From: utzoo!decvax!yale-com!leichter
Newsgroups: net.misc
Title: Re: A question about records speeds
Article-I.D.: yale-com.866
Posted: Fri Feb 11 11:32:59 1983
Received: Sat Feb 12 05:46:57 1983
References: arizona.1325

The odd-ball speeds of records - 78, 45, 33 1/3, 16 2/3 - are holdovers from
an earlier day and a desire to make cheap turntables.  The easiest, cheapest
way to make a cheap constant-speed electric motor is to use a synchronous motor,
whose speed is determined by the line frequency - 60 cycles/second.  The
easiest, cheapest way to get convert a 60 rotations/second motor speed into
a useful turntable speed is with an idler-wheel drive to the turntable.  The
speeds given - except for 78 - come from simple integer ratios for the motor
axle, the idler wheel, and the turntable itself.  I think you'll typically
see a 60/1 ratio in the motor axle to idler wheel, and then a 3/4 reduction
for 45 rpm, a 5/9 for 33 1/3, and a 5/18 for 16 2/3.  (Actually, even 78 isn't
that far off; it's a 10/13 ratio.  However, 78 was chosen as a compromise; there
were a lot of old mechanical-age records around that used a wide variety of
speeds (when you use a spring drive, the exact number isn't significant as far
as design goes); 78 was easy to get from 60 cps, and near the middle of the
range for then-existing records.)

BTW - an item of historical interest.  You sometimes see turntables described
as "transcription".  This once had a special meaning.  Transcription-style
records were made for radio-station use.  They were 15 inch disks, played at
(I think) 16 2/3 - although faster rather than slower would make more sense,
maybe it was 45 - and played from the inside out!  The reason for this oddity
was simple:  Record distortion is at a maximum near the inner diameter (where
you have less linear distance to spread a given time-units of signal over),
minimal at the outer edge.  Most classical pieces (what the radio stations back
then were playing) are relatively soft in the beginning, but have a grand
finale.  So...you put the loudest music where it will distort the least.
							-- Jerry
						decvax!yale-comix!leichter