Bootleg Radio (538)

A lo-fi bootleg tribute to Music Thing Modular's Radio Music — not a replacement, a cheaper, grubbier cousin. Load a USB stick with WAV files, plug it into the Workshop Computer's front USB-C jack, and tune through them like radio stations. The name comes from 538, the AM frequency the pirate station Radio Veronica used off the Dutch coast; Bootleg also nods to the Botlek, the Rotterdam docklands where sub- and pirate culture ran thick.

The DAC is 12-bit and files are resampled with simple linear interpolation, so expect character rather than fidelity — which is rather the point.

Quick start

1. Format a USB stick as FAT32 or exFAT. The usual macOS/Windows defaults are fine — see Supported USB sticks.
2. Copy uncompressed 16-bit PCM WAV files (mono or stereo, any sample rate up to 192 kHz) into the root directory, and/or into subfolders. Each subfolder that contains WAV files becomes a bank.
   • The official Music Thing Radio Music Suggested Audio page is a great set of loops and samples to load onto the stick first — it is the recommended starting content for trying the card.
3. With the Computer powered off, plug the stick into the front USB-C jack. This needs a Rev 1.1 or later board (the Computer acts as USB host), and nothing else may share that socket.
4. Insert the card and power up. While a stick is present but not yet readable, all six LEDs blink together; once the filesystem is readable they stop and LED 4 lights. (Reading the stick can take a moment after it enumerates.)
5. Turn Main (and/or patch CV in 1) to pick a station. Hold the switch up to change bank, or flick it down to retrigger the loop.

The root directory is always bank 0, even if it holds no WAV files. If it is empty the card starts silent — hold the switch up and turn Main to reach your folders.

Controls

Tape speed links pitch and time: slowing the card plays lower too. There is no separate time-stretch or "90s mode".

Inputs and outputs

• CV in 1 — station/bank selection, added to Main. This is how you tune by voltage.
• CV in 2 — tape speed, added to X.
• Audio in 1 — loop start point, added to Y. It is DC-coupled, so a steady voltage works just like a CV.
• Audio in 2 — loop length, as a fraction of the file. Unpatched, the loop runs to the end of the file.
• Pulse in 1 — clock for the two CV outputs. Unpatched, they use the internal 120 BPM clock.
• Pulse in 2 — external retrigger, the same as a short switch-down tap.
• Audio out 1 / 2 — left and right. Mono files appear on both.
• Pulse out 1 — loop gate: high while the loop plays, low during the wrap gap.
• Pulse out 2 — loop wrap tick: a short pulse each time the loop wraps.
• CV out 1 — clocked random stepped CV.
• CV out 2 — clocked bipolar triangle LFO.

The CV inputs are bipolar: a unipolar 0–10 V source reaches only the upper half of the range. The audio inputs used for loop start behave the same way.

Banks

Files are organised into banks: the root directory is bank 0, and each subdirectory that contains WAV files is another bank (up to 32).

• To change bank, hold the switch up and turn Main (or patch CV in 1). The new bank starts at its first station, and nothing changes until the knob moves. Release the switch to go back to selecting stations.
• The card powers up on the bank setting from settings.txt (default 0 = the root), and starts on that bank's first station. The Main knob does not override the start bank until you actually turn it.
• The root is always bank 0, even if it holds no WAV files.
• Banks and the stations inside them are natural-sorted: case-insensitive, with runs of digits compared as numbers, so station2 comes before station10 — the same order you see in Finder. Root stays first.
• A bank holds up to 64 stations; any beyond that are ignored.

Looping

A station loops over its window: it plays from the loop start (Y / Audio in 1) for the loop length (Audio in 2), then wraps back to the start. Unpatched, the length is the whole file, so the loop runs from the start point to the end and back.

The card never advances on its own. You move it with Main/CV in 1 (station), switch up (bank), or switch down (retrigger).

Loop seam. Wrapping re-reads the file from the USB stick, so there is a very short gap at the moment of wrap. A click-free crossfade is a future improvement. There is also no reverse playback or crossfade between stations yet.

Tune modes (Loop vs Sync)

Both modes loop the station. The mode only decides where a new station starts when you tune (Main/CV1) or change bank (switch up):

• Loop — the new station starts at its loop start.
• Sync — the new station starts at the same proportional position as the old file (synchronised station jumping), clamped into its window. Sweeping the station knob or CV glides through the stick's material in place. This is the default.

To toggle, hold switch-down for about a second. The whole panel blinks 2 times for Loop and 3 times for Sync. A short tap still retriggers.

The built-in LFO and random CV

The two CV outputs are a clocked modulation pair, so you can self-patch the card with no external LFO. They share a clock: the internal 120 BPM by default, or an external clock patched into Pulse in 1 (the tempo glides to the new rate, and falls back to 120 BPM if the clock stops). They run whether or not a file is playing.

