# 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. ![rspektrum spectrogram view with mLnL annotation overlay](resources/Screenshot.png) --- ## 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`](mlnl_chunk_spec.md) for the annotation format. --- ## 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. --- ## Building The build is driven by a checked-in Makefile. `premake5` is **not** required to build — only to regenerate the makefiles. ```bash make -f rspektrum.make config=debug_x64 # -> bin/Debug/rspektrum make -f rspektrum.make config=release_x64 # -> bin/Release/rspektrum ``` Web (WebAssembly) build: ```bash ./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. --- ## Usage (desktop GUI) ```bash ./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: ```bash ./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, …). --- ## 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): ```bash ./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. ```bash # 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. --- ## 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`](AGENTS.md); the working reference implementation is [`shot_input.sh`](shot_input.sh). The loop in one breath: ```bash 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. --- ## 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. --- ## 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`](raylib_for_desktop_applications.md) for the performance / idle-CPU lessons behind the desktop build, and [`AGENTS.md`](AGENTS.md) for the headless-testing playbook.