Does anyone know how the text-to-speech used on the Commodore, i.e. Software Automatic Mouth, or SAM, actually synthesized the voice? Did it have any sort of recorded sample data? Did it just use additive synthesis? Or formant synthesis?

There's a github of someone's port of it to C here, although many of the variables, functions, and labels are just named based on their location in memory, not their function, so it still appears a black box.