Movie projector synchronizer - in one day. Encyclopedia of radio electronics and electrical engineering

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More recently, many enthusiastically engaged in amateur filming. And today they transfer films accumulated in their personal archives to modern media - magnetic tape or CDs. It is easy to do this even in amateur conditions. It is enough to shoot a film projected on the screen with a video camera. Considering the low resolution of XNUMXmm film, the possibilities of the VHS standard are more than enough. However, for reshooting, you need to take care of the synchronization of the film projector and video camera ...

The film projection speed should be such that the image flicker frequency is equal to the TV signal field frequency - 50 Hz.

Violation of this condition will affect the quality of the video. When viewed on a TV screen, periodic changes in the brightness of the image will be observed.

Widespread projectors "Luch-2", "Volna", "Rus" provide the required flicker frequency at the standard frame rate for 16,7mm films (1 s-XNUMX). Unfortunately, if you do not take special measures, the projection frequency "floats" under the influence of the mechanism warming up, changes in the mains voltage and other factors that are difficult to take into account. You can, of course, precisely maintain it manually with the projector control, constantly watching the strobe marks on the frame change knob. This technique is suitable if you have to quickly reshoot a couple of five-minute films. But when the volume of the archive is several hours of projection, automation is indispensable.

The projection speed stabilizers described in [1, 2] would perfectly cope with the task, for example. However, the feasibility of assembling a device from more than a dozen chips in order to perform a generally one-time job is doubtful. Meanwhile, back in 1969, an article [3] was published in which a simpler solution was proposed. It was, of course, not about re-recording the movie on video, but about synchronizing the projector with a tape recorder for recording and playing sound.

During the demonstration of each frame, a cam mounted on the shaft of the clamshell mechanism of the projector closed three times the contacts of the electromechanical switch, which is a kind of phase detector, included in the power circuit of the electric motor that drives the projector. As a result, the equality of the contact closure frequency to the mains frequency (50 Hz) was maintained automatically.

A similar device was made by the author and used successfully for a number of years. Practice has shown the insufficient reliability of the contact sensor, and it was not easy to place it in the projector. It is better to use a non-contact sensor. This one was also described in [3], but its performance on the element base of those years raised great doubts. A simple synchronizer was manufactured and tested in operation, operating according to the above principle, but with a non-contact sensor based on modern elements. It can be made and adjusted in one day.

The synchronizer circuit is shown in the figure. There is also a diagram of the "Luch-2" film projector that has been preserved for few people (circled with a dash-dotted line, the positional designations of the elements correspond to the factory ones), with which the synchronizer works.

The projection frequency sensor is an optocoupler from an emitting diode of the IR range of the VI and a BL1 photodiode. The optical connection between them is interrupted three times during the demonstration of one frame by the shutter of the projector. The emitting diode VI is connected through a rectifier (diode bridge VD1) to the secondary winding of the transformer Tr1 of the projector. The diode current, limited by resistor R2, must not exceed 100 mA.

A direct voltage drop (approximately 1,5 V) on the green LED HL2 is not enough to open the composite transistor VT1. But the EMF generated under the action of IR radiation by the BL1 photodiode, in total with the voltage on the LED, is already enough to open the transistor. Thus, the transistor VT1 is open if the IR radiation reaches the photodiode, and closed if there is an obturator blade between the photo- and emitting diodes.

When the open state of the transistor coincides with the positive half-cycle of the current of the excitation winding of the motor M1 of the projector, its resistor R2 is shunted by the VT1VD3 synchronizer circuit, the motor excitation current increases, the rotational speed of its shaft increases. When the tripled shaft speed is equal to the mains frequency, the system is set and automatically maintained in equilibrium. Zener diode VD4 limits the voltage at the collector of transistor VT1, protecting it from breakdown.

Neon lamp HL1 serves for visual control of synchronization. When illuminated by this lamp, the stroboscopic marks on the single-frame projection handle should appear stationary or "wobble" slightly in synchronous mode.

Transistor VT1 - composite, with a permissible collector-emitter voltage of 80 V and a current of 1 A - can be replaced by another similar or appropriately connected transistors KT815G and KT817G. If the allowable collector-emitter voltage of the applied transistor is more than 100 V, the VD4 zener diode can be excluded.

As an IR emitter and a photodetector, you can use the corresponding elements from TV remote control devices, and replace the HL1 neon lamp with any two LEDs, turning them on in anti-parallel.

All parts of the synchronizer, with the exception of a neon lamp and resistor R1, are mounted on a board made of foil fiberglass, soldering their leads to the "patch" cut out of foil. The optical windows of the emitting diode BI1 and the photodiode BL1 should be directed at each other with a distance between them of no more than 5 mm. The value of the resistor R2 installed in the projector should be reduced to 75 ohms.

The neon lamp is placed in a separate small housing and provided with connecting wires long enough to be held close to the strobe marks.

After installing and connecting the stabilizer to the projector, make sure that the projection speed decreases when there is an opaque barrier between the IR emitter and the photodetector, and increases when it is removed. If this does not happen, try changing the distance between the elements of the optocoupler, replace the HL2 LED with a similar one of a different type or glow color, select the values ​​​​of the resistors R3-R5.

Having adjusted the sensor, the synchronizer board is glued to the lower part of the projector case in such a way that the obturator blades, during rotation, interrupt the flow of IR radiation between the VI and BL1 diodes.

After turning on the projector, achieve synchronization by rotating the speed control knob - variable resistor R1. To stop the synchronizer, it is enough to connect sockets 1 and 8 of the projector's SHZ socket to each other. This jumper is included in the stock plug supplied with the projector.

If the synchronization is accidentally broken during the shooting, it is clearly noticeable by ear, it can be quickly corrected by a slight turn of the speed control knob. Short-term desynchronization practically does not affect the quality of the video.

It is very important to reshoot a movie on videotape with the camcorder's autofocus turned off. Otherwise, attempts by automation to compensate for the lack of clarity of the movie image will drastically degrade the quality of the video. After disabling the machine, temporarily place some drawing or other clear contrast image on the movie screen and focus the video camera manually. Before you start shooting, do not forget to lubricate the moving parts of the projector and clean the manifold of its engine.

The described device can also be used with other film projectors equipped with collector electric motors, for example, "Volna" or "Rus".

The proposed device, of course, synchronizes the projector not with the scanning frequency of the video camera, but with the frequency of the voltage in the AC network. However, experience shows that the latter, at least in large cities, is close to the nominal (50 Hz) and stable to a degree sufficient for the problem being solved.

Literature

1. Zhurkin I. Film projection frequency stabilizer for re-recording on video. - Radio, 1997, No. 10, p. 46-48.
2. Biryukov S. Film projection frequency stabilizer for re-recording on video. - Radio, 2000, No. 10, p. 35, 36.
3. Vovchenko V. Synchronization of the film projector. - Radio, 1969, No. 3, p. 40-42.

Author: N. Khlyupin, Kirov

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