Well, this is my take on this topic:
From the beginning of the electric-powered turntables, it was very difficult to have a 33 (or 78) RPM direct drive motor. For 33rpm this means 120 poles (synchronous), which creates an enormously complex motor. If the engineers used an induction motor, then they would need to have had a power line frequency of a few hertz, not 50 or 60Hz. So direct drive was out of the question. Induction motors typically had 1500rpm or 750rpm...
Some sort of reduction needed to be done, and i recall having seen some Garrard electric motor that used gears as reduction. But gears are not too silent...
The most obvious solution was, then, an idler wheel mechanism that takes advantage of the big (>30cm) diameter of the platter, to achieve reduction from 1500RPM to 78 or 33rpm. So this was the state of the art. Studio quality idler wheel turntables such as the EMT 927 or the Garrard 301 having technically perfect parameters (rumble, wow, flutter) even compared for today's turntables. Those are turntables where the plinth and construction assures that any vibration from the motor will never reach the platter.
But consumer gear is different, and it's obvious that a mundanely suspended 1500RPM motor on a light wooden plinth means rumble... But for consumer gear of the 50s this didn't mean much since bass reproduction was limited in the common household. That is until, in the late 60s, reasonably-sized speakers with decent bass response appeared (think Acoustic Research)... Then perhaps rumble started to be a problem.
So in the late 60s the suspended-chassis belt drive turntable (think AR again -- surprise
) appeared and, indeed, compared to consumer idler wheel turntables, it had far less rumble, and this was a significant advantage. So practically all manufacturers jumped on the belt-drive bandwagon (Dual, Thorens, Sony, etc), and, what is more important, from a marketing point of view idler wheel drive turntables were now perceived as "outdated", "bad", "mediocre", etc.
The Garrard 301 and 401 remained unchallenged in the studios. Same for the EMTs.
Finally on the early 70s, electronics was sufficiently advanced to create a direct drive motor, so DD turntables appeared... All japanese TT makers jumped on the bandwagon. Also Dual with a different patent for DD. These times saw the Garrards probably being replaced with the professional Technics SP-10s. EMT brought a direct drive studio turntable (948, 950), Telefunken too (PS 81 DD). Cutting lathes (Neumann) were retrofitted with the Technics direct drive motor specially built for them.
Forward to the mid 80s and 90s. The major Japanese manufacturers moved on to CD players. That means no more DD offerings. The cottage audiophile industry manufactured almost exclusively Belt Drive turntables, since they weren't covered by any patents, and were easy to make. Dual also returned to belt-drive turntables, probably because that's what the market wanted to buy. Idler wheel drive turntables? Those are for the museums and pawn shops.
Only lately, with the 'rediscovery' of the Garrard 301 and the Lenco 75, the idler wheel turntables started making a comeback. Indeed, if it not were for the internet, i would have passed on the offer for a free Lenco L75 that a friend of mine was about to put into the dumpster :tears: . I, too, thought that the Lenco was an outdated machine, incapable of challenging my nice Technics DD turntables.
Needless to say, my knowledge of TTs was limited back then.
Bottom line, physics are physics and if you have a massive platter, enormous inertia and torque, and smooth rotation, then BINGO, you have an excellent turntable. You can achieve them with any drive system (ID, BD, DD), but IMHO i find more examples of these goals being achieved with the classic, well-regarded idler wheel turntables.
