Vinyl Polarity Relationships in the Westrex 45/45 System

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Wayne Kirkwood
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Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 43122Unread post Wayne Kirkwood
Mon Jun 27, 2016 9:21 pm

Vinyl Polarity Relationships in the Westrex 45/45 System

This discussion ties together several loose ends regarding mechanical, electrical, absolute and crosstalk polarity in the Westrex 45/45 system. It is not a discussion on the audibility of absolute acoustic polarity.

The motivation for this investigation stems from the observation that a single channel stereo tone played by a STR-100 test disc produces mid-band crosstalk that is in the opposite polarity of the modulated channel.

This phenomena has only been observed on one STR-100 disc but with three different cartridges, an AT-95, AT-120 and S681. Based on my analysis, I'm of the opinion that polarity-inverted crosstalk is inherent in the Westrex system.

A stereo source resistively blended to measure 20 dB separation with a meter will not sound the same as a cart, or any source, when the cross-talk becomes polarity-inverted “anti-crosstalk.” The anti-crosstalk source will sound wider than the restively blended one because it has gain in the Side channel. Increased Side channel gain creates width.

Left and Right in the Westrex 45/45 system are at 45 degrees to the disc plane. Stereo cuts produce a combination of Lateral and Vertical mechanical modulation. Lateral and Vertical are also Mid and Side or Sum and Difference.

Some Mid/Side Facts

Left and Right can also be expressed as Mid and Side:

L=M+S
R=M-S

When:

L=0 R=0; M=0 S=0
L=1 R=0; M=1 S=1
L=0 R=1; M=1 S=-1 (Same as Left only but S is inverted.)
L=1 R=1; M=2 S=0

(It's often more convenient to express L=0.5M+0.5S and R=0.5M-0.5S so that L+R=1. In this context keeping L+R=2 keeps things simple.)

When only Left is modulated, Mid and Side are in polarity. When only Right is modulated Mid and Side have opposite polarity.

In terms of Vinyl, Mid is Lateral and Side is Vertical modulation.

In order to avoid confusion with Left being “L” and Lateral, also beginning with “L,” we'll call Vertical “V” and Lateral “H” (for Horizontal) to establish the following:

L=1 R=0; H=1 V=1
L=0 R=1; H=1 V=-1
L=1 R=1; H=2 V=0

When only Left is modulated, Lateral (H) and Vertical (V) are in polarity.

When only Right is modulated, Lateral and Vertical are out of polarity.

The Vertical component for Right-only modulation is mechanically-reversed from that of Left-only modulation.

It should be pointed out that Left and Right are at right angles to each other but rotated 45 degrees from the disc surface. As a result “1” unit of L or R modulation translates to 0.707 units of vertical or horizontal modulation.

Thus when L=1 and R=0 H=0.7 and V=0.7. When conditions are reversed, with only right modulated, the only difference is that V is -0.7 units.

L+R modulation of 1 unit each produces a Lateral displacement of 1.414 units.

The NAB64 specification dictates that mono causes lateral modulation and that the right channel groove wall is on the outside of the disc.

See: http://www.lathetrolls.com/viewtopic.php?f=1&t=5526

To permit lateral modulation for mono, one cutterhead voice coil has to push and the other pull. To accomplish this, one cutterhead's channel's voice coil polarity is reversed. (It can be the electrical or the magnetic circuit's polarity.)

See: http://www.vinylrecorder.com/stereo.html

It is believed that Neumann had an internal specification that the positive peaks of lateral modulation drive the cutterhead to the disc edge. (http://www.lathetrolls.com/viewtopic.php?f=1&t=5526)

Consider the speaker analogy that connecting a battery to the positive terminal will push the cone outward from the basket. Connecting a positive voltage source to the right cutterhead voice coil will push the chisel to the disc edge. A positive voltage on the left channel voice coil has to pull the chisel to the edge.

Refer to the figure. The right channel voice coil points down; the left voice coil points up.

Image
Cutterhead and Cartridge Orientation. Image Courtesy of Vinyl Recorder and Steve Hoffman

Left only positive modulation pulls the chisel laterally to the edge and vertically up; right channel only positive modulation pushes the chisel to the edge and forces it down.

