Apple's text-to-speech uses diphones. Compared to other methods of synthesizing speech, it is not very resource-intensive, but limits how natural the speech synthesis can be. American English and Spanish versions have been available, but since the advent of Mac OS X, Apple has shipped only American English voices, relying on third-party suppliers such as Acapela Group to supply voices for other languages (in OS X 10.7, Apple licensed a lot of third-party voices and made them available for download within the Speech control panel).