specrender

Render spectrogram images from audio files, or interactive HTML players that play the audio over its spectrogram. Images come out at exactly the pixel size you ask for, in any palette, as PNG, JPG, WebP, BMP, TIFF or GIF. Hour-long recordings render without loading the whole spectrogram into memory.

• Exact output size. width × height is what you get, including stereo channels stacked with a gap between them.
• Any palette. 11 built-in palettes that need no matplotlib, any matplotlib colormap name, or your own colors.
• Linear, log or mel frequency axis, with fmin/fmax limits and start/end time cropping.
• Comparable levels. Levels are measured in dBFS, so a fixed db_range puts many files on the same brightness scale.
• Short events stay visible when you scale down. Max-pooling keeps a 1 ms click in a 10-minute file on screen.
• Most audio formats. WAV needs nothing extra. FLAC, OGG and AIFF use soundfile. MP3, M4A, Opus and video files use ffmpeg. Numpy arrays also work as input.
• Interactive player. A single HTML file with play/pause, volume and speed controls. Click to jump, drag the playhead to scrub, and drag across the spectrogram to select a region and loop it.
• Library and CLI. Use sr.render(...) from Python, or run specrender *.wav -d out/.

Install

pip install "specrender[all]"

The core install needs only numpy, scipy and Pillow. The extras add more:

Extra	Adds	For
annotate	matplotlib	annotate=True (axes, labels, colorbar) and matplotlib colormap names
formats	soundfile	FLAC, OGG and AIFF without ffmpeg
all	both

For MP3, M4A and similar formats, install ffmpeg and make sure it's on your PATH.

Quick start

import specrender as sr

# Load, analyse, render and save in one call
sr.render("recording.mp3", "recording.png", width=1200, height=400, palette="magma")

# Speech: mel scale up to 8 kHz, saved as JPEG
sr.render("voice.wav", "voice.jpg", width=800, height=300,
          scale="mel", fmax=8000, palette="viridis", quality=90)

# Labeled axes, title and dB colorbar
sr.render("chirp.flac", "chirp.png", width=1200, height=600,
          scale="log", annotate=True, title="Test chirp")

Compute once, then render as many ways as you like:

spec = sr.compute("track.wav", scale="log", width=1600, height=500)

spec.save("dark.png",  palette="inferno")
spec.save("print.jpg", palette="gray_r", dynamic_range=60)
png = spec.to_bytes("PNG", palette="waterfall")   # bytes, e.g. for a web response
img = spec.to_image(palette="cividis")            # a PIL.Image you can compose further

spec.db      # (channels, rows, cols) in dBFS; row 0 is the lowest frequency
spec.freqs   # Hz at each row
spec.times   # seconds at each column

Numpy arrays from an SDR, a microphone or generated signals work too:

sr.render(samples, "iq_audio.png", sample_rate=48000, width=1000, height=300)

To render a folder of clips on a shared brightness scale:

from pathlib import Path
sr.render_batch(sorted(Path("clips").glob("*.wav")), "out/", format="webp",
                width=600, height=200, shared_levels=True)

Interactive player

sr.player("recording.mp3", "recording.html", scale="mel", fmax=8000, palette="magma")

specrender recording.mp3 -o recording.html

This writes one self-contained HTML file, with the spectrogram, the audio and the player script all inside it. Open it in any browser; no server or internet connection is needed.

Do this	To
Click	Jump to that point
Drag the white playhead	Scrub
Drag across the spectrogram	Select a region. Playback loops inside it
Drag a selection edge	Resize the selection
Double-click or Esc	Clear the selection
Space · ←/→ (Shift = 5 s) · L	Play/pause · seek · toggle loop

Hover over the spectrogram to read the time and frequency under the cursor. There are also a volume slider, a mute button and a playback-speed menu.

On your own website, write separate files and use the <spectro-player> tag:

sr.export_player("talk.mp3", "site/players/")   # specrender-player.js, talk.png, talk.json, talk.mp3

<script src="players/specrender-player.js" defer></script>
<spectro-player meta="players/talk.json"></spectro-player>

Large files can stay where they're hosted: pass audio_url="https://…/talk.mp3" and only the image and settings are generated. Try docs/player-demo.html (download it, then open it), or see the player guide.

Command line

specrender talk.mp3 -o talk.png -W 1600 -H 500 -p inferno -s mel --fmax 8000
specrender *.wav -d thumbs/ -f webp -W 400 -H 120 --db-range -90:0
specrender capture.flac -o annotated.png -a --title "{name}" -s log
specrender session.flac -o session.html -s log      # interactive player
specrender --list-palettes

Palettes

Add _r to any palette name to reverse it ("gray_r" gives dark-on-light for print). To register your own palette:

sr.register_palette("brand", ["#0b0b2b", "#3355ff", "#ffffff"])

Documentation

• User guide: choosing FFT size, frequency scales, dynamic range, channels, long files, and recipes for speech, music and radio.
• API reference: every function, method and parameter.
• CLI reference: every flag, with examples.
• Changelog

Development

git clone https://github.com/kk4cnm/specrender && cd specrender
pip install -e ".[dev]"
pytest
python examples/make_gallery.py   # regenerates docs/images/

See CONTRIBUTING.md for details.

License

MIT. See LICENSE.