read so that a bouncing contact never counts as a click either way.

Click knob 1, knob 4 or the volume knob to step through their pages (knob 4 has four). The click comes when you let go, and not at all if you turned the knob while it was pushed (a single detent while pushed, as pressing an encoder can jog it, doesn't count, and doesn't change anything); each page has its own colour, and so does each CHOMPI function (the light changes colour while CHOMPI is held). A knob never goes darker than a fifth, so you can always see which page it's on. The knobs accelerate: clicked slowly, each click moves a setting about 1 % (fine control); turned at a normal pace, the whole range takes a turn or two; spun, under a turn. Env mod, decay, accent and slide time go a quarter faster still. Stepped settings move one position a click, and tempo 1 BPM a click turned slowly, up to 5 spun. CHOMPI + click sets both functions of that knob's page back to their defaults (pattern length back to 16); on knob 4 it resets all the effects, every page. On the volume knob it's the metronome instead: CHOMPI + click turns it on or off (the knob flashes green for on, red for off). While the pattern runs it clicks each beat, higher on the first beat of each bar, about 12 dB under the bassline and following the volume knob; it keeps to the beat on the triplet scales too, isn't recorded or sent over MIDI, and is off again at power-on. Hold CHOMPI and push knob 1, 2 or 3 for 2 s to reset every page of that knob at once (the knob flashes white).

The two lights above the big purple knob show cutoff (resonance with CHOMPI held). While the pattern plays they flash yellow instead, on the eighth notes, alternating sides: left on step 1, right on step 3, left on step 5, and so on; the side of the current eighth flashes red when a note is recorded. For a moment after you turn the knob they show its value again. PLAY is steady green while running, and the CHOMPI button's light flashes brighter on each step that plays a note (brightest on the beat); the transpose mode key (C#4) is dim yellow, the transpose amount bright yellow.

Big purple knob click: tap tempo. Volume knob click: steps through volume / drive, tempo / swing, the bass / drums mix and the compressor, and stops any stuck live notes.

The voice

One oscillator (saw, or square with pulse width) into the TB-303's 4-pole diode ladder low-pass, after Tim Stinchcombe's analysis of the 303 filter: four one-pole stages at 0.128, 1.04, 2.33 and 3.24 times the cutoff inside one resonance loop. The spread stages give the 303's slope (around 18 dB/octave above the cutoff, 24 only far above) and its broad resonance, which stops well short of self-oscillation; a high-pass in the loop and saturation at its input, run at twice the sample rate. A high-pass at 120 Hz after the filter stands in for the 303's coupling capacitors, which thin out its lowest notes. Then a decay-only filter envelope and a gated amp envelope. As on a 303, the cutoff knob moves about an octave in its bottom half and nearly 2.5 in its top. Env mod does little until 12 o'clock and a lot after it: even at its minimum the envelope sweeps about half an octave, and at full it sweeps over 5. Turning env mod up raises the top of the sweep and lowers where it rests, about 70/30. So the lower the cutoff, the sooner a long note's resonance sinks into the bass and dies away. The cutoff and env mod curves, the decay and the bass loss were fitted to recordings of a real TB-303. Accent makes a step louder, gives the filter envelope its shortest decay, and charges an accent "capacitor" that drains slowly, so accents in a row build up. Slide holds the gate into the next step and glides there. Gates are half a step; ties and slides hold them.

The resonance amount, cutoff range and envelope times are first estimates, to be fitted to recordings of a real TB-303.

The effects

In this order after the voice, each off at zero:

• Shared delay. The delay is one effect, shared with the drums. The bass's delay knob sends the bass into it (the lower half brings the echoes in, the upper half also fades the dry bass out); turned down, the echoes already in it ring out rather than stopping.
• Delay time. Synced (CHOMPI + knob 4) or free (push knob 4 and turn). A new synced time crossfades to it over 80 ms, so it changes without a pitch bend; a free time glides there like a tape machine's rate control, bending the repeats' pitch up to a third (a swoop, never backwards). Push and turn starts the free time from the synced one, so nothing jumps; CHOMPI + turn goes back to synced, crossfading to the setting it had.
• Drive (CHOMPI + volume): a pedal-style hard clipper in the spirit of a DS-1: a high-pass tightens the low end and a treble lift puts the highs in front, up to ~60x gain hits a nearly hard, slightly asymmetric clip, and a tone low-pass (8 to 6 kHz) takes only the harshest fizz off. The level is evened out as it turns up.
• Bit crusher (knob 4, page 4): bit depth 16 down to 4, and sample rate down to 1/32, separately.
• Chorus / flanger (knob 4, page 3): the first half of the turn is chorus, the second half flanger with rising feedback; width spreads the sides apart and deepens it.
• Tape delay (knob 4, pages 1-2): tempo-synced ping-pong. Dry/wet: the middle is 50/50, the top all echoes. Every repeat goes through tape EQ (a low-pass set by tone, a high-pass) and saturation, so repeats darken as they fade, with a little wow and flutter. Feedback goes just past self-oscillation at the top, where the saturation holds it.

