Narrow-Bandwidth Television (NBTV) was pioneered by John Logie Baird in the 1920s-30s using spinning Nipkow disks to mechanically scan images.
Images were encoded as audio signals — each sample's amplitude representing pixel brightness — transmitted over radio or telephone lines at just 30 lines of resolution.
This decoder emulates that process: audio amplitude → pixel brightness, scanned column by column, recreating the ghostly images of television's dawn.