GB2351198A

GB2351198A - Telecine system with afterglow correction.

Info

Publication number: GB2351198A
Application number: GB0006385A
Authority: GB
Inventors: Stuart William Arundell Hunt
Original assignee: Innovation TK Ltd
Current assignee: Innovation TK Ltd
Priority date: 1999-03-16
Filing date: 2000-03-16
Publication date: 2000-12-20
Anticipated expiration: 2020-03-16
Also published as: GB2351198B; GB0006385D0; GB9906025D0

Abstract

A method and apparatus for correcting afterglow of a cathode ray tube (CRT) in a flying spot scanner wherein light output from the CRT is measured during a scanning operation at a time or position when information (i.e. film) is not being recorded. An afterglow correction value is then derived from the measured output. In the embodiment shown , light from CRT 1, unmodulated by film 3, is detected by burn cell 2. The output of this cell is then delayed by delay 6, with the output of the main scanning detector 4 also being delayed by a delay 5. Signals from delays 5, 6 are then passed to respective afterglow correctors 9, 10: the afterglow corrected main signal is then divided by the afterglow corrected burn signal in a divider 11, providing an output signal. A further embodiment is shown in Figure 5.

Description

2351198 Telecine Systems The present invention relates to cathode ray

tubes (CRTs), particularly to cathode ray tubes used in flying spot scanners, for example in telecine machines and film scanners. In general, the invention relates to methods and apparatus for compensating for deviations of the output of a flying spot scanner, in particular a telecine machine, from an ideal signal due to inherent properties of the cathode ray tube and detection system, such as afterglow.

Telecine machines and film scanners are examples of devices for converting images stored on photographic or cinematographic film to corresponding electrical signals. In general, such devices operate by detecting the light from an illumination source transmitted through a region of a film frame and converting the detected light to an electrical signal corresponding in amplitude to the intensity of the transmitted light, by means of a photodetector, such as a photomultiplier tube. A plurality of photodetectors may be provided to produce electrical signals corresponding to the intensity of particular wavelengths of light. Usually, respective photodetectors are provided for each of the red, green and blue wavelength components of the transmitted light.

Telecine machines have been known for at least 50 years in one form or another, and are currently manufactured by several companies, including Cintel International Limited, of Ware, Hertfordshire, UK.

Telecine machines which have been manufactured by Cintel International Limited include the Rank Cintel Mk III, the Cintel URSA, and the Cintel C-Reality, all of which use a cathode ray tube (CRT) as an illumination source in the scanning system, in a well-known manner.

In such telecine machines, the cathode ray tube generates a beam of electrons which is directed -o the faceplate of the CRT, the inside of which is coated with a phosphor. When an electron strikes the phosphor, the phosphor emits light, and thus the beam of electrons causes a bright spot of light (a "flying spot") to appear on the faceplate of the CRT. The position of the flying spot on the faceplate of the CRT is controlled by applying a horizontal and vertical magnetic field of varying strength to the electron beam in order to deflect the electron beam in the horizontal and vertical directions. By applying a gradually increasing horizontal magnetic field to the electron beam, it is possible to direct the flying spot to scan a horizontal "line,, across the face of the CRT. By repeating the horizontal deflection with an increased vertical deflection, it is possible to scan a series of parallel lines, until a "scan patch" has been scanned on the face of the CRT.

It should be noted that the terms "horizontal" and "vertical" used herein are not intended to imply any strict requirement as to the orientation of the CRT, but are merely used as convenient labels for two substantially orthogonal directions on the CRT face.

Ideally, the phosphor of the CRT faceplate would emit a full quantity of light instantly when hit with electrons, and would instantly cease to emit light when the electrons are not hitting the phosphor. In practice, it takes a finite time after the electrons are first applied to a particular region of the phosphor for the phosphor to reach full brightness. Likewise, after the electron beam has been turned off, there is a significant period of time before the level of emitted light reaches zero. The phenomenon of continued emission from a particular region of the CRT even after the electron beam has ceased to be incident on that region is known as "afterglow". The decay time of practical phosphors can be so long that it takes many seconds for the light level to return to zero after the electron beam has been removed.

