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In the Forum: Analog Playback
In the Thread: Turntable speed analysis.
Post Subject: More thoughtsPosted by N-set on: 6/11/2012
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 Romy the Cat wrote:
 N-set wrote:
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.
Actually there is not. When I was taking “irregularity lower than 0.1%” I did not mean THD but the rotational irregularity, or Wow&Flutter. I am not familiar with TT Wow&Flutter but in my past I dealt a LOT with reel tape and I know that if we are beyond 0.1% then we are out of dander.


Of course we are talking about 0.1% or whatever of W&F! Sorry if I have confused you, but I use THD only as an example: a bare numbers tells you little, you have to see the spectrum behind that number. Getting 0.1% is easy, virtually every reasonable deck gets it,just like getting 0.1% THD is easy. Yet all those <0.1% decks sound different...I'd be careful...

 Romy the Cat wrote:

 N-set wrote:
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 themotor-belt interaction (is your belt stiff or soft?).
This is the whole point – even if we do see the problem we still have no knowledge where this problem came from and is responsible for it. Did the record was cut properly. Did they cut the whole on exact middle? Did the platter during the record cutting had any speed regularity?


Some of those problems can be +/- identified: eccentricity, wraps, etc show up as 0.555Hz fundamental and  it's harmonics.
As for the cutting quality of the test LP that's why it would be very educative to record your deck with a known LP, like the Ultimate Analog one.
It has been used in several test so far and seems +/- ok cut, although the purity of 3.15kHz tone is disputable (see some eraly Paul's analysis of this record).

 Romy the Cat wrote:

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... I disagree. The THD are deterrent from W&F as W&F are pretty much liner distortion existing in nature. In THD the order of the harmonics is important and the reason that creates the distortion is important. In case of Wow&Flutter the reason is NOT important but only the fact.


I don't know what kind of linearity of W&F you mean? Again, I was building THD <-> W&F anaolgy to illustrate a big step being made in measurements: from a single number (THD or WF) to much more detailed information (harmonic spectrogram and WF spectrogram respectively). No more than that.
I would very strongly disagree that with WF the reason is not important!!! What do you mean? I definitely want to know such reasons ! For example in my case the plot is slightly pentagonal with a bigger dent and a bit elliptic. This means that something relatively small is happening 5x per revolution and something bigger 1x. The only thing that in my case rotates 5x the speed of the platter is the idler. Looking at the spectrogram I see a strong peak at 2.7Hz. Bingo, one of the reasons for my speed variation is identified: the idler (either eccentric or uneven or both). Now I can think of measures to improve
the situation. You get my point?

 Romy the Cat wrote:
What I was trying to say was that comparing the same topology give an explanation WHY the analyses looks like it looks. If you have a TT with analyses done on a given record then you can run the same analyses again but juts for instance loose the belt tension or supplying more air pressure in platter suspension. This single change would give an idea of how the analyses be different in case of one single change was done. Doing it for loooooooong time we might understand the reasons and predict what was responsible for this or that bad analyses.


I can't do any better than to agree with the above. But apart from such optimization of a given topology/realization, IMHO comparing different topologies, with a good selection of samples withing each class (belt, idler, DD, ...) has a lot of sense as to give a sort of "topology fingerprint" (again using my THD-WF analogy, each PP and SET topologies have their specific harmonic patterns--their topology fingerprints; same may be here: e.g. belt and idler may have their own topology-specific spectra of WF).


Cheers,
N-set

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