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Re: [ARSCLIST] New online publication: Manual of analogue audio restoration techniques, by Peter Copeland
George Brock-Nannestad wrote:
From: Patent Tactics, George Brock-Nannestad
Hi all,
Mike Biel wrote
...... I am also glad on pages 88 and 89 he explains the strobe disc as
being the basis of 78.26 and 77.92 (leaving out the asinine contention
from Warren Rex Isom that gear-drive teeth ratios were the original
causal determination.)
[a lot of other comments left out here]
Mike, in what way were Warren Rex Isom's ideas asinine - I always had a good
impression of his understanding of the technical matters?
Best wishes,
George
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. The question
came up as to why 78.26 instead of 78.00. The answer is the strobe
disc, of course, which started being used in the mid-20s. The question
should really be why *78.anything* instead of 80, 76, 75, or even 77 or
79. But somewhere in the pre-strobedisc era 78.00 had been chosen, at
least for public consumption. Now that the strobe disc technology was
available, it had to be 78.26 in the 60 cycle world and 77.92 over in
your land of 50 cycles. Fine. But then in the gala anniversary issue
of the AES Journal, Isom claimed as the reason 78.26 was SELECTED was
because that is what results when you reduce a 3600 RPM electric motor
by a 46 to 1 gear ratio for worm gears. Of course you could get 80.00
with a 45 to 1 gear ratio, but he used an excuse that Maxfield and
Harrison was interested in getting Victor to adopt their electrical
recording system so they chose 78.26. But they were also courting
Columbia which used 80. (And he completely overlooks that Victor really
was recording at 76 and Columbia at 78.)
There are many problems with his theory that the speed was selected
because of a gear drive ratio. The first one is that WE did not supply
the turntables to the record companies, only the recording head. They
were affixed to the turntables used by the recording companies which
were mainly weight driven. Another problem with the gear theory is
that ALL turntable mechanisms at that time, whether using spring,
weight, or electric motors, were governor controlled. ALL were designed
with motors that would drive the turntable faster than needed with the
speed reduced to the proper one by a governor. Even gear drive tables in
the 20s had a governor. When I originally researched this I had over a
dozen books spread out here with the details of almost every turntable
mechanism from the era and none fit his description. A short
description of the situation can be found in the same AES Journal, pages
750-51 by James H. Kogen. Most electric turntables at the time used
idler wheel drives which did not require an even number in their ratio
like gears do. It is possible that the RCA 70 series table would fit
Isom's worm-gear description, but it was not designed until the mid-30s,
long after 78.26 was established. Therefore it was designed to fit a
previously selected speed, not the reason the speed was selected.
OK, I hear you asking, what about the sync turntables for motion picture
sound. First of all, they were not 78.26. Secondly, his description of
how 33 1/3 was selected is bogus. He talks about minimum groove speed,
but that was not the total consideration. Reducing the difference
between inner and outer groove speed was. It was vital that there not
be much noticable difference when the discs were switched. They could
have used a faster revolution speed to get higher groove speed at
smaller diameters, but that would make the outer groove speed that much
faster and more noticably different. One of the engineers on the
project, Stanley Watkins, discusses that the speed was selected after
they were told what the largest size could be pressed by Victor. It was
16 inches. In Isom's AES article he several times states that the disc
was 20 inches in size. That screws up all his calculations in that
section.
Likewise his description of how 45 was chosen is wrong. Once again the
primary design feature was keeping the inner and outer groove speeds as
close to each other as possible. I have the RCA ledger sheets of the
Madam X project from the early 1940s which show they started with 6 1/2
inch discs at 40 rpm in 1942 and in June 1943 raised it to 45 RPM for
7-inch discs. 45 at 7-inch provided the closest match with the proper
time length with a high inner diameter surface speed that was not much
different from the groove speed at the outer diameter. The RCA
engineers scoffed at the Columbia LP because not only was the inner
groove speed too slow, the ratio between the outer groove speed and the
inner groove speed was enormous. To overcome this there was an attempt
over at Columbia to have the first LP masters maintain a large inner
diameter, but these records looked wasteful with all that non-used
area. This aesthetic reasons is one of the reasons why the LP label is
larger than the 78 label, but it really should have been even larger!
The 45 label had to be the large size it is because of the loss of
printing area where the hole is. It had the secondary advantage that it
kept people from questioning all of the blank space on them. Columbia
used a small label on their 7-inch 33s in the early 50s, but kept the
records from looking too empty by using those larger-than-necessary
gripping lines outside the label.
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