Turntable speed stabilty

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How did you record the angle/x axis
It's based on 'correct' time, ie the time when perfectly constant 33 1/3 rpm would present at that angle/location on the rotation. I think that's fine, even if it means there's always some error in exact angle/location - we're not that interested in finding exact position. Whereas we are interested in accurate frequency relative to real time, reasonably locally on the rotation.

LD
 
So, e.g. in your graph from #4 the eccentricity would move in contra-rotational direction (assuming that the record is spinning a little slow) if you'd record more than one turn?

Best regards!
Yes. However, in a healthy TT error is often small enough to tolerate and I try to do that. However, I also have the option to vary start point of 2nd revolution display based on average frequency of the first. This is why I only display 2 revolutions, BTW, since errors accumulate.

It's also why I standardise on 3150Hz test tones, though 3kHz is supported. Actually, pretty much any frequency would work, but to avoid this issue one has to know what the original frequency was meant to be in the first place...

LD
 
Thanks Bill and LD.
Also, focus on accurate motor speed control is typically misplaced, IMO, because that's seldom a dominant factor in overall pitch stability, it seems.
LD
I like the idea of displaying a servo signal where there is one, if nothing else, for a health check......!

But pitch stability is only partly about platter speed stability. Typically factors like spindle centering, platter vertical run-out, warp, and cart-arm stability contribute far more in an otherwise healthy TT. So this is where bang-for-the-money is, and is well within most DIYers capabilities methinks.

LD
Kindly explain platter vertical run-out. Also what do you mean by Cart-arm stability affecting constant speed ?
TTs having good sonics are not related to pitch stability is little surprising. Do cutting lathes have perfect speed? How is it achieved ? I think beyond certain point it does not matter as much as rumble noise. But achieving it or measuring it is interesting. Do you or anyone else have polar plots of classic tables mentioned earlier ?
Thanks and Regards.
 
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Here's FFT of my turntable PL910(kyocera late1980).Testrecord is "stereo reviiew's model sr12".When FFTsize is 32768, calculated data is about 0.3second.When 8192,about 0.085second.

I can't measure speed drift from FFT.But I suppose peak level of successive small size FFT(8192) is referrence to average turntable speed.They are fairly stable more than predicted.:hypno2:
 

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Some thoughts about the correlation of sonics and pitch stability...

Pitch stability is measured with a discrete frequency and amplitude, so there is a (more or less) constant drag on the motor.
I think any TT motor can handle a constant drag satisfactory.

With music playing the drag on the needle changes permanently. So maybe we should look at the dynamic behaviour of the motor and the motor controller. A testrecord with a frequency that changes the amplitude abruptly could probably give a better prediction of the sonics...
 
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With music playing the drag on the needle changes permanently. So maybe we should look at the dynamic behaviour of the motor and the motor controller. A test record with a frequency that changes the amplitude abruptly could probably give a better prediction of the sonics...

I agree on this Boris.
But also remember that there is vinyl –mat interface, mat material and mat-platter interface.
The more lossy the two interfaces are and the more compliant and thick the mat is, the more decoupled the vinyl is from the platter.
This is good for acoustic decoupling but not good for torque transfer under dynamic drag changes.

George
 
To LD,

Your plot is very clever and informative. I wish you can add user interface so we all can measure our own tables.

I am thinking if I can export a set of data, i.e. time and frequencies, from recording of a test tone 31Hz. Then, I may import the data into some kind of polar plot software. I may get same measurements as yours. Do you know any polar plot software I can use?

Thank you!
Jim
 
It works with any test record that provides a continuous sine tone at about 3kHz or 3.15kHz, and just needs a short recording of 10s or so in any lossless format at any reasonable level.

So, over a few years, I collected together recordings of test sample tones that people have posted on forums, but usually I don't know and it doesn't matter what the actual test record was.

I don't mind running a few examples here, if anyone likes?

The software is a set of VB modules I wrote a few years back for myself to diagnose and improve pitch stability on my rigs. I never got round to making a proper user interface, so I still have to run them them manually.

