Here's the latest schematics for our invisible instrument.
This version hasn't even been built yet (not even on a breadboard) but is an idea for improving on the current version.
We're acutely aware that four inputs isn't a massive number for an instrument - we could use "doubling-up" to create up to seven inputs (e.g. accept 1000 as the left-most position, 1100 as the next position towards the right, 0100 as the next, 0110, then 0010, 0011, 0001) but this would stop the instrument being poly-phonic (since 0110 would mean "play sound four" rather than "play sounds 2 and 3 together"). It's swings and roundabouts really.
Despite only four inputs, this does allow a large number of pop (and rock) songs to be played - 90% of popular music consists of three chords, four if you include a relative minor for the middle eight (if none of this means anything to you, don't panic!). Changing the sequence or overplaying chords with different lead lines is what gives most pop music depth. After all, too many chord changes and the song becomes unpredictable: and it's the predictability of music that gives it its "hook" (or, easy-to-recall melody).
Triggering riffs/chord samples with a single input allows songs to be built up from simple, repetitive sequences
This new instrument includes a dial (for selecting the current key/chord/scale if we ever get around to midi output) as well as some buttons and LEDs. The buttons are used during playback to "layer" multiple sounds from a single input:
The idea is to allow the user to load up to four different sounds to be played back whenever an input is trigger. It's a bit like having up to four channels, which can be muted/unmuted. It could be a bassline, guitar chord(s), synth parts and a melody line. When an input is triggered, all four sounds can be played at once. The user can turn off which of these are played by pressing the buttons which latch the sounds on and off. The LEDs show which "channels" are selected for playing when an input signal is detected.
So players can start with just the guitar part playing (for example, channel one active) - playing up to four different chords/riffs/patterns either in sequence or together - then turn on the bassline (channel two) and whenever a single input is triggered, both the guitar and bass parts play. The key to the success of this approach of course is that sound1 on channel 1 and sound1 on channel 2 are in the same key or at least sound harmonious when played together.
By combining these simple channels, it should be possible for someone (with a little practice) to create some rather complex sounding tunes.
I guess the only way we'll ever know, is to build one of these things and try it out!