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18 December 2020
Any Point in Wow & Flutter? Part II
We have ended our previous post with a question: can the momentary (as opposed to average) speed performance of a turntable be reliably estimated by playing back the test tone?

Today, we will first see what happens when we analyze the test tone playback using the high resolution mode of our HP 3561A signal analyzer. And then we'll have a closer look at two different test tones: would they highlight the same speed variations?

What Can the HP Tell Us?
We have exported data out of the HP analyzer to be able to visually compare different readings. It never ceases to amaze me how easy it is to connect a 35-year-old piece of equipment to today's computers.

In particular, I would like to use HP's resolution to answer two questions:

1
Do the harmonics we see on the plot come from the drive irregularities, or are they introduced in the playback chain?
2
Would the extra resolution enable us to meaningfully differentiate between turntable drives, or does the LP excentricity dominate the picture?
To answer the first one, let's compare the plots made on the same turntable but with different pickup cartridges. If the harmonics stay more or less the same, then their source is the drive.
Above is the plot of Micro Seiki RX-1500G playing back a 3150 Hz track on a Dr Feickert test LP. The green trace was obtained by using a Sumiko Pearl cartridge, and the green one with an Audio Technica AT11E.

And while there are quite prominent sidebands on the green trace, the red one is quite different. As I long suspected, these are the result of the playback (or recording) chain imperfections.

This doesn't mean that all such sidebands originate in the pickup or downstream of it. But in my opinion, it's impossible to reliably separate the influence of the drive from that of the playback chain.
Now for the second question. The red trace on the plot below is a Micro Seiki RX-1500G, and the blue is a Rega P3 (model year 2000). Can we draw any conclusions?
Well, obviously Rega runs too fast, like all the Regas of that vintage. The "top of the mountain" is quite a bit wider, pointing to greater speed deviation. But, looking at the "sides" of the spectrum peak that are supposed to tell us the secret inside story, there's no change. Nothing interesting to explore.

So, although HP's superior resolution allows us to see some difference between the turntable drives, the insight we get is still quite limited. Maybe the test record itself is the limitation?

Back to FM

Rummaging through our stack of test LPs, I found a never opened "Hi-Fi Sound" one that has a 3000 Hz tone on it. We'll play it back on the same Micro Seiki RX-1500G, with the same tonearm, headshell and cartridge as the already mentioned Dr Feickert's... and then what? How do we know what irregularities are there on the test LPs, and which are the product of the turntable's drive and the reproduction chain?
There are many ways, but to me the simplest is the original wow & flutter meter approach: let's FM demodulate the carrier signal and see what is left. I have used Virtins Multi Instrument again because it has FM demodulation built right into its oscilloscope feature.
This is the result. The low frequency area where the traces overlay are the turntable drive's performance plus LP excentricity. The areas where they don't are individual test LP artifacts.

You can see that the Dr Feickert LP has sidebands around 3150 Hz cut right into it. While the Hi-Fi Sound one has just one sideband below its 3000 Hz main frequency.

I have no idea what happens around 600-700 Hz. Some kind of a cutter resonance? Tonearm scrubbing? This is a good question for the future research. I'm tempted to run both through an active linear tracker tonearm and see what comes out.

What Test LPs Can and Cannot Do
A different approach is needed to measure the turntable drive's performance
If you have a high resolution spectrum analyzer like our HP 3561A, a wow & flutter test LP isn't entirely useless. It highlighted quite clearly (some) difference in performance between a budget Rega and a top of the line Micro Seiki.

Is this all the performance difference there is? Of course not. But to me, it feels like a different approach should be used to measure the turntable drive's performance. One that reads the rotation sensors directly and does not involve the cutting and playback of a signal.

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