Is Direct Drive Really Better?

Here's something that's always made me wonder. Folks talk about quartz/PLL-controlled DD as constantly "hunting" for the correct speed, or attempting to compensate for deviations after they've happened.

What I'd like to know is this: In a BD, I'd assume that the "kick and coast" action of the motor, or any dynamic drag flucuation caused to the platter, would cause the elastic belt to be stretched a bit on one side and relaxed on the other, because then the drive-pulley and the platter would be turning at slightly different speeds. The belt of course would attempt to regain a state of equilibrium in tension, but this would set up an oscillation between sides that would take a while to die out, before which another disturbing discrepancy would have come along, etc. So the belt would constantly be in a state of "hunting" for the correct speed, n'est-ce pas?

On an SL-1200 there is a built-in strobe, so you can see speed deviation and recovery behavior. If you use your finger or a brush to momentarily apply some extra friction to the turning platter (enough to noticably slow the platter -- in other words hundreds of times more friction than a stylus playing a record could ever apply), it will come back up to speed in a controlled, deliberate fashion without visible overshoot or oscillation.

If you put a strobe on a BD and do the same thing, does it appear to behave the same way, or is there some degree of "bouncing around" visible in its recovery behavior? This may be a "trick" question -- any differences might well be too small and/or fast to be visible either way.

Turntables drive systems is an interesting topic and one that I have been contemplating and experimenting with a great deal lately.

I have concluded that speed stability is one of the most important factors in turntable sound quality. For that matter it is also one of the key performance factors in digital audio. It is well known and accepted that digital jitter significantly degrades sound quality. What is remarkable about digital jitter is that such extraordinarily small timing errors could be audible at all. The message here is that our ears are far more sensitive to timing errors than with amplitude errors. With analog the principle and effects of jitter/timing errors are essentially the same. In both cases waveforms are being reconstructed and timing errors create similar distortions. Distortions that for some reason are much more audible than one might expect.

So when it comes to turntable speed stability it is a much more complex issue than many would think. Technically there is no such thing as “constant speed”. Any drive system will have micro variations in speed. As with digital jitter, both the frequency and amplitude of these variations are important. Wow and flutter measurements only quantify large, low frequency variations and don't seem to correlate well with sound.

The main source of speed variation is the motor. All motors cog, or have variations in torque as they rotate. The correct term is torque ripple. In general it is beneficial to isolate the torque ripple to reduce the effect it has on platter speed. Belts and idler wheels provide some degree of isolation. How much of course depends on how compliant the material is. At the same time it is beneficial to have the motor tightly coupled to the platter and rely on the motors torque to keep the platter speed constant. Tight coupling of the motor is best way to reduce the effects of stylus drag. So we have two opposing objectives, coupling and isolation. It would seem that for any motor, platter combination there would be an ideal compromise between isolation and coupling. For example AC motors have a lot of torque ripple so the best compromise is usually a lot of isolation using a stretchy belt. DC motors have far less torque ripple so they typically sound better when used with more rigid coupling (½ tape). With direct drive there is no isolation so the torque ripple must be very low to get acceptable sound.

Beyond the isolation and coupling issue belt and idler drives are both are susceptible to oscillation. We end up with two rotating masses connected with a compliant medium. The worst case is when both the motor and the platter have the same inertia. At first glance it would seem that the inertias are quite different. However, the motor typically spins much faster than the platter so that the inertia actually ends up being similar despite very different mass. Increasing platter mass and decreasing motor inertia helps reduce oscillation. I believe that this is one of the reasons that heavy platters tend to sound better.

As I said earlier both the frequency and amplitude of speed variations is important. There is considerable evidence that very small, higher frequency variations are particularly audible. Power regenerators for AC motors and batteries for DC motors have consistently provided better sound. My own experimentations has also shown that efforts to reduce high frequency noise results in better sound. This would indicate that higher frequency speed variations are more detrimental to good sound. This kind of error sounds remarkably like digital jitter. It sounds harsh, edgy and smeared. Not an artifact that would typically be attributed to speed stability.

Various techniques may be used to reduce these effects but they are never completely eliminated. A heavy platter will reduce high frequency variations more than a light platter. However, lower frequency problems are less effected. For example the effect of stylus drag is different but not less with a heavy vs. light platter. With a light platter a heavily modulated passage will reduce the speed but because of low inertia it will quickly recover. On the other hand a heavy plater will be slowed less but it will take longer for the speed to recover. So platter mass only changes the frequency of stylus drag effect and does not eliminate it. However, it would seem that lower frequency variations from a heavy platter would generally be more benign.

So after all the ramblings the question is what is the best approach? As many have surmised it mostly boils down to implementation. It also is a matter of compromises. Some will prefer one set of compromises over another based on their tastes. However, I do believe that direct drive has the greatest potential. With careful design and implementation direct drive can result in less compromises. With sufficiently low torque ripple and noise the results are remarkably good. Good enough that a direct drive offering from Teres is in the works.

Chris,

do you think you'll have a DD prototype ready for next month's open house? That would make for some more fun!

Will the Teres offering be anywhere near the 1200 price point?

Gosh and we thought that digital wasn't as good as analogue! Why digital even manages to 'create' the same speed problems as vinyl LP turntables, as jitter!
Bob P.