leichter (02/11/83)
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
floyd (02/12/83)
The 15 inch discs were used primarliy to get more programming onto a single disc in order to minimize interruptions. Not too long ago, if not any more, you could buy broadcast turntables with 16-inch platters and ~14-inch tonearms. The selectable speeds were 33, 45 & 78.
hal (02/13/83)
Another obscure fact about 15" transcription disks. Because they were
only intended for professional use, they often didn't have the "bead" on
the outside edge that commercial records have. (The bead is the rounded
off edge.) Without the bead, these disks had razor sharp edges. I once
threw one of these things like a frisbie at a tree (with no one around,
of course), and it imbedded itself about 1" into the tree. Makes Odjob's
hat seem like a safe children's toy.
Hal Perkins
{etc...}!cornell!hal
hal@cornell (arpa)