Afterglow is a significant problem in the field of flying spot scanners, as it is generally assumed that the light detected by the photodetectors at any given time is generated only by the flying spot at its current position on the faceplate of the CRT. The reduction in intensity of the light received by the photodetectors is therefore taken to be due to the opacity of the film frame in the film gate at the position (x,y) corresponding to the current position of the flying spot.

However, in practice, the light collected at time t not only includes the light transmitted through the film at position (x, y) but also the decaying light from the previous illumination point (x-1, y) in the scan patch, which was illuminated at time t-1. Furthermore, there will also be contributions to the signal level due to the decaying light from the sample at the pre-preceding position (x-2,.y) taken at time t-2 and from other preceding positions depending on the decay time of the phosphor.

Thus, at time t, the photodetectors not only detect the amount of light transmitted by the film frame at the current scan position (x,y) of the flying spot but also a proportion of the light transmitted at the preceding scan position (x-1,y) and the pre-preceding position (x2,y) and so on. Effectively, the spatial resolution of the flying spot scanner in the x-direction (horizontal) is reduced due to afterglow.

Many solutions to the above problem have been proposed. As long ago as at least 1975, Rank Cintel included analogue afterglow correction circuitry in the Mk III telecine machine. The analogue circuits performed a "delay and decay" function in an attempt to emulate and correct for afterglow.

Furthermore, GB-A-2239572 (Rank Cintel) proposes a digital implementation of a system which examines neighbouring picture elements around the current picture element and attempts to derive a combined "flare" (phosphor emission over an illumination area greater than that desired) and afterglow correction.

According to a first aspect of the present invention, there is provided a method of correcting for afterglow of a cathode ray tube in a flying spot scanner, wherein the light output from the cathode ray tube is measured during a scanning operation of the flying spot scanner at a time during the scanning operation when image information is not being recorded, and an afterglow correction value is derived from the measured light output.

In a preferred arrangement, the light output of the cathode ray tube may be measured in the line blanking and/or frame blanking period of the scanning operation. The line blanking period is the period of time notionally taken by the flying spot to return to the start of the next horizontal line in a raster pattern. Similarly, the frame blanking period is the period of time notionally taken by the flying spot to return to the start of the first horizontal line of the next frame to be scanned.

The method of the invention has significant advantages over previously known methods.

Firstly, according to embodiments of the invention differences in the afterglow characteristics of the cathode ray tube when the film to be viewed is stationary and when the film is being transported through a telecine machine are automatically compensated. In general, when the scanned film is stationary, the CRT raster "patch" is of the same aspect ratio as the film area to be scanned, even when there is some optical magnification or reduction of the film image. However, when the film is transported through the telecine in a continuous motion transport, by the time the bottom line of the film frame is scanned, it has moved towards the top of the film gate. Thus the required CRT raster patch is effectively compressed in the direction of travel of the film compared to the patch for stationary scanning of the film.

Alternatively, the film can be scanned from the bottom line of the film frame up to the top. In this case, the required CRT raster patch is effectively stretched in the direction of travel of the film compared to the patch for stationary scanning of the film. The scanned image can then be held in a frame store and read out of the store in reverse order so that the image will be the right way up. The size of the scan patch affects the afterglow characteristics of the CRT during scanning.

According to conventional afterglow correction methods, separate calibration of the afterglow correction coefficients would be required for the stationary scanning of the film and for each transport rate of the telecine machine. However, according to embodiments of the invention, afterglow correction is carried out in the course of the scanning operation by reference to the light output from the cathode ray tube at a time during the scanning operation when image information is not recorded by the scanner. In effect, therefore the afterglow correction according to embodiments of the invention is constantly calibrated, so that the afterglow compensation is unaffected by the transport rate of the telecine machine.

Furthermore, in a conventional telecine, afterglow adjustment facilities are provided for the engineering personnel of the end user to adjust. Commonly, an afterglow calibration film is used, which has horizontal "bars" of progressively increasing length thereon. The engineer sets the values of the, usually five, time constants used to emulate the effect of afterglow to produce the best correction for each of the bars. The inventor has realised that this needs to be done frequently, because the afterglow characteristics of CRTs differ even between CRTs from the same manufacturer. Thus the afterglow calibration adjustment is required at least once per installed CRT. In practice, the afterglow adjustment needs to be done much more frequently. This is commonly thought to be due to one of several mechanisms including "drift" in the afterglow circuitry. However, the inventor has realised that the variation in the afterglow compensation of known systems after initial calibration is predominantly due to the change in the afterglow characteristics of the CRT due to an "ageing" process. According to this process, the response of the CRT phosphor to the electron beam changes over time as the CRT is used, such that the phosphor tends to emit a lower intensity of light for the same intensity of the electron beam as the CRT ages. Consequently, the afterglow characteristics of the CRT vary as the CRT ages.

