Michael Wessel's Talker/80 Brings Back the Classic TRS-80 Voice Synthesizer Add-On

Maker Michael Wessel has developed a modern voice synthesis add-on for a somewhat less-modern microcomputer: the Tandy-RadioShack TRS-80 Model 1, from 1977.

"I am proud newbie owner of a Model 1 since January 2020. I always wanted to have one of these — a member of the 1977 Trinity that started the home computer revolution," Wessel writes. "Being a fan and collector of vintage speech synthesizers (I must have one of the biggest collections in the world by now...) I REALLY wanted to add the TRS Voice Synthesizer to my setup.

"But they are scarce, and the last one I saw sold for over $400 on Ebay... I am crazy, but not that crazy. So, I thought, if I cannot get the real thing, then at least let's try to port my LambdaSpeak speech synthesiser for the Amstrad CPC to the Model 1. Both the TRS-80 Model 1 and the Amstrad CPC are Z80 machines, but the official vintage speech synthesizers used different chips."

The original TRS Voice Synthesizer was based on the Votrax SC-01 speech chip, which is long out of production — and when system-pulls or new-old stock does show up, prohibitively expensive. The solution: Emulation, the same approach Wessel had taken for his earlier LambdaSpeak add-on.

"Now, for LambdaSpeak the SP0256-AL2 emulation was a bit easier to achieve, given that I have the original SP0256-AL2 (and Amstrad SSA-1 speech synth that uses it) at hand," Wessel explains. "But I never even saw a Votrax SC-01. Not even from the distance.

"But I have the TRS Voice Synthesizer Software, and the VS-100 software. Thanks to the guys on the Vintage Computer Forum, I got some technical information regarding the SC-01 phoneme set, and the mapping of the TRS Voice Synthesizer "ASCII phoneme character set" (a TRS invention) to the SC-01 phonemes. Now I only needed to figure out how to map the SC-01 phonemes to DECtalk phonemes, which required a bit of trial and error."

Mapping complete, Wessel turned to the hardware - and reusing GAL22V10B chips purchased at a low cost in bulk, using one GAL for IO decoding, another as a databus latch and tri-state buffer, and an ATmega644-20PU running at 20MHz interfacing with Mikroelektronika Text2Speech Click Board and its on-board S1V30120 speech chip running DECtalk firmware.

"A PCB was made with KiCAD, and OSH Park produced the PCB prototypes in the usual perfect quality," Wessel says. "After fixing some bugs and iterations, I now have PCB revision 3, and I am happy with it. I will probably produce a small batch for other enthusiasts. And also make the firmware open source, so others can build it themselves."

Full details of the build can be found on Wessel's project page, while files are available on GitHub.