rspektrum

rspektrum is an interactive spectrogram viewer for inspecting radio captures and arbitrary audio. It loads a WAV file, computes a short-time Fourier transform (STFT), and draws the result as a zoomable, pannable time–frequency image. Its distinguishing feature is support for mLnL annotations — labelled regions (TX frames, assertion outcomes, impairment fires, …) carried inside the WAV file itself — which it overlays on the measured spectrogram so you can compare what a modem intended to transmit against what actually hit the air.

You can box a time/frequency region, hear it back through a bandpass filter, and export either the picture (PNG) or the isolated audio (WAV). rspektrum runs three ways: a native desktop app (C + raylib), a headless command-line renderer, and a WebAssembly build in the browser.

Click for Video Demo

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What it's for

The primary use case is reviewing captures from the mLink radio stack: a WAV recording of an over-the-air signal with an embedded mLnL chunk describing what the modem/daemon believed it was transmitting at each instant. rspektrum renders those annotations on top of the measured spectrogram, frame by frame, so intent and reality sit side by side.

It also works as a general-purpose spectrogram tool for plain WAVs with no annotations. See mlnl_chunk_spec.md for the annotation format.

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Features

• STFT spectrogram — selectable colormaps, adjustable dB floor / dynamic range, absolute (dBFS) or relative amplitude scaling.
• mLnL annotation overlay — labelled boxes from the WAV's embedded annotation chunk; hover a box (or its region on the scope) for per-frame detail (sequence, channel, rate, scheduling offset…).
• Zoom & pan the time/frequency view.
• Region selection — box a time and frequency range with the mouse.
• Filtered playback — play just the selected region, band-limited to the selected frequency box via an FFT bandpass. What you hear is what you'd export.
• Waveform scope — toggleable time-domain view beneath the spectrum.
• Marker / ruler and a spectrum slice (PSD) readout.
• Export — save the view as a PNG, or the selected region as a WAV.
• Headless render mode — produce an annotated PNG from the CLI with no window, no GL, and no X server. Pure CPU; runs in CI, containers, or over SSH.
• Broad input — WAV directly (8/16-bit PCM, 32-bit float; stereo downmixed to mono); other formats transcoded via ffmpeg if it's on PATH. Drag-and-drop.
• Cross-platform — Linux/desktop, Windows, and a WebAssembly build.

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Building

The build is driven by a checked-in Makefile. premake5 is not required to build — only to regenerate the makefiles.

make -f rspektrum.make config=debug_x64      # -> bin/Debug/rspektrum
make -f rspektrum.make config=release_x64    # -> bin/Release/rspektrum

Web (WebAssembly) build:

./build_web.sh      # emscripten; emits the WebAssembly bundle

The release build enables -O2, which turns on extra warnings (-Wformat-truncation) that the debug build doesn't. Build release before declaring a change clean.

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Usage (desktop GUI)

./bin/Debug/rspektrum [input.wav]

Load a file by passing it on the command line, dragging a .wav onto the window, or pressing O for the file browser. Try the bundled sample:

./bin/Debug/rspektrum mlnl_samples.wav      # in-repo WAV with an embedded mLnL chunk

Controls

Input	Action
O	Open file browser
Mouse wheel	Zoom time/frequency
Alt+drag / middle-drag	Pan the view
LMB drag	Select a time + frequency region
Space	Play / stop the selected region
Hover an annotation	Tooltip with that frame's mLnL detail
P	Show / hide the waveform scope
M	Marker / ruler tool
S	Spectrum slice (PSD)
E	Export PNG
W	Export selection as WAV
Home	Reset view (fit all)
End	Zoom to start
F11	Toggle fullscreen
F1	About / help
Esc	Clear selection / close dialog

Most controls are also available as buttons in the left sidebar (colormap, floor, dynamic range, annotation opacity, grid, …).