Meantime, here's another good example from the archive. This is from a Micro Seiki SX-777. Red and Blue traces are for 2 consecutive revolutions.

LD


Here is a 3150hz sample from a Phonosophie No3 turntable.

Phonosophie P3 3150Hz Tone /Phonosophie_P3_3150Hz_Tone.wav

msdin
 
Here is a 3150hz sample from a Phonosophie No3 turntable.

Phonosophie P3 3150Hz Tone /Phonosophie_P3_3150Hz_Tone.wav

msdin
Here's the polar plot for that file from your Phonosophie P3. Also, another plot from a Phonosophie P3 file I had in the archive, for reference which happens to be at 4kHz but with the same scale so it's easy to compare. The archive one is the 2nd attachment.

Overall I would say your P3 is good and healthy. If you wish to improve it, something seems going on around 6Hz or so, which also occurs on the archive P3 so might be drive related. Also you have something going on at just over 2Hz which doesn't seem to crop up on the archive P3.

You have far more stable 'once per revolution' eccentricity like rotation speed variation than the archive P3, and also your cart/arm resonance seems more stable.

BTW, the y-axis on the inset spectrum auto-scales. So when there's a lot of eccentricity, as in the archive P3, it looks as though all other contributions are smaller, whereas that's just an artefact of the scale.

Hope this is interesting/helpful ?

LD
 

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For those wishing to read more on flutter analysis and diagnostic methodologies I can recommend Dale Manquen's site, M A N Q U E N . N E T - A U D I O

Dale isn't with us anymore but his site lives on. In it (among other things) he explains how to take the data like you have on the last post and use it to identify the cause(s).

Cheers
Alan
 
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Is it "turntable" speed stability or is it "record player" speed stability?

This LP12 example is actually quite good as these things go. One can readily see mostly there's a once per platter revolution 'eccentricity', which might well be record mis-centering. Otherwise, one can see contributions around the 8-12HHz region which might well be cart-arm resonance - often this can dominate stability in a good TT, IME, as it appears here.

So in this case although the underlying TT seems good, the cart-arm resonance might well be dominant in determining pitch stability, or there might be something else TT drive-wise that contributes the 8-12Hz instability. LD

Stylus-groove friction isn't steady, it varies with time in a 1/f flicker noise like profile about an average value IME. This variation causes variation in skate force in an offset arm, and in downforce due to VTA on all arms. This is a stimulus that can cause the headshell to move which is a source of FM or pitch variation. What happens next depends on the cart-arm resonant system, and how much damping there is around. LD

With music playing the drag on the needle changes permanently. So maybe we should look at the dynamic behaviour of the motor and the motor controller. A testrecord with a frequency that changes the amplitude abruptly could probably give a better prediction of the sonics...

The effects of speed stability cause a pitch fluctuation or Frequency Modulation (FM) of the audio signal in the groove. A bent motor shaft on an idler or belt drive turntable will have a similar effect on the sound as an eccentric capstan on a tape recorder or a bad drive component in a movie film projector. These are all mechanical reproducers and have rotating parts which can cause cyclic and/or transient speed variations. The topic has been extensively researched and documented in technical papers and journals. The British Broadcasting Company published an excellent research paper “The Subjective Discrimination of Pitch and Amplitude Fluctuations in Recording Systems” in 1955 in Britain, and reprinted in the US by the AES in 1957. The BBC research built on other research references dating back to at least 1941. The BBC researchers found that the sensitivity threshold for FM among groups of listeners was broadly centered in the range of about 4Hz to 10Hz. Many arm/cartridge combinations have a mass/compliance resonance frequency within the 4Hz to 10Hz range. Bruel & Kjaer published a paper “Audible Effects of Mechanical Resonances in Turntables” in 1970. B&K mounted three different tonearms on the same turntable. Using the same turntable/motor and same cartridge moved in turn to each of the three arms, the weighted % wow and flutter B&K measured varied by a factor of 3 to 1, depending on the tonearm being used in the test setup. If you were the manufacturer of that turntable, which wow and flutter number would you use for the spec’s in your sales brochure?

