Andrew Menadue Brings Back GI's Speech-Synthesizing SP0256-AL2 — Powered by a Raspberry Pi RP2040
Having built speech synthesis boards for the Psion Organizer II, Menadue found stock of the 1980s-era chip short — so made his own.
Maker and vintage electronics enthusiast Andrew Menadue has a solution for anyone looking to add a little 1980s speech synthesis to their projects but struggling to find the parts: a pin-compatible drop-in replacement for the General Instruments SP0256, powered by a Raspberry Pi RP2040 microcontroller.
Released in 1981 by General Instruments, the SP0256 family of chips were designed to give microcontrollers and microprocessors the gift of speech. The best known of the parts, the SP0256-AL2, came with a 2kB mask ROM pre-loaded with 59 allophones which could be assembled jigsaw-like into English speech. It found a home in a range of kits and microcomputer add-ons, many of which are highly sought after by collectors today — which is where a problem lies: the SP0256 is hard to come by.
"The SP0256-AL2 is almost unobtainable, and if you can get a non-fake device then it will cost you lots," Menadue explains. "What do I do with my Psion speech adapter PCBs that I have no [ICs] for? Make an emulated IC, that's what."
Menadue's emulated IC builds on two existing projects, one from Extreme Electronics which sought to create a C-based emulation of the SP0256-AL2 which could run on a Raspberry Pi Pico or other RP2040-based device and an effort by fans of the Amstrad CPC family of microcomputers to gather together the allophone data. Where it goes beyond both is in its footprint: the entire circuit, including the RP2040, fits in the footprint of a DIP-packaged SP0256-AL2, making it a low-cost drop-in replacement for the hard-to-source part.
There are only a couple of catches: "It does not," Menadue writes, "handle an external ROM [nor] handle the 'play allophone on address line going to 1' mode." As a result, some devices designed for a real SP0256-AL2 may not work as expected — though it's certainly good enough for the Psion Organizer II speech adapters Menadue was aiming to populate.
The KiCad project files and firmware source code are available on Menadue's GitHub repository under an unspecified open source license.