You've Got A Message Through The Stratosphere

A WAV file carries a picture sent over digital SSTV. The audio is not analog SSTV but HamDRM (DRM mode B) OFDM, so it needs a DRM-capable decoder to recover the image with the flag.

Solve

1. Inspect the file. It is a mono 16-bit PCM WAV at 48000 Hz, about 151 s long.

   file message.wav
   python3 -c "import wave; w=wave.open('message.wav'); print(w.getnframes()/w.getframerate())"
   

2. Look at a spectrogram. There is silence up to about 2.3 s, then a short multi-tone sync burst from about 2.3 to 4.0 s, then a continuous broadband OFDM block from about 4.5 s to the end occupying roughly 1500-2500 Hz. That leading DRM sync burst plus the flat continuous data block identifies HamDRM digital SSTV, so an analog decoder will not work.
3. Set up audio routing so the WAV can feed QSSTV. Create a virtual sink and play the file into it:

   pactl load-module module-null-sink sink_name=sstv
   paplay --device=sstv message.wav
   

   In pavucontrol set QSSTV's recording source to "Monitor of Null Output".
4. In QSSTV open the Receive tab and select the DRM sub-tab. Start the receiver, then start playing the WAV. The DRM decoder locks onto the OFDM frames, the FAC/MSC constellations populate, and the picture renders until END PIC. Observed parameters: Mode B, bandwidth 2.3 kHz, QAM-16, Interleave Long, Protection High, callsign NIKI, 296 segments, payload re.jp2 (JPEG2000).
5. Read the flag off the decoded image.

Flag: SVIUSCG{m3ss4g3_fr0m_th3_st4r5_:3}