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Re: [ARSCLIST] New online publication: Manual of analogue audio restoration techniques, by Peter Copeland
From: Patent Tactics, George Brock-Nannestad
Mike Biel wrote
> Hi George.
> We fought this out several times in the 78-L in past years, and someone
> might be able to come up with my more detailed answers.
----- thanks for the response that is as lucid as your replies are always!.
I only have a few comments.
1) AC, which is the prerequisite for stroboscope (and induction motor) use,
was by no means universal. And even when it was, the frequency was not
standardised. This obviously improved after Lamme wrote his paper "The
Technical Story of the Frequencies" in 1918 (American Institute of Electrical
Engineers, Washington D.C. Section, reproduced in "Electrical Engineering
Papers" by Benjamin G. Lamme, Westinghouse Electric & Mfg. Co. 1919, pp. 569-
589), but certainly Great Britain had DC, 25 Hz, 40, 50, 60, 83 (44,000
persons had this!!) and 85 Hz well into the 1930s. Central Copenhagen
(Denmark) had DC until 1962. I have seen a disc record stroboscope for use
with an a=435 Hz tuning fork as a light modulator, which avoided the issue
The universal instruction for Gramophone Company recording experts and
engineers was to place a thin white slip of paper (cigarette or onion skin)
under the wax and count (GC correspondence and interview with the Denmark-
based EMI recording engineer from 1936-45) for a minute, aiming at 78. This
was in those locations that used the Hayes portable. In Oslo (Norway) they
had a synchronous motor, so the speed was 77.92 by default, if the mains was
stable at 50 Hz, which it was not (hydroelectric with slow regulation).
2) whenever you have a pinion gear train, there are unavoidable flutters that
are smoothed out by the inertia of the turntable and the brake on the
centrifugal governor, and if you used an asynchronous motor (the induction
disc motor from General Electric 1921 is also of this kind) it would be the
same. In a synchronous motor the speed of rotation would be dependent only on
the mains frequency, and that cannot be braked down, and it is such an
advantage that you would not introduce an element that causes slip, such as a
friction wheel. Here the mathematics is precisely the same as for the
stroboscope - just as you describe it. A two-pole motor for 60 Hz mains would
have a speed of rotation of 3600 rpm, just like a lamp (in particular a neon
lamp) would have 120 flashes per second (7200 flashes per minute, which is
why you need 2 x 46 lines at 78.26 rpm). The paper is not to hand, but
Blattner and Elmer described in the Transactions of the Society of Motion
Picture Engineers in 1929 how a very complex mechanical filter comprising
slitting the 46-pinion gear wheel that cooperated with the worm into four
thinner wheels, each contributing to the turning of the turntable.
I would be very interested indeed in a precise reference to a 1920s use of
the stroboscopic disc for turntables.
I am glad you took the time to comment on the other of Isom's failings - it
is a necessary reference and comment, just like your previous one on Peter
............ a lot stricken - see Mike's original post on 17 September 2008 in view of
the earlier from 15 September.
> This is going beyond my original statement that strobe discs, not gear
> ratios, was the reason it is 78.26 and 77.92. But your question
> concerned Isom, and although much of what he wrote is good, this entire
> section on speed is bogus.
> Mike Biel mbiel@xxxxxxxxx