Again, conventional afterglow correction methods, require an initial calibration step which must be repeated on the user's initiative as the CRT ages.

According to embodiments of the invention, however, afterglow correction is carried out in the course of the scanning operation by reference to the light output from the cathode ray tube at a time during the scanning operation when image information is not recorded by the scanner. Thus, the afterglow correction according to embodiments of the invention is constantly calibrated, so that the afterglow compensation is unaffected by ageing of the cathode ray tube.

Moreover, the inventor has realised that the afterglow characteristics of a CRT are also related to the recent thermal conditioning of the phosphor. When the CRT is operating, the temperature of the phosphor increases over time due to bombardment by the electron beam. As the phosphor heats up, its emission characteristics and thus the afterglow characteristics of the CRT change. It is therefore expected that a different afterglow setting is required during transport of the film when the scanning patch covers a smaller or larger area of the CRT face and the energy of the electron beam is more or less concentrated than for stationary scanning of the film. This can be demonstrated by setting the afterglow characteristics on a conventional telecine machine to be correct in the stationary position, and then transporting the film. It will be noticed that the afterglow characteristics cease to be correct.

In order to deal with this problem, it might be possible for a conventional telecine machine to store separate afterglow settings for stationary scanning and transported scanning of the film. However, the size of the raster patch during transported scanning depends on the film transport speed, the "cropping" and/or "zoom" that the operator wishes to use, the format of the film used (i.e. whether 35mm, 16mm or another format of film is used), the aspect ratio of the television output frame (4:3 or 16:9), and the amount of parallax correction, among other operator selectable functions. There are in practice an almost infinite number of raster patch shapes and sizes, making the manual determination and storage of the afterglow characteristics for all of these combinations effectively impossible.

According to embodiments of the invention, afterglow correction is carried out in the course of the scanning operation by reference to the light output from the cathode ray tube at a time during the scanning operation when image information is not being recorded. In effect, therefore the afterglow correction according to embodiments of the invention is constantly calibrated, so that it is ensured that the afterglow compensation is appropriate to the scanning settings of the telecine machine.

The inventor has realised that the best way to correct for this variable afterglow is to calculate the optimum afterglow correction for each and every frame.

The optimal correction values may then be used according to a conventional afterglow compensation technique, for example in a conventional afterglow corrector.

The afterglow corrector may be implemented by analogue circuitry, for example as in a Cintel Mk III telecine machine. Preferably, however, the afterglow corrector is implemented by digital circuitry. For example, a general purpose microprocessor may be used. Digital methods have the advantage of lower noise and drift and greater thermal stability.

In a preferred embodiment therefore the invention provides a circuit that performs real time measurement of afterglow for each frame of film. The circuit is active in the line blanking and/or the frame blanking intervals that occur respectively at the end of each scan line and each frame. The circuit measures the decay of the light level at the end of each scan line, and/or also after the last scan line.

In general, at least one additional photodetector is provided to measure the decaying light level. An additional photodetector is provided as the light received by the main photodetectors of the scanner is modulated by the film usually also during blanking, which presents difficulties in accurately and consistently measuring the light decay from the CRT using these photodetectors. The additional photodetector is therefore preferably arranged to receive light from the face of the CRT which has not been modulated by the film.

For many years, commonly available telecine machines such as the Rank Cintel Mk III have incorporated a so-called "burn" cell. This photocell (or photomultiplier) receives light directly from the face of the CRT, and measures the amount of light coming from the CRT. This measured amount is used to compensate the video signals that are produced by the photomultipliers that receive light which has passed through the film. Thus, the actual level of light emitted by a particular point on the CRT face is taken into account in calculating the effective transmission of the film at that point. Such "burn" compensation techniques are well known in the field of telecine machines, and are described for example in DE 2525073 (Rank Organisation Ltd.).

