APF Imagination Machine data cassette
The APF Imagination Machine was a late-1970s home computer, or actually a game console that could be expanded into a computer with an optional add-on containing a keyboard and tape drive.
The tape drive used normal audio cassettes, but was a dual-track drive which read both sides of the tape at once, using one of the tracks for program data and the other for audio. This permitted it to play sound included on the program/data tape while the data loaded, which could be used for introduction or instructions of the program, or for music played while you wait (like "on-hold" music on the phone).
The data consists of a series of bits, recorded as a 500 microsecond cycle for a zero, and a 1 millisecond cycle for a one.
As stored on cassette with the CSAVE command, a program will be preceded by a 512-byte block taken either from bytes 0-511 or 512-1023 of memory (depending on whether memory location 41452 was set to 255 or 0 respectively), which will be placed in screen memory when the cassette is loaded (allowing the display of a title screen or instructions while the program finishes loading). Then all used memory is saved, including the program and variable values.
It was also possible to create a data cassette storing other ranges of memory, using appropriate peek/poke/call commands instead of CSAVE.