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8mm Telecine Film to Video Conversion
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Here is a project that describes what an
electrical engineer does for fun. This telecine uses a servo loop to sync the
speed of a 8mm film projector to the 29.97Hz frame rate of a VCR camera. I've
found that nailing the frame rate allows the automatic VCR focus and color
adjustments to be much more stable resulting in very good images. |
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The VCR camera looks at about 7" x 7"
image shown by the variable speed projector. The projector is about 42 inches
from the screen, the VCR camera is about 30 inches from the screen. The screen
is nothing more than a bright white heavy copy paper. The VCR camera is
offset from the projector-screen axis an inch or two. This results in some
parallax, but I never notice this in the captured image.
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The servo loop has three components:
movie projector frame rate measurement, stepper motor used to control projector
motor speed and control software.
A HP 34401A multimeter measures projector shutter frequency. A photocell placed
near projection lamp picks up the light intensity variation produced by the
shutter. My photocell shows a sawtooth with 200mV p-p amplitude offset by about
1.5VDC. The observed frequency is 2x the frame rate.
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The photocell can be seen at end the
orange and white wires. I mounted it on a small wooden arm which is in turn
velcro'd to the projector. |
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The servo motor is really a stepper
motor which belted to to the variable speed control of the projector.
The software control loop runs the following sequence over and over.
-Read the frequency from the HP34401A If the frequency is within the deadband do
nothing (i.e. close enough)
-If the frequency is less than the setpoint turn the speed knob up
-If the frequency is more than the setpoint turn the speed knob down
-Command the stepper
-Wait for change
-Loop to top
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