In a particularly convenient arrangement according to the present invention, the additional photodetector is arranged as a burn detector. For example, while the main photodetectors are detecting light modulated by the film, the signal from the additional photodetector which represents the unmodulated light level from the CRT face is used to compensate the signals from the main photodetectors for variations in the light level produced by the CRT. However, when image information is not recorded by the scanner, for example during a blanking period, the signal from the additional photodetector is used to derive an afterglow correction value.

The signal from the burn detector is usually only monitored in a telecine whilst picture scanning is taking place. When the video signals are "blanked" at the end of scan lines and at the end of the scanned frame, the burn signal is usually ignored. The inventor has realised that the burn detector is ideally suited to monitoring the phosphor decay for a scan line or frame.

Many arrangements for burn detection are known, varying from a monochrome burn detection system which monitors only the intensity of the burn, to systems which measure the burn in three discrete regions of the spectrum to provide three "burn" signals for correction of the three (RGB) video signals.

whilst for optimal correction of the afterglow it is preferable to measure the afterglow characteristic in each of the three colours, and apply the correction to each of the three video signals independently, a compromise can be made by detecting a monochrome afterglow signal, and applying the deduced correction equally to each of the three video signals.

According to one embodiment of the invention, the optimal afterglow correction parameters are determined at the end of a given frame (frame "M,,) and this correction is used to correct the next frame to be scanned (frame 1IM+111). In practice, this is a reasonable approximation, as the scanning patch for one frame is very similar (if not the same) as the previous frame. Even in the case of a,dynamic,, (where the scanning patch is dynamically moved from one part of the CRT to another, to simulate a camera "pan" or "tilt"), the incremental move from one frame to another is usually very small.

Thus, the afterglow correction applied to image information generated by the flying spot scanner may be determined by reference to the light output of the scanner at a time during the scanning operation which precedes the time at which the image information is recorded by the scanner.

The only case in which this does not work well is where there is a "cut" from one scene in the film to another. It may be that at the end of one scene the operator requires a large zoom, utilising a small CRT patch. For the next frame, which is the first frame of a subsequent scene, the operator may require a "normal" picture, and inappropriate afterglow correction parameters will be applied to this first frame.

According to another embodiment of the invention a "frame store" is utilised to delay the picture output from the telecine by one complete frame. This allows the derivation of the optimal correction parameters for 11 - frame 'W' of the film, and then the application of these parameters to the delayed frame M.

Thus, the image information may be temporarily stored to allow a corresponding afterglow correction value to be determined, and applied to the image information.

The system may include a buffer of one or more lines, in addition to or instead of a frame store. This system would allow the afterglow coefficients measured after the first line or several lines of the scanning patch, to be applied to the lines of the scan in relation to which they were determined. This system offers a cost saving over the necessity to store whole frames of picture information, but does not allow is application of the correct afterglow coefficients determined at the end of the current frame.

In another embodiment, feedback is utilised to ensure that the afterglow coefficients are correctly selected to compensate the video signal for the effect of afterglow. According to this embodiment, initial values for the afterglow coefficients are selected and applied to the video signal. The compensated video signal, or the burn signal, is then monitored to ensure that the afterglow has been correctly compensated. If not, the afterglow coefficients are adjusted to achieve optimal compensation for the effect of afterglow. The necessary adjustment may be predetermined by adjusting, e.g. increasing, the coefficient and observing the effect. If the effect is reduced compensation, the adjustment is made in the opposite direction, e.g. decreasing the coefficient. Such a feedback technique may also be used to initially determine the afterglow coefficients.

Thus, the afterglow correction value may be derived by applying an initial afterglow correction value to a signal representative of the light output to generate a corrected light output signal and adjusting the initial afterglow correction value by reference to the corrected light output signal.

In a further embodiment, "short term" afterglow may be measured in the line blanking period, for example at the end of some of the first lines of the scanning patch. "Long term" afterglow may be measured in the frame blanking period, as the frame blanking period is generally significantly longer than the line blanking period. The short term afterglow represents the decay of the light from the CRT over a relatively short period of time, whereas the long term afterglow represents the decay of the light over a longer period. In order to fully characterise the afterglow of the CRT, the light level due to afterglow should be quantified over a is sufficient period of time that the light has decayed to a level at which it will not affect accurate scanning. Hence, the long term afterglow should be measured over as long a period of time as possible. However, in practice, a compromise will be required between the period over which the long term afterglow is measured and the overall speed of the scanning operation.

