Re speed stability.
As has been noted, the numbers produced by the platter speed app vary from test
to test. This depends upon how the tester placed the record on the spindle,
there is a small amount of clearance, so the record can appear to have
different amounts of eccentricity each time it is placed on the platter. Also
it depends upon when the test is started relative to the point at which the arm
is swinging in and out due to the eccentricity. There will be a different
reading if the test started when the arm was at mid swing compared to if it was
at the inner or outer extreme. You can see the impact of this on the green
trace at the start of the plots. Some show an uptick or downtick at the zero
second mark due to the arm being at or near full excursion, (AF0, XD1, AC-2,
1000R, K3), where one is neutral with the arm’s swing more or less at its mid
point (TT-101).This biases the low pass numbers in favour of the TT-101. The
length of time the test runs also impacts on the final outcome. Then there is
the low pass filter that is applied to the raw data, this designed to remove
the impact of the records eccentricity. It is somewhat a blunt instrument in
that it also removes key data about the platters micro speed stability within
each revolution.
It is possible to do multiple tests and pick the best or worst numbers to
highlight whatever point you are trying to make. A reasonable conclusion
is that beyond a certain point, the numbers produced are of interest but are
not particularly robust data.
But what is pretty consistent and robust from test to test is the yellow trace.
(amplitude aside as mentioned above due to record/spindle positioning) The
platter speed app is interesting in that it first plots the CARTRIDGE OUTPUT as
a function of frequency with respect to time.
In a perfect world where the signal generator sent a constant 3150hz to a
perfect cutting lathe, we stamped a perfectly concentric record and played it
back on a perfectly speed stable TT, we would see the yellow trace as a
straight line at 3150hz
But of course, this is impossible and one of the major deviations from
perfection is that the record spindle hole is not in the centre. So if all
other parameters were still perfect and we considered a non concentric record,
we would see the cartridge output a clean sine wave, symmetrically
centred about 3150hz with a constant amplitude per cycle. IOW, a
TT producing a yellow trace that is a badly distorted sine wave is NOT speed
stable, although its speed could average 33.333. I hope that this is self evident.
The program then puts this raw trace thru a low pass filter to remove the effect
of eccentricity, we now have the green trace. But what the low pass filter does
is filter distortion as well. This distortion is a graphical
representation of rapid speed changes. It is no longer visible and what we are
left with is a smoothed average which is used to compute the low pass filtered
numbers.
A far more accurate and useful metric of the speed accuracy of a TT is to
simply look at the raw yellow trace, ignoring the numbers. How close is it to
the ideal symmetrical, clean, constant amplitude, sine wave? Some of the
TTs mentioned in this thread, plot significant deviation from this ideal with quite
rapid and frequent speed changes.
Remember that the yellow trace is the cartridge output plotted as frequency
with respect to time, any distortion of the sine wave is a change in frequency,
thus a change in speed. These speed changes would be superimposed on any
music that was being played. With this in mind, I invite you to look again at
all of the speed plots
.
On top of this conundrum, the platter speed app does not load the system
dynamically since the 3150hz tone is of constant amplitude. This is a
whole different set of equally important measurements. We do not listen
to constant amplitude, single frequency tones.
BTW two of the TT plots posted have a low frequency oscillation with a period
of approximately 9 seconds. A low frequency oscillation with a multi second
period like this is not uncommon and is very difficult to eliminate.
Cheers,
