GB2354088A

GB2354088A - Controlling film tension in film scanner

Info

Publication number: GB2354088A
Application number: GB9921151A
Authority: GB
Inventors: Terence William Mead
Original assignee: Cintel International Ltd
Current assignee: Cintel International Ltd
Priority date: 1999-09-07
Filing date: 1999-09-07
Publication date: 2001-03-14
Anticipated expiration: 2019-09-07
Also published as: GB2392739B; GB9921151D0; GB2392739A; GB0322008D0; GB2354088B; GB2392740A; GB2392740B; GB0322005D0

Abstract

A control system for controlling the tension of film in a film scanner such as a telecine, has a guide member 8 around which film is guided and which, with guide member drive 6, forms a torque bar to oppose tension in the film. Encoder 5 produces a guide member position signal which serves as feedback to control film drive 3. Other variations disclosed include determining a target (central) position of the guide member at which the tension of the film is a minimum for a given drive applied to the guide member by the guide member drive, and using a film position detector 13, 14 to produce a film position signal, and using this signal to control film tension using the film take up or film feed systems.

Description

2354088 Method and Apparatus For Film Tenslon Control This invention

relates to the scanning of cinematographic film to produce electrical signals corresponding to the images stored on the film. Telecine or film scanning equipment used to produce such signals from cinematographic film have been known for many years, and are described for example in "TV and Video Engineers Reference Book" Chapter 39 Butterworth and Heinemann ISBN 0- 7506-1021-2. In particular the invention relates to a continuous motion telecine, or film scanner, or film writer, using a servo system containing torque motors to control the film tension. A commercially available system using torque motors is manufactured and sold under the name C-Reality telecine, by Cintel International Limited.

It is well known that the stability of the picture image from telecine and film scanning equipment of the constant motion type is dependent on the film moving past the imaging gate at a smooth and constant velocity. The constant velocity is generally related to the spacing of the sprocket holes on the film. In this type of equipment it is common practice to use a capstan drive system where the film is driven by a capstan tyre, and the drive to the capstan motor is carefully controlled by a servo system. The servo system maintains a constant capstan velocity by comparing the time separation between pulses from lines on a capstan tachometer, which is solidly attached to the capstan.

The "aim velocity" of this servo loop is controlled by measuring the position of a sprocket wheel tachometer at reference points in time.

The film velocity is thereby adjusted so as to position each frame of the film in proper registration with the image sensor or scanning beam.

The capstan tachometer usually consists of several thousand lines per revolution allowing continuous and accurate correction. There may be provision to correct for cyclical errors in the sprocket wheel, sprocket teeth, or tachometers for example as shown in GB-A-2,266, 169. The sprocket position sensing may also be done by other means than a sprocket wheel, e.g. by scanning across the sprocket hole and detecting its position from the resulting video signal in the fashion of US-A-4,104,680.

For ease of description a system will be considered where a sprocket wheel with a tachometer giving one pulse per film frame is used. In this case it is usual that the servo loop which controls the "aim velocity" of the capstan, by comparing the time of the sprocket pulse with that of a reference frame pulse, has a weaker effect and reacts more slowly than the main capstan velocity servo.

The capstan velocity servo can therefore be considered to substantially control the film motion, and the point of contact of the film with the capstan can be considered as a point of precisely and properly controlled film motion. Mechanical constraints dictate that the point at which the sensors or scanning beam view the film is generally a little distance from the capstan, for example 170 mm. Providing that the film maintains a constant and unchanging dimension between film frames then the film motion will also be precisely and properly controlled at the viewing aperture. This relationship will not hold in the event that the film tension changes; if the film is considered to be attached firmly to the capstan, and the tension on the film is increased, the film will stretch causing a different part of the film frame to appear in the viewing aperture.

A particular cause of changing film tension is variation in the tension applied by the auxiliary servo systems controlling the film supply and wind spools.

For example the film-viewing aperture may be between the capstan and the supply spool such that a variation in tension from the supply spool servo will cause film stretch in this region. It is known to use a sprung compliance arm around which a loop of the film is passed to detect such tension changes, and control the power to the supply spool so that the compliance arm is maintained in a relatively constant position. Sensible operation of this servo requires that there will always be some movement of the compliance arm with an attendant change in the spring pressure and consequently film tension. It is also known to use techniques such as a very long spring to minimise this effect.

An alternative configuration, which is used in the commercially available C-Reality telecine mentioned above, is to use a constant torque compliance motor instead of the sprung compliance arm, this makes it simpler to change the tension applied to the film for different film gauges or operating conditions, and also makes lacing of the film easier. The spooling servo system in this arrangement attempts to maintain the angular position of the torque motor constant, but for sensible operation of this servo there will always be residual changes in this angular position.

