elfchart — ELF Spectrogram Chart Generator

Generates a black-on-white FFT spectrogram of the infrasound / extremely-low-frequency (ELF) range from video or audio recordings. Charts 5 pixels per second at 1080 px height, showing the FFT below 100 Hz with horizontal markers at 50 Hz and 20 Hz, and appends a source filmstrip at the exact time markers for visual context to infrasound artifacts. Uses SoX, ffmpeg, ImageMagick, and GNUPlot. ---

Output

┌──────────────────────────────────────────────────────┐  ← 1080 px tall
│  filename — HTTPS://ESUI.CC | ELF Chart (0–100 Hz)  │
│  ── ── ── ── ── ── ── ── ── ── ── ── ── ── ── (50 Hz)│
│                                                      │  (black = high mag)
│  ·· ·· ·· ·· ·· ·· ·· ·· ·· ·· ·· ·· ·· ·· (20 Hz)│
│                                                      │
├──────────────────────────────────────────────────────┤  ← 2 px spacer
│ 🎞 🎞 🎞 🎞 🎞 🎞 🎞 🎞 🎞 🎞  (filmstrip thumbnails) │
└──────────────────────────────────────────────────────┘
  ← time (5 px/sec) →

• X axis — time (horizontal); default 5 px per second
• Y axis — frequency (vertical, 0 Hz at bottom, 100 Hz at top)
• Colour — pure black/white; black = high magnitude, white = low
• Markers — dashed lines at 50 Hz and 20 Hz labelled in grey
• Filmstrip — video thumbnails aligned to the same time axis, one frame per 72 px interval (one still every ~14.4 seconds at default PPS)
---

Dependencies

Tool	Purpose
SoX	Audio resampling, low-pass filter, and raw FFT spectrogram generation
ffmpeg	Audio extraction from MTS/MP4 and video thumbnail extraction
ImageMagick (convert, mogrify)	Frequency-marker annotation, label overlay, filmstrip compositing
GNUPlot	Fallback annotator when ImageMagick is unavailable

Install on Debian/Ubuntu:

sh
sudo apt install sox ffmpeg imagemagick gnuplot

---

Usage

elfchart.sh [OPTIONS] [FILE ...]

When no files are given the script scans the current directory (or --dir) for .mts, .mp4, and *.wav files.

Options

Flag	Long form	Default	Description
-H	--height NUM	1080	Chart height in pixels
-p	--pps NUM	5	Pixels per second on the time axis
-f	--max-freq HZ	100	Maximum frequency displayed (Hz)
-r	--sample-rate HZ	1000	SoX resample rate (Nyquist = rate ÷ 2)
-w	--max-width PX	8000	Maximum output image width (ImageMagick policy guard)
-d	--dir PATH	.	Directory to scan for media files
-o	--output-dir PATH	.	Directory for generated PNG files
-i	--interactive	—	Prompt for each setting; defaults shown in []
-h	--help	—	Print help and exit

Examples

sh
Process every MTS/MP4/WAV in the current directory with all defaults

./elfchart.sh

Interactive setup — walks through every setting

./elfchart.sh -i

Single file, 10 px/sec, 720 px tall

./elfchart.sh -p 10 -H 720 recording.mp4

Batch scan a footage directory, write charts elsewhere

./elfchart.sh -d /mnt/camera -o ~/charts

Higher time resolution for a short clip

./elfchart.sh -p 20 clip.mts

---

How it works

• Audio extraction — ffmpeg pulls the mono audio track from MTS/MP4 files. WAV files are used directly.
• ELF filter — SoX resamples the audio to 1 kHz (configurable) and applies a low-pass filter at MAX_FREQ, isolating the infrasound band.
• FFT spectrogram — SoX generates a monochrome, axes-free PNG at the exact target pixel dimensions (-m -l -r).
• Marker overlay — ImageMagick draws dashed horizontal lines and labels at 50 Hz and 20 Hz, then adds the title and scale annotation. GNUPlot performs the same step if ImageMagick is not installed.
• Filmstrip — For video files, ffmpeg extracts thumbnails at intervals matching the time axis scale; ImageMagick assembles them into a strip and appends it below the chart.
---

Output files

Each input file produces one PNG named <filename>elfchart.png in the output directory. ---

Notes

• The 10 Hz empirical frequency offset in the Y-position calculation compensates for the way SoX aligns FFT frequency bins relative to the image boundary.
• For recordings longer than MAXOUTPUTWIDTH ÷ PPS seconds the pixels-per-second value is automatically reduced to keep the image within the configured width limit.
• Minimum image width is enforced as OUTPUT_HEIGHT + 100 px to guarantee landscape orientation.
• All intermediate files are written to a temporary directory and cleaned up automatically on exit.