TL;DR
Jonathan Rowny has created a unique digital audio player housed within a cassette tape, allowing it to play music directly through a tape deck without needing an external device. This innovative design uses a stereo tape head salvaged from a cassette adapter, which is repurposed to generate a magnetic field that mimics the signal from actual magnetic tape. When inserted into a tape deck, the playback head picks up this field, enabling the playback of MP3 files stored on a microSD card.