Figure 1 shows an example of a known motor servo system. The film 1 is supplied from the film supply spool 2 on the left of the drawing and collected by the film take-up spool on the right of the drawing. The supply and take up servo systems are identical since the transport must operate in either direction, and only the supply system will be described here. Hold-back tension is applied to the film by the left hand supply spooling motor 3 acting against the pull of the capstan drive 9. As the film is rolling through the system this hold back tension is maintained substantially constant despite variations in load of the film spool. This is achieved by detecting movement of the torque arm (8) which indicates a tension change, and varying the drive to the spooling motor to correct the change. The spooling servo system 4 monitors the pulses from the torque arm tachometer 5 and applies drive to the spooling motor 3 such as to maintain the torque arm 8 in a fixed position. Under these conditions the tension applied to the film should be constant and related to the torque applied to the torque motor 6 by the torque signal input 7. This torque signal input is a constant value that is chosen to given the best performance taking into account variables such as the film dimension and condition, and the film speed.

The film is moved through the system by the capstan drive 9, which is powered by the capstan motor 10 under control of the capstan servo 12. In the capstan servo pulses from the capstan tachometer 11 are measured and compared with the desired velocity, to provide a feedback control system to the capstan motor. In this way the speed of the film is maintained constant. The phasing of the film to the scan image is controlled with reference to the film sprocket holes by means of pulses from the sprocket tachometer 14, which is driven by the sprocket wheel 13. The speed of the capstan is adjusted by the capstan servo in response to the pulses from the sprocket tachometer to correctly phase the film image.

Between the capstan and the sprocket the film passes the viewing gate 15 where the scanning beam is imaged onto the film. It is most important that the film is accurately synchronised with the scan image at this point.

Concentricity errors or variation in the load of the film supply spool may cause variation in the power required by the spooling motor to maintain tension constant and thus the compliance arm in the correct position. The spooling servo will substantially maintain the compliance arm or torque arm 8 in a constant position but there will inevitably be some residual movement. We have appreciated that if the torque applied by the torque motor is maintained constant, then the film tension will change in response to this residual movement of the compliance or torque arm. This change in the film tension will result in a strain in the film, causing the dimension between the capstan and the viewing aperture to change, and the position of the film frame in the viewing aperture to move up or down.

We have further appreciated that a compliance arm or torque arm can introduce a variation in film tension because of angular displacement of the arm. In particular, we have appreciated that a disadvantage of the torque motor system is that, for a constant torque from the motor, the film tension will change with the angular position of the torque motor and further that the change can be predicted from the geometry of the system.

Accordingly, in a first aspect, there is provided a control system for controlling the tension of film in a film scanner having a guide member around which film is guided which is biased to oppose tension in film and which is displaceable by variations in film tension, a guide member drive for applying a drive to the guide member to oppose the tension in film, and a guide member position detector for detecting the position of the guide member to produce a guide member position signal, the control system comprising a guide member position signal input for receiving the guide member position signal and for providing an indication of the guide member position, 6 - means for determining the drive to apply to the guide member as a function of the indication of the guide member position, and a control signal output for providing a control signal to the guide member drive for controlling the guide member drive to vary the drive applied to the guide member as a function of the indication of the guide member position.

The first aspect of the invention provides the advantage that the guide member does not introduce variation in tension which can occur in known film scanners as the guide member is displaced. This is achieved by varying the drive applied to the guide member to ensure uniform tension is maintained. In known film scanners the tension can vary with position.

In an embodiment of the first aspect of the invention the detector is a tachometer attached to the drive which is a torque motor to measure its angular position. The position is the angular position of the torque motor and is used to address a look up table, which provides a correction value corresponding to the change in film tension caused at that angular position. The correction value forms a position signal and is added to the torque motor drive signal in a control in such a fashion as to compensate for the change in film tension, thereby maintaining constant film tension. More precisely the correction value corresponds to the drive to the torque motor required to compensate for the film tension error that would otherwise result at an angular position of the torque motor and is a function of the position signal.

We have also appreciated that a compliance or torque arm requires alignment for best operation. In particular, the arm should be positioned such that rotation thereof has a minimum effect on tension variation of the film.

Accordingly, in a second aspect, the invention provides a control system for controlling the tension of film in a film scanner having a film feed arranged to feed film under tension, a guide member around which film is guided which is biased to oppose the tension of the film and which is displaceable by variations in film tension, a guide member drive for applying a drive to the guide member to oppose tension in the film, and a guide member position detector for detecting the position of the guide member to produce a guide member position signal, the control system comprising a guide member position signal input for receiving the guide member position signal, and means for determining a target position of the guide member at which the tension of the film is a minimum for a given drive applied to the guide member by the guide member drive.