Right only modulation produces a “-” vertical term in the matrix equations. It pushes the chisel down. Thus, the polarity of the Vertical channel is such that positive peaks produce upward vertical motion; negative vertical peaks are downward.

The playback cartridge also has it's coils oriented with opposing polarity. This corrects the polarity inversion in cutting, Lateral modulation, mono, creates equal in-polarity outputs at the cartridge terminals.

Vertical modulation produces opposing outputs at the + cartridge terminals.

Drop tests, where a stylus is dropped on a non-rotating disc and the response recorded, should produce a positive peak on the left's + output terminal.

See: http://forums.stevehoffman.tv/threads/finally-an-absolute-polarity-test-for-vinyl-playback.473528/

Refer again to the figure. The cartridge drawing also uses the “speaker” convention that movement from the negative coil terminal to the positive produces a positive output. It is drawn exactly like the cutterhead: The left coil points up, the right coil down.

Image
Cutterhead and Cartridge Orientation. Image Courtesy of Vinyl Recorder and Steve Hoffman

Dropping the stylus onto a blank portion of the disc produces upward vertical motion. The left coil is driven from “-” to “+” and produces a positive output pulse. The right coil, responding to upward motion, is driven from “+” to “-.” The right channel output will pulse negative.

Vertical motion (drops) will always produce opposing outputs because, by definition, Vertical is Difference.

We now have a definition of absolute vinyl polarity defined by “specification” and geometry.

Lateral positive peaks drive the cutterhead to the edge.

Vertical positive peaks drive the cutterhead upward.

A cartridge has the "correct" electrical polarity when the Left terminal's positive output pulses positive on a drop test.

A cart having a positive pulse on the left channel + output is one that agrees with the Neumann “standard” specifying positive lateral modulation being toward the disc edge.

Crosstalk Polarity

So you may be asking "Why does playback crosstalk have the opposite polarity of the driven channel?"

That's the question which caused this mental exercise.

Refer back to the figure. Consider left-only modulation value of “1” where the Left vector at 45 degrees is pulling Laterally to the disc edge by 0.7 units and Vertically upward by 0.7 units. These vectors should produce no output in the right channel.

The recording, disc production and playback system have mechanical errors. Some Vertical and Lateral movement get converted, by accident, into the opposite plane.

What if the values of Vertical and Lateral Modulation aren't exactly 0.707 for 1 unit of left-only modulation? When Lateral modulation gets converted into excess Vertical modulation things get interesting...

If the left channel link wire in the cutterhead is pulling upward, and some of it's movement, more than 0.707, can be converted to vertical. Lets say it moves by 0.77. That movement will be upward – it can't go in the opposite direction (down) due to physics. The same effect may occur in the playback cart or an intermediate production process.

The excess vertical upward movement – which is gain in the Side channel - get's converted to right channel information. Because the movement is upward, it's polarity is reversed relative to the left channel because the movement, relative to the coil, is from “+” to “-.”

Thus, left-only modulation, bleeding into the right, has its polarity inverted.

Cross-talk from right to left also has it polarity reversed because the excess vertical motion is downward and inverted with respect to Left.

The above is theory and is the best "proof" I can come up with based on observation and a lot of thinking.

I knew that high school geometry would come in handy some day.

In any case, comments are appreciated.

Wayne

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markrob
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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 43128Unread post markrob
Tue Jun 28, 2016 11:08 am

Hi Wayne,

Really nice job of getting to the answer.

You might be interested in these short excerpts from the Ortofon DSS731 manual. This is a rocker based cutting system, so it has different geometry issues as compared to the Westrex system. In this system, the triangle formed by the two driver pivot points and the tip of the cutting stylus must form a right triangle (see the first excerpt below). If the tip protrusion is adjusted such that this is not the case, crosstalk is created (see second excerpt). In this geometry the phase of the crosstalk can be either polarity depending on how the tip deviates from the correct length.

Fig 1
Ortofon DSS 731 Instruction Manual Fig 1.pdf
Appendix 1
Ortofon DSS 731 Instruction Manual Apendix 1.pdf
Mark
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Wayne Kirkwood
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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 43130Unread post Wayne Kirkwood
Tue Jun 28, 2016 12:40 pm

Thank you very much for that.

