Positional updating v Linear Frequency Generator

[Epicurus]

Having read this (http://www.kabusa.com/myth2.htm) article on direct drive's ability to provide accurate platter rotation speed and the two servo control systems that are used, I want to ask the following:


Can anyone provide some examples of DD turntables that use either system (OK, I know my Techie uses the latter method)?
Can someone explain how they work?
What about belt-drive decks? What servo control type do they use?

[Beobloke]

The positional updating system that the article refers to is most likely the type of setup used by the likes of Sony, amongst others, where the inside of the platter has a magnetic coating and the rotation of the platter is 'read' by what is effectively a tape head. This is then fed back to the motor controller and the speed adjusted as necessary.

I'm afraid the article's generalisation that this is a bad system is not true. Yes, some of the cheaper DJ units use this type but on a small disc attached to the spindle and so the level of control is very coarse. Sony's system is far from being coarse and works very well indeed, as anyone who has heard or owned the likes of the PS-8750 or the PSB-80 can testify.

As with so many things in hi-fi, sweeping generalisations of 'Method A= Bad, Method B=good' do no-one any favours. Again, it is the implementation that counts, rather than the methodology per se.

As to belt drives, I don't know of any that use the tape head method but there may well be some. The old Fons CQ30 had a sensing coil inside the motor itself that was used to keep the speed steady. This worked well on one example I owned but, on the other, the coil was faulty. Result - platter ran at about 200rpm!

[Epicurus]

I saw one Clearaudio model that has a photocell underneath the platter - perhaps this is some sort of positional update implementation? At any rate, in theory at least, continuous servoing has better chances to deliver the goods; at least, that's what I learned in university.