An embodiment of the second aspect of the invention automatically adjusts the nominal alignment of the angle of the torque motor attached to a guide member in the form of a torque arm to be correct, by measuring the power applied by the spooling servo system to the spooling motor, which indicates the tension of the film, and changing the nominal angle of the torque motor by varying the tension provided by the spooling servo system so as to achieve a minimum power value whilst maintaining a constant value of torque from the torque motor.

We have also appreciated that, notwithstanding improvements to the way in which tension control is done using a torque arm, residual variations in tension can occur. We have appreciated that such residual variations can be detected.

Accordingly, in a third aspect, the invention provides a control system for controlling tension of film in a film scanner having a film feed system arranged to feed film under tension, a film drive arranged to drive film through the scanner from the film feed system, and a film position detector arranged to detect the position of film between the film feed system and the film drive to produce a film position signal, the control system comprising a film position signal input for receiving the film position signal and, means for controlling the film feed system in response to variations in tension of the film detected by the film position detector.

This aspect of the invention provides improved control on the basis that, for a given drive speed of film, the position of the film should be constantly moving. However, if there are variations in film tension, these appear as variations in the speed and hence position of the film. The film feed system is thus varied to counteract such tension changes.

An embodiment of the third aspect of the invention automatically adjusts the scaling of the film feed system tension control servo for optimum cancellation of the tension variation. This automatic adjustment is performed by monitoring the pulses from the film position detector such as a sprocket wheel tachometer and computing the variation in speed of these pulses, this speed variation corresponds to the residual stretch errors in the film and indicates the degree of scaling error in the tension control servo loop of the film feed system. The scaling error is corrected by applying this signal to a multiplier within the film feed system that changes the gain of the tension control servo loop accordingly.

Since cyclical errors of the sprocket wheel could produce speed variations which may confuse this process it is advisable to first remove the cyclical errors for example as taught in GB-A-2,266,169. This is within the scope of an embodiment of the invention.

A further possible source of confusion to the scaling process is the variation in film shrinkage (unstressed film length per frame), which would also appear as a speed variation in the sprocket pulses. This film shrinkage would in itself result in film position movement and so could be corrected by the automatic scaling process. There is however a risk of exceeding the permissible range of film tension applied and in some circumstances it may be advisable to avoid or reduce correction of film shrinkage by making the automatic scaling process respond only to effects alternating more rapidly than the film shrinkage. As an example a cut off frequency for the loop may be chosen at 0.1 Hertz, however this value can be selected for optimum performance, and is also within the scope of an embodiment of the invention.

The invention in the various aspects is defined in the independent claims to which reference is directed. Preferred features are set out in the dependent claims. It is noted, for the avoidance of any doubt, that the three aspects of the invention may be used in conjunction or separately. In the embodiment described, all three aspects of the invention are embodied.

An embodiment of the invention will now be described, with reference to the figures in which:

Figure 1: is a schematic diagram of a known film scanner; Figure 2: is a schematic diagram of a known film scanner embodying the invention; Figure 3: is a schematic diagram of a controller embodying the invention for use in a film scanner; and Figure 4: is a graph of the inverse cosine function.

The embodiment of the invention described is a control system used within a film transport mechanism within a film scanner or telecine. It is not necessary to describe a complete film scanner or telecine as these are well known to those skilled in the art. The embodiment described is particularly suited to so called "continuous motion" telecines such as CCD line array or CRT machines in contrast to intermittent motion telecines.

The key parts of a film scanner embodying the invention, particularly the film transport and control, are shown in figure 2 which uses the same numbering as Figure 1 for like parts. The film scanner broadly comprises a film feed system and film take up system for supplying and gathering film to the scanning position, a film drive to control the speed of movement of the film through the scanner, a film position detector and a control.

The film feed system (and film take up system) comprises a film feed in the form of supply spool 2 which holds a reel of film 1 and a spooling motor 3 which provides hold back tension on film fed from the spool. The hold back tension is governed by a spooling servo system 4.

The film feed system also comprises a guide member in the form of a torque arm mounted on a guide member drive in the form of a torque motor 6. A position detector in the form of a torque arm tachometer 5 measures the angular displacement of the torque arm which is an indication that the tension of the film has varied. This is because the torque arm is biased against the tension of the film by the torque motor 6, so that any increase or decrease in the film tension will cause the torque arm to move. The displacement of the torque arm from its usual position is measured by monitoring pulses from the torque arm tachometer. The film tension is maintained by applying a drive to the spooling motor 3 to increase or decrease the tension until the torque arm is returned to its usual position. In this way the tension of the film is governed in the manner previously described and using the spooling servo system 4.

