Here's a new one for those of you in a technical frame of mind. I copied this from a thread over at Vinyl Engine.Pinch effect is a term describing the consequence of a real stylus with a radius tracing in a groove made effectively by a line cutter. In which contact point height on the stylus varies with groove angle because the stylus is an imperfect fit in the groove. And so the stylus is 'pinched' and moves vertically even on a purely lateral groove.
Why pinch effect is not frequency dependant Step 1 Peak pinch effect occurs at the instant of peak groove modulation angle (for some reason I can't get the picture to show-up here, so click below to see this issue visually)http://www.vinylengine.com/phpBB2/album_picm.php?pic_id=14272 Groove is cut by line edge cutter head, has same line length at C and D. Stylus size B, the maximum radius that fits at peak groove angle, is smaller than maximum size A, the maximum size that fits when groove angle is zero.
So pinch effect has a maximum at B, when groove angle is maximum, and is zero at A. The pinch effect at A, zero, is the same as a silent groove.
Step 2 Peak groove modulation angle is not frequency dependant Peak modulation angle at B is determined solely by the peak modulation velocity, and the linear velocity. Neither of which is frequency dependant. Modulation velocity depends on level (post RIAA), and linear velocity depends on radius from spindle.
Modulation angle is simply arctan(modulation velocity/linear velocity).
I built a spreadsheet that draws a groove and calculates peak angles, given frequency, level (pre RIAA), and radius from spindle. It's here :
www.luckydog.demon.co.uk/images/groovy.xls You can play about and see the shape and size of curves. Realistic tracking limit is an angle of 25-30 degrees, i reckon, based on observations of real trackability. Velocities that exceed 20 cm/s or accelerations that exceed 2000G are 'hot'. But 30 degrees seems a realsitic typical limit for groove angle.
Step 3 Pinch effect does not vary with frequency. It follows that pinch effect minima at A and maxima at B are not frequency dependant, just dependant on the groove angle at B. So the plot above is generic, it applies to any frequency. Then pinch effect does not vary with frequency !
As to Jim Lesurf's and other similar web descriptions, I believe diagrams are not quite representative, and in many cases impossible to track. It matters that realistic angles and dimensions are used. Just about all of the prior publications I've found deal with stylus fit at maximum groove curvature (the peak of amplitude) rather than maximum groove angle (zero of amplitude). As far as I have checked using the spreadsheet, realistic permitted levels of high frequencies yield curvatures that accomodate normal stylii. In other words pinch effect at A is zero.
Comments, corrections, questions etc ?
Interesting eh ?
