Votrax Type 'n Talk (1980)
Yay! I finally obtained a TNT on eBay after 34 years of waiting. Amazon even had a new-old-stock, "O1P-1224B," power supply for it. I've put up the contents of the owner's manual, which is impossible to find online in this format: Owner's Manual (PDF)
Type 'n Talk owes its existence to the speech synthesizer design created in 1970 by Richard T. Gagnon. This was after coming up with a viable design scheme in his basement laboratory during his spare time. The older I get the more I find that I like looking at patents. Here is Gagnon's patent from 1973: US3908085
The TNT uses a Motorola 6802 CPU with 4K ROM and 1K RAM. It interfaces to the outside world via an onboard RS-232C connector capable of receiving data at eight different baud rates from 75 to 9600 (8-N-1).
In 1980, Votrax designed and manufactured an integrated circuit speech synthesizer called the SC-01. It was essentially Gagnon's discrete design mapped to silicon. The TNT leverages the SC-01. I find that a lot can be learned by looking at the data sheets for ICs, and here is the one for the SC-01.
The TNT does not have an internal speaker, but the jack on the back is pre-amplified and not a "line out" signal, so headphones should work fine.
In 1981 two Votrax employees, Kathryn Fons and Tim Gargagliano, published an article in BYTE regarding the topic of voice synthesis. This is a great source of history. BYTE republished it 15 years later, but now it is almost impossible to find easily. BYTE Magazine - February, 1996 (PDF)