The SN-Voice Project
The Texas Instruments SN76477N
If you're new to Synth DIY and have never heard of the SN76477 IC, there's a good reason - it's long been out of production. But it still exists out there in the wild - a few supply houses on the internet still list it, and it's been known to inhabit parts bins all over the world waiting for just the right application.
The SN76477 appeared in many a video game of the late '70s and early '80s, supplying players with bleeps, bloops and swept noise as they vanquished pixellated aliens dropping from the skies. It was also featured in other items, more often than not, toys. There were a few kits out there for hobbyists. Some were music related, others were for just making revving engine, steam engine, explosion and gunshot effects. Somewhere between the mid '80s and early '90s (I'm not sure when) Texas Instruments terminated the SN76477. With prejudice.
The SN76477N didn't deserve such an unceremonious fate; if ever there was a feature packed DIY wet dream, it was the SN76477N. In fact, the SN76477 is a fully loaded synthesizer on a chip. Within its silicon boundaries lie the following functions:
- A Voltage Controlled Square Wave Oscillator
- A Super Low Frequency Oscillator (a fancy term for LFO)
- A VCA-like 'modulator'
- A Sweepable Digital Noise Generator (shift register based)
- A Filter for the Digital Noise Generator
- An Envelope Generator (Attack/Decay)
- A programmable Mixer
The SN76477 didn't die its ignoble death in vain. There were a couple of serious synth applications developed for it. In his book "Build a Better Music Synthesizer", Thomas Henry published a design for a controller module, dubbed "The SuperController" based around the SN76477. In this application, the SN76477 was primarily used as a source for syncable gates, triggers, and LFO signals, but it did also feature the sweepable digital noise generator as a signal output.
John Blacet brought the SN76477 into the commercial synthesizer world with his excellent "Dark Star Chaos" module, a discontinued and still much-sought-after design produced by his company, Blacet Research. The DSC2000 took great advantage of the SN76477's capabilities. Its application was for creating sounds ranging from the sublime to the monstrous. One can still read about its functionality on the Blacet "Cool Old Stuff" history page.
Other than the DSC2000 and the SuperController, I've heard of a few 'cigar box' synthesizers mentioned on the Synth DIY list, but, other than the hobbyist items mentioned above, nothing else.
So why didn't more synth based designs using the SN76477 pop up everywhere - why did it shuffle off of this mortal coil remembered mainly as an IC that was great for making unusual sound effects and little else? After all, it has all of these features......
The one feature that the SN76477 lacked, when viewed from the perspective of us analog synthesizer diehards, is that there was no terribly effective means for controlling it in a scaled musical sense. The control for the VCO is 'linear' and not in that nice Korg V/Hz context, either. What 'mini-organ' hobby projects I've seen for it invariably involve a few keys with specific resistors controlling the amount of voltage each key sends. Even the Blacet Dark Star chaos is intended more for sound effect generation - a specific note is made that the VCO is not intended for accurate keyboard use.
I'm sure there are a few other reasons. For example, the SN76477 produces only two audio signals - a square (or pulse) wave and a digital noise signal. It doesn't have a full-on ADSR envelope generator, and its envelopes are not the snappiest in the world. Those are minor, workable things, but that VCO control voltage - well that's a show stopper.
More specifically, it was a show-stopper until 2006, long after the SN76477 passed into oblivion and three years after the last DSC2000 left Blacet's doors. Thomas had one of those "Hey, wait a minute" moments and began calculating, breadboarding, and calculating even more. In a nutshell, Thomas came up with a means to control the SN76477 exponentially - true V/Oct for a range of five octaves or so with reasonable accuracy.
To put what he accomplished into perspective, listen to this demo of the SN-Voice that Thomas produced. It consists of a composition by JS Bach using an organ voice, crystal controlled, from Thomas' sound card and the SN-Voice singing along with it. No filter is used on the SN-Voice (PWM), and it uses the internal EG, LFO and VCA of the SN76477N, which lies at the heart of the SN-Voice. A light touch of reverb was used.
So, there you have it - the Texas Instruments SN76477's final revenge, doing things no data sheet or application note ever hinted it was capable of.
Once Thomas had established the breakthrough of controlling the SN76477's VCO exponentially, attention was turned to exactly what to do with it. Ideas began to come together, and over the space of week, through a flurry of communication and breadboarding, a design began taking shape. In typical Thomas Henry fashion, a lot of functionality was gleaned out of very few parts, thus was born the SN-Voice module.
The SN-Voice provides the following:
- V/Oct VCO with Square or PWM (modulated by either LFO or EG) output. This output can be switched between pulse/square output or digital noise output. The amplitude of this output can be controlled by the envelope generator or be locked to a constant output amplitude. Output is 10Vp-p max, with an output volume control.
- VCO Triangle wave output. This output is a constant 10Vp-p and is not controlled by the envelope generator.
- VCO is controlled by a V/Oct control voltage input, an expo modulation input (with associated level control), a linear modulation input which can be either AC or DC coupled and also has a level control, a coarse tune control and a fine tune control.
- Sweepable Digital Noise Generator
- The Digital Noise Generator can be controlled by a keyboard control voltage input and an expo modulation input, which has an associated expo modulation level control.
- Manually adjustable Noise Filter.
- Wide range triangle wave LFO (less than .2 Hz to over 200 Hz over three ranges) and 10Vp-p output. Frequency of the LFO is set manually.
- A gated ASR (attack/sustain/release) envelope generator that is active when the pulse/noise output is set for EG. Output is 0 to 5V. Maximum attack and decay time is approximately 6.5 seconds each. The envelope generator accepts +5V gate signals and will remain high as long as the gate is applied.