The short term coefficients determined from the currently scanned frame may be used in combination with the long term coefficients determined at the end of the previous frame to apply afterglow correction to the current frame. Alternatively, the image information may be temporarily stored in a frame store so that the short term and long term coefficients determined during scanning of the current frame are applied to the current frame.

Yet another implementation may include the measurement of the short term coefficients at the start of the picture frame to be scanned, and the comparison of these values with the previous frame short term coefficients. If these coefficients are within a defined tolerance of the previous frame, it may be legitimate to use the long term coefficients from the previous frame. If they are not within this defined tolerance, the percentage change of the short term coefficients may be calculated and applied to the long term coefficients.

Thus, a first afterglow correction value may be derived by reference to a measurement of the light output from the CRT over a first period of time, and a second afterglow correction value may be derived by reference to a measurement of the light output from the CRT over a second period of time, the second period being longer than the first period. The second afterglow correction value for a first frame may be adjusted to provide a second afterglow correction value for a second frame by reference to a comparison of the first afterglow correction value for the first frame and the first afterglow correction value for the second frame.

In general, the line blanking period and the frame blanking period are defined by the format of the video output from the scanner. Since the time periods for determination of the long term coefficients are generally considerably longer than the line blanking interval of most standard video formats, these coefficients can only usually be determined during the frame blanking interval. Even then, the period of time over which the long term coefficients can be sampled is limited to the length of the frame blanking period.

In one embodiment, the first part of the scan patch is scanned at a data rate faster than the data rate required by the output video format. The accelerated scan data is stored in a frame store. Thus if for example the first quarter of the picture is scanned into the frame store at 1.5 times the usual data rate, there is time to blank the CRT screen, effectively generating a virtual frame blanking interval a quarter of the way through the frame. The second quarter of the picture may then be scanned at the same data rate, followed by 14 - another "blanking break", and so on. After all of the frame has been scanned, the image information can be read out of the frame store at the normal data rate in accordance with the output video format. The overall time taken to scan the frame at the higher data rate, including the imposed blanking breaks is equal to the time required to transmit the frame according to the output video format. However, during the blanking breaks the light output from the CRT can be measured to generate afterglow correction parameters. The blanking breaks can be made as frequent and as long as desired, limited only by the maximum data rate at which the scan can be performed and the format of the output video.

The length of the blanking break may be set to allow is afterglow measurements to be taken over a sufficiently long period of time to allow accurate characterisation of the afterglow. In this way, the longer time constant values of the afterglow may be measured many times per frame.

Thus, the scanning operation may be conducted at a data rate higher than the data rate at which image information is output from the scanner, and the scanning operation may be interrupted for a period of time during which the light output from the CRT is measured in order to determine the afterglow characteristics of the CRT.

Afterglow correction values may be averaged over a plurality of frames or a plurality of measurements.

Furthermore, afterglow correction values may be processed to characterise an afterglow function for the CRT. For example, real time determination and correction for each scanned frame may be effected, with a constant set of correction parameters for that frame.

Alternatively, a different set of correction parameters may be used for different parts of the picture frame, for example each line.

One further development of the system involves the afterglow correction of the burn signal which is used during scanning to correct the detected image information, the video signal, for variations in the light level from the CRT. It is desirable to correct the burn signal, as well as the video signals, for the effect of afterglow before the burn signal is used to correct the video signal.

The above method will work well with conventional picture shapes, namely rectangular-shaped areas. The URSA range of telecine machines, however, is capable of a range of "scan effects". These effects are accomplished by scanning the picture with a nonrectangular scanning patch. Thus, to correct for a trapezoidal shape of the original film image, and to make it appear rectangular, the film frame is sampled with a correspondingly trapezoidal scanning patch.

In this case, there may be a significant difference in the afterglow between one area of the CRT face and another. For example, if a horizontal line of the scan patch is shortened, but still includes the same number of pixels, the concentration of thermal energy along the line is increased, because the points of incidence of the electron beam are closer together. In the case of a trapezoidal scanning patch, for example, there is a much greater concentration of thermal energy on the CRT face at the short parallel side of the trapezium than at the long parallel side thereof.