So it's not completely universal because we have the Ortofon cutterhead whose recording crosstalk can be in either polarity depending on mechanical adjustment. I had wondered if there were pivoting systems...

My reasoning that Left vertical movement in the 45 degree plane would cause added vertical movement in the Right 135 degree plane is that crosstalk, whether mechanical or electrical, is usually additive. Because the mechanical crosstalk is inside the double-inverted electrical circuits, the mechanical crosstalk sees only one inversion - the one in playback. The rocker system turns that assumption on its head.

I've had several inquiries about using my Width Controller to provide slight mono blending to "improve" vinyl playback. No one ever came out and told me "why" other than to say a slight mono blend tightened up imaging.

When I started doing level calibration on the PTS and was looking at single-channel tones on the scope, seeing that they were out-of-polarity the light switched on and I knew why people were asking.

I could make a 20 dB separation cart measure 30 dB. I didn't listen long enough to determine which was "better." My gut tells me slightly wider (unblended) is "better." Warp is the opposite. On 'phones a healthy amount of playback LF EE is far better particularly if there's a lot of warp. There's a huge queasiness without it.

Needle drop was almost universally "up" in the left channel. (Though of course there's no guarantee at all someone's preamp doesn't invert.)

So I have to ask fellow trolls the question: Is the cross-talk component on playback more often in-polarity or out-of-polarity? How many of us actually look for that?

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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 43442Unread post Sillitoe
Mon Aug 01, 2016 10:37 pm

Thanks for this Wayne!
Im seeing this left vertical crosstalk in channel separation tests for my heads.
Great stuff!

1kHz at 7cm/sec panned
https://www.dropbox.com/s/b3gu3oopihig5kk/9th%20head%20cut.aif?dl=0

Cheers
James

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Wayne Kirkwood
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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 43464Unread post Wayne Kirkwood
Wed Aug 03, 2016 8:14 pm

Thanks for sending the samples James.
They sound really good.

I noticed during the tones that the L>R cross talk was in-polarity and the R>L out-of-polarity.
Trying to wrap my head around that.

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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 49097Unread post jesusfwrl
Wed Jan 24, 2018 3:39 pm

Drop tests are only meaningful if you will be observing the resulting waveform on a system of verified absolute polarity!
An A/D converter and DAW are not immune to incorrect polarity. If the waveform display system can be verified for polarity, then the drop test will indeed be useful, if a test record is not available.

Wayne has correctly defined absolute polarity according to the several standards, although he took the scenic route to the answer.

