slowrx

A decoder for Slow-Scan Television (SSTV).

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slowrx is a shortwave listener's Linux tool for receiving SSTV using a sound card. It's still in alpha.

Written in C99 by Oona Räisänen OH2EIQ.

  • Adaptive noise reduction. DSP window length changes based on SNR. Even very noisy pictures can be decoded, albeit at a lower resolution.
  • Automatic frequency shift. The Rx window will shift based on the detected VIS frequency. No need to manually fine-tune the radio.
  • Automatic slant correction. But manual readjustment is also simple.
  • FSK ID decoding. Displays the station's call sign under the picture.

Screenshot

Brief usage

Capture device

  • Select the sound card to be used. An icon will appear next to the combo box telling if there was a problem. The icon has a descriptive tooltip.

Saved pictures directory

  • Location to save the received PNG images in.

Rx control

  • AutoStart toggle: Start decoding automatically upon receiving a VIS code.
  • AutoSlant toggle: Try to fix slant automatically after receiving image.
  • AutoSave toggle: Save received pictures as PNG files.
  • NoiseAdapt toggle: Use a wider windowing function when SNR is poor.
  • FSKID toggle: Decode FSK callsign after image.
  • Abort button: Stop receiving immediately and return to VIS listening mode.
  • Clear image button: Clears the last received picture. Saved PNG will not be deleted.

Manual slant adj.

  • Set left edge button: Set the left edge of the image with two mouse clicks. Slant will then be recalculated.

Manual start

  • Mode select combobox: The SSTV mode used for decoding the image. Does not affect AutoStart.
  • Shift Hz spinbox: Number of Hertz to shift the reception window up or down.
  • Start button: Start decoding audio immediately.

Signal level

  • FFT: Simple spectrum analyzer to aid in setting up the audio.
  • SNR: Estimated signal-to-noise ratio. Only active during decoding.

Supported modes

  • Martin 1, 2, 3, 4
  • Scottie 1, 2, DX
  • Robot 72, 36, 24, 24 B/W, 12 B/W, 8 B/W
  • Wraase SC-2 120, 180
  • PD 50, 90, 120, 160, 180, 240, 290
  • Pasokon TV 3, 5, 7

Requirements

  • Linux
  • Alsa
  • Gtk+ 3.4
  • FFTW 3

And, obviously:

  • shortwave radio with SSB
  • computer with sound card
  • means of getting sound from one to the other

Future plans

  • Support for Redundant Digital File Transfer (RDFT i.e. "Digital SSTV")?
  • Automatic timing-based mode detection (such as in QSSTV) in case of missed VIS?

References

The program was inspired by several papers:

Licensing

Copyright (c) 2007-2013, Oona Räisänen (OH2EIQ [at] sral.fi)

Permission to use, copy, modify, and/or distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.