It definitely sounds like synth playing the low notes in the song's intro (on the first beat and on the second eighth of the second beat of each bar).
The tonic note F# is doubled with the kind of distorted synth.
In the verses it sounds as if synth takes over completely, with bass guitar coming back for the bridges.
Yes, you are correct.
Thriller
I wanted to say that the basic riff was played on Yamaha DX-7 tone, but...
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Here are the keyboards used to record Thriller.
Yamaha CS-80
Roland Jupiter 8 (x3)
NED Synclavier II
DK Synergy
Roland Jupiter 4
Roland MC-4 microcomposer/sequencer
Yamaha GS-1 FM synth
Oberheim Four-Voice
SCI Prophet 5 (x2)
Yamaha CE20 FM preset synth
Yamaha portasound keyboard
Roland VP-330 vocoder/strings
Bode Vocoder
E-mu Emulator I
plus a couple of Minimoogs.
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Here is what Bill Wolfer said about recording Thriller:
"I used a CS80 to play the chords that move against the bass line on Bille Jean. The bass part was already there when I recorded, so I can't say who played it (I'd guess Phillangaines) or what synth it was. Sounds like a Minimoog to me. I can't believe it's been 25 years. Wow."
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Now, here is an interview with Alex Al:
"Who decided what bass or keyboard you played on each song?
That was my job: to find out what was going to work best. Again, my approach was: let’s preserve what’s on the record and make it better. I’d listen to the Jackson 5 records and have my Fenders ready. I called Greg Phillinganes to find out what synth sound he used on “Thriller”; he’d say, “
A Minimoog with two oscillators instead of three,” and so on. Every night at home I’d do at least an hour of programming to get the right keyboard sounds. Also, because bass and synth bass together were a key part of Michael’s sound, there were even sections of “Wanna Be Startin’ Something” and “Billie Jean” that I was playing bass and Minimoog together! I’d turn up my bass volume and really dig in with my left hand, hammering the notes, like on upright, while playing keyboard with my right hand."
Men in the Mirror: The Bassists of Michael Jackson How Alex Al And His Predecessors Pumped Up The King Of Pop
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Let's take a look how superb R.I.P. Louis Johnson tries to play that riff without those overdubs on the bass and keyboards.
The Thriller riff starts around 1:08 after a magnificent slap intro by the Master.
Mr. Johnson plays that low note with his thumb (right hand) on the 5-stringer.
Here is more discussion about Billie Jean at TB.
Micheal Jackson's Billie Jean Bass tone question.