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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
___________________________ 

    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
    ____________________________________________________________

    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