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8mm Telecine Film to Video Conversion
     
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.
 
 
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.
     
 
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.
     
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.
     
 
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

 
The servo loop is more of a incremental controller, if there is an error we'll increment the stepper forward or back.  This helps overcome stiction of the belt and projection speed knob since we're moving the stepper at least 3.6 degrees per bump.

Frequency readings from the HP34401A are fairly noisy.  Reading to reading variation can be as much as 0.5 Hz.  I'm not sure if its due to mechanical variation or the analog signal presented to the multimeter. An averager attempts to smooth the data.

Stepper motor driver is from Weeder Technology.  The control program is written in 'C' using LabWindows/CVI development tool.
 

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