In such a situation, the afterglow correction value determined in the line blanking period at the end of each line may be used to correct the image information from that line (or an adjacent line), without performing an averaging operation. Thus, different afterglow correction values are provided for different parts of the picture.

In one embodiment, the afterglow correction value(s) are determined at the end of several scan lines, but are not averaged. Instead, the determined afterglow correction value(s) are applied directly to corresponding parts of the frame. For example, the values may be applied to "nearest neighbours", i.e. the set of available afterglow parameters measured at aposition in the scan that is closest to the pixel in question. In practice, the applied afterglow correction value may have been determined in the blanking period at the end of the preceding scan line.

Preferably, interpolation is carried out between the neighbouring afterglow correction values that bound the line in question.

In one arrangement, short term afterglow correction values are determined at the end of each line, and the image information from that line is delayed by one line time period, allowing time to apply the determined coefficients to the image information. Alternatively, the afterglow correction coefficients determined at the end of one line are applied to the image information from the subsequent line.

In an alternative embodiment of the present invention, light output from the cathode ray tube during the scanning operation may be measured at any time during the scanning operation, i.e. including a time at which image information is being recorded. Thus, from a further aspect, the present invention provides a method of correcting for afterglow of a cathode ray tube in a flying spot scanner, wherein the light output from the cathode ray tube is measured by an image information recording means after it has passed through a film in the scanner and by an additional light recording means before it has passed through the film, and an afterglow correction value is derived from the light output measured by the additional light recording means.

This embodiment of the invention has the advantage that light levels for deriving an afterglow correction value may be measured at any stage during the scanning operation as the light output of the cathode ray tube is measured by an additional light recording means provided specifically for this purpose. Thus, afterglow correction values may be obtained even in situations where image information is being recorded throughout the scanning operation.

The invention also has the advantage as described above in relation to the first aspect of the invention, that the afterglow correction may be constantly calibrated so that the afterglow compensation is unaffected by ageing of the cathode ray tube or the transport rate of the telecine machine.

Preferably, an afterglow correction value is determined for each frame of film being scanned. The values determined may then be used according to a conventional afterglow compensation technique.

In one implementation of the invention, the optimal afterglow correction parameters are determined at the end of a given frame (frame "M") and this correction is used to correct the next frame to be scanned (frame 11m+111).

According to another embodiment of the invention a "frame store" is utilised to delay the picture output from the telecine by one complete frame. This allows the derivation of the optimal correction parameters for frame "M" of the film, and then the application of these parameters to the delayed frame M.

In another embodiment, feedback is utilised to ensure that the afterglow coefficients are correctly selected to compensate the video signal for the effect of afterglow. According to this embodiment, initial values for the afterglow coefficients are selected and applied to the video signal. The compensated video signal is then monitored to ensure that the afterglow has been correctly compensated. If not, the afterglow coefficients are adjusted to achieve optimal compensation for the effect of afterglow. The necessary adjustment may be predetermined by adjusting, e.g.

increasing, the coefficient and observing the effect. If the effect is reduced compensation, the adjustment is made in the opposite direction, e. g. decreasing the coefficient. Such a feedback technique may also be used to initially determine the afterglow coefficients.

is Thus, the afterglow correction value may be derived by applying an initial afterglow correction value to a signal representative of the light output to generate a corrected light output signal and adjusting the initial afterglow correction value by reference to the corrected light output signal.

It will be appreciated that the light output from the whole of a scan patch on the face of the cathode ray tube could be used to derive the afterglow correction value. Preferably however, the light from only a part of the scan patch is used.

Still more preferably, the light from one quarter of the scan patch is measured by the additional light detecting means. This will allow the afterglow correction value to be determined during the time interval in which the remaining area of the scan patch is scanning so that the method can be achieved in real time.

It will be seen therefore that in broad terms the invention provides a method of determining an afterglow characteristic of a cathode ray tube of a flying spot scanner, wherein the light output from the cathode ray tube is measured during scanning and at least one 19 - afterglow characteristic is derived from the measured light output.

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

Figure 1 is a schematic representation of an embodiment of the invention; Figure 2 is a schematic representation of the signal from the burn cell of the embodiment of Figure 1; Figure 3 is a schematic representation of a telecine according to a second embodiment of the invention; 1P Figure 4 is a schematic representation showing a scan patch as used to derive an afterglow correction value in the embodiment of Figure 3; and Figure 5 is a schematic representation of the embodiment of Figures 3 and 4.

