Off center vinyl pressing

Where do you get one of these speed compensators?

 

My dog sure didn’t seem to like that video...

 

My AP test record for measuring azimuth was pressed 'off center' and screw up my measurements somewhat - I just wish I would have noticed it sooner so I could have returned it.

 

It’s appalling that test records don’t have impeccable quality control. They are sold as imperical testing equipment, and priced accordingly, but the things are apparently off-center and/or warped pretty routinely nowadays.

 

Hi AutomatedElectronics,

I didn't get it, I made it. It's centered around an Atmel AVR 8-bit controller, which takes the TT's quartz oscillator output as reference, and uses a timer to frequency modulate it with the horizontal oscillations of the tonearm. And it is this new "wobbly" reference which I feed to the servo circuit instead of the original "steady" one.

 

Neither did my girlfriend...

 

I think it is nice and had a similar idea of this long time ago. But simce I am poor at electronics I had no idea where to start. I wonder if this also can be applied to a belt drive given if the platter is not too heavy. In the end you would need quite small adjustments in speed. Care to share the details?[/QUOTE]

 

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Hi Thomas,

I've been thinking about whether it would work on a belt drive too, and there is no reason why it wouldn't. It all comes down to differences in how the motor is controlled, and interfacing with that particular servo. It could even be made to work with a "servoless" mains frequency locked synchronous motor, but in this case, it's the 50/60Hz power supply to the motor that has to be modulated (and synthesized), but by the exact same relative amount as a PLL servo reference would be (like in my TT).

What exactly would you like to know? If you prefer, you can write to me at [email protected].

 

The Technics has the advantage of having a motor who's speed control is directly based on a free-run oscillator/tone generator at a particular frequency. The idea is quite logical, using a tonearm positional sensor to track the "wobble" of the tonearm and calculate a frequency correction. My linear tracking tonearm even has an optical sensor that already does this - it instead ignores the wobble, and uses the low-pass position tracking angle sensor information to advance the tonearm pivot when needed.

The programming of intelligence, math, and hysteresis is the hard work, Edison's 1% inspiration, 99% perspiration. The degree and position of off-center can be discovered within a few rotations with enough sensor input. One has to write the learning algorithms to perform this optimally, especially on a belt drive, as the turntable motor and its platter mass doesn't respond instantly, and we need to use more informed correction for an ideal result.

It still can't understand the audio to know if its doing a good job.

 

That is an interesting thing, but only a partial solution, or half a hack and I do not mean hack in a negative way here.
There would be unequal groove wear, as for half a revolution there would be additional pressure on the left groove wall, and half a revolution would have additional pressure on the right wall.
There would be higher stylus wear for the same reasons, although that could equal out with half a revolution each of higher pressure on the left and right edges of the stylus.
Similarly or additionally for antiskate - for a half revolution the pressure would effectively increase the antiskate compensation, and half a revolution would work against the antiskate, amplifying the above effects.
There also remains a phase problem, as the angle of the stylus would be correct at only two points per revolution. Imagine a 50,000 Hz tone recorded on the vinyl in mono, played with a stereo cart summed to mono or a mono cart - that tone would go in and out as the phase cancels and aligns, looking like a sine wave. That might not be audible in the audible range but it would be there somewhat at all frequencies with all records.

It's better, and simpler, to get the record centered without regard to the wrong position of the spindle hole.

 

Have a look at this new pressing of Van Morrison 'The Healing Game' pressed at MPO in France Vinyl LP Care - www.vinyllpcare.com

It is probably the worst record I have seen in years! It is not just an off centre spindle hole either as I have successfully 'fixed' many hundreds of those down through the years. My feeling is that the stampers in the press were not correctly aligned with each other leading to one side (Side 2 in this case) being as much as one centimeter off centre on rotation. No intervention as to 'filing' the spindle hole will fix this one either. This is my second copy too from the same store so that certainly bears out the point you make. Depending on stamper life, there could be as many as 500 to 1000 identical copies in circulation. And to add insult to injury, if you look to the left of the record as it rotates, you will see an inner warp or glitch at a certain point. I refer to this 'glitch' as a 'speed bump' on the record. I am taking this back to the store again and I don't want another copy. The store got a bad consignment and yet nobody, as of a few days ago at least, has complained? I am 99.999% certain that the entire stock is the same. I even contacted SONY MUSIC in Dublin about it and am waiting on their reply. I have really come to dislike MPO in France in the past three years. I have had torn centre labels, ridiculous warps (which I fixed) and non-fill on a slew of titles pressed in North West France. People here on another thread have got 'perfect' copies of 'The Healing Game' though, but I will have to mail order it from Germany, most likely, and hope for the best.

