view and obscuring the black earth background? That's why most direct drive turntables and puck drive turntables make the music sound grundgy and veiled.
     It's worth noting that this kick and coast problem exists even before we turn to further problems in some turntables of servo error, servo correction, and servo drift. These further problems can make the speed irregularities even worse. But servo correction systems cannot cure the kick and coast problem of multipole motors, for the simple reason that the correcting servo cannot deliver the speed correction by any mechanism other than a kick of a motor pole; even the fastest, most perfect servo correction system has to wait until the next motor pole kick in order to actually deliver any speed correction.
     It's also worth noting that turntable rumble, even if spec'd to be very low or at an inaudibly low frequency, can contribute to FM distortion of your music by a turntable. Some vector component of rumble (and indeed of any vibration) is bound to be along the groove line direction, which defines the time axis of the music waveform being recreated by your turntable and cartridge. Thus, this rumble or vibration vector component modulates the time axis of your music waveform, creating FM distortion sideband byproducts around all musical notes, as surely as if the speed of the motor were changing in sympathy with these vibrations. That's why some direct drive turntables with very low claimed rumble "within the audible spectrum above 20 Hz" might still make music sound grundgy and veiled, since they are literally distorting music, creating modulation distortion sidebands spaced less than 20 Hz from every musical note (plus harmonics spaced further away).
     The speed variations (and so also the distortions) introduced by this kick and coast problem can be ameliorated somewhat by making the platter rim heavier (yielding a higher moment of inertia), but only up to a point, and unfortunately that point is still a compromise. The speed variations can be reduced by making the platter's moment of inertia high relative to the strength of the motor pole kicks, so that each motor kick doesn't disturb the platter speed as much. But, if the turntable designer makes the platter massive enough so that each motor kick can't significantly disturb the platter speed, then the turntable will take forever to get up to 33 rpm from a standing start -- precisely because each motor kick can't significantly disturb the platter speed (i.e. it can't disturb it enough to get it up to 33 rpm from a standing start). This puts the turntable designer in a hopeless compromise dilemma. Look at his dilemma from another angle. As soon as he makes the platter light enough, relative to the strength of the motor kicks, to be able to get the speed disturbed all the way up to 33 rpm from zero by the motor kicks in a reasonably short amount of time, then the platter is already more than light enough for the same strength motor kicks to disturb its speed a much smaller amount (say from 33 to 34 rpm) in a much shorter amount of time.

Automodulation Distortion

     The turntable designer might instead try to deliberately use a very weak motor, with very weak kicks. But that still doesn't solve the above compromise dilemma (he'd still have to move to a lighter platter, if he wants reasonable startup times). And it also creates a new problem, relating to automodulation distortion.
     What's this new distortion? Turntables get slowed down by outside forces, such as the drag of the stylus in the groove. This drag changes as the groove angle changes with large amplitude music signals. The more that the stylus is yanked side to side by a larger groove excursion representing a louder music signal or transient, the more its passage is impeded in the direction along the linear vector of the groove travel representing the time axis.
     What's doing the impeding? The turntable is moving the groove under the stylus in a linear direction along the time axis. If there's zero music signal amplitude, the groove is relatively straight, and the groove walls are parallel to this direction of travel, and so these groove walls offer minimal resistance to the stylus gliding past them. But when there's large signal amplitude, then the groove swings wildly from side to side, so the groove walls are more nearly perpendicular to the ultimate direction of the stylus' travel, which is the time axis direction. Thus, the stylus slams into the nearly perpendicular groove wall, which naturally offers more impediment to its travel in the time axis direction. Think of it as the difference between gliding along on an ice rink, parallel to its surface, versus slamming into an ice wall perpendicular to your direction of travel. Pretty dramatic difference, right?