An embodiment of the invention is shown in Figure 1. In Figure 1, only a single channel, for example the red channel, of the system is shown for reasons of clarity. In practice, identical red, green and blue channels are provided connected to respective main detectors and burn cells.

According to the embodiment of Figure 1, a scan patch is generated on the face of the cathode ray tube 1. The light from the CRT 1 passes through the film 3 and is detected by a main photodetector 4. A burn cell 2 directly detects light emitted by the CRT 1 without the light being modulated by the film 3.

The detected signal from the main photodetector 4 passes through a frame delay 5 (or frame store) where it is stored for a period corresponding to one frame of the film in order to give sufficient time for the afterglow correction parameters to be calculated. The signal from the burn cell 2 is also passed to a corresponding frame delay 6. However, the signal from the burn cell 2 also passes to a multiplier 7. The multiplier 7 is controlled by a controller 8 which is arranged to operate the multiplier only during line and frame blanking intervals.

During active scanning of the film 3, the delayed signals from the frame delays 5, 6 are passed to respective afterglow correctors 9, 10 which correct for the afterglow of the CRT as described below. The afterglow corrected main photodetector signal is then divided by the afterglow corrected burn cell signal in a divider 11 to produce an afterglow and burn corrected output signal.

During line blanking, the signal from the burn cell 2 is scaled by the multiplier 7 according to a scaling factor determined by the controller 8. During line blanking, the burn signal represents the decaying light level from the CRT 1 due to afterglow. As shown in Figure 2, this light level is sampled over two distinct periods Al and A2. During the first period Al, when the light level is relatively high only, 16 samples are taken, under the control of the controller 8. During the second period A2, when the light level is lower, 64 samples are taken to improve the accuracy of the measurement at the lower light level. Similarly, in the frame blanking period, an additional three sampling periods A3, A4, A5 are used, respectively with 256, 1024 and 4096 samples each. The scaling value provided to the multiplier 7 by the controller 8 is inversely proportional to the number of samples in the current period, so that when the samples are accumulated, the total value is not skewed by the number of samples.

The inventor has realised that the afterglow values to be measured by the burn cell 3 may be very small signals. By sampling and accumulating many samples of picture elements, the signals can be measured with greater accuracy. Figure 2 shows the measured signals from the burn cell 3, covering the last few lines of the frame, and the frame blanking. In the example shown, it is possible to measure the first two afterglow values Al, A2 at the end of some or every line. Similarly, in this example the last three values A3, A4, AS can be measured only at the end of the scanning frame in the frame blanking interval. Because these values are even smaller, greater accuracy is achieved by measuring them over much greater sampling periods. It is possible to measure the first two coefficients Al, A2 at the end of the last line (which is also the end of the frame) followed immediately by the measurement of the last three values A3, A4, AS.

The number of afterglow correction values and the corresponding number of sampling periods given herein is by way of example only and these numbers may be selected appropriately in order to achieve the desired afterglow correction.

For ease of implementation the afterglow coefficients are selected to correspond to the afterglow periods already implemented in telecine systems such as the Rank Cintel Mk III or URSA. These in practice are:

Al 0.7 microseconds, A2 2.0 microseconds, A3 6.0 microseconds, A4 20.0 microseconds and A5 60.0 microseconds.

The URSA telecine in scanning of standard definition television pictures has a pixel sampling interval of 55 nanoseconds, and a line scanning rate of 48 microseconds. Thus, according to this embodiment of the invention, the light level from the burn cell 2 is sampled over the required number of samples around the selected coefficient points Al, A2, A3, A4, AS to obtain the correct coefficients.

There are two afterglow correctors 9, 10, one for the main signal afterglow correction and one for the burn afterglow correction. Each of these is capable of being loaded with the five coefficients Al to A5 which represent the proportion of full scale signal measured at that particular time constant after the turning off of the beam.

During line or frame blanking, the signal from the multiplier 7 is passed to an accumulator 12 which adds all the samples of the light level taken for the current afterglow coefficient. The total of the accumulated samples is divided in a divider 13 by the signal level stored in latch LO, which corresponds to the signal level from the burn cell 2 during scanning, to obtain the coefficient as a proportion of the full scale signal. The proportional coefficients are stored respectively in latches Ll to L5 under the control of the controller 8. From the latches Ll to L5, the afterglow coefficients are passed to the afterglow correctors 9, 10 for correction of the delayed signals from the main photodetector 4 and the burn cell 2.

