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Old 14th September 2017, 08:33 AM   #441
luckythedog is offline luckythedog  United Kingdom
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I don't think there's any suggestion of loose headshells etc as a cause of pitch instability - perhaps confusion with 'headshell movement' meaning that the tonearm is free to pivot and so the headshell is always free to move?

In response to headshell displacement, eg due to a click or patch of different friction, there is pitch variation due to scrubbing. This, IME from polar plots, typically follows a natural oscillatory decay, with spectral content at the cart/arm resonant frequency or frequencies. It shows up on polar plots as a classic decay pattern, following a log dec path.

Its effect can be significant, and is due to an interaction of cart/arm/record which is fairly random. When other things are going on at the same time, it can be tough to separate visually, but sometimes it's obvious. Also can be difficult to separate from what variation there is cut into the test record, and that's why rigs like JP's which show it only to a small extent should be used in test record reference recordings, IMO. It will still be there, of course, to a small extent - but shouldn't be there from the lathe.

I'll try to dig out polar plots of a bad example of cart/arm stability or two....... [EDIT: check out post #89 et seq on this thread for a clear demo and discussion]

Tonearm/cart stability typically can be a significant or dominant contributor to pitch instability, moreso than platter speed variation in a healthy TT, IME. As discussed earlier in the thread.

LD

Last edited by luckythedog; 14th September 2017 at 08:55 AM.
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Old 14th September 2017, 11:00 AM   #442
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by davidsrsb View Post
The 12oclock of the polar plot is determined by the start of the capture file, nothing to do with physical
Pretty easy to mix in a once per rev signal off the table. Maybe a small reflective opto or tiny neo-magnet and hall sensor? I would just either mix it in or record it on one channel.

Also last night I realized that just to get to a certain point quickly I used the code I wrote for my article. The Scipy package probably has a far richer set of options for filtering built in.

EDIT - Yes Scipy has the back and forth IIR filtering built in, which is linear phase. Also the internal routines will be compiled and get closer to real time processing. On my machine two revs takes 3 or 4 seconds as is but that includes drawing the plot.
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Old 14th September 2017, 11:09 AM   #443
billshurv is offline billshurv  United Kingdom
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I was thinking last night that we generally are using mono test records here so we could use the spare channel?
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Old 14th September 2017, 11:18 AM   #444
scott wurcer is offline scott wurcer  United States
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A small piece of tape sticking out a little on the bottom of the platter would make a fine interrupter for an opto switch.
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Old 14th September 2017, 11:23 AM   #445
luckythedog is offline luckythedog  United Kingdom
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Quote:
Originally Posted by scott wurcer View Post
Pretty easy to mix in a once per rev signal off the table. Maybe a small reflective opto or tiny neo-magnet and hall sensor? I would just either mix it in or record it on one channel..
Good idea.... however we are victims of success in that amplitude mixing won't show up in the FM domain, ie the polar plots, because the detectors are too good at AM and baseband noise rejection..... (!?!)

So one might need to manually mute a section of suitable duration where the marker should be.......? That should do it.

I should say, I haven't found much need to know absolute rotational position, but for sure there will be times.


LD

Last edited by luckythedog; 14th September 2017 at 11:25 AM.
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Old 14th September 2017, 11:25 AM   #446
Pano is offline Pano  United States
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OK, what do you want to do with the opto-coupler signal? AC couple it along with some attenuation, then go straight into channel 2 of the sound card?
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Old 14th September 2017, 11:30 AM   #447
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by luckythedog View Post
Good idea.... however we are victims of success in that amplitude mixing won't show up in the FM domain, ie the polar plots, because the detectors are too good at AM and baseband noise rejection..... (!?!)

So one might need to manually mute a section of suitable duration where the marker should be.......? That should do it.


LD
As Pano said, I was thinking use the second channel as a guide for grabbing your frames. Should be easy to get a few msec 1V blip every rev. Thinking of doing this real time is getting ahead of ourselves for now. Or adding the marker to the plot is trivial since I can read both channels, or in fact use the marker to rotate the plot to 0.
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Last edited by scott wurcer; 14th September 2017 at 11:33 AM.
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Old 14th September 2017, 11:40 AM   #448
Pano is offline Pano  United States
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Using the marker to rotate to 0 seems a good idea. Then if the test LP is rotated for another pass, we'd be able to see what's what.
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Old 14th September 2017, 11:42 AM   #449
Hiten is offline Hiten  India
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Would a time scale representation on plots help with the marker thing you all are discussing.
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Old 14th September 2017, 11:43 AM   #450
Hiten is offline Hiten  India
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Quote:
Originally Posted by luckythedog View Post
I don't think there's any suggestion of loose headshells etc as a cause of pitch instability - perhaps confusion with 'headshell movement' meaning that the tonearm is free to pivot and so the headshell is always free to move?
Yes probably confusion from my side. I will re read those post again to understand.
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