A Few Turntable Measurements using the RPM Android App

Dear @kevemaher  : As you I bougth my units at blind on specs and were ( two units ) coming from an audiophile that I respect, so second hand at very good price: 2 top of the line ( in those times. ) TTs with 3 outboard motors and one motyor controler that I had to modify to choose between one TT or the other and till today I have not a single issue with those AS and the " belts " I use are simple silk thread amnd yes I have to push-start. In reality a critical issue with my model TTs was that came with no kind of damping footers so what I did it is to put 3 AT-616 pneumatic Audio Technica footers and a top these footers goes a centered an inverted aluminum tip-toes that for me makes very good job.

Here you can read some real time measurements in one of AS today model:

https://www.acoustic-signature.com/files/downloads/tests/2021-08-stereoplay-double-x-neo-ta-500-neo-mcx1-en.pdf

R.

@rauliruegas Hi, Thanks for the link. Surprisingly, the wow and flutter reported for the Double X by the author of the article you referenced is the same as the wow value I reported above for the AS WOW XXL using the RPM app. This result is encouraging, but not enough for me to trust completely, because the method of analyzing the data may be different.

There is no denying that the Double X measured wow value is 5x higher than the 0.02% I measured for the DP-57L table using the RPM app. Even more damning, the Double X it is worse than the wow reported for the Micro Seiki DD-40, an "entry level"  turntable.

The rumble spectrum is also quite informative. Although I can't make a 1:1 comparison with my (unpublished) results because gain and reference voltages may differ, I can say that the rise in noise from the 500 Hz baseline to the peak at 10Hz is about 60 dB for the Double X table in the article. I'm using the blue curve. This may be an incorrect trace, but there is no legend to indicate what the conditions were for each curve. My results indicate only a 30 dB rise for the DD-40. This is a 30 dB (30X) difference!!! Perhaps the region around the Double X was particularly high in vibration on the day of measurement. Or the line filtering in the Double X power supply is poor design or faulty.

One can also see the on the same chart a 50ish Hz peak.frequency. It rises 60dB above the baseline. The source is not identified, but is probably the AC line frequency. My measurements used a rumble track on the Hi-Fi News test record. The AC peak for the DD-40 is not visible, buried in the noise, which is already lower than the Double X.

It is very obvious now why Acoustic Signature does not publish these specifications. It could be very difficult to explain why a 70s "entry level" table beats their Double X for wow, flutter and rumble.

With all this now under our belt, can one say which table sounds better?

No.

But if one had to pick a table based on these results, the choice is clear.

I am also boggled to read that the reviewer summarized that the Double X performance as "high end".

How low do wow, flutter, and rumble have to be before they become inaudible in a "typical" listening situation? If the value is higher than that of every measurement result shown here, then the better performance of the DD-40 and DP-57L is not really needed. The Double X performance will be indistinguishable from those tables. But I think the answer is that since the manufacturer cannot control the conditions that a buyer will use its product, the manufacturer must strive to equal or beat the performance of the other "high end" tables.

If none of those publish these specs either, then my entire discussion is meaningless, since there's no way to determine from measurements which table is "better". Listening is always a very good method, but is usually highly compromised in any showroom environment. Perhaps listing is the only spec to test for. But how many people purchase a table without extensive listening to it prior to that purchase? I'd wager most do.

@kevemaher  

With all due respect, keve, I think that you are putting the cart before the horse. Before one aggregates observations, one must make observations. The difficulty with making speed observations is that for practical purposes they must be averages: distance travelled divided by elapsed time. This is not an instantaneous quantity.

Even aggregating a finite number of observations implies discrete, not continuous. That means sampling frequency.

This is not a moot point because modern controllers tend to correct speed at about 10KHz. If it were easy, they would correct more frequently. I doubt that an app is better; in any case, it's important to quantify the limitations of the observations; in this case the granularity  imposed by sampling frequency.