• CV out 2 is a bipolar triangle, one cycle every 40 clock pulses (20 seconds at 120 BPM).
• CV out 1 is a random stepped voltage, a new value on every clock pulse (2 Hz at 120 BPM).

Patch them back into any input to bring the card to life — see Patch ideas.

Settings (settings.txt)

The card has no screen or editor (the USB socket is the host port for the stick), so a few "set and forget" behaviours live in an optional settings.txt file in the root of the stick. A ready-to-copy example is included as settings.example.txt. It is read once each time the stick is mounted (only after the filesystem mounts and the root directory can be listed); all lines are optional:

# settings.txt - all lines optional
mode = sync        # power-on tune mode: loop | sync
bank = 0           # bank to start on (0-31)
clockBpm = 120     # internal clock tempo (30-240)
triPulses = 40     # clock pulses per triangle cycle (1-256)
pulse1 = gate      # Pulse out 1: gate | tick | clock | off
pulse2 = tick      # Pulse out 2: gate | tick | clock | off
led = verbose      # LED mode: verbose | quiet

Rules and defaults:

• key = value lines; # and ; start comments. Unknown keys and blank lines are ignored.
• A missing file, or an out-of-range/unknown value, keeps the default — so with no file the card behaves exactly as described here.
• Pulse-out modes: gate = high while looping; tick = a short pulse each wrap; clock = the active clock pulse; off = always low.
• LED modes: verbose shows level, playing, station-change and fault; quiet shows only level, playing and mounted (LEDs 3 and 5 stay dark).

LEDs

Reading the LEDs when something is wrong

There is no console on a USB-host card, so the LEDs carry the diagnosis:

• LED 4 + 3 — mounted, but no .wav files were found in the selected bank. (If you started on an empty bank 0, hold the switch up and turn Main to reach a folder.)
• LED 4 + 3 + 5 — WAV files found, but none could be decoded (they must be uncompressed 16-bit PCM, mono or stereo).
• LED 4 + 5 — the stick was detected but its filesystem would not mount.
• LED 4 + 2 — playing normally.

Supported USB sticks

The card reads an unusually wide range of sticks:

Notes:

• macOS Disk Utility defaults to a GUID Partition Map, which is fine.
• For a GPT stick, the data volume must be the Microsoft Basic Data partition type — that is what macOS and Windows use for FAT/exFAT volumes, so a normally formatted stick already qualifies.

Patch ideas

• Slow station sweep (self-patched). Patch CV out 2 (triangle) into CV in 1. The card sweeps through the stations in the current bank, one pass every 40 clock pulses (20 s at 120 BPM). Turn Main to choose the centre of the sweep.
• Clock-synced random scanning. Patch CV out 1 (random stepped) into CV in 1, and clock the card via Pulse in 1. A new station is picked on every clock pulse — a random radio scanner locked to your patch.
• Tape wobble / loop jumps. Patch CV out 1 into CV in 2 for stepped speed changes, or into Audio in 1 to jump the loop start point around on every clock. Slow CV out 2 into CV in 2 gives a smooth speed sweep.
• Rhythmic wraps. Patch Pulse out 2 (wrap tick) into a percussion trigger, or back into Pulse in 1 to clock the CV outs from the loop itself. Patch Pulse out 1 (loop gate) into a VCA to mute the seam gap.

Links

• Radio Music Suggested Audio — the recommended set of loops and samples to load onto the stick.
• Music Thing Radio Music page

Credits

• Radio Music is a Music Thing Modular module by Tom Whitwell. Thank you, Tom, for Radio Music and for the suggested audio sets. This card is an unofficial, lo-fi bootleg tribute to the idea, not a replacement for it.
• This card is adapted from Chris Johnson's usb_msc_host ComputerCard example, added to the Workshop_Computer repository in PR #437. ComputerCard.h is © Chris Johnson, MIT licensed.
• fatfs/ is FatFs by ChaN; see fatfs/LICENSE.txt.

Building from source

This card needs TinyUSB 0.21 or later (older versions read sticks too slowly for audio). From this folder:

make

The built firmware is copied to UF2/bootleg_radio.uf2. Flash with the usual Workshop Computer workflow (debug probe/OpenOCD or UF2 drag-and-drop). If you build outside the repo dev container, point CMake at a newer TinyUSB with -DPICO_TINYUSB_PATH=/path/to/tinyusb.

Files of interest

• main.cpp — core 1 runs the 48 kHz audio and knob/CV/switch handling; core 0 runs USB, FatFs and WAV parsing, and fills the ring buffer.
• diskio.c, fatfs/ — FatFs glue for a TinyUSB host mass-storage device (built with FF_LBA64 = 1 and a 4k FF_MAX_SS so GPT and 4096-byte-sector sticks mount).
• tusb_config.h — host-only TinyUSB config (mass storage, single device).
• ComputerCard.h — vendor copy of Chris Johnson's header-only card library.