The drive system of direct drive turntables concists of either a DC motor or an AC motor directly connected to the center spindle where the platter rests. The motor is then carefully controlled and monitored by an electronic speed control system. This system is far superior in reliablility to most belt drive systems. Obviously, it is also a much simpler and more straightforward way of making the platter rotate. Also, high quality DD's are in the real world much more accurate in keeping perfect speed than belt drive systems. Furthermore, since many belt drive systems don't have any means of either verify the actual speed (stroboscope) or more importantly, adjusting it you could never be sure it's actually playing your records at the right speed! On numerous occassions I've put a stroboscope platter on belt drive TT's just to discover that I've been playing my records either too fast or too slow and there was no way of adjusting it! For that reason, the only belt drive TT's I would even consider using are models with stroboscope and pitch controls. One, could ask, why are most TT's manufactured today belt drives then? Well, there are probably several reasons. First, it's much easier to design and manufacture a simple belt drive TT. Second, there has been a notion among audiophiles that belt drive TT's sound better than DD TT's. However, I have never come across any serious scientifical research or testing that establish this notion and in my own experience DD turntables sound as good as comparably priced belt drive models. Of course, a high quality belt drive TT can be a very good TT and have a very exact speed control (Linn LP12 Sondek with Valhalla speed control is one such very high quality model) no doubt about it but usually they are very expensive. However, in my point of view, DD TT's offer so many other advantages it's an easy choice for me.

Some of the advantages with DD turntables versus belt drives:

1. Speed accuracy - advanced electronic control means the speed accuracy is far superior to most belt drives, especially UNDER LOAD and OVER TIME. Furthermore many DD TT's use Quartz control which make them even more accurate and this is very unusal on belt drives.  

2. Fast start and stop of the platter possible.

3. Strong DD motor unsensitive to high loads such as a cleaning brush - doesn't slow down, or more importantly, no belt skip.

4. No speed variation over time and under different temperatures.

4. Much more reilable system

5. No maintanance - NO belt change!

6. DD motors are of the LOW rpm type which is much more desireable than the HIGH rpm types used on belt drives for obvious reasons (low rpm means less noise and interference).

Disadvantages:

1. No isolation between motor and platter.

2. Requires much bigger R&D investment to design a high quality DD system from scratch. 

The only real disadvantage I can see is the first one, that there are no islolation between the motor and platter. That means the motor has to be of a very high quality to be even considered. Also, the electronics involved is much more complicated than on most belt drive TT's. Those are some of the reasons a good DD TT is very expensive to design and manufacture.

This is a very high quality DD motor system on a Pioneer PL-L1000 tangential arm TT. The Rotor is the only moving part and can be seen at the left. The Stator - the  stationary part is also clearly displayed here and the windings can be seen

In the early days of DD turntables, there were motors which had a "cogging" rotation which could in the worst cases be heard. That disadvantage was soon eliminated by the mid 70s. The best DD drives use a heavy platter (at least 2 kg) for high rotational mass/torque. This necesitates a very strong motor that also has to be coggless and quiet - not an easy task! Technics, Denon, JVC and the other big Japanese players spent countless of $ millions in research and development of these DD drives. They were the only ones that had the financial clout to do it, the rest had to get help from specialist companies and /or go with belt drives.    

In order to keep the speed accurate with minimal fluctuations (wow and flutter) under diverse loads a speed monitoring system is needed. The manufacturers continually developed and refined these systems and in the mid to late 70s they also started to add Quartz control. The most common way to monitor the rotating platter is to use a device called a FG generator which generates a frequency that varies according to the speed of the platter. This way, the electronics can sense if the platter is rotating to fast or to slow and compensate by adjusting the drive voltage to the DD motor. Many FG generators use a disk with slits in it which is mounted directly on the motor. A lamp or a led is then placed on one side of the disk and a light sensor on the other. As the disk rotates, the light sensor then senses the light as the slits are rotating by and thereby prducing the frequency that the electronics can detect. Another common solution is a metal rim with groves that rotates past a special sensor.

There are other FG generators also, one of the best systems is Denon's where a magnetic reading head is used to read the inner rim of the platter which is coated with 1000 magnetic fields/pulses. The picture here shows the aluminium chassies of a Denon DP75M with the platter removed and the recording head can be seen on the right side. This system is much more accurate than a disk as the disk might only have around 150 to 200 slits. Sony also used this system in their better players. All belt drives except very few has no way of detecting the actual speed of the platter for the drive electronics. On the ones that do have it there is no way of fast correction of the speed due to the belt drive so only the nominal speed can be corrected. Most common though is that higher quality belt drives rely on the heavy platter/high rotating mass principle where the platter is so heavy that it will by itself maintain a smooth and steady rotation. However, these TT's are by nature very heavy and thus very expensive. Furthermore if they don't use a sofisticated speed control then they just can't rival good DD TT's when it comes to speed accuracy especially under load and over time.

There are Belt drive TT's that use sofisticated electronics and even Quartz control but they are more of an exception than a rule, and usually very expensive.