     What happens when the stylus slams into the groove wall? Who gives way? Well, the stylus might be smaller than the groove wall, but he's a sturdy little bugger, being made of diamond and being anchored firmly to the pickup arm fixed on the plinth. So he hardly budges longitudinally, in the time axis direction (although he's free to swing from side to side, to track groove modulations). Instead, it's the groove wall that yields. Its soft vinyl gets momentarily deformed. As the compressed vinyl reacts to and springs back from this deformation, some of the energy gets transformed into heat, while another part causes the groove wall to partially shudder to a stop, i.e. to slow down. Thus, the groove wall slamming into the immovable stylus, at a nearly perpendicular angle to the groove's time axis motion, causes the groove wall itself to recoil from this collision and slow down in its motion along the time axis direction. When the groove slows down, of course the whole record slows down and the whole turntable platter slows down.
     In sum, when the music gets loud, the larger side to side groove modulations become more nearly perpendicular to the stylus' steady travel along the time axis direction of the groove. The groove wall has acquired a vector component that effectively collides with the stylus, instead of gliding along parallel to it. Along this time axis direction, the stylus position is fixed, so it is the groove wall that loses in this collision, and the groove (hence turntable) slows down in the time axis direction.
     This means, quite simply, that loud music slows down your turntable.
     But any variation in the speed of the turntable is a distortion of the time dimension itself, which is one of the two axes defining the music waveform (the other being the instantaneous amplitude, furnished by the groove's side to side modulations). Distort the time dimension and you distort the music waveform, as surely as if you had distorted the amplitude dimension.
     Therefore, loud music makes your turntable distort the music. Loud music distorts itself, when played on most turntables. The type of distortion is still FM distortion, but it is a particularly cruel joke because the music is causing its own degradation. Thus this is automodulation distortion. Incidentally, there is a similar phenomenon in some digital systems, where the amount of and nature of jitter (which ultimately causes distortion) is correlated with the amplitude and nature of the music itself -- again, music causing its own distortion.
     With a turntable, an increase in the amplitude dimension of the music waveform causes a distorted stretching of the time dimension of that same waveform. The time axis of the music waveform is modulated by what is happening on the other (amplitude) axis, instead of ticking away steadily, constantly, and independently on its own.
     Note that this automodulation distortion is different from the more ordinary type of nonlinear distortion, wherein cartridges, amplifiers, speakers simply become more nonlinear as signal amplitude gets larger. In that ordinary type of nonlinear distortion (AM distortion), the amplitude axis directly becomes distorted at various signal levels, due to the nonlinear amplitude response of some system. But this automodulation distortion is more sneaky and indirect. Here, it is the time axis rather than the amplitude axis which gets distorted at various signal levels. And it is only indirectly that distortion of the time axis leads to distortion (via FM distortion) of the music signal. Even though it is an indirect distortion, it is just as real, and in fact is even more pernicious, since FM distortion sounds uglier and is more detectable and objectionable in smaller amounts than AM distortion (some of which can even sound benign or actually even euphonic, such as second harmonic distortion).
     How can the turntable designer keep the platter from slowing down every time the music gets louder, thus causing FM automodulation distortion? The only available tactic is to make the platter heavy, with a large moment of inertia, so that its large angular momentum tends to keep the groove going at a constant speed along the time axis. But this then implies that the turntable must have a relatively powerful motor with relatively fast response, in order to adequately power a heavy platter, and in order to restore its speed losses due to stylus drag within the same fast time frame that music's louder transients occur.
     And this in turn means that one whole turntable design approach, weak motor with light platter, is ruled out. This design approach might be one way to address the multipole kick and coast problem, but it cannot adequately address this new problem of automodulation distortion.
     So that puts us back with a heavy platter, a powerful motor, and a relatively fast motor response. But this also puts us squarely back into the dilemma of the multipole kick and coast problem. As soon as we make the platter heavy enough to avoid automodulation distortion, then we also have to make the motor powerful enough so that the periodic pulses of its kicks will be felt.
