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