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!

Structure

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).

A8CAS-FSK<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>

where:

• <signal> is a single character equal 1 or 0 - the value of the current signal
• <length> is an integer number representing the signal's length in "samples"
• <rate> is an integer number representing the signal's "sample rate"
• <comments> is an optional string with any contents, that is ignored. This field needs not be separated from <rate> with a blank space, however it has to if it starts with a number.

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.