     As if this weren't bad enough, there's also a problem of how fast to make the servo correction circuitry that drives the motor. After learning about automodulation distortion, we now see that we have to make the motor response fast enough to respond promptly to heavy music modulation within the time frame of music. But if a powerful motor is fast enough to respond to music, then it is also fast enough to directly intermodulate with the frequencies of music. Thus, we are now faced with yet another new dilemma in designing a turntable. We can make the servo correction fast to avoid automodulation distortion, but then we run the risk of directly intermodulating with musical frequencies. Or we can make the servo response slow, to avoid directly intermodulating with musical frequencies, but then the turntable will always be slowly drifting and hunting in speed, hardly ever being right on target (as is indeed the case for many direct drive turntables).
     And, in either case, regardless of whether we make the servo response fast or slow, we still must face the basic problem that any multipole motor can only deliver its power via discrete kick pulses. So we're still stuck with the basic kick and coast problem.
     Where does that put us? It appears that multipole motors are the devil's own instruments, distorting music from analog turntables because of their kick and coast problem, so long as the motor is rigidly coupled to the platter (via direct drive or indirect but rigid puck or idler drive). We've explored a number of possible turntable design approaches (light vs. heavy platters, weak vs. strong motors, fast vs. slow servo loop correction), and nothing really cures the basic problem of distortion and smearing that the multipole motor brings to the half of the music waveform for which it is responsible, the time axis dimension.
     Clearly, we need help, big time. And, just in the nick of time, here comes our purported savior out of left field, riding a white horse. It's the elastic belt to the rescue! The elastic belt furnishes an elastic, non-rigid coupling between the motor and the turntable platter. The elastic stretchiness of the belt acts as a filter, filtering out higher frequency vibrations, so they are not transmitted from the motor to the platter.
     Different belt materials and constructions have differing amounts of elastic stretchiness along their length direction. Some are relatively stiff and unyielding when you try to stretch them along their length, while others stretch easily. Generally, the easier a belt stretches elastically, the better it is at reaching to lower frequencies to filter out unwanted vibrations. By making a belt sufficiently elastic, we can make it reach down to the frequencies of a multipole motor's kicks, and filter them out. Incidentally, once a belt has been made elastic enough to filter out these low frequencies, it usually pays to also make it elastic enough to also filter out the motor's rumble vibrations, which start at the motor's primary rotational frequency. Belt drive turntables still use multipole motors that put out the dreaded kick and coast problem, but the belts are generally tuned to filter out these multipole problems.
     The kicks from the multipole motor may be regarded as unwanted vibrations, which we don't want the platter to feel. A belt of sufficient elasticity filters out vibrations above a certain frequency, which could include all frequencies from the multipole kicks (including the fundamentals of say 22 Hz or 43 Hz as above, plus higher frequency overtones). If the platter can't feel the vibrations of the kicks, then it also can't feel the temporary speed increases that those kicks would tend to induce. Thus, the platter wouldn't feel the kick and coast problem from the multipole motor, if the energy from the motor is transmitted through an elastic belt. If (thanks to the elastic belt) the platter doesn't feel the kick and coast speed variations that the multipole motor is still putting out, then its speed will tend to remain more constant. And, if the platter speed remains more constant, there will be less distortion and smearing of your music signal, because a full half of the waveform will have been created more accurately by the turntable. That, in a nutshell, is why most belt drive turntables sound better than most rigid coupled drive turntables. The turntable is responsible for recreating half of your music waveform, and the belt drive can recreate this half more accurately than a rigid drive, because it can filter out the dreaded kick and coast problem inherent in every multipole motor.
     However, on closer inspection, the elastic belt is not quite the savior on a white horse that it purports to be. He's more like Don Quixote riding an old grey mare one step away from the glue factory. In short, the elastic belt brings with it a whole fresh set of problems and engineering compromises.
