Wednesday, March 05, 2008

Stroboscope

In radio today music is most often played from digital files. Sometimes CDs are still employed but the turntable has largely been phased out. Modern turntables deliver consistent speed, and require little maintainence. A good Technics 1200 if not abused, will last decades without a single adjustment. In turn the records themselves have become more consistent, now standardized at 33 and 45 rpms. In the past variations were a problem. Victors LPs were 71 RPM until the teens. Then both Brunswick and Edison LPs were recorded at 80 rpms well into the 1920s. What is a DJ to do?Previous to this era of high torque direct drive motors, turntable speed was irregular. Shade-pole motors were inappropriate for this. In that type of motor variations in voltage cause variations of speed. Modern turntables use an induction motor, that is tuned to the 60 cycle alternation of our AC power supply. This allows them to produce consistent rpms.

The Belgian professor Joseph Plateau invented the stroboscope in 1832. Plateau created the first Phenakistoscope by cutting slits into a disc which he turned while viewing images on a separate rotating wheel. It was almost simultaneously invented by the Austrian Simon von Stampfer. Stampfer called his a "Stroboscope"the nomenclature we use today.

The older hand crank spring motors required adjustment even during the play of a single side of a single record. An experienced user could do this by ear, but sometimes a Stroboscope was employed. A stroboscope is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. The light gun used to adjust your timing belt works on a similar principle.

A stroboscope for phonographs is a paper disc placed at the center of the platter. On the disc is a circle of equally spaced lines. When viewed under an electric light the record is spinning at the correct speed (in this case 78 rpm) the lines on the stroboscope appear to be still.

What is happening is that the electric light is actually blinking on and off 3600 times a minute (3000 at 50Hz.) Each time the light goes on, the black lines on the stroboscope have rotated one space if the stroboscope is running at the proper speed. The book The Compleat Talking Machine by Eric L. Reiss has some simple instructions on making one or you can just print out the one on the W.A.M.S. webpage here.