A quick look at the Korg Digital Voice Processor (DVP-1) from 1986, which was used by Air on numerous productions, including "Kelly Watch the Stars" from Moon Safari.
The DVP was originally designed to give programmable voice effects at an affordable price, but you can run any audio signal you like through it. There's four main engines or "modes":
Vocoder (as the name suggests, a digital vocoder)
Harmonize (add five intervals to an audio signal, also has chord memory)
Int Wave (internal digital waves that can be played via midi)
Pitch Shift (digital pitch shifting)
There's a built-in chorus effect, unison and poly modes and patch memory.
Editing is tedious, but easy; select the parameter button and use the legend on the top of the unit to find the number of the required parameter, then use the value editor to change it. There's various settings for the different modes including formant shift, waveforms, pitch envelope, modulation generator (LFO), harmonizer intervals, chorus speed, intensity, type, MIDI etc etc
00:00 | Intro
00:59 | The Modes
02:53 | Summary & Sexy Boy