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From: Patent Tactics, George Brock-Nannestad

Background.
About 1980 I set about to compare several sets of Edwin Fischer's recording
with Karl Boehm conducting the Saxon State Orchestra in Beethoven: Concerto
no. 5 Op. 73 that I had been fortunate to obtain previously. German wartime
pressings were easy to find in Denmark at the time and generally under-
priced. According to my notes made at the time, first of all I noted that all
published sides except side 1 were made in the "Transfer Room", i.e. they
were re-recorded from an original master disc to the side used in
manufacture. Secondly I found four supplementary "takes" (really a misnomer
in the case of re-recording, unless the particular original was really a
different take) that had not been identified in the authoritative discography
at the time, Henning Smidth Olesen: "Edwin Fischer A Discography", Danmarks
Biblioteksskole Copenhagen 1974. There may well be better discographies
available now. In distinguishing between the various "takes" I measured the
outside diameter of the recorded area and the width of the recorded area.
This was a first approach to using the physical layout of the recording to
document an individual copy of a record in order to compare it to other
records having the same catalogue number. This approach was not unique at the
time - I certainly had collector colleagues who did similar things. And there
is no doubt that sometimes you need to go into such details in order to
distinguish sides. There is a need to find identifiers.

The recording lathe.
In 1997 I digested a lot of material I had collected about EMI practices and
published it as "The EMI recording machines, in particular in the 1930s and
40s", The Historic Record & AV Collector No. 43, pp. 33-38, April 1997. In
the article, I noted that the four groove pitches obtainable from Columbia-
derived recording equipment were all different from the four groove pitches
obtainable from Gramophone-Company-derived equipment, and that one might then
distingiuish unambiguously between the two types in a particular recording
session.

Since then I have been increasingly occupied with the actual mark left on a
recording by a particular type of recording lathe - a part of the so-called
ancillary or secondary information carried by a mechanical sound recording
(in which the recorded sound is the desired or primary information). The
reason is that there are frequent cases where it is uncertain what went on
during recording, what was the equipment, and settling the issue of equipment
may be the clue to a better understanding of a recording session and its
placement geographically or in time. A recording lathe might be identified at
a location where it was not expected to have been. We do not have recording
ledgers for everything. The most important mark is certainly the groove pitch
- the number of revolutions needed for the cutting stylus to travel one inch
or the number of complete groove-land cycles you pass on a linear inch along
a radius. The groove pitch is set by a gearbox, and you really need major
conversions to go outside the normal selection in a particular recording
lathe. For puzzle or race-track records you needed a groove pitch that was
for instance 3 or 6 times as steep.

The recorded area information I have mentioned above, but that is not per se
an identifier of a particular recording lathe type. Another important mark is
the rumble, in particular the vertical rumble (from a lateral recording),
because that is frequently related to teeth on cogwheels not meshing
properly, and cogwheels are individual to a particular model of recording
lathe. Also, the gear ratio between the governor shaft and the turntable may
be measured in the rumble.

Recent developments
I have of lately increasingly measured the groove pitch on records that I
inspect, and I consider that information to be just as essential as the
matrix number or other markings at various clock-face locations on a record.
And, in contrast to the markings that may be entered manually subsequent to
inspection into the meta-data of a recording, nobody to my knowledge is
concerned with the groove pitch. But that goes for the recorded area
information mentioned above as well. So much essential discographical work is
dependent on the possibility of comparing physical record sides that we must
consider mass transfer of mechanical records into a digital format to be
deficient, unless these types of information are made available too. The same
goes for mass destruction of apparently identical recordings (duplicates) -
unless they have been compared to this detail, information will have been
destroyed.

The groove pitch is measured in grooves per inch (gpi) and so is the screen
ruling or screen frequency in half-tone printing (but called lpi in this
trade). For this reason I am using an adaptation of an implement used in the
graphics arts for the measurements. It will only work simply if the record
does not display twinning, which a surprising number of early recordings did.
Twinning is caused by a stick-slip phenomenon: the movement across the record
was not entirely even, but stopped, until the pressure from the transport
mechanism was sufficient to overcome the static friction, and then the cycle
started again. Good lubricaton was one way of avoiding the problem, but it
did not always work. Certain recording lathes had special provisions to
completely avoid the problem. And I warn you: some lathes I know use 96 gpi
and some others use 97 gpi (at the setting just below 100 gpi), and that
requires quite some precision in working. Another way would be to have a
precise linear travel indicator on your tonearm and a revolutions counter for
your turntable.

The groove pitch information may possibly not be directly measurable in the
photographs that form part of the new process called VisualAudio developed in
Switzerland, because that relies on a lens forming an image of a record side
on a film (which is later scanned in order to convert the image to a linear
sound file), and unless that lens is designed to preserve linear distances in
the image formation, the groove pitch will vary from the rim and towards the
centre. Only if some calibration is introduced will it be possible to enable
the scan to detect the linear frequency of the groove image along a radius.

Now, I would like to see if the groove pitch information could be a shortcut
to distinguishing beetween VTMC recordings that were repeats of earlier
recordings without change of catalogue number or matrix no. They might have
changed their recording lathe. And the approach could conceivably also be
used to check the origin of reissues of smaller label recordings by larger
labels. However, the approach does not work for variable pitch recordings,
such as Fuellschrift or Variable Micrograde.

If anybody would like to use the groove pitch comparison approach, then
please note that you read about it first here (or in 1997). Unless, of
course, if anybody has knowledge about prior use of the approach, then I
would like to hear about it, and I will give due credit in future
communications.

I hope that you have been inspired by the above.

George.