The embodiment of Figure 1 is implemented using ALTERA logic elements from the EPF 10K family of logic.

Each of the logic chips used contains 4992 logic elements and they are programmed using ALTERA Max+II software. Code to perform the required functions is downloaded into the chips using the Joint Test Action Group protocol.

Figure 3 shows a telecine machine according to an alternative embodiment of the invention. As shown, light from the face of a cathode ray tube 1 passes through a film 3 and is detected by a main photodetector 4 as in the first embodiment. Additionally, the light from the face of the CRT taken before it has been modulated by the film is imaged through a lens 20 onto an afterglow cell or photodetector 21. The lens is configured such that only the top left hand quarter of the area of the scan patch on the face of the CRT is imaged onto the afterglow cell 21. The remaining three quarters of the area of the scan patch are imaged onto a matt black light sink 22. Thus the light from these three quarters of the scan patch will not be detected.

This is also shown in Figure 4 in which the top left hand quarter of the scan patch is imaged onto the afterglow cell 21 and the remaining area of the scan patch is imaged onto the light sink 22.

The light measured by afterglow cell 21 is sampled during line blanking intervals over time periods Al and A2 in the same manner as described above in relation to the first embodiment of the invention. Similarly, in the picture interval corresponding to the time in which the bottom half of the scan patch is scanning, the further three sampling periods A3, A4 and A5 are used.

As shown in Figure 5, many elements of this embodiment of the invention correspond to those of Figure 1. As in Figure 1, only a single channel of the system is shown and like reference numbers have been used for corresponding parts.

In the embodiment of Figure 5 (as previously described in relation to Figure 3) a scan patch is generated on the face of the cathode ray tube 1. The light from the CRT 1 passes through the film 3 and is detected by a main photodetector 4. An afterglow cell 21 directly detects light emitted by the CRT 1 without the light being modulated by the film 3.

The detected signal from the main photodetector 4 passes through a frame delay 5 (or frame store) where it is stored for a period corresponding to one frame of the film in order to give sufficient time for the afterglow correction parameters to be calculated. The signal from the afterglow cell 21 is passed to a multiplier 7. The multiplier 7 is controlled by a controller 8 which is arranged to operate the multiplier only during line and frame blanking intervals.

During active scanning of the film 3, the delayed signals from the frame delay 5 are passed to an afterglow corrector 9 which corrects for the afterglow of the CRT as described below.

The signal from the afterglow cell 21 is scaled by the multiplier 7 according to a scaling factor determined by the controller 8. The afterglow signal represents the decaying light level from the CRT 1 due to afterglow. During line blanking, this light level is sampled over two distinct periods Al and A2. During the first period Al, when the light level is relatively high only, 16 samples are taken, under the control of the controller 8. During the second period A2, when the light level is lower, 64 samples are taken to improve the accuracy of the measurement at the lower light level. Similarly, in the time interval when the bottom half of the scan patch is scanning, an additional three sampling periods A3, A4, A5 are used, respectively with 25G, 1024 and 4096 samples each. The sampling periods Al to A5 correspond to those described above in relation to the embodiment of Figure 1. The scaling value provided to the multiplier 7 by the controller 8 is inversely proportional to the number of samples in the current period, so that when the samples are accumulated, the total value is not skewed by the number of samples.

The afterglow corrector 9 is capable of being loaded with the five coefficients Al to A5 which represent the proportion of full scale signal measured at that particular time constant after the turning off of the beam.

The signal from the multiplier 7 is passed to an accumulator 12 which adds all the samples of the light level taken for the current afterglow coefficient. The total of the accumulated samples is divided in a divider 13 by the signal level stored in latch LO, which corresponds to the signal level from the afterglow cell 21 during scanning, to obtain the coefficient as a proportion of the full scale signal. The proportional coefficients are stored respectively in latches Ll to L5 under the control of the controller 8. From the latches Ll to L5, the afterglow coefficients are passed to the afterglow corrector 9 for correction of the delayed signal from the main photodetector 4.

The embodiment of Figure 5 is implemented using ALTERA logic elements from