Digitizing of 8mm Films

Synchronization of Film Projector and Camcorder

Abbreviated Version

 

1. Introduction

Different aspects have to be considered in connection with 8 mm film transfer to video. For hobby users, in my opinion, the best method is to use a camcorder to directly take a picture of each frame of the film. This discourse will be limited to the aspects of synchronization between film projector and camcorder for this purpose.

2. Basics

8 mm film cameras normally run at about 18 frames per second while (PAL) video cameras or camcorders use 25 frames per second respectively 50 fields per second. 8mm film projectors, also running at 18 frames per second, normally use a three-blade shutter to show each frame three times in a row. So we get 3 x 18 = 54 frames per second. To scale these 54 frames per second to the 50 fields per second of the camcorder, the projector speed has to be reduced from 18 to 16,7 frames per second as 16,7 x 3 = 50.

A drift of the projector speed would result in the projector shutter periodically varying the effective comcorder exposure time, leading to flickering videos. Therefore, synchronization of film projector and camcorder is necessary.

There is also another kind of synchronzation between projector and camcorder: the phase synchronization. This means to make sure that the shutters of projector and camcorder are open at the same time respectively closed at the same time. In this context, there are two choices of adjusting the exposure time of the camcorder:

a) 1/50 sec
Caused by the width of the projector shutter blades, the projector shutter opens for a noticeably shorter period of time than 1/50 sec, thus determining the effective camcorder exposure time. In case of 1/50 sec and synchronism, this time stays constant, no matter when - within the camcorder exposure period of time - the projector shutter is open or closed. Therefore here it is possible to avoid flicker just by adjusting the correct projector speed, without taking care of the phase.

In principle this could cause an interference. In worst case the projector shutter would close in the middle of the camcorder exposure period of time. When this coincides with a change of frame in the projector two different projector frames would be superimposed in one camcorder frame, each with half the exposure time. But for several reasons this effect is not visible during playback. Therefore it would also be difficult to judge whether the phase is adjusted synchronously or not.

b) 1/1000 sec
A short exposure time however requires a stable phase control because otherwise the projector shutter blade could blind out a complete frame, causing maximum flicker.

3. Retrofit of Synchronization Devices

This chapter briefly lists different possible synchronization methods for different types of projectors.

4. Realized Controls to synchronize Projectors

Electronic synchronization controls for two types of projectors will be described.

4.1 Closed Loop Sync Control using a Phase Control Dimmer

This control is suited for types of projectors with one or two fixed speeds, using dc motors. As the sync control uses a dimmer I would recommend that you first test whether your projector speed can be adjusted using a commercial dimmer, capable of inductive load.

When this works you will soon find out that the adjusted speed is not stable. It would be difficult to manually readjust the speed during the film transfer in such a way that no flicker occures. Therefore the Sync Control was designed to do that work.

The advantage of this control is that no modification of the projector wiring has to be made. Only an optical sensor monitoring the blade shutter has to be installed.

4.1.2 Function

The details of the sync control funcion are as follows (see wiring diagram):

- The camcorder output AV signal is fed into the 'Video Sync Separator' chip (IC5 - LM1881N) which filters one square puls per frame out of this signal.

- Another pulse sequence is generated by the optical sensor monitoring the three-blade shutter. One pulse corresponds to one blade of the projector shutter, resulting in three pulses per frame.

- The phase comparator of the 'PLL' chip (IC4 - 4046N) compares the frequencies and the leading edges of both if these pulse sequences. It delivers a dc output voltage depending on the differences in terms of frequencies and phase angles of the input signals. For this purpose it detects which input pulse positive edge leads or lags.

- The four operational amplifiers of IC3 - LM324N together with the phase comparator operate as a closed loop combined speed and phase control. The camcorder dictates the speed and phase, while the blade shutter signal monitors the actual speed and phase. With IC3C and potentiometer R1 the phase comparator output voltage is shifted down so that zero output voltage represents zero phase shift (e.g. zero error signal), (respectively a phase shift different from zero - see below).

As the phase comparator has a very narrow operating range, the projector speed operating point has to be preadjusted with R2 (like with a commercial dimmer).

IC3A and IC3B generate the integral and proportional control gain. The integral part is tuned to compensate for disturbances with long time constants, like the projektor running warm.

