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In the Forum: Analog Playback
In the Thread: Turntable speed analysis.
Post Subject: More on the plots and the whole philosophyPosted by N-set on: 6/11/2012
 Romy the Cat wrote:
I have no idea why you are surprised that I submitted my test. Anyhow, I do not know what to look at and how to interpret what I see when I analyze the result of this test. The feedback that Paul R forwarded to me did interpret the result differently then you N-set do but I do not know whose interpretation is more reasonable I have no opinion on the subject.

First of all I'm surprised by the results of the test not by you supplying the sample, of course! Please read carefully.
The spectrum diagram represents the low frequency components of the the rotational speed variation.
There is a certain analogy between THD number and Wow&Flutter number: bare THD tells you very
little, if anything at all, about the amp, one has to dig into that number deeper and see a detailed spectrum behind it.
Those tools of Paul do the same with W&F--this is a detailed analysis of different components
of the speed instability. The circular plot is one smart way of representing the speed spectrogram:
the distance of each point on the plot represents the instantenous speed as a function of the
angle in the course of the rotation of the platter (I think Paul puts circa 5 full rotations on one plot).
In the ideal world the figure should be a circle: a constant speed during the whole turn of the platter.

I'm not sure if Paul R gave you more output than what he wrote on the PF, but I think neither of us really gave
a solid interpretation as it is difficult at this moment: too little experience with the tool. One clearly sees a regular waviness,
there is something happening approx 16 times per revolution. This may be induced by the motor and/or amplified by the
motor-belt interaction (is your belt stiff or soft?).
 Do you know at what speed does the motor go? This waviness is also superimposed on an oval rather than
a circle, so this suggests some eccentricity (you can see it at the very low end of the spectrogram, there are some
peaks there; 0.55Hz peak corresponds to the LP eccentricity).

 Romy the Cat wrote:
This test is very interesting but it still has from my view 4 main questions:

1)    How what we measure by this test impact sound. The irregularity lover then 0.1% are not auditable and with all TT that they measure we are in .002% realm.

Oh, this is a huge question and a big work to be done: correlate measurements to the sonic results. As I see it at this moment the tool is still in its teething fase, identifying merely some "hardware" mechanics in the plots. This is needed to learn the tool, but I hope with time the hard work of correlating with the sound will start (again an analogy to the advent of spectrum analyzers in amp analysis is in order IMHO). As you imagine a time (and a good will of the community!) is needed to establish enough statistics. The statements "irregularities below 0.X% are not auditable" I'd treat rather carefully, just like parallel statements made thoughout the years that THD below X% is not auditable...

 Romy the Cat wrote:
2)    If one TT measure 0.5% with horrible result and another measure 0.0005% with perfect result and the first TT (being absolutely the same topology presumably) sounds better then what does it mean?

There was an example in the thread of Voyd 3 motor machine showing a rather far from ideal plot (BTW it seems to support your view on multimotor machines: it seems impossible in the real world to synch the motors and keep them synced; look how at different revolutions of the platter
the Voyd speed instability gets shifted in the phase), but still the owner preferred it to the other deck with seemingly better plot. I think
no sane body believes that those plots will tell you everything about the TT and more importantly about it's sonic. Until one starts to correlate
plots with sonics it's difficult to judge, the only thing we have are hypotheses (i.e. the closer to the circle the better the sound), common sense and other intellectual crap.
 Romy the Cat wrote:
3)    How to plug the subjective or sonic result into the analyses and correlate it with measurable data?
4)    Doing the analyses we compare apples and oranges as 2 TT of different topology have very little in common.  I think it would makes senses to do the analyses of this type on the same TT and to see how different action on this TT would impact the analyses.

As per 3) this is the question of methodology. One has to develop it. By trial and error.
I do not 100% agree that comparing different topologies is meaningles. Quite to the opposite: instead of intellectual fantasy arguments
(like invoking "laws of physics" and other crap), one can measure different topologies and see how they differ between each other
in reaching what seems at the moment to be the objective: a constant rotation of the platter. Same with e.g. different amp topologies:
why wouldn't it make sense to spectrum analyze a PP amp and a SET and compare?
Obviously, what you propose in the second part has already been going on: perform a different action on the TT and then see the results,
both measured and sonical. People try different PSU, oils, dampings etc etc.


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