RISAT

RCA-Image Stereo Analog Transport converts an image into a stereo audio signal that can pass through two RCA cables or be recorded onto an analog audio medium, then reconstructs the image with channel correction and error recovery.

This repository contains the first working RISAT-1 reference implementation:

• risat-enc-tx: image → stereo WAV / live sound-card output
• risat-dec-rx: stereo WAV / live sound-card input → recovered image
• risat-gui: desktop TX/RX application for Windows, macOS, and Linux
• continuous-phase 4-FSK at 600, 1200, or 2400 baud
• Mid/Side diversity over the two RCA channels
• calibration tone and chirp
• global tape-speed correction
• left/right gain and delay correction
• per-FSK-tone equalization learned from the training sequence
• Reed–Solomon correction, CRC-32 validation, frame repetition, and interleaving
• SHA-256 validation of the final image

RISAT-1 is an experimental transport. Preserve the complete calibration preamble when recording or trimming audio.

Install

python -m venv .venv
source .venv/bin/activate        # Windows: .venv\Scripts\activate
python -m pip install -e .

For direct playback/recording through an audio interface:

python -m pip install -e ".[audio]"

Desktop GUI

Install the audio extra when you want direct sound-card playback or recording:

python -m pip install -e ".[audio]"
risat-gui

You can also launch it through the unified command:

risat gui

The GUI provides separate TX and RX tabs:

• TX image preview, resolution/format/quality controls, baud and repetition settings
• WAV generation, optional JSON report, and direct stereo sound-card playback
• RX decoding from an existing WAV or a timed stereo recording
• automatic 600/1200/2400 baud detection with the selected value used as the first attempt
• automatic recovered-image extension, image preview, diagnostics, and saved recordings
• stereo input/output device selection with a system-default fallback
• live GUI language switching: Traditional Chinese, English, Japanese, and Catalan

The GUI selects a language from the operating-system locale on first launch. You can switch between 繁體中文, English, 日本語, and Català from the header; the selection is saved for the next launch. The aliases zh-tw, eng, ja, and cat are also accepted through the RISAT_LANG environment variable.

Tk is included with normal Windows and python.org macOS Python installations. On Debian/Ubuntu, install it with sudo apt install python3-tk when necessary.

Encode / transmit

risat-enc-tx photo.png -o photo-risat.wav

Higher robustness for cassette tape:

risat-enc-tx photo.png -o tape.wav \
  --resolution 320x240 \
  --format jpeg \
  --quality 60 \
  --baud 600 \
  --repeats 5 \
  --report tx-report.json

Generate the WAV and immediately play it through the selected stereo interface:

risat-enc-tx photo.png -o tx.wav --play --device 3

Connect the interface's left and right line outputs to the two RCA inputs. Avoid microphone inputs, automatic gain control, spatial effects, bass enhancement, Dolby processing, and lossy audio codecs.

Decode / receive

risat-dec-rx recorded.wav -o recovered.jpg

Record a 90-second stereo RCA input and decode it:

risat-dec-rx --record 90 --device 2 \
  --save-recording capture.wav \
  -o recovered.jpg \
  --report rx-report.json

The RX report records the detected baud rate, measured speed ratio, synchronized channel candidates, training accuracy, equalization weights, frame count, and embedded image metadata. The decoder also locates the calibration chirp, so a modest amount of silence before the RISAT signal is tolerated.

Suggested analog settings

For a normal cassette deck, start with 600 baud, a 320×240 JPEG, five repeats, and manual record level. Keep peaks below clipping and disable all enhancement/normalization. For direct RCA cable transport, 1200 or 2400 baud is appropriate.

RISAT uses a common-mode/main stream and a differential/side diversity stream:

L = 0.75 M + 0.45 S
R = 0.75 M - 0.45 S

The decoder also tries the raw left and right channels. A mono capture may therefore still recover the main stream, while a proper stereo capture offers packet diversity.

Development

python -m pip install -e ".[dev]"
pytest

Protocol details are in docs/RISAT-1.md.

License

Apache License 2.0.