A mechanism used in 1980s-era synthesizers to provide for external storage of patch data, prior to the availability of low-cost disk drives (it was also common on personal computers of the era). It worked by encoding data in the form of varying audio tones, in the same manner as a computer modem. (And in fact, most synths that had a cassette interface used off-the-shelf modem circuits to implement the feature. The "Bell 202" modem circuit was a common choice.) The result was that the user could record data on an audio cassette or some other form of audio recording media.
To use the interface, the user connected the synth’s cassette interface to an ordinary audio cassette deck. To store patches, the user put the cassette deck into record and then pressed a “SAVE” button on the synth; the synth then encoded the patch data into the encoded audio signal, and sent it to the cassette deck to be recorded.To reload the patches, the user pressed the “LOAD” button on the synth and then set the cassette deck to play; the synth received the audio signal and decoded the patch data from it. Some units also had a “VERIFY” function that checked the tape after a save by reading back the saved data and comparing it to the data in memory.
Cassette interfaces were always slow and not very reliable. The readability of the data depended a lot on the recording level, the type of tape used (there were three major types of tape used in cassettes in the 1980s, and the deck had to be set properly for the tape in use), and to what extent noise reduction circuits in the cassette deck interfered with the process. Often, tapes were unreadable after a few years of storage.
Cassette interfaces disappeared from synths (and computers) as soon as reasonably priced floppy disk drives became available, around 1987. (Nowdays, the floppy drive is also obsolete, and most synths transfer patch data to/from external storage via MIDI or USB interfaces.) However, for synths that have them, the interface is still useable. The signal can be recorded and played back with high reliability by modern digital recorders, and patch banks are sometimes distributed by sound files that can be emailed and downloaded. For many such synths, the factory patches are available on the Web as a sound file.