A8CAS - software for reading/writing Atari 8-bit cassettes

FSK file format - description

FSK is a simple text-based format that stores Atari tapes as sequences of MARK (logical 1) and SPACE (logical 0) signals. Signals are stored sequentially, along with numbers describing length of each signal.

Note: everywhere in this document, "newline" means Unix newline character, 0x10. Only Unix newlines are supported.

Here is an example of a file in the FSK format: Bang! Bank!


The file in FSK format starts with a line containing the file's header. Length of the line is precisely 10 bytes (with the terminating newline).


The second line contains the tape's description, finished with a newline. The description may be empty - then this line contains only a newline character.

<description string><newline>

From now on, each line corresponds to a single logical signal (1 or 0), as written on the tape. Format of the line is:

<signal> <length>/<rate><comments><newline>


Dividing <length> by <rate> gives the signal's length in seconds.

Normally A8CAS produces FSK files in which signals alternate between 1 and 0, but having consecutive signals of the same value in the file is accepted when reading.