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Usage (headless render)

--render writes the spectrogram straight to a PNG with no window, no GL context, and no X server. It computes the STFT, colorizes the bitmap, bakes the annotation overlay onto it, and exports — all on the CPU — so it runs anywhere (CI, a bare SSH session, a container with no display):

./bin/Debug/rspektrum --render OUT.png INPUT.wav [options]

The output is the real spectrogram bitmap at native STFT resolution (not a screenshot of the UI), so it carries no sidebar/scope chrome — just the time–frequency image with the annotation overlay.

Flag	Effect
-r, --render OUT.png	Render to OUT.png and exit (no window/GL/X)
-a, --annotations	Force the annotation overlay on
--no-annotations	Force the overlay off
--annotation-opacity=V	Overlay strength 0..1 (default 0.5)
--annotation-kinds=LIST	Comma-separated kinds to draw (default: all)
--width N	Resize output to N px wide (default: native STFT size)
-h, --help	Usage

Annotation boxes are drawn outline + label only (no translucent fill): mLnL captures contain many overlapping full-band boxes whose fills would alpha-stack to opaque and bury the signal, so the outline marks each region while the spectrogram reads through.

# everything, brighter overlay
./bin/Debug/rspektrum --render /tmp/all.png mlnl_samples.wav --annotation-opacity=0.7

# only on-air frames and failed assertions
./bin/Debug/rspektrum --render /tmp/tx.png mlnl_samples.wav \
  --annotation-kinds=tx_frame,assertion_failed

Annotation kinds: tx_frame, tx_burst, control, channel_up, channel_down, assertion_passed, assertion_failed, impairment_fire, gain_change, unknown.

The hover tooltip only appears with a live mouse over a box, so it cannot show up in a static --render. To verify tooltip behaviour you need a real (or virtual) display driving the GUI — see below.

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Driving the GUI headlessly (agents / CI)

The app can be run, screenshotted, and clicked on a virtual X display with no monitor or GPU (Mesa software GL under Xvfb). The full playbook lives in AGENTS.md; the working reference implementation is shot_input.sh.

The loop in one breath:

Xvfb :99 -screen 0 1280x800x24 >/tmp/xvfb.log 2>&1 &     # 1. fake screen
DISPLAY=:99 ./bin/Debug/rspektrum mlnl_samples.wav \
        >/tmp/app.log 2>&1 &                              # 2. run on it
sleep 2                                                    # 3. reach a steady frame
DISPLAY=:99 import -window root /tmp/shot.png             # 4. grab the frame

Prerequisites (Debian/Ubuntu): sudo apt-get install xvfb imagemagick xdotool (plus libgl1-mesa-dri and LIBGL_ALWAYS_SOFTWARE=1 if GL fails / frames are black). Synthesize input with xdotool against DISPLAY=:99 to exercise UI paths.

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Technical notes

• STFT — Hann-windowed, 2048-point FFT with 50% overlap by default; frequency resolution sampleRate / fftSize Hz per bin. Amplitude in dB.
• Axes — X = time (s), Y = frequency (Hz, scaled to the file's Nyquist), colour = amplitude.
• Playback / WAV export share one processing path: the selected time span, FFT-bandpassed to the selected frequency box, peak-normalised.
• mLnL parsing — walks the WAV's RIFF chunks for the four-CC mLnL chunk (UTF-8 JSON Lines); unknown chunks are skipped, so annotated files stay standards-compliant audio everywhere else.

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Source layout

src/
  spectrogram.c        # entry point, main loop, CLI args, headless render
  stft.c / fft.c       # STFT + FFT
  render.c             # spectrogram, annotations, tooltips, scope
  ui.c                 # sidebar, file browser, buttons
  audio.c              # WAV load (ffmpeg fallback), bandpass, playback, WAV export
  mlnl.c / mlnl.h      # mLnL annotation chunk parser
  platform_*.c         # per-OS shims (linux / win32 / web)

See raylib_for_desktop_applications.md for the performance / idle-CPU lessons behind the desktop build, and AGENTS.md for the headless-testing playbook.