 

Although it is always best to fix it at the source, the method takes away the FM related modulation which is audible. The other effects of the other things are there no matter if you do the adjustment or not. The uneven pressure on the wall is minimal unless you have to low resonance frequency of the tonearm-cart combination or too high friction in the tonearm bearings. The same with the position of the stylus.

 

Another solution is to use the tonearm sideway to some kind of mini stepper motor with a small rod that gently pushes the record into position the first revolutions. This requires a play between spindle and the hole and a mat that is slippery, like a felt mat. The method works in principle by doing manual pushes with a gentle tap with the finger.

Now how can should such a device look like? The measurement should be done similarly as above, but the push needs to be aligned with the signal, and corrected by phase depending on where the device sits.

 

Thanks, indeed, the idea is a very "logical"one to have, and most of my credit is believing in it and proving that it really does work. I've also the had the idea for a long time before I realized I have a good shot at implementing it myself. And so far, I haven't found proof of anybody else doing this. And yes, there are a few "tricks" that I used in the programming; this is where my embedded programming experience came in handy. The algorithm doesn't apply the correction based on high-pass filtering the angle information (I'm using an MR angle sensor which detects the orientation of a magnet strapped to the LEGO frame which rotates with the tonearm) alone, instead it locks onto the phase of this signal and applies an analytically computed sine function. This way I can control the phase of the correction signal in relationship with the phase of the oscillations. My DD motor also needs some "phase-ahead" to obtain optimum results.

"It still can't understand the audio to know if its doing a good job." - true, the learning was not automatic, and other types of turntables would need calibration of some parameters. And adding a simple frequency counter of the cartridge output would enable some sort of automatic calibration using a special test record - that's also a neat idea

 

That is true of course, my solution is not an "absolute" one like Nakamichi's with their Dragon CT and TX1000 TTs, however, the impact of phase errors is extremely small when compared to the speed aberrations. They are no bigger than the normal angle errors of any non-tangential tonearm. Same goes for the pressure variations caused by the inertial momentum of the tonearm - they can be calculated, and proven to be negligible (against vertical variations caused by record warps, for example). On my test record I also have a track with "maximum" amplitude signal for trackability testing, and with my current cartridge I am at the limit of that trackability when playing it at 45RPM. That means, if I set the stylus force just a little bit lower, I get distortion. Also if the antiskating is off by just a little, on the respective channel. So I can confidently state that offsetting the record center even by a lot has a minimal impact on trackability. But once again, your observations are absolutely correct.

 

Appreciate the effort, but there's way simpler solution that every turntable should come with:
Spindle for Technics SL-1200 SL-1210 that helps correct off centre records

 

I agree. And regarding lateral tonearm friction, it is why I consider the counterweight method of providing antiskating force to be suboptimal: the frictional force of the wire against the wire frame supporting it will change its direction depending on the direction of the tonearms lateral motion. In other words, you get less antiskating when the tonearm is moving inwards, and more antiskating as it is moving outwards. Having a rotating spinde (as some better ones do) does help, of course. But the simple spring loaded solution is better in my opinion.

 

I wonder if they couldn't make a Nakamichi Dragon CT style automation to the hole punching process. A tonearm or laser that measures the groove movement and then can center the stamper before the hole is made. That way, it's electronically verified that they have found the exact center of the stamper.

 

This would be great with every turntable, yes, or just removable spindles as a standard.

 

I beg to differ: you need to manually center every record, every side, every time. My system is automatic, and it is sensitive enough to track and compensate for off-centering less than visually discernible with the naked eye. It also has a BOM which costs far less than this custom spindle. It's really just down to which method offers you most satisfaction. Of course, it does pay the effort to center manually when digitally archiving.

 

The fact that the sides are generally off-center to each other is one reason, but still, you could have 2 individual stamps overlapping into an "oval" hole, with an arrow on each side indicating which side of it should touch the spindle.

 

Was about to add that. Like some tables had it in the past.

 

The manual method works fine if you have a hole that is little larger compared to the spindle, even if you have a standard sized one. One need to have a good eye watching the tonearm movement and a slippery felt mat, and then gently push the record into position.

 

I'm not sure I understand what you mean. Right now, the stampers have their hole punched by an operator who looks through a scope and the operator lines up the grooves to a guide by eye. I'm just suggesting that if they automate that process somehow, they could conceivably eliminate off center pressings. Of course the stamper also needs to be properly installed in the press, and I'm not sure if further error could be introduced there. I think an oval hole would just lead to much operator error in the home.

 

What you describe would not necessary form an oval, as the off-centeredness between sides is not necessarily opposite directions. So you’d have some weird holes.

There certainly has to be a way to line the two sides up correctly first, though. The technology of pressing vinyl records and punching hols in the center of them is not exactly recent...