The simple explanation:
The circuit lets the motor come up to the desired sync speed, then when it reaches the proper RPM, it "grabs" the motor and locks it to the camera sync.
For techies, the details:
Counters U3 and U4 are configured as "synchronous one-shots"--they use the output from a crystal oscillator to produce pulses that are 10mS long. At the output of U3, these pulses are in sync with the vertical sync pulses from the camera; at U4, they are in sync with the output of the Hall-effect device that is part of the motor.
Flip-flop U2B is a speed detector. It compares the output from the Hall-effect device (which produces two pulses per revolution of the motor) to the output of U4. While the motor is coming up to speed, U2B allows the Hall-effect signal to be fed directly back to the motor, as if the motor were free-running. When the motor is at sync speed (1500RPM), the Hall-effect output pulse is 20mS long (high for 10mS, low for 10mS). If any positive pulse from the Hall-effect is less than 10mS, U2B switches the drive signal from the Hall-effect to the output of U3, which is in sync with the camera pulses. It does this on a pulse-by-pulse basis, so this switching is occurring very rapidly and continuously. The switching is effected through gates U5A, U5B and U5C.
If allowed to run continuously in this mode, the motor SPEED is precisely in sync with the camera--one revolution per field--but the POSITION of the shutter with respect to the camera sync pulse will not be correct. So, after the proper speed is attained (which takes about 4 seconds), timer U6 latches the motor drive to the camera sync. This causes the motor/shutter to shift its position and lock to the camera sync in both speed and position.
Note that the crystal clock does NOT control the shutter timing. It is there simply to provide fairly precise pulse widths needed for the synchronous one-shots.
At the lower left of the schematic, U2A looks at the Vertical and Horizontal sync pulses from the camera to identify the odd and even ("upper" and "lower") fields. The output drives two LEDs to identify the fields. Because the motor has four poles, it can lock into one of two positions, the "correct" one, or 180° out of phase. The lock position is random each time the motor is turned on. So the LEDs indicate which position the motor has locked into. If it's "wrong," the shutter motor assembly is simply rotated 180° to the correct position.
Of course, field identification is only required when the footage is going to be transferred to film; transferring with the fields reversed would re-introduce an interlace artifact and it would look like video again!
Comments? Questions? Email me.
