Editing Apple II (original) (section)

==Sound==
Rather than a dedicated sound-synthesis chip, the Apple II contains a toggle circuit that can only emit a click through a built-in speaker or a line-out jack. More complex sounds, such as music or audio samples, are generated by software manually toggling the speaker at an appropriate frequency. This technique requires careful and precise timing, rendering it difficult to display moving graphics while sound is playing. Third party [[Apple II sound cards|expansion cards]] were later released that addressed this problem.

A similar technique is used for cassette storage: cassette output works the same as the speaker, and input uses a simple [[Zero crossing threshold detector|zero-crossing detector]] as a 1-bit audio digitizer. Routines in machine ROM encode and decode data in [[frequency-shift keying]] for the cassette.