The film 1 is moved through the system by a film drive in the form of a capstan drive 9 powered by a capstan motor 10 under control of a capstan servo 12. In the capstan, servo pulses from the capstan tachometer 11 are measured and compared with the desired velocity to provide a feedback control system to the capstan motor. In this way the speed of the film is maintained constant. A film position detector comprising a sprocket wheel 13 and sprocket wheel tachometer 14 detects the position of the film, and provides pulses from the tachometer 14 to control the phase of the film with respect to the film scan from a CRT or other scanner. To do this the speed of the capstan is adjusted by the capstan servo in response to pulses from the sprocket tachometer.

The control unit embodying the invention is a tension control servo 16 which has inputs from the sprocket wheel tachometer 14, the torque arm tachometer 5 and a torque signal input 7, and provides an output to control the torque motor 6 and spooling servo system 4, as shown in Figure 2.

The tension control servo 16 applies the torque signal input to the torque motor and also adjusts the torque in response to the position of the torque arm to maintain constant film tension. Since the above effects can occur in similar fashion in the film take up spooling and torque system, it is preferable that a tension control servo is also applied to the take up torque arm as shown.

The importance of the tension control can be seen by considering the effect of tension variation on film.

For 16 mm film assuming film cross sectional area of 16 mm x 0.15 mm, a nominal film tension of 3.9 Newtons, a distance from the capstan to the film viewing gate of 170 mm, and Young's modulus of elasticity for the film of 449 N/mm 2. Then the nominal film stretch between the capstan and the viewing gate will be:

3.9 x 170/(16 x 0.15 x 449) = 0.615 mm.

Aiming for a picture stability better than 0.025% of picture height (7 mm) permits less than 1.75 microns of positional registration error of the film, this requires the film tension to be controlled to better than 0.29% of nominal. Considering the geometry of a typical torque motor compliance arm used in a telecine, we have appreciated that the film tension will change in accordance with the inverse cosine of the angle of the torque motor. Figure 4 shows a graph plotting film tension against angular position of the torque motor; it can be seen that the desired 0.29% variation of tension would correspond to an angular movement of about 5 degrees, or if the nominal alignment of the motor were displaced by 10 degrees then this would correspond to less than 1 degree of change of motor angle.

The reason why the tension is affected by angular position of the torque arm, and varies as the inverse cosine of the angle of the torque motor can be seen with reference to Figure 2, even though this is a schematic view.

The purpose of the torque arm is to provide an indication that the tension of the film has changed to provide feedback to the spooling servo system 4. With the torque provided by the torque motor 6 constant, the force at an end of the torque arm 8 is the torque divided by the length of the torque arm, which is also constant. In the usual position of the torque arm, the film feeding from and to the torque arm is substantially perpendicular thereto, and so the force on the film is the force at the end of the torque arm. As the torque arm rotates, however, only a component of the force provided by the film tension at the end of the torque arm is in the direction rotation of the arm. That component is proportional to the cosine of the angular displacement of the torque arm. Accordingly, the tension of the film in equilibrium is proportional to the inverse cosine of the torque.

The amount of tension variation corresponding to a given rotation of the torque arm is a minimum when the torque arm is nominally located perpendicular to the film path. To ensure this condition the power being applied to the is spooling motor is monitored, and the aim position for the torque arm (used by the spooling servo) is adjusted for a minimum value of applied spooling motor power. This is in accordance with the second aspect of the invention.

Figure 3 is a schematic diagram of the tension control servo 16. When the equipment is first actuated the torque motor is driven to a reference position which may be one end of its travel. The spooling servo is then actuated and will apply tension to the film causing the torque arm to rotate.

The counter 161 will count the number of tachometer pulses as the torque arm rotates towards its new position (there may typically be 10,000 pulses per revolution). This pulse count is compared with a target count in the comparator 162, if the difference is positive it decreases the drive to the spooling motor. In this way the torque arm is positioned substantially at the target rotation.

The target count is provided by the centre position calculator 169. The centre position calculator can provide a predetermined value or can automatically provide the optimum position.

This automatic procedure measures the mean value of the drive to the spooling motor servo using the low pass filter 168, and then stores this first mean value. The target count is then advanced by a small amount and the measurement repeated, if this second mean value is lower than the first stored value then the target count is advanced in the same direction, otherwise the target count is moved in the opposite direction. This process is repeated continuously thereby maintaining the target count, and the torque arm position, at the point where minimum drive current to the torque motor is required. This process is arranged to operate very slowly (e.g. one increment per second) so that it is not unduly affected by the variation in spooling motor drive due to other effects. A variation of the procedure would be to run the automatic adjustment process for a while with the film stationary (in this way these other effects are reduced in significance), then store the achieved value for subsequent use when the film is running.

