Analog-controlled speech synthesizer

Back in the 1980’s, there was an awesome speech synthesizer chip available from Radio Shack, the SP0256-AL2 NARRATOR Speech Processor. Among the other speech chips out at the time, it is the easiest to interface with. It produces a funny classic computer voice, very lo-fi and droney, but decently intelligible if used correctly. Basically, it has a menu of 64 short speech noises called “allophones” which are simple consonant and vowel sounds. They are strung together to produce words. You can view the chip’s datasheet here.

The SP0256-AL2 chip is now relatively rare in 2008, and commands a price of around 30 USD. I was lucky to find one in an old Tandy computer cartridge called Speech/Sound Program Pack.

I carefully desoldered the speech chip and considered what to do with it.

My idea was to make an analog-synth compatible speech module with inputs for pitch CV, speech gate, and allophone LFO. I designed a minimal circuit using only discrete logic, i.e. no microcontroller involved. So far, it is alive and talking, w/ speech gate and allophone inputs working. (The allophone LFO input is connected to an 8-bit ADC that converts it into digital code, which represents the various allophones.)

photos coming soon…

SP0256-AL2 pseudorandom speech (6:41)

It’s clear that it’s not speaking English… but if you are familiar with other languages, will you tell me if its speaking yours? : )

Next I’ll tackle the pitch CV problem. I’m looking at ways to give it a variable-speed clock, controllable by a standard 1V/octave CV input… this would give a singing machine that speaks its own “language”. This requires a trick, because the SP0256-AL2 is designed to use a 3.12MHz crystal oscillator, which unfortunately gives it a monotonous pitch. The chip has two pins that connect directly to the crystal. I’ve found a way to eliminate the crystal and use a variable-speed oscillator in its place.  Now it has a tuning knob that covers just over an octave.  (I found that above about 3.5MHz, the chip glitches out and makes horrible blasts of noise)  Here’s a layered recording to demonstrate the pitch range…

SP0256-AL2 singing a chord

Hopefully when the design is more concrete I’ll package it up and make it available… maybe as a kit, and you will have to supply the SP0256-AL2 chip.