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With a silent projector, you can get comparable results by careful operation of the speed control knob. Stick a stroboscopic disc on the side of one of the sprockets and illuminate it with a neon lamp. If you adjust the speed control to keep the stroboscope bars apparently stationary, then the speed will be proportional to the mains frequency. A rise in frequency will accelerate the recorder motor. It will also make the stroboscope pattern appear to slip backwards. When you re-set the speed control to counteract this, the film and tape speeds will be substantially in the correct ratio once more.
You can calculate the number of stroboscope bars by using the equation on p. 22. However, the table below gives the number required in the most common cases.
STROBOSCOPES FOR PROJECTORS (For 50-cycle mains only)
Nominal Speed
Disc on flywheel
Disc on sprocket with t teeth t=8 t=IO t=l2 t=l4 t=l6
Nominal 16 f.p.s. Nominal 24 f.p.s.
No. of bars Actual f.p.s. No of bars Actual f.p.s.
6 167
4 25 0
50 62 75 87 100 160 161 160 161 16-0 33 42 50 58 67 24-2 23-8 240 241 23-9
On 16-mm. projectors, count only the teeth on one side of the sprocket
Watching a stroboscope continuously is a tedious business. The projector speed tends to rise as the machine
Used in the same way as those shown on page 89, these patterns provide stroboscopes with (A) 76, 72, and 68 bars; (B) 38, 36 and 34; and (C) 19, 18, and 17 bars. Intermediate numbers can be obtained by cutting along a radius and lapping to form a shallow cone with an overlap of one, two or three bars and spaces, as necessary.