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If the recorder uses friction clutches on the two reels, the tape tensions will vary as the clutch surfaces wear. However, most recorders use separate torque motors to drive the two reels, one restraining the feed reel and the other driving the take-up. With this arrangement, there are no friction clutches, but mains variations will affect the motor torques and hence the tension on the tape.
You can largely eliminate all these causes of poor synchronisation by controlling the projector speed from the capstan speed. You can most readily do this by fitting a stroboscopic disc to the capstan spindle. Then, instead of lighting it by a neon lamp, you use part of the projector light output. This flashes rapidly due to the action of the shutter, and by adjusting the projector speed you can adjust the rate of flashing to arrest the stroboscope pattern. As long as the pattern is stationary, the projector speed is in a definite relation to the capstan speed.
You will have to experiment a little to determine the correct number of bars for the capstan stroboscope. You can, however, make a good approximation by consulting the table below. First of all, you must find how many times the projector shutter cuts the light right off for every frame projected. The majority of projectors use a shutter obscuring the beam three times per frame. Some machines produce only two obscurations, others four.
STROBOSCOPES FOR TAPE RECORDER CAPSTANS
Number of bars per inch Diameter of Capstan Shutter Projection with tape moving at:
Obscurations Speed 375 4-8 7-2 7-5
per frame (fps.) ins. /sec. ins. /sec. ins. /sec. ins.lsec.
210 — 13-4
— 210 201 31-4 — 201
— 31-4 30-2 41 9 — 26-8
— 4! 9 40-2
92
16
26-8
24
40-2
16
40-2
24
60-3
16
53-5
24
80-4