OffAir

AM/Shortwave/Longwave radio simulator for the Workshop Computer.

OffAir lets you tune across a virtual band the way you tune a real shortwave receiver. As you approach a station you hear the signature heterodyne whistle slide in pitch, the audio pull into tune, and the background static duck away. Tune past it and the audio garbles, the whistle rises again, and it fades back into the noise.

Simulating the modulating and demodulating an RF carrier, OffAir gives the sound of detuning each station directly from how far off-tune you are:

• a heterodyne whistle whose pitch slides with detune — the signature SW sound;
• on SW/LW, the audio is single-sideband frequency-shifted by the detune amount, so harmonics break and voices go "wrong pitch" / metallic — exactly the near-tuned SSB sound (this is genuine product-detector behaviour: a detuned local oscillator shifts the recovered audio);
• on AM, the audio stays at correct pitch (envelope detection) but distorts as you detune;
• the station fades out as you move off, and at zero-beat you get clean audio with no whistle.

There are two tunable Stations (Station 1 and Station 2 — fed by the live audio inputs, or by baked recordings in baked-in audio mode) plus three interference signals (morse / data / numbers). They're scattered across the dial and re-randomised on every band change — so the layout is different each time and a Station and an interferer sometimes overlap, like a crowded band. A separate bank of short one-shot events ("Insta-ference") can be fired in on demand.

See offair_overlay.jpg for the panel layout.

Bands

Tap the switch Down to cycle AM → SW → LW. The band sets both the noise character and the demodulation behaviour:

Band	Demod near a station	Noise character
AM	Correct-pitch audio, distorts off-tune (envelope detection)	Mellow mid hiss, steady
SW	Directional pitch-shift (SSB)	Bright, busy, crackly wash that swishes
LW	Directional pitch-shift (SSB)	Deep slow rumble, barely swishes, heavy fade

The dial layout (Station and interference positions, and which clips play) is re-randomised each time you change band.

Inputs

Jack	Panel	Function
Audio In 1	Station 1 In	Station 1 source (live audio, in audio-input mode)
Audio In 2	Station 2 In	Station 2 source (live audio, in audio-input mode)
CV In 1	Tuner	Tuning offset (1:1, added to the Main knob; ±5V ≈ ±half the dial). Patch an LFO/sequencer to scan, or CV Out 2 to home on Station 1
CV In 2	Noise	Noise level — adds to Knob Y (voltage-controlled static)
Pulse In 1	Shuffle Signals	Rising edge — re-randomise the Station / interference layout. (In audio-input mode with Switch Up it becomes the Morse In key instead — see Controls)
Pulse In 2	Insta-ference	Rising edge — fire a one-shot event from the curated Insta-ference bank

Outputs

Jack	Panel	Function
Audio Out 1	Output	Full mix — tuned audio, whistles, static, Insta-ference
Audio Out 2	Just Noise	Noise / static only
CV Out 1	Signal Strength	An envelope that rises as you tune onto a Station
CV Out 2	Station 1 CV Offset	Station 1's offset from the main knob
Pulse Out 1	Station 1 Tuned Gate	Gate HIGH while tuned to Station 1
Pulse Out 2	Station 2 Tuned Gate	Gate HIGH while tuned to Station 2

Controls

Main Knob — Tuning. Scans across the dial. Sums with CV In 1 (Tuner).

Knob X — Brightness (IF bandwidth). Sets both how wide a Station's capture window is and the audio brightness. Fully CCW = narrow, muffled and selective (fiddly to tune); fully CW = wide, bright and easy to find.

Knob Y — Noise Level. Silent fully CCW; raises the static floor toward full CW. The static slowly swells and swishes on its own (random-walk level and filter sweeps), heaviest on LW. Noise ducks away when you tune onto a Station — fully on AM, strongly on SW/LW. CV In 2 adds to this.

Switch Down tap — Cycle band (Radio). AM → SW → LW, re-randomising the layout each time.

Switch Up hold — mode-dependent. What it does depends on how you booted:

• Baked-in audio mode: mutes Station 1 & 2. The interference signals carry on, and the Stations still "exist" — they keep driving CV Out 1/2 and Pulse Out 1/2 at their dial positions — they just make no sound. Release to bring them back.
• Audio-input mode: Station 2's audio is replaced by a ~600 Hz morse tone keyed by Pulse In 1 (Morse In) — feed a rhythm or gate pattern into PU1 and it becomes an audible keyed signal at Station 2's position (heard when tuned to it, pitch-shifting off-tune like any Station). While the switch is held up, Pulse In 1 no longer shuffles the layout.

Insta-ference (one-shot bank)

Pulse In 2 (Insta-ference) fires a short event from a curated bank — clicks, dropouts, crashes, single morse bursts: things that make sense heard in isolation. Each trigger plays a random clip once through and stops; re-triggering restarts it. The Insta-ference bank is separate from the looping interference clips, so the two can be curated independently (see Making your own version below). If the bank is empty the firmware falls back to the looping interference clips.

Boot modes

Audio-input mode — power on with the switch in any position other than Down (or release it within the first ~200 ms). The two live audio inputs become Station 1 & 2.

Baked-in audio mode — hold the switch Down at power-on until all six LEDs flash. The two live inputs are replaced by baked recordings, turning the module into a self-contained radio. Everything else works identically.

LEDs

LED	Function
0	Station 1 signal strength
1	Station 2 signal strength
2 / 3	Band — both off = AM, LED 2 = SW, LED 3 = LW
5	Tuning position

Patch ideas

• Auto-tune to a Station. Patch CV Out 2 (Station 1 CV Offset) through a slew into CV In 1 and the dial tunes onto Station 1, whatever the knob is set to. Trigger Pulse In 1 to re-randomise and it slowly hunts to the new position — a self-playing radio that keeps finding the station (cleaner on AM band, LW and SW get 'near').

