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.
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.
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
