In article ,
David Looser wrote:

I've heard it a number of times over the years on first play of a vinyl
record, on both 33 and 45 rpm discs, and the pre-echo has always been
*exactly* one revolution ahead of the peak, both on the lead-in and
between tracks or in quiet passages. I'd put it down as an artifact of
either the material of the master deforming slightly while being cut, or
distortion of the plastic while the hot record was cooling after coming
out of the mould. Unless, of course, the feed reel on the tape deck being
used just happened to rotate at the same speed as the turntable.

The latter is much more likely. Can't see how the vinyl could deform that
much and still retain an accurate image from the mold.


I've met that as well, pre-echo exactly one revolution of the disc ahead. It
would seem to be a remarkable co-incidence if all tape spools revolve at
exactly 33 1/3rd rpm! I'm not sure that the fact that you don't understand
the process is important.

I've read that this sort of LP pre-echo can occur as a result of the
original lacquer-cutting process during the making of the master. The
cutting stylus cannot do a perfect, side-effect-free job of cutting
the groove - that is, it cannot magically remove the unwanted portion
of the lacquer while having no effect at all on other portions of the
lacquer. There's some amount of "pushing aside" action, which
deforms the un-cut part of the lacquer - pushing some of it "inwards"
(towards the as-yet-uncut part of the master disc) and pushing some of
it back "outwards" (towards the groove that was cut one revolution ago).

If the pitch between the grooves is too narrow, this pushing of the
lacquer will have the effect of deforming the inner wall of the
previous groove... in effect, "imprinting" the modulation from this
groove upon the previous groove.

This unwanted adjacent-groove modulation effect becomes a permanent
part of the lacquer, is carried over into the metallization process
and the the creation of the mold/stampers, and is molded into every
piece of vinyl created from those stampers.

--
Dave Platt AE6EO
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