The comparator 162 provides a signal representing the deviation from the target rotation of the torque arm. This signal is applied to a look-up table 163 that gives a signal representing the additional torque required to maintain constant film tension. For the torque arm considered here the film tension is proportional to the applied torque divided by the cosine of the angle of deviation from the optimum position of the torque arm.

The look-up table therefore needs to contain the cosine function of the deviation signal. (Required torque constant x required tension x cosine of angle). The correction signal from the look-up table is adjusted to the required proportion by the multiplier 164.

The scaling of the additional torque signal needs to be adjusted for best results, this could be done by a manual pre-set adjustment, or as shown in Figure 2, by control of a signal from the sprocket wheel tachometer. The phase of the pulses from the sprocket wheel tachometer correspond with the stretch of the film between the capstan and the sprocket wheel, and so give a measure of any residual tension errors. This measure of residual tension error is applied to a scaling multiplier that multiplies the value of the additional torque signal to reduce the residual error. This is in accordance with the third aspect of the invention.

The scaling multiplier input can be a predetermined fixed value or can be obtained from the scaling calculator 165.

is The scaling calculator may be operated dynamically whilst the film is being scanned, or is preferably operated in a pre-operational sequence where the sprocket wheel input to the capstan servo is disabled. With the sprocket wheel input disabled the capstan servo maintains the film motion at a constant speed, but not in phase lock with the scans.

Under these conditions the pulses from the sprocket wheel tachometer should be of constant speed, any deviation which would correspond to varying film stretch, is detected by the scaling calculator 165 using the sprocket tachometer input. If the scaling input to the multiplier is too great then the tension control servo will overcorrect the tension errors so the multiplier gain needs to be reduced, and vice versa. The scaling calculator compares the polarity of the stretch errors detected from the sprocket wheel tachometer with those predicted by the comparator and slowly increments the scaling input to the multiplier towards its correct value. The multiplier output signal is converted into analogue form by the digital to analogue converter 166 and then the amplifier 167 adds it to the torque signal input 7 to produce a drive signal for the torque motor.

Claims

1. A control system for controlling the tension of film in a film scanner having a guide member around which film is guided which is biased to oppose tension in film and which is displaceable by variations in film tension, a guide member drive for applying a drive to the guide member to oppose the tension in film, and a guide member position detector for detecting the position of the guide member to produce a guide member position signal, the control system comprising:

a guide member position signal input for receiving the guide member position signal and for providing an indication of the guide member position, means for determining the drive to apply to the guide member as a function of the indication of the guide member position, and a control signal output for providing a control signal to the guide member drive for controlling the guide member drive to vary the drive applied to the guide member as a function of the indication of the guide member position.

2. A control system according to claim 1, wherein the guide member position detector indicates the angular position of the guide member, the guide member position signal input provides an indication of the angular position of the guide member and the means for determining the drive to apply to the guide member comprises means for determining the drive as a function of the indication of the angular position of the guide member.

3. A control system according to claim 2, wherein the means for determining the drive as a function of the indication of the angular position of the guide member comprises means for determining the required drive as a function of the cosine of the indication of the angular position of the guide member.

4. A control system according to claim 1, 2 or 3, wherein the guide member is a pivotable arm, the guide member position detector is arranged to detect the angle of rotation of the pivotable arm, and the means for determining the drive comprises means for determining the required drive as a function of the angle of rotation of the pivotable arm.

S. A control system according to any preceding claim, is wherein the means for determining the drive comprises a look up table.

6. A control system according to any preceding claim, wherein the guide member position signal input comprises means for determining the displacement of the guide member from a target position and the indication of the guide member position is an indication of the displacement of the guide member from the target position.

7. A control system according to claim 6, wherein the guide member position detector comprises a guide member tachometer, the position signal comprises pulses from the guide member tachometer and the guide member position signal input comprises a counter for counting the pulses from the guide member tachometer.

8. A control system according to claim 7, wherein the guide member position signal input comprises means for determining the difference in position of the guide member from the target position by comparing the number of pulses counted by the counter with a target count to produce the indication of the guide member position.

9. A control system according to claim 8, wherein the means for determining the difference in position of the guide member from a target position comprises a comparator.

10. A control system according to any of claims 6 to 9, wherein the guide member is pivotable, and the difference in position is the angle of displacement of the guide member from the target position.

