# 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](https://nicecrew.tv/w/2w9Y5qvKuDz6mwrzAweryW)



![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

You need only **`make` and a C compiler** (`gcc` or `clang`) plus the X11/OpenGL
**development** headers (see below). raylib is vendored in this repo and compiled
from source — there is no separate raylib install step, no `premake`, no network
access required. A plain clone builds:

```bash
make                # release -> bin/Release/rspektrum   (-O3 -ffast-math, AVX2/FMA)
make DEBUG=1        # debug   -> bin/Debug/rspektrum     (-g, no optimization)
make run            # build + launch
make test           # build + run the DSP correctness tests
make bench          # FFT benchmark over mlnl_samples.wav
make clean
```

Useful overrides: `make CC=clang`, or `make ARCH=-march=native` to tune for your
own CPU (the default `-march=x86-64-v3` targets any ~2013+ x86-64 chip; drop it
with `make ARCH=` for an older CPU).

### System dependencies

The compiler needs the X11 and OpenGL **dev** headers (the runtime libs are
already present on any desktop; only the `-dev`/`-devel` packages are usually
missing). The X11 extension libraries (Xrandr, Xinerama, Xcursor, Xi) are opened
at runtime via `dlopen`, but their **headers** are still required to compile.

If `make` stops with an error like `fatal error: X11/Xlib.h: No such file` or
`GL/gl.h: No such file`, install the dev packages for your distribution:

| Distro | Command |
|--------|---------|
| **Debian / Ubuntu / Mint** | `sudo apt install build-essential libx11-dev libxrandr-dev libxinerama-dev libxcursor-dev libxi-dev libgl1-mesa-dev` |
| **Fedora / RHEL / Rocky** | `sudo dnf install gcc make libX11-devel libXrandr-devel libXinerama-devel libXcursor-devel libXi-devel mesa-libGL-devel` |
| **Arch / Manjaro** | `sudo pacman -S base-devel libx11 libxrandr libxinerama libxcursor libxi mesa` |
| **openSUSE** | `sudo zypper install gcc make libX11-devel libXrandr-devel libXinerama-devel libXcursor-devel libXi-devel Mesa-libGL-devel` |
| **Alpine** | `sudo apk add build-base libx11-dev libxrandr-dev libxinerama-dev libxcursor-dev libxi-dev mesa-dev` |

On Debian/Ubuntu the single metapackage `xorg-dev` pulls in all of the X11 `-dev`
packages above, if you'd rather not list them.

Run `make check-deps` to probe for the required headers before building — it
prints the install hint for your platform if anything is missing.

### Web (WebAssembly) build

```bash
./build_web.sh      # emscripten; emits the WebAssembly bundle to bin/web/
```

---

## Usage (desktop GUI)

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

### Controls

| Input | Action |
|-------|--------|
| **O** | Open file browser |
| **Mouse wheel** | Zoom both axes (preserves aspect ratio) |
| **Shift+wheel** | Zoom the time axis only |
| **Ctrl+wheel** | Zoom the frequency axis only |
| **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.
- **Time zoom limit** — the tightest visible window is derived from the STFT hop
  (`fftSize / HOP_RATIO` samples), not from a fixed fraction of the file, so time
  resolution does not degrade as files get longer: a 30-minute recording zooms in
  just as far as a 30-second one. At 48 kHz / 2048-point FFT the floor is ~85 ms
  across the viewport; a smaller FFT zooms correspondingly tighter. Past that
  point there are no further STFT segments to show, so the view would only
  interpolate.
- **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.

Known rough edges — behaviour that is unspecified or awkward rather than simply
broken — are tracked in [`known_bugs.md`](known_bugs.md).
