"Brian Gaff" wrote in message
m...
I often used to wonder about the out of phase effects on some records,
because in theory, these make the groove deeper and shallower, I'd
imagine. As for grooves actually intersecting, this would surely have
shown up on the master, I'd think. I had some issues in the 70s with that
Pye plant pressing records with bits of paper clearly poking out in some
places. So much for quality control of recycled Vinyl.

Left and right channels out of phase will cause vertical groove modulation,
making the groove deeper and shallower. If you ever looked at stereo on an
oscilloscope in x-y mode, which pretty much displays the motion of the
stylus (if you're using a stylus ;-) you will see that the stylus moves in
all directions with normal stereo. The audio looks like a cotton ball on the
screen. In x-y mode, L=R (in phase) is normally a diagonal line going up to
the right; L=-R (out of phase) is normally a diagonal line going up to the
left.

Fortunately, long wavelengths and large excursions are mostly in the low
frequencies, which tend to have minimal out of phase characteristics (which
is why subwoofers are practical). But just in case, I do believe that record
cutters did blend left and right channels in the low frequencies, to reduce
the risk of the cutter leaving the surface entirely.

I have seen grooves intersect on records that I own, but the stylus rides a
long ways below the surface, so it is really only a problem if the *bottoms*
of the grooves overlap. Then the stylus would have a "choice" wouldn't it?
That would be bad...