- The closed loop speed and phase control output is connected to the 'Phase Control' chip IC6 - TCA785 which controls the Triac TIC226N via the opto-coupler MOC3023. (Phase control in this context is related to phase angle control using a triac to adjust the projector motor voltage and has nothing to do with the phase synchronization mentioned before.)

4.1.4 Start-up

Separate projector from supply.
Switch on the sync control.
Safety advice: keep safety rules as parts of the circuit are on 230 volts!
Turn blade shutter manually, voltage across R11 shall jump between zero and supply voltage (12 volts).

Adjustments

1. Adjust peak saw tooth voltage (IC6 pin 10) to supply voltage minus 2 volts using R3. (In case no scope is available, turn this voltage so high that you still can dim an electric light bulb by adjusting R2. After connecting the projector, it might be neccessary to decrease the saw tooth voltage.)

2. Adjust voltage at TP2 to half of supply voltage using R1

3. Connect and switch on camcorder and projector (still without film). Preadjust projector speed using R2, slowly reduce speed starting from maximum until voltage at TP3 is slowly oscillating between zero and supply voltage without hitting one of these limits (use voltmeter with pointer).

4. Find final R2 setting to avoid IC3A getting saturated in cold or warm condition of the projector (avoid short circuit across C13,C14!).

5. With R1 the phase can be varied, e.g. the frames of projector and camcorder can be timed in relation to each other within certain limits given by the system stability. This might be necessary when using an exposure time of 1/1000 sec. In case the adjustable range is too small, the optical sensor monitoring the shutter blades must be repositioned.

6. With a film in the projector, the voltage at TP3 becomes less stable due to the mechanical influences of the film transport. Item 4 should now be checked again.

4.1.5 Hints

1. Warm up the projector

2. Provide sufficient leader so that the sync control has enough time to become stable in the beginning.

3. Switch on the projector first, then start it with the sync control. This protects the control from getting saturated befor the start.

4.2 Impuls Control of an ELMO Projector

4.2.1 Idea

The ELMO 'SP Hi-Deluxe' type projector is designed to run synchronously with frame pulses from a tape recorder (see projector schematic here). This allows synchronization between film and sound. But it also allowes synchronization between camcorder and projector. The adapter described below was designed for this purpose.

4.2.2 Function

- The camcorder output AV signal is fed into the 'Video Sync Separator' chip (IC7 - LM1881N) which filters one square puls per frame out of this signal, e.g. 50 pulses per second with a PAL camcorder.

- IC6 (CD4018N) together with IC5A and IC5B (CD4011N) are connected to build a 'devide by three-counter' which lets pass only every third pulse. This is neccesary to dictate a projector speed of 50 / 3 = 16,7 frames per second. As the projector 3-blade shutter causes each frame to be shown three times in a row, we get again 16,7 x 3 = 50 fps whereat the camcorder will record each projector frame three times in a row.

- By adjusting R15, timer IC3 (NE555N) can delay the pulses by up to 20 milli secs. Thus the frames of projector and camcorder can be timed in relation to each other. This is important when using an exposure time of 1/1000 sec.

- The second timer IC4 determines the puls length of 3 milli secs.

- The pulses are amplified by the P-channel MOSFET Q2 and adjusted to about 60 milli amps with R9.

- This adapter can be connected to the ELMO pulse input via a 6-pin DIN connector using pin6 (+) and pin 5 (-).

- LED2 will flash with the 16,7 hertz rythm to show that the device is working well.

After you have turned the projector motor switch to 'run' while the E.S.S. switch is set to the position of 'sound rec./sound proj.', the projector can be started or stopped by switching the pulses on or off with S2.

5. Addendum

Remark:
In case of NTSC, the adapter ('ELMO-Steuerung') should work without modifications. Connecting the adapter to an NTSC type camcorder will synchronize the ELMO projector to the field refresh frequency of approximately 60 Hz of that camcorder. The adapter will make the projector run with 1/3 of this frequency, in this case 60/3=20fps. Thus, 24fps footage will run too slow, 18fps footage too fast. If this is not acceptable, the speed can be corrected by an editing software afterwards.

 

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Experiences and ideas? Don't hesitate to send comments or questions.

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