• Fine-tune to a Station. Self-patch CV Out 2 (Station 1 CV Offset) straight into CV In 1 with no slew. The dial snaps to Station 1, and shuffling the layout (Pulse In

  1. or changing band keeps tuning locked near Station 1 — so the Main knob becomes a fine-tune around it.

• Rhythmic morse instrument. Boot in audio-input mode, hold Switch Up, and clock Pulse In 1 (Morse In) from a sequencer or trigger pattern. Station 2 becomes a keyed ~600 Hz tone in time with your rhythm — tune onto it for a clean beep, or off it for pitch-shifted, whistling morse. A signal that's also a musical part.

• Radio as an envelope/gate source. CV Out 1 (Signal Strength) rises as you tune in — patch it to a VCA or filter so other voices swell as you find a station. Use Pulse Out 1 / 2 (Station 1 / 2 Tuned Gate) to clock or gate envelopes only while a Station is locked.

• Hands-free band scanning. Patch a slow LFO to CV In 1 (Tuner) to drift across the dial on its own — stations and interference drift past, whistling in and out. Add Pulse Out triggers to fire events each time a Station passes.

• Tuned drum trigger. Sweep slowly and take Pulse Out 1/2 as triggers: each pass through a Station fires a hit — the dial layout (re-randomised by Pulse In 1) sets the rhythm.

• Stab punctuation. Clock Pulse In 2 (Insta-ference) from an off-beat to drop in short clicks/dropouts/morse bursts as percussion under the tuned audio.

• Process external audio. In audio-input mode, feed your own sources into Audio In 1 / 2 (Station 1 / 2) and use the radio as a tuning-based degrade/pitch-shift/gate effect — tune in for clean, off for the SSB-mangled version.

• Dual-output split. Take the full mix from Audio Out 1 and pure static from Audio Out 2 (Just Noise) into separate channels — process the static independently (reverb, gate) for an atmospheric bed under the stations.

Technical notes

• Behavioural demodulation: per-Station detune drives a sliding heterodyne whistle, an SSB frequency-shifter (IIR Hilbert quadrature pair, SW/LW) or envelope detection with off-tune distortion (AM), and a strength fade. All processing stays in the audio band, so there is no aliasing.
• Integer-only DSP, runs on core 1; an RF PWM path runs on core 0.
• Interference / Insta-ference clips: 8 kHz unsigned 8-bit mono.
• Station clips: 11025 Hz 12-bit packed signed (2 samples per 3 bytes).
• Looping interference clips are trimmed to ≤12 s to leave flash headroom for the Insta-ference bank; Station clips run ~32–34 s.
• 200 ms startup holdoff before audio begins, with a short linear fade-in.

Using the prebuilt firmware

Download offair.uf2 from the releases page (or grab it from this folder), hold BOOTSEL on the Computer, and drag the file across. No build needed; the baked-in audio (clips.h) is already compiled in the firmware.

Making your own version with custom sounds

All the radio audio is baked into clips.h, generated from source audio files by convert_clips.py. The audio sources themselves are not included in this repo — supply your own. To build a version with your own Stations / interference / events:

1. Install the Python tools (one-off): pip install miniaudio numpy

2. Prepare your audio. Any format/sample rate works (WAV, MP3, AIFF, FLAC, mono or stereo, any rate — the script resamples and downmixes). There are three pools:

• Stations — the two main signals you tune in (played in baked-in audio mode).
• Looping interference — continuous beds (numbers, morse, data) that sit at dial positions and loop forever.
• Insta-ference — short isolated events (clicks, dropouts, crashes, single bursts) fired by Pulse In 2. Curate these to make sense heard alone.

3. Point the script at your files. Edit the top of convert_clips.py:

• FOLDER — the directory holding your Station and looping-interference files.
• The CLIPS table — list each Station / interference file (filename, start second, max length). LOOP_MAX_SEC caps interference loop length; MAX_BCAST_SEC caps Station length. Keep 2 Station + 6 interference entries unless you also change the counts in main.cpp.
• ONESHOT_FOLDER — a folder of Insta-ference files; all files in it are auto-discovered (any count). ONESHOT_MAX_SEC caps each one's length.

4. Generate and build:

python convert_clips.py          # regenerates clips.h, prints sizes + flash budget

Then build with the Pico SDK (CMake / Ninja) from this folder and flash the new offair.uf2. The script warns if your audio exceeds the ~2 MB flash budget — trim clip lengths or drop Insta-ference if so.

Credits

By Andy Jenkinson (uglifruit), developed with Claude Code.

Built on the Workshop Computer platform by Tom Whitwell (Music Thing Modular), using the ComputerCard framework.

The RF PWM transmission path — generating an AM carrier on the DEBUG_1 pin receivable by a nearby MW radio — is directly inspired by and based on AM Coupler by Chris Johnson.

Inspired by RadioMusic by Tom Whitwell and the Music Thing Radio Music workshop.

Interference recordings sourced from Numbers & Oddities, a shortwave monitoring archive. Individual recordings are in the public domain.

The baked-in Station recordings are Protect and Survive (UK Government, 1970s civil defence broadcast) and the Shipping Forecast (BBC Radio 4). These are included for personal and educational use only. Protect and Survive material is Crown Copyright; Shipping Forecast is © BBC. Neither is licensed for commercial use or redistribution. If you build a derivative work for release, replace these recordings with material you have the rights to use.

Licensed under Creative Commons Attribution-ShareAlike 4.0 — except where third-party copyright applies as noted above.