Method and Apparatus for Compensating for Film Registration Weave in Film Scanning Systems

This invention relates to a method and apparatus for compensating for positional instability of film passing through a film gate in a telecine or film writer.

The picture frames on cinematographic film are conventionally registered with the sprocket holes by register pins. A steady picture display is ensured in such systems as film projection equipment can display each frame using a similar registration mechanism. However, in telecine and film writers for example, the film conventionally moves through the film gate at a steady speed and thus register pins cannot be inserted.

In conventional telecine equipment, the cinematographic film is positioned in the film gate by pressing one edge against the guide skid as illustrated schematically in FIGS. 1 and 2. As the picture frames are located relative to the sprocket holes and not the film edge, a problem arises when the edge of the film is uneven and not accurately positioned relative to the sprocket holes. In such an instance, the frame with the picture image will wander according to the irregularities in the film edge.

The CRT beam scans the same area of the film gate for each frame. Thus, if the frame is slightly misplaced on the film, its actual position will not coincide exactly with the CRT scan. This will have the result that the displayed image will weave slightly from side-to-side. Because film manufacturers register picture frames with respect to sprocket holes, little care is used in maintaining the taughtness of the film edge, thereby increasing film weave.

It is desirable to minimize this film instability effect, and various methods for so doing have been proposed. For example, U.S. Pat. No. 4,104,680, teaches moving the CRT scan to compensate for the film motion and the error is measured by optical detection of the sprocket hole.

A disadvantage of such a system is that further errors may be caused by the presence of dirt or dust in the sprocket hole.

An object of the present invention is to provide a weave compensation system which does not suffer from the disadvantages of prior art systems.

Broadly, the invention develops a compensation signal based on variations in the distance between the sprocket holes and the film edge.

A system embodying the invention may have a number of advantages. Most notably, the problems incurred in optical sensing techniques caused by the presence of dust or dirt in the sprocket holes or damaged sprocket holes are overcome as the film is registered against the teeth of a sprocket wheel, this being analogous to the register pins in a conventional film projector, and the position of the film edge is measured at this point relative to the sprocket teeth. Preferably the distance is measured using a stylus positioned to contact the film edge.

Embodiments of the invention will now be described by way of example, and with reference to the accompanying drawings, in which:

FIG. 1 is a conventional guidance system showing a film as it passes through a telecine film gate;

FIG. 2 is an enlarged view of a portion of the conventional guidance system of FIG. 1;

FIG. 3 is a schematic diagram of a guidance system embodying the invention;

FIG. 4 is an enlarged view of a portion of the system of FIG. 3;

FIG. 5 is a block diagram of the compensation circuit including the delay unit shown in FIG. 4; and

FIG. 6 is a block diagram of a further development of the compensation circuit of FIG. 5.

The guidance system in FIGS. 3 and 4 shows film 1 passing over a sprocket wheel, the teeth 6 of which are engaged in the sprocket holes 3 of the film. A first pressure roller 8 ensures that the film is located pressing against the inside edge of the sprocket teeth 6 on the side of the film adjacent to a positional transducer 2. This may be seen from the enlarged view of the system shown in FIG. 4. A stylus 16 of the positional transducer device 2 bears on the film edge, and acts to measure the distance between the film edge and a first sprocket hole 4, and deviations therein for subsequent sprocket holes, to provide signals representative of the positional errors at the sprocket holes. The output of the positional transducer system 2 is coupled to a delay unit 10 shown in FIG. 3 which delays the signal corresponding to the error of a particular sprocket hole until that sprocket hole reaches the film gate, at which point it is applied to the horizontal scan shift as a compensation signal. The film passes through the film gate where it is scanned by the CRT raster. The film is maintained in position laterally at this point, by a second pressure roller 14 which presses the film edge against a guide skid 12.

The output from the positional transducer shown in FIGS. 3 and 4 is digitized by a/d converter 20 as shown in FIG. 5. The digitized signal is then applied to a pipeline register delay 22. This error signal is clocked through the pipeline register delay 22 by pulses obtained from the rotation of the sprocket wheel caused by movement of the film, so that the compensation signal corresponding to a particular sprocket hole is delayed and arrives at the output of the register at the same time that the sprocket hole arrives at the scanning area. The signal is then either applied directly to the scan shift to compensate a digital scanning system, or converted to an analog signal in a d/a converter 26 and applied to compensate for horizontal weave of the film image.

A second embodiment is shown in FIG. 6. This circuit derives a further compensation signal which compensates for cyclic errors caused by, for example, imperfections in the sprocket wheel such as the relative lateral displacement of the sprocket teeth.

A proportion of the output of the pipeline register delay 22 is added in an arithmetic logic unit 28 to a proportion of the value stored in a memory 30 corresponding to previous outputs relating to particular sprocket teeth. As the film runs, the ALU 28 will build up in the memory 30 stored values corresponding to the errors in the location of the teeth in the sprocket wheel. The memory locations are addressed by the counter 32 which is advanced or retarded by pulses from the sprocket tachometer according to the direction of film motion, each address corresponding to one sprocket tooth. The output from the memory 30 provides signals representative of the cyclic errors and is applied to a subtractor circuit 24, which subtracts the output of memory 30 from the compensation signal from the output of the pipeline register delay 22 to take account of the cyclic errors, thereby providing an overall compensation signal.

Various alternatives to the embodiments described are possible and will occur to those skilled in the art. For example, although the system embodying the invention has been described with reference to a flying spot telecine, it would be possible for the invention to be included in a flying spot film writer system. Also, the stored signals representative of the cyclic errors present may be precalibrated or automatically entered by taking the long term average of measurements for each tooth.

In some cases, it may be advantageous for the output of the pipeline register to be sampled at a faster rate than once per sprocket tooth as described. This may be achieved by providing more pulses from the sprocket tachometer.

The positional transducer may be a record player type stylus and a piezo-electric transducer may be mechanically, modified to fit the telecine viewing gate.