11. A control system according to any of claims 6 to 10, wherein the means for determining the drive to apply to the guide member comprises means for determining the drive as a function of the cosine of the angle of displacement of the guide member from the target position.

12. A control system according to any preceding claim, wherein the guide member drive is a torque motor, the drive is torque from the torque motor and the control signal output comprises means for generating a control signal to apply to the torque motor to provide torque as a function of the guide member position signal.

13. A control system according to claim 9, wherein the means for generating a control signal comprises a multiplier.

14. A control system according to claim 9 or 10, wherein the means for generating a control signal comprises a digital to analogue converter.

15. A control system according to any preceding claim, wherein the guide member is a torque arm.

16. A control system according to any of claims 6 to 11, further comprising means for determining the target position of the guide member.

17. A control system according to claim 16, where the means for determining the target position comprises means for determining the position of the guide member at which tension in film is a minimum for a given drive applied to the guide member.

18. A control system according to claim 16 or 17, wherein the film scanner has a film feed arranged to feed film under tension, and the means for determining the target position of the guide member comprises means for controlling the film feed to vary the feed of film and means for determining the position of the guide member at which the film feed provides a minimum tension.

19. A control system according to claim 18, wherein the film feed comprises a spool and spooling motor, the means for determining the target position comprising means for measuring the drive to the spooling motor.

20. A control system according to claim 18, the means for measuring the drive comprising a centre position calculator for determining the target position of the guide member at which the drive to the spooling motor is least.

21. A control system according to any preceding claim, further comprising a feedback control arranged to receive the indication of the guide member position from the guide member position signal input and a film position signal, and arranged to provide a scaling signal to the control signal output to vary the control signal as a function of the film position signal.

22. A control system according to any claim 21, wherein the film scanner comprises a film feed system arranged to feed film under tension, a film drive arranged to drive film through the scanner from the film feed system, and a film position detector arranged to detect the position of the film between the film feed system and the film drive to produce a film position signal, the control system comprising a film position signal input for receiving the film position signal.

23. A film scanner including a control system for controlling the tension of film comprising a guide member around which film is guided which is biased to oppose tension in film and which is displaceable by variations in film tension, a guide member drive for applying a drive to the guide member to oppose the tension in film, and a guide member position detector for detecting the position of the guide member to produce a guide member position signal, the control system comprising:

24. A film scanner according to claim 23, wherein the guide member position detector indicates the angular position of the guide member, the guide member position signal input provides an indication of the angular position of the guide member and the means for determining the drive to apply to the guide member comprises means for determining the drive as a function of the indication of the angular position of the guide member.

25. A film scanner according to claim 24, wherein the means for determining the drive as a function of the indication of the angular position of the guide is member comprises means for determining the required drive as a function of the cosine of the indication of the angular position of the guide member.

26. A film scanner according to claim 23, 24 or 25, wherein the guide member is a pivotable arm, the guide member position detector is arranged to detect the angle of rotation of the pivotable arm, and the means for determining the drive comprises means for determining the required drive as a function of the angle of rotation of the pivotable arm.

27. A film scanner according to any of claims 23 to 26, wherein the means for determining the drive comprises a look up table.

28. A film scanner according to any of claims 23 to 27, wherein the guide member position signal input comprises means for determining the displacement of the guide member from a target position and the indication of the guide member position is an indication of the displacement of the guide member from the target position.

29. A film scanner according to claim 28, wherein the guide member position detector comprises a guide member tachometer, the position signal comprises pulses from the guide member tachometer and the guide member position signal input comprises a counter for counting the pulses from the guide member tachometer.

30. A film scanner according to claim 29, wherein the guide member position signal input comprises means for determining the difference in position of the guide member from the target position by comparing the number of pulses counted by the counter with a target count to produce the indication of the guide 15 member position.

31. A film scanner according to claim 30, wherein the means for determining the difference in position of the guide member from a target position comprises a comparator.

32. A film scanner according to any of claims 28 to 31, wherein the guide member is pivotable, and the difference in position is the angle of displacement of the guide member from the target position.

33. A film scanner according to any of claims 28 to 32, wherein the means for determining the drive to apply to the guide member comprises means for determining the drive as a function of the cosine of the angle of displacement of the guide member from the target position.

34. A film scanner according to any of claims 23 to 33, wherein the guide member drive is a torque motor, the drive is torque from the torque motor and the control signal output comprises means for generating a control signal to apply to the torque motor to provide torque as a function of the guide member position signal.

35. A film scanner according to claim 34, wherein the means for generating a control signal comprises a multiplier.

36. A film scanner according to claim 34 or 35, wherein the means for generating a control signal comprises a digital to analogue converter.

