SSTV – Transmitting Images

The types of radio communications, the things you can send over thin air, is remarkable. If you can convert it to audio, you can send it. With the ISS Slow Scan Event coming up, I decided to experiment with Slow Scan TV or SSTV. What you end up with is what feels like downloading photos over the Internet circa 1995. Considering no Internet or direct cable connection is involved, it’s pretty darn cool.

Since my Windows PC tower generates so much RFI or Radio Frequency Interference, I am using my Linux Hamtop. However, I will note that there are similar Windows applications that perform the same functions.

Receiving with SDR

Since I’m limited on low power radios that I can experiment with, I leveraged my RTL-SDR dongle as a listening device, and would transmit the test images with my Android attached to my Baofeng.

  • In Linux, I’m running the GQRX application, installed from the Ubuntu Software “Store”. Setup is simple, and it easily detects my NooElec NESDR Nano 3 SDR dongle.
  • On Windows, I would use the free SDR# (Sharp) application from AirSpy. Setup is a little more difficult, but not insurmountable. I had to run an executable buried in the installation folder to create a custom driver for the dongle.

Windows typically has a recording device called “What You Hear”, where anything piped over the speakers may be recorded by applications like Audacity. In Ubuntu, you need to install a proper audio mixer for Pulse Audio:

# sudo apt-get install pulseaudio pavucontrol mplayer -y
pavucontrol

Once installed, launch the application by tapping your Start button on the keyboard or the apps list in the dock and enter “vol”. PulseAudio Volume Control will present itself. Go to the Recording tab and select “Monitor of Built-In Audio…”. This will record whatever spews out the speakers. Note that if you mute the audio to avoid annoying your significant other, the application listening for or recording this audio will go deaf. I recommend plugging in headphones or a cut audio plug.

The materials I referenced to set this up are here:
ref: https://www.rtl-sdr.com/tag/sstv/
ref: https://www.rtl-sdr.com/tutorial-setting-up-a-low-cost-qrp-ft8-jt9-wspr-etc-monitoring-station-with-an-rtl-sdr-v3-and-raspberry-pi-3/

PC Encoding & Decoding

There are two excellent free applications for automatically detecting the type of encoding and decoding the signal, as well as transmission of your own images. For listening purposes, set the source to the default recording source in Linux, or “What You Hear” or similar in Windows.

Now that you have the SSTV software up and running, you can use any sound source, like demos on YouTube and get output like that in the image below:
https://www.youtube.com/watch?v=Ke5UB2HLaZ4&t=186s

SSTV on Android

Thus far in my exploration of SSTV, I’ve found Android apps that either encode or decode:

SSTV Encoder by Olga Miller

Select a photo from your camera roll or the camera itself, and click the Play button. You can set different modes in the menu, but the default Robot 36 works well. The application will spit the audio out of the default device.

I find the default audio output device on my phone, a Samsung S7 Active running Android Oreo, is always the speaker unless actual headphones are attached, thanks to an OS update not long ago. An app such as Audio Router works well to force audio over the headphone jack, regardless of what’s connected or not. The bulk of the screen is an advertisement, but the app works. Just set the Audio Jack field to On. Don’t worry about any of the other options or features. You’ll know it’s working if you see the Audio Output button in the notification shade.

The results of my SSTV experiment, using QSSTV listening to the laptop’s SDR, my Android producing the audio over the headphone jack connected to the Baofeng on SSTV frequency 430.95 MHz, VOX set to 1.

Robot36 – SSTV Image Decoder by Ahmet Inan

The Robot36 app works well, even by the built-in mic on the phone. Hold it near an SSTV audio source, and it’ll decode for you. Images received can be downloaded to the camera roll on your device.

More to come…

I’ll continue to expand this article, as I experiment with mobile radios and transmitting photos over longer distances. In the meantime, the setup is working well for receiving SSTV images from the International Space Station.