Dear All,
Sorry to be even later-than-usual in responding to this question.
A previous correspondent correctly noted that the
centre-and-outwards motion of a pickup arm could be used to trigger an
auto-changer. But no-one seems to have posted information about *how* an
eccentric was achieved.
When discs were mastered on wax, a small degree of runout-groove was
cut as the song (or whatever) ended, and then the cutter was raised from
the wax. Here in the UK, this runout-groove may be seen on His Master's
Voice acoustically-recorded 78s in late 1924 and early 1925, followed by
a *con*centric to catch the reproducing needle before it ploughed-up the
label.
After about March 1925, the wax would be taken to a factory well
away from the actual studio, where it could then be loaded onto a
machine dedicated to cutting eccentric grooves. I have never seen such a
machine myself, but the one used by the British "Gramophone Company"
(for His Master's Voice and Zonophone records) evidently had a locating
pin for the wax which was deliberately off-centre. The cutting tool
would then be aligned - probably with a micrometer-type mechanism - so
that the cutting blade was exactly at the position of the run-out
groove. I imagine there would be a mark at the outside edge of the wax,
to indicate precisely the direction in which the axis of the eccentric
should be located, to reduce the risk that part of the eccentric might
overwrite the sound. Then the eccentric would be added to the wax. I
imagine precisely the same system was used at Victor, since the few such
records I've seen from about 1925 to about 1932 have precisely similar
eccentrics - "double eccentrics", to be pedantic.
From the mid-1930s, a number of American record-makers adapted the
idea of a special turntable for the eccentric, to cut "eccentric"
*run-out grooves* as well as the eccentric itself. I imagine this would
mostly have been done on cellulose nitrate lacquer, rather than wax; but
this meant that any mechanism to detect a pickup moving outwards could
respond even faster.
After the war, at least one disc mastering machine had the ability
to cut the main groove, plus run-in and run-out grooves, plus an
eccentric, and do it all in one operation. I think it was Lyrec, but I'm
not sure if other machines were capable of the same trick.
From about 1950, juke boxes began playing 45rpm pop "singles",
triggered by the pickup reaching an exact radius. The label was
specified as having a diameter of three and a half inches for seven-inch
45s, with the concentric being just outside the label. So having
eccentrics close to the end of the sound (to save time in the middle of
a symphony pressed with "auto-couplings", essentially at arbitrary
radii), was no longer necessary.
Peter Copeland
Former Technical Manager,
British Library Sound Archive
----- Original Message -----
From: Jody
To: [log in to unmask]
Sent: Thursday, May 19, 2005 2:10 PM
Subject: [88sOn78s] Exit Grooves on Shellac and Early Vinyl
Hi Folks:
I've never been able to find a clear answer on this, though I suspect
it has something to with hardware co-operation on old record changers
(ie. velocity trip).
My question is: Why do electrically recorded shellac discs, and early
vinyl LPs have an exit groove that zig-zags back and forth, when it
spins? Why did they go to a stationery exit groove in the early 60s?
Can anyone accurately/technically answer this.
Much appreciated,
Jody Thornton
(Hamilton, Ontario)
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