As an exercise, I modeled my SME 3009 in a CAD program and ‘flexed’ the arm sideways to empirically measure what the stylus point does in the groove under dynamic conditions:



For a sideways displacement of about the thickness of a credit card, give or take, about 40% of the sideways displacement of the stylus goes into scrubbing motion along the groove, causing FM distortion. At mass/compliance resonance frequencies the scrubbing-induced FM shows up as measurable, and often audible, flutter. The amount and character of the scrubbing depends on the magnitude of the displacement, arm geometry, and stylus shaft length.



The usual measure of turntable speed stability, i.e. wow and flutter, includes the mechanical stability effects of the tonearm/cartridge, and is a speed stability measure of the record player, not just the turntable by itself. The two contributions need to be analyzed separately. A single numerical value for the wow and flutter that’s going into the preamp is meaningless. Dynamic stylus drag variation could cause a transient pitch variation (FM) in the turntable drive or does the dynamic stylus drag variation cause a sideways displacement of the cantilever which translates into transient pitch variation (FM) from the resultant scrubbing? Each of these sources requires a different remedy.

Thoughts?

Ray K

AES E-Library Subjective Discrimination of Pitch and Amplitude Fluctuations in Recording Systems

https://www.vinylengine.com/turntable_forum/viewtopic.php?f=46&t=58101
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Any other method other than test records ? Since offcenter and warps can not be avoided any modern method to measure speed stability ?
A Postscript printer can print precise dots (or markings) we can make perfectly center thick white sheet with such markings and optically measure the speed variations. Right ?
How do classic tables like Thorens, Garrard, Lencos and Technics compare with regards to speed stability. Any one has measured them and have data on them ?
Regards.

The advantage to your optical dots method is that it measures the pitch stability of the turntable, and is free from the mechanical stability effects of the tonearm/cartridge. By comparing the results of an optical method signal with the results from the arm/cartridge signal one could segregate the contribution of the turntable speed stability effects from the arm/cartridge resonance and mechanical stability effects. Put the printed dots pattern on the Feickert disc while running the test and both method signals could be measured simultaneously, as an added measure of controlling all the test variables. Now, if the Feickert discs were pressed with the dots printed right on the disc label, or stamped or laser etched on the outer perimeter lead-in area, hmmm…:rolleyes:

Ray K
 
That it all tape related though? Can't see any vinyl information.

He explains the effects of AM modulation and FM modulation generated by a mechanical reproducer. When you read through his website, focus on the discussion and portions of his graphs that cover the spectrum “Lower bandpass - .5 Hz to 250 Hz to isolate components due to rotating tape drive members”. His website is more comprehensive than the AES paper that it was built upon, plus the technical info on his website is free.
A record player is a mechanical reproducer, just as a tape deck is. The FM effects from an eccentric tape deck capstan at 10 Hz is the same thing as FM effects of stylus scrubbing with an arm/cartridge system excited at a 10Hz resonance frequency. Take advantage of the research that's already been done by others and build on it. Don’t spin your wheels re-researching something that’s already been done.

M A N Q U E N . N E T - A U D I O

Oh, and while you’re at it, check out his electronics bench and model train set. He has DIY skills too:

The Magic Portal

Ray K
 
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As the lowest tech possible :D, no math approach, I tried heterodyning a single frequency vinyl playback file with a computer generated test tone of the same frequency and same level.
I made a new file. L channel is the vinyl playback tone, R channel is the synthesized tone.
I synchronized the two tones at the beginning of the file.
Then I saved the file as a mono (L+R) file.
See the outcome for a 3150Hz track from the JVC QL-A2, for which Lucky kindly provided the polar plot in post #1257.
I measured the period T in sec of each pattern .
1/T is the frequency component of this pattern.
The more times the same pattern appears, the stronger it’s freq component is.
1.8sec frame length is one revolution of the platter.
This way I can locate all the modulating components, i.e. the Cartesian Freq-level inset from Lucky’s analysis.

George
 

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