The RS talking calculator sounds as if it uses stored human speech
rather than any kind of true speech synthesizer.  Stored-speech systems are
getting cheaper and better so that is a good option for a small
stored vocabulary system.  It is a shame, however, that there isn't a modern
version of the SPO256 and its matching text-to-speech converter.  They sound
funny, but they can do a fair job of speaking free-form text which is something
that stored vocabulary systems can't do.

Martin McCormick