37. A film scanner according to any of claims 23 to 36, wherein the guide member is a torque arm.

38. A film scanner according to any of claims 28 to 33, further comprising means for determining the target position of the guide member.

39. A film scanner according to claim 38, where the means for determining the target position comprises means for determining the position of the guide member at which tension in film is a minimum for a given drive applied to the guide member.

40. A film scanner according to claim 38 or 39, wherein the film scanner has a film feed arranged to feed film under tension, and the means for determining the target position of the guide member comprises means for controlling the film feed to vary the feed of film and means for determining the position of the guide member at which the film feed provides a minimum tension.

41. A film scanner according to claim 40, wherein the film feed comprises a spool and spooling motor, the means for determining the target position comprising means for measuring the drive to the spooling motor.

42. A film scanner according to claim 38, the means for measuring the drive comprising a centre position calculator for determining the target position of the guide member at which the drive to the spooling motor is least.

43. A film scanner according to any of claims 23 to 42, further comprising a feedback control arranged to receive the indication of the guide member position from the guide member position signal input and a film position signal, and arranged to provide a scaling signal to the control signal output to vary the control signal as a function of the film position signal.

44. A film scanner according to claim 43, wherein the film scanner comprises a film feed system arranged to feed film under tension, a film drive arranged to drive film through the scanner from the film feed system, and a film position detector arranged to detect the position of the film between the film feed system and the film drive to produce a film position signal, the control system comprising a film position signal input for receiving the film position signal.

45. A control system for controlling the tension of film in a film scanner having a film feed arranged to feed film under tension, a guide member around which film is guided which is biased to oppose the tension of the film and which is displaceable by variations in film tension, a guide member drive for applying a drive to the guide member to oppose tension in the film, and a guide member position detector for detecting the position of the guide member to produce a guide member position signal, the control system comprising:

a guide member position signal input for receiving the guide member position signal, and means for determining a target position of the guide member at which the tension of the film is a minimum for a given drive applied to the guide member by the guide member drive.

46. A control system according to claim 45, wherein the means for determining the target position comprises a film feed controller for controlling the film feed system to vary the film tension, and means for determining the position of the guide member at which the tension is a minimum.

47. A control system according to claim 45 or 46, wherein the guide member position detector indicates the angular position of the guide member, and the target position is the angle of the guide member at which the tension of film is a minimum.

48. A control system according to any of claims 45, 46 or 47, wherein the guide member position detector comprises a guide member tachometer, the position signal comprises pulses from the guide member tachometer, and the guide member position signal input comprises a counter for counting the pulses from the guide member tachometer.

49. A control system according to claim 48, wherein the guide member position signal input comprises means for determining the difference in position of the guide member from the target position by comparing the number of pulses counted by the counter with the target count.

50. A control system according to any of claims 45 to 49, the film feed comprising a spool and spooling motor, the means for determining the target position comprising means for measuring the drive to the spooling motor.

51. A control system according to claim 50, the means for measuring the drive to the spooling motor comprising a centre position calculator for determining the target position of the guide member at which the drive to the spooling motor is least.

52. A film scanner including a control for controlling the tension of film comprising, a film feed arranged to feed film under tension, a guide member around which film is guided which is biased to oppose the tension of the film and which is displaceable by variations in film tension, a guide member drive for applying a drive to the guide member to oppose tension in the film, and a guide member position detector for detecting the position of the guide member to produce a guide member position signal, the control system comprising:

53. A film scanner according to claim 52 wherein the means for determining the target position comprises a film feed controller for controlling the film feed system to vary the film tension, and means for determining the position of the guide member at which the tension is a minimum.

54. A film scanner according to claim 53 or 54, wherein the guide member position detector indicates the angular position of the guide member, and the target position is the angle of the guide member at which the tension of film is a minimum.

55. A film scanner according to any of claims 52, 53 or 54, wherein the guide member position detector comprises a guide member tachometer, the position signal comprises pulses from the guide member tachometer, and the guide member position signal input comprises a counter for counting the pulses from the guide member tachometer.

is

56. A film scanner according to claim 54, wherein the guide member position signal input comprises means for determining the difference in position of the guide member from the target position by composing the number of pulses counted by the counter with the target count.

57. A film scanner according to any of claims 52 to 56, the film feed comprising a spool and spooling motor, the means for determining the target position comprising means for measuring the drive to the spooling motor.

58. A film scanner according to claim 57, the means for measuring the drive to the spooling motor comprising a centre position calculator for determining the target position of the guide member at which the drive to the spooling motor is least.

