A Few Turntable Measurements using the RPM Android App

@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.

Thanks for the discussion, @kevemaher .

@kevemaher 

I was never an AS fan. I do not care for units that go out of their way to look flashy without obvious benefit. Having said that there are four sources of noise when playing vinyl, the turntable, the record, the environment and lastly electronic. I assume you used the same record for all measurements. The environment is basically the same for each table and to my knowledge none of them have isolating suspensions. Electric noise would also be the same for each table. The conclusion you came to is correct. The AS has a Rumble issue. Rumble is a problem if you can hear it or if it affects the performance of speakers particularly subwoofers. I also think it is a significant marker of overall quality. It is also possible this turntable was damaged.

I had an 80s era MS turntable which was wonderful. Its only failure was lack of a suspension. The apartment building I lived in had the worst environmental rumble problem I have ever lived in. The AC compressors were clearly audible in spite of concrete floors, all 19 stories. Because of this I returned to the Linn LP12 which was much quieter as long as you tip - toed around it.

Measuring is always the best way to confirm anything. You also discover that “ears” have limited sensitivity when dealing with certain things as it sounds you only identified the problem by measuring.  Measuring is also a great way to train your ears. If you really want to have fun get yourself a Dayton Omnimic V2. They are about $300 and worth every cent. You need a PC. The program is free but you need to have a serial # to access it. In the process you download the calibration file for your microphone. You will find it essential for possitioning loudspeakers, adjusting room treatments and writing target curves if you ever get into digital signal processing.