A soft limiter keeps the output in range however hard they're pushed.

MIDI (DIN and USB)

Channels and on/off switches are in /X0X/options.txt: midi_drum_channel (default 10) sets the drums' channel, in and out, and drums_in 0 stops incoming notes playing them.

On the card (/X0X)

Patterns save 2 s after the last edit and settings 3 s after the last change. A new card (no patterns.txt or current.txt yet) starts with a demo set: patterns 14A-16A and knob settings to go with them, written to the card at once. They live in defaults/ (edit those, then run python3 defaults/make_header.py). Existing files are never replaced. CHOMPI blinking red three times at power-on means the card didn't mount; it then runs without saving.

CHOMPI + PLAY + LOOP held at power-on puts the battery in shipping mode, as in the stock firmwares.

Building

Toolchain: Arm GNU Embedded Toolchain 10.3-2021.10, on your PATH. Build the libraries once, then the firmware:

make -C code/libs/libDaisy
make -C code/libs/DaisySP
cd code/src && make

The output is code/src/build/CHOMPI.bin.

Testing on the desktop

code/src/x0x/ is the whole instrument without the hardware: plain C++ that also builds on a computer. host/ tests it and renders it to WAV:

make -C host
host/tests
mkdir -p host/out && host/render host/out

tests checks sequencer timing to the sample, slides, ties, nudged steps, swing, the external clock, pattern queueing, the shared middle C, every step-mode and pitch-mode panel behaviour, real-time recording with each quantize setting, note entry, the knob pages, tap tempo and the file formats. render writes demo patterns (saw, square, pulse width, accents building up, slides, each effect, swing) and checks they stay finite and in range.

Flashing

The .bin is a release download (chompi-x0x-v1_0_1.bin and so on) or, if you built it yourself, code/src/build/CHOMPI.bin; the name doesn't matter.

With the Multi Firmware Launcher: add the .bin to the FIRMWARE folder in the root of the SD card, alongside your other firmwares, and pick it at power-on. A number in front of the name puts it on that key (04_X0X.bin: key 4); without one it takes a free key. x0x keeps its files in its own /X0X folder, so it shares the card with the others.

With the standard bootloader:

1. Copy the .bin to the root of the SD card. It must be the only .bin there.
2. Power on. The rainbow shows while the bootloader installs it, then a red sweep across the keys.

A CHOMPI that has never had the bootloader needs bin/install_bootloader.sh once first. One running any stock firmware already has it. To go back, use a card with a stock firmware's .bin.

Either way, your patterns and settings live in /X0X on the card, apart from the firmware.

Layout of this repo

The first commit is TEMPO v1.0 as released, so git diff against it shows everything this firmware changes.

Credits

• Built on CHOMPI Club's open-source TEMPO firmware (MIT); see LICENSE, THIRD_PARTY.md and TRADEMARKS.md. The CHOMPI name belongs to CHOMPI Club, and this is a community firmware, not an official release.
• The fix that keeps the firmware from occasionally starting at 64 MHz (libDaisy's boot_info placed in backup SRAM) is from sfaber02/CHOMPI-MULTI-FIRMWARE.
• Key, LED and knob tables, and fixes for SD card cache alignment, codec start-up and button edges, from hiwatts' POLY (sfaber02/chompi-poly, MIT). TEMPO's MidiManager is the model for the DMA MIDI out.
• This was a total "vibe" job and majority of credit goes to Claude for doing the actual coding allowing me to put this together in three days. I may have given considerable direction when shaping UI/UX behaviour and mechanics and modelling the filter behaviour off my RE-303 (also consulting Tim Stinchcombe's page (https://www.timstinchcombe.co.uk/index.php?pge=diode2) about building a 303 diode filter) but translating this into useable and bug-free code was 100% done by Claude.