Late Era Lateral

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Late Era Lateral

Postby Fonotec » 31 Jan 2015, 18:30

In this late era for gramophones, it's nice to consider the behavior of lateral modulation with respect to frequency and signal intensity (which starts out in the reel of tape as magnetic remanence, is measured in nano-Webers per meter, and is transduced by the cutting head into a record of the signal's velocities, measured in cm/second). Since the original disc cutting done for Emile Berliner's gramophone records was lateral (rather than "Hill and Dale," as was the case with Edison's phonographs), we still use the lateral reference peak velocity for calibrating the intensity of signaling with Alan Blumlein's stereo cutting systems, even though those grooves include both the Berliner and Edison types of modulation at the same time, whenever the complex signals of stereo music require. This was standardized in the United States as 7.0 cm/sec, peak (lateral) velocity at 1 kHz (The velocity achieved depends on signal strength and frequency).

To achieve the reference intensity lateral groove during calibration with a stereo cutting head, each of the two drive coils is made to produce 5.0/cm/sec, peak, diagonal velocity from a 1 kHz sine wave (at nominal intensity (e.g., +4 dBm)) that is in phase (with itself, however, the drive coils are deliberately wired with one channel in opposite polarity to the other in order to force them to "take turns" for lateral vectors) and of the same intensity on both channels. When 5 cm/sec, peak, diagonal velocity is summed with another 5 cm/sec, peak, diagonal velocity, in orthogonal vectors (45 degreees + 45 degrees = 90 degrees), the result is 7 cm/sec, peak, lateral velocity. The irrational number, root2 (i.e., the square root of 2), is used to derive the lateral from the diagonals, as well as to derive the peak from the RMS... {In Europe, one often sees 8.0 cm/sec, peak, lateral velocity (which is achieved with stereo cutting heads when both channels are each producing 5.5 cm/sec, peak diagonal velocity...).}

Shown, below, are pictures of grooves cut into lacquer that were magnified approximately 150 times and projected through a television camera onto a monochrome t.v. set.

Image
200 Hz modulation, above, is shown being faded up from silence. At its peak (on the right) it is being cut from tones that are at +4 dBm, just like the 1 kHz stereo test tone that produces 7 cm/sec/peak, lateral velocity. The right half of the screen (which corresponds to an earlier location of the cut on the disc), shows segments of the same 200 Hz test tone-modulated groove as was used for the segments of the groove shown on the left, but with the level still very low, being pushed up, via the master fader of a Sonic Studio HD session. Four channels are ganged to the master fader and comprise the two stereo test tones in use - one, for the cutting amps, and, another, for the lathe automation, which arrives one full turntable revolution in advance of the channels for the cutting amps, in order for the lathe to have enough time to respond. It looks ahead but also remembers what it cut and applies that to the feedscrew motor acceleration/deceleration calculations.




Image
200 Hz modulation at reference intensity (i.e., the same intensity as would have caused 1 kHz to be cut at 7 cm/sec, peak, lateral velocity). The grooves are approximately 2 mils, across the top, leaving approximately 20% land, between groove edges.


Root2 relationships:

7 cm/sec, peak, lateral velocity = 5 cm/sec, peak, diagonal velocity + 5 cm/sec, peak, diagonal velocity, when the vectors are like the sides of an upside-down, equilateral triangle. (The lateral motion is along the base of such a triangle.)

7 cm/sec, peak, lateral velocity = 5 cm/sec, RMS, lateral velocity...

5 cm/sec, peak, diagonal velocity = 3.54 cm/sec, RMS diagonal velocity.

Common theme? 1.414... (i.e., root2)
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