59. A film scanner or control system according to any preceding claim, wherein the guide member is a torque arm.

60. A film scanner or control system according to any preceding claim wherein the guide member drive is a torque motor and the drive is torque.

61. A method of determining a target position of a guide member in a film scanner having a film feed arranged to feed film under tension, a guide member around which film is guided which is biased to oppose the tension of the film and which is displaceable by variations in film tension, a guide member drive for applying a drive to the guide member to oppose tension in the film, and a guide member position detector for detecting the position of the guide member to produce a guide member position signal comprising:

applying tension to the film using the film feed thereby causing the guide member to be displaced, measuring the tension provided by the film feed and the position of the torque arm, varying the tension provided by the film feed and measuring the new tension and position of the torque arm further times, and determining from the position and tension measurements the position of the guide member at which the tension is a minimum.

62. A method according to claim 61, wherein the guide member position detector indicates the angular position of the guide member, the step of determining the position comprises determining the angular position and the target position is the angle of the guide member at which the tension of film is a minimum.

63. A method according to claim 61 or 62, wherein the guide member position detector comprises a guide member tachometer, the position signal comprises pulses from the guide member tachometer, and the guide member position signal input comprises a counter for counting the pulses from the guide member tachometer and the step of measuring the position of the guide member comprises counting the pulses.

64. A method according to claim 63, wherein the step of determining the difference in position of the guide member from the target position comprises comparing the number of pulses counted by the counter with the target count.

65. A method according to any of claims 61 to 64, the film feed comprising a spool and spooling motor, the step of measuring the tension comprising measuring the drive to the spooling motor.

66. A control system for controlling tension of film in a film scanner having a film feed system arranged to feed film under tension, a film drive arranged to drive film through the scanner from the film feed system, and a film position detector arranged to detect the position of film between the film feed system and the film drive to produce a film position signal, the control system comprising:

a film position signal input for receiving the film position signal and, means for controlling the film feed system in response to variations in tension of the film detected by the film position detector.

67. A control system according to claim 66, the film feed system comprising a film feed and a guide member around which film is guided which is biased to oppose tension in film and which is displaceable by variations in film tension, the means for controlling the film feed system comprising means for determining the drive to apply to the guide member as a function of the film position signal.

68. A control system according to claim 67, the film scanner having a guide member drive for applying a drive to the guide member to oppose the tension in film, and a guide member position detector for detecting the position of the guide member to produce a guide member position signal, the control system further comprising:

a guide member position signal input for receiving the guide member position signal and for providing an indication of the guide member position, and means for determining the drive to apply to the guide member as a function of the indication of the guide member position.

69. A control system according to claim 68 further comprising a control signal output for providing a control signal to the guide member drive for controlling the guide member drive to vary the drive applied to the guide member as a function of the indication of the guide member position.

70. A control system according to claim 69, wherein the means for determining the drive is arranged to receive the indication of the guide member position from the guide member position signal input and a film position signal, and arranged to provide a scaling signal to the control signal output to vary the control signal as a function of the film position signal.

71. A control system according to any of claims 66 to 70, wherein the film position detector comprises a film sprocket hole detector.

72. A control system according to any of claims 66 to 71, wherein the means for controlling the film feed system comprises a scaling calculator.

73. A film scanner including a control system for controlling tension of film the film scanner having a film feed system arranged to feed film under tension, a film drive arranged to drive film through the scanner from the film feed system, and a film position detector arranged to detect the position of is film between the film feed system and the film drive to produce a film position signal, the control system comprising:

74. A film scanner according to claim 73, the film feed system comprising a film feed and a guide member around which film is guided which is biased to oppose tension in film and which is displaceable by variations in film tension, the means for controlling the film feed system comprising means for determining the drive to apply to the guide member as a function of the film position signal.

75. A film scanner according to claim 74, comprising a guide member drive for applying a drive to the guide member to oppose the tension in film, and a guide member position detector for detecting the position of the guide member to produce a guide member position signal, the control system further comprising:

76. A film scanner according to claim 75 further comprising a control signal output for providing a control signal to the guide member drive for controlling the guide member drive to vary the drive applied to the guide member as a function of the indication of the guide member position.

77. A film scanner according to claim 76, wherein the means for determining the drive is arranged to receive the indication of the guide member position from the guide member position signal input and a film position signal, and arranged to provide a scaling signal to the control signal output to vary 25 the control signal as a function of the film position signal.

78. A film scanner according to any of claims 73 to 77, wherein the film position detector comprises a film sprocket hole detector.

79. A film scanner according to any of claims 73 to 78, wherein the means for controlling the film feed system comprises a scaling calculator.