You can read a detailed report on this subject on my blog, which also points out a test record that can be used to quickly and easily check the polarity of your reproduction system:
https://agnewanalog.blogspot.com/2018/01/absolute-polarity-for-disk-records.html
~~~ Precision Mechanical Engineering, Analog Disk Mastering ~~~
Agnew Analog Reference Instruments: http://www.agnewanalog.com

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Wayne Kirkwood
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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 49098Unread post Wayne Kirkwood
Wed Jan 24, 2018 4:10 pm

I agree with everything you said in your blog.

This is priceless:
Part of the problem is that mastering engineers and cartridge manufacturers do not like talking to each other. I have personally tried communicating with several cartridge manufacturers, being a disk mastering engineer myself, regarding compatibility issues, and most of them refused to talk to me. The few who did, appeared to have no idea how records are made. Most did not seem interested in changing that fact. I am not only talking about cheap cartridges, by the way. I am also talking about companies claiming to be offering the ultimate cartridge at the price of a luxury car, who appear to be putting them together by chance...
That has been my experience.

I might add that positive electrical polarity needs to ultimately produce acoustic pressure so the system needs to be verified end-to-end.

I hear absolute polarity on some instruments.
Compare these two .wav files. Both identical except...

http://www.proaudiodesignforum.com/content/Renee_Olstead-Someone_To_Watch_Over_Me_Cut_1.wav

http://www.proaudiodesignforum.com/content/Renee_Olstead-Someone_To_Watch_Over_Me_Cut_2.wav

Does one sound a little brighter?

A good test waveform is male speech. A sustained "ahh" or "eee" have high acoustic asymmetry with positive pressure bias.
We have a built-in acoustic generator we can use for verification of the microphone/preamp/scope we use to check final acoustic polarity.

BTW I asked the question with regard to polarity inversion in converters over at GS and someone who had tested a great many of them had never observed it.
I was surprised by that finding.
That of course doesn't prevent inversions in numerous other places.

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Wayne Kirkwood
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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 49099Unread post Wayne Kirkwood
Wed Jan 24, 2018 4:14 pm

Forgot to ask. What has been your experience observing playback crosstalk polarity?

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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 49151Unread post jesusfwrl
Mon Jan 29, 2018 6:35 am

Wayne, many thanks.
I agree about the need for end-to-end system verification, I am planning on writing more soon on absolute polarity for magnetic tape, loudspeakers, etc.

I have not tested "a great many" converters for polarity, but in the ones I have, I did come across some that did not conserve polarity. In my view the best test is analog-to-analog: Start with an analog asymmetric waveform, feed it to A/D converter, connect this straight to a D/A converter and hook up a scope to see the analog signal at the output. If this works with correct polarity, then you can verify if adding a data storage interface between the ADC and DAC would change anything. I have also seen the DAW inverting, even when the ADC ad DAC directly did not. I do not have enough data on this to be able to talk about percentages, but I can confidently say none of it can be taken for granted. Verify it, to be sure.

As for disk reproduction crosstalk polarity, this is an interesting subject, but a very long discussion. I have of course seen inverted polarity crosstalk, and also correct polarity. It can even change with frequency.

The same can be observed on the Feedback Monitor output while cutting a sweep on one channel only. However, when subsequently reproducing that disk, the crosstalk and its polarity/phase with respect to frequency will be VERY different to what was observed on the Feedback Monitor output. There will also be differences between different reproduction systems.

To fully understand this phenomenon, one must follow the entire signal path and examine all the possible places where crosstalk can occur, and what the phase characteristics would be at every stage, from cutting to reproduction. By signal path, I also mean the mechanical stages.

Part of the crosstalk occurs while cutting (I would say the smaller part) and is recorded on the disk. Part of it occurs during reproduction (the larger part). Part of it is due to electrical shortcomings and part of it due to mechanical shortcomings.

But, even if we could make cutting and reproducing equipment with no shortcomings (theoretically ideal), a significant amount of crosstalk would still be observed as a result of the geometric "incompatibility" between cutting and reproducing.

For instance, I have measured the channel separation on my amplifier rack, modified Caruso preamps, Magnetovolt monoblock power amps, Caruso nr. 135 head, though the Feedback Monitor output, while cutting a 1 kHz tone on one channel, as being 43.5 dB. This is a figure that will never be possible to measure upon reproduction from the disk. However, it does match (to the extent permissible by the accuracy of the method) the figure obtained by optical means. The SME tonearm with an AT MM cartridge, however, only measured 26 dB! Fine tuning the geometry and using a super expensive cartridge with a super expensive tonearm, carefully wired to a super expensive preamp can get you to around 32 dB. This is about as good as it can get, from cutting and subsequent reproduction of an actual disk.
~~~ Precision Mechanical Engineering, Analog Disk Mastering ~~~
Agnew Analog Reference Instruments: http://www.agnewanalog.com

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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 49161Unread post Gridlock
Mon Jan 29, 2018 3:54 pm

You guys are the best. Who even talks about this stuff? Maybe you should all move to Eugene and start a militant record cutting compound with me, prestofan and epicenterbryan. That's really the only viable solution here. We already have a field extension office located in Tucson. That's only like a ten hour drive In the Lamborghini
<\\\\\\\\\\\\\\\\\\\\\[[[[[[[\/]]]]]]]\\\\\\\\\\\\\\\\\>
Recordette Sr.......Presto K-8

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Re: Vinyl Polarity Relationships in the Westrex 45/45 System

Post: # 49166Unread post jesusfwrl
Tue Jan 30, 2018 5:40 am

Gridlock, we need a secret bunker buried deep inside a mountain! It must have at least one big red button, for emergency purposes. Can't say more in public. This message will self destruct...if it doesn't just hit the red button.
~~~ Precision Mechanical Engineering, Analog Disk Mastering ~~~
Agnew Analog Reference Instruments: http://www.agnewanalog.com

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