Most of the speech hubbub around Mountain Lion has centered on the OS’s marquee Dictation feature, which happily accepts your spoken words as a substitute for typing them in. Dictation works in almost any text entry field, and it’s surprisingly effective; Steve even dictated his entire post about Dictation. Speech-to-text is only one side of the coin; there’s also text-to-speech. OS X Leopard introduced a single high-quality voice named Alex. “He” sounded so natural compared to the previous generation of Mac synth voices that it was a little bit disconcerting

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Mountain Lion 101: Updated high-quality voice synthesis