Slow Scan Television or SSTV is a picture transmission method used by amateur radio operators. A lesser used application for SSTV is the ability to encode images into audio which can be stored on low-tech formats such as vinyl and compact cassette. In my case, I have done some experimenting with taking a selection of my photos over the last 20 years, encoding them into SSTV signals using free software and recording the signals to a cassette. The tape recording is then played back through a loudspeaker. It is picked up by the microphone on my smartphone where an app simply decodes the original image. Most of these have used the Robot36 mode but I have played around with others.
The resulting images have a oddly fuzzy, ghostly and lo-fi quality. They look like haunted satellite TV signals from the late 1980s. This makes storing and sharing photos on cassettes possible! Depending on the format, each 240p colour image uses about 40 seconds of audio, so that's about 60 per side of tape. Interestingly, this means a standard 90 minute cassette tape could store about 120 frames of video (or 4 seconds at 30fps.) Video signals have been stored on compact cassette before, but it usually only works with high-bias type II/type IV tape. The advantage of SSTV is that standard type-I tapes aren't a problem as SSTV signals can be very weak and noisy but still be decoded with good results.