     Past articles published on belt drive have oversimplified the benefits, and have underplayed the problems. If you ask a typical belt drive turntable designer or technical proponent what the elastic belt achieves, chances are he'll say that the filtering action of the belt averages out the speed irregularities of the multipole motor's kick and coast problem, thereby sending a constant speed drive to the platter. That's a true statement, generically speaking, as far as it goes. But it doesn't go nearly far enough. It only scratches the surface of the way that the elastic belt really works, and thus it fails to reveal the many problems and tradeoff compromises. It gives a rosy generic oversimplified summary description of what a belt is ideally supposed to do, but it ignores the details of what a belt actually does.
     We'll have to dig deeper, if we are to understand why belt drive turntables are compromised. What does a turntable belt do, actually, not just generically?
     The motor of a belt drive turntable has a pulley, with a certain circumference. The platter of the turntable has a rim (perhaps tucked underneath the platter), with a certain circumference. The ratio of the pulley circumference to the platter rim circumference will determine the rotational speed of the platter, as a fraction of the rotational speed of the motor. For example, if the multipole motor had a rotational speed of 333 rpm and the ratio of the circumferences were 1:10 (the platter rim being 10 times larger than the motor pulley), then the rotational speed of the platter would be 33.3 rpm.
     Thus, the turntable platter speed depends upon the speed of the motor (naturally), and upon the ratio of these two circumferences (also known as the reduction ratio). And, as we now know, the turntable speed is fully responsible for recreating half of your music waveform, the time axis dimension. So, the time axis half of your music waveform depends on the turntable platter speed, which in turn depends upon the ratio of these two circumferences. Therefore it follows that the time axis half of your music waveform depends directly on the ratio of these two circumferences.
     In particular, if we want the time axis half of your music to remain stable, unwavering, and undistorted, then we also want the ratio of these two circumferences to remain fixed, stable, and unwavering. So far, so good. That seems clear, straightforward, and obvious enough.
     Now, where does the belt figure into all this? The belt moves around the circumferences of both the motor pulley and the platter rim. It transmits the rotational motion of the motor pulley to the platter. Specifically, the rotating motor pulley circumference engages and moves a certain length of belt, say 1 inch worth, and then the moving belt in turn engages and moves 1 inch worth of the platter rim over the same period of time. The belt's primary responsibility is to faithfully communicate 1 inch's worth of circumferential travel by the motor pulley into exactly 1 inch's worth of circumferential travel at the platter rim. This is how the ratio of the two circumferences is communicated between the motor and the platter. For example, if 1 inch represents the complete circumference of the motor pulley, then (assuming our 1:10 ratio) 1 inch would represent 1/10 of the circumference of the platter rim, so a complete rotation of the motor pulley would produce exactly 1/10 of a complete rotation of the platter, and thus the platter would rotate at exactly 1/10 the rotational speed of the motor. Note that the belt is responsible for accurately maintaining a constant, unwavering ratio of motion between the pulley and platter rim.
     As a visualization exercise, imagine that a 1 inch portion of the belt's length is painted a different color, say red. Ideally, that red 1 inch portion of the belt's length will be driven by a matching 1 inch portion of the motor pulley circumference, for a given period of time we'll call t. As the belt moves, this red 1 inch portion will gradually travel over toward the platter rim, travelling at whatever speed the belt happens to be moving. Now, the belt's primary responsibility is to ensure that this red portion engages and moves exactly 1 inch worth of the circumference of the platter rim, and accomplishes this in exactly the same duration of time t. If the belt succeeds in doing this, then it will faithfully communicate the correct ratio of circumferences and the correct ratio of speeds, and then the belt drive turntable will have a chance to run at the correct speed, and accurately recreate the time axis half of your music waveform.
     But, if the belt cannot fulfill this primary responsibility, then the belt drive turntable is doomed to run at incorrect speeds, and is doomed to distort your music by incorrectly recreating the time axis half of your music waveform. These music distortions will be different than the distortions brought on by the multipole motor's kick and coast problem, discussed above for turntables with rigid drive coupling. These music distortions will be brought on by the belt itself, failing to act ideally. So already we can see that a belt, introduced to solve one turntable problem (kick and coast), might

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