Explainations of why certain speeds were chosen for certain types of phonograph recordings...
s p i n s d o c t o r e d
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q. Does anyone know why 78 revolutions per minute was chosen
as the standard rotation speed of old-fashioned gramophone
records, rather than a round number such as 75 or 80 rpm?
And are there convincing explanations for the choice of
speeds for later EPs and LPs of 33 1/3 rpm and 45 rpm?
JAMES LEE
Oisterwijk
The Netherlands
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a. It was Emil Berliner, the inventor of the gramophone, who
determined roughly how fast old disc records should spin. He
avoided Edison's need for a stylus made from precious jewels
by using points which could be made from steel sewing
needles and pins. The size of the stylus effectively
determined the size of the grooves in a record and the
recordable frequency range limited by this groove size
determined a speed between 70 and 90 rpm.
Standardisation did not begin until 1912, when the British
Gramophone Company conducted listening tests on their back
catalogue. They settled on the average (or possibly the
median) of these tests, which turned out to be 78 rpm. Other
companies adopted this, but the process was not complete
until the early 1930s. Even after this date rogue rpm
records still appeared. After standardisation problems still
occurred. Because of electrical mains frequencies
differences on opposite sides of the Atlantic, stroboscopic
speed testers and synchronous motors meant a nominal speed
of 77.922 rpm in countries that used 50 hertz and 78.261 in
countries that used 60 hertz. These were later fixed in
national (but not international) standards.
Records of 33 1/3 rpm were developed in conjunction with
films. A 12-inch 78 with Berliner-type grooves could hold
between 4 and 5 minutes per side. The first practical sound
films produced in the US in the late 1920s had their sound
on separate disc records and it was more important for the
sound to be continuous. A reel of film might run for 11
minutes, so a rotational speed of about 32 rpm was required
to make the sound match the picture. History doesn't tell us
why precisely 33 1/3 was chosen, but in retrospect it was a
very good choice because stroboscopic speed testers can be
made for this speed which will work on both sides of the
Atlantic.
It seems CBS engineers (who developed the first LPs in
1948), simply experimented with one of the old machines
hanging around in their workshop. They then developed new
groove dimensions which gave an acceptable signal-to-noise
ratio with the new plastic material "vinyl".
The 45 rpm speed was the only one to be decided by a precise
optimisation procedure (by RCA Victor in 1948). Calculus was
used to show that the optimum use of a disc record of
constant rotational speed occurs when the innermost recorded
diameter is half the outermost recorded diameter. That's why
a 7-inch single has a label 3 1/2 inches in diameter. Given
the CBS vinyl groove dimensions and certain assumptions
about the bandwidth and tolerable distortion, a speed of 45
rpm comes out of the formula.
PETER COPELAND
British Library National Sound Archive
London
a. From 1894 to around 1930, there were many different
record speeds ranging from 65 to 90 rpm, each case being a
compromise between playing time and the need for a clean cut
in the original wax. The Victor company used 76 rpm for many
years for its recordings but instructed buyers to reproduce
at 78 rpm, the record's durability was improved that way.
The standard of 78 rpm arrived by default, although the
actual speed depended on the electrical mains frequency.
Constant linear speed, or varying the rpm, was
commercialised but did not prove to be a success (until the
arrival of the CD).
The speed of 33 1/3 was introduced in 1927 after theoretical
analysis of the compromise between signal-to-noise ratio and
playing time (3 minutes per radial inch) by J. P. Maxfield
of Bell Laboratories for sound films produced on the
Vitaphone system. And it was a professional de facto
standard before it became commercialised by CBS in 1948. It
has been suggested that 78 minus 33 equals 45 was the reason
for the emergence of 45 rpm records but, in fact, Maxfield's
analysis still applies: the 45 "single" was RCA's equivalent
to a 10-inch, 78 rpm record, only smaller.
GEORGE BROCK-NANNESTAD
Copenhagen
Denmark
a. Emil Berliner's first disc gramophones were wound by hand
at somewhere between 60 and 100 rpm. The 7-inch discs lasted
a minute or so and had low sound quality. Berliner and his
assistant Fred Gaisberg realised that unless the speed was
governed, the gramophone would never be more than a novelty.
Gaisberg visited a young mechanic who was making clockwork
machinery, hoping to use it for sewing machines. This
machinery was never successful in sewing machines, but was
ideal for gramophones, and it rotated at 78 rpm. The
mechanic, Eldridge Johnson, became a millionaire. Columbia
made all its discs to run at 80 and HMV had its pioneer
recordings produced between 68 and 92 rpm with the key of
the piece marked on the label. You then tuned it on your own
piano, using the gramophone's governor. These speeds all
gradually settled into the standard of 78.
When talking pictures first arrived in the late 1920s, the
sound was recorded separated on discs and had to be
synchronised by the projectionist at each showing. Every
cinema projection room had a pair of projectors, each taking
1000-feet reels of film, whose running time was about 10
minutes. The projectionist switched projectors after each
reel. Ideally, this meant that the sound should last 10
minutes as well, as it would be impossible to synchronise a
sound changeover in midreel. At the time, however, a 12 inch
78 rpm record lasted for only about 4 minutes, so the
Vitagraph company simply slowed down the 78 until it lasted
10 minutes and recorded all their masters on that, starting
each disc in the middle, as it was easier to drop a needle
there than the outer edge. This new speed was 33 1/3 rpm,
adopted for other records in the late 1940s when Columbia
introduced its first vinyl, long-play discs with
microgrooves, giving a play time of about 30 minutes on each
side.
However, the long-play disc wasn't particularly suitable to
popular music, as the public wanted its records as singles
with good sound quality even at high volumes. RCA Victor
came up with a 7-inch vinyl disc with microgrooves, rotating
at 45 rpm, a speed chosen specifically to make the most of
the music, unlike 78s or 33 1/3s. And does no one remember
the 16s?
ROGER WORSLEY
Haverfordwest
Pembrokeshire
© Copyright New Scientist, IPC Magazines Limited 1997
