Let's Groove Tonight by Earth, Wind and Fire

I would guess it's a vocoder.

 

Sounds to me like a "vocoder" - so you have a special synth/keyboard type thing which has an integral microphone. You sing into it and the keyboard provides the pitch and envelope of the note which is mixed with the voice, which provides the sibillants and consonant sounds.

There were many variations, but at this time I don't think it was available as an "effect" as such and vocoders were very expensive and difficult to use!!

 

One of my bands covered this song once. If your vocalist sings flat, and in tune with the synth and bass, you'd be amazed at the closeness to the record.

 

Anyone(besides Brad)remember the hit by Yarbrough & Peoples called "Don't Stop The Music"?
Classic example of a vocoder...

Worthless trivia-
There's a tune on Lee Ritenour's Rit II album("Cross My Heart")...same damn bass figure as EW&F's "Let's Groove Tonight".

 

Obsure reference #3.

Rent.

Act I.

La Vie Boheme song and scene. (Ode to the
"death" of Bohemia.)

....
"Maureen Johnson, back from her spectacular
one-night engagement at the eleventh street
lot, will sing native american tribal chants
backwards through her vocoder, while
accompanying herself on the electric cello-
which she has never studied.