Then one could aggregate the data into moments, from which the central moments of mean, standard deviation, skewness, kurtosis, etc. could be calculated, although perhaps a spectral or Fourier analysis would be more revealing.

There is no problem with the data arising from a process which is not perfectly random - it is data. The point at which randomness enters the picture is in identifying which inferences are valid, and which are not.

Yes, I think that speed stability is a problem. The question then is, "Which frequency?" I strongly suspect that most of the audible differences between turntables arise from small speed variations in the lower kilohertz range, which we hear as  brilliance or sibilance. It is obvious to suspect speed instability of causing this - but at greater frequencies than are usually measured. For example, bearing noise affecting speed.

By way of evidence, a common criticism of Nottingham turntables (high mass, low torque) is that they are 'dark', that is, without brilliance. I prefer the term 'smooth'. Also, that is how the bearing noise from my 1.8W motor manifests itself. Yes, it's repeatable.

As you point out, "Measuring performance parameters of instruments is crucial to product development." But unfortunately, associating numbers to phenomena is one of the most difficult human activities - doing it right, that is. It is therefore of cardinal importance to report the limitations of the measurement before reporting the results of the analysis. In my opinion.

@terry9 There is a well-defined method for measuring wow and flutter and rumble, which only differ in frequency and source, but not in nature. This method turns out to be quite complicated. Whether the RPM app developer used this method or not is not disclosed. This makes comparison between published results and the RPM app results dubious. I do think that comparisons between tables using the same app (and phone) are valid qualitatively. That's why I feel sanguine about the comparisons I've made and reported.

If a manufacturer clearly states the method used (there are standards), then it can be used as a valid comparison between this table an others measured using the same method. It happened that I did measure the same wow 2 sigma as the spec sheet for the DP-57L and the DD-40. This may be coincidence. If a manufacturer does not provide a spec based on a standard protocol, then no valid conclusion can be made, obviously.

I find it difficult to believe that a manufacturer would not measure their product according to the well-accepted protocol. Engineering must have used some quantifiable method to characterize their creation. Engineers need a goal to reach. They need a goal that tells them that they've reached the goal and can freeze the design, stop development and move on to the next problem.Sales and marketing can decline to publish the results for a variety of reasons. Whatever the reasons, I feel a bit queasy without reported specs. I made this mistake with the WOW XXL. I will never do it again.

The test only goes to 200Hz. Any software and hardware (computer with A/D if chosen to do digitally) will have plenty of bandwidth (sample rate) to provide accurate results. The actual test uses a different frequency (3kHz or 3.15kHz), This frequency is detected, not speed. This makes the measurement more accurate because integration (averaging) times can be very short.

Implementation of the standardized protocol with all due attention to the accuracy of the test gear can accurately reproduce kHz frequencies. But only the errors that show up below 200Hz will be used to develop the result. If your hypothesis is correct, there is no method known today to capture errors from frequencies higher than 200Hz.The spec would have to change.

I have been in Engineering my entire career developing products for medical research, defense and communication at the highest level. What I've stated above is a part of how the development process works. Specs are designed so that they are measurable. If it is found that a method does not capture all the errors known, then the spec method is changed. Development cannot proceed without a measurable goal. In fact, turntable engineers may have an in-house spec to reach, knowing that meeting the in-house spec will guarantee that the industry standard method will produce the desired performance.

I don't presume to be an expert in measurement methods for characterizing turntables. I'm learning. Meanwhile, I call on my work patterns to inform me how I would proceed if developing a table. Differences in my approach to turntable engineers approaches may differ in details, but are the same in nature.

I do agree that some things are difficult to quantify. Turntable performance is not one of them. Whether one prefers this or that table is an emotional decision, a valid concern.Wow, flutter, and rumble measurements are quantifiable and repeatable (and agreed on).

@kevemaher I agree that wow, flutter, and rumble measurements are quantifiable and repeatable and agreed upon. I think, however, that this is not the whole story and that a 50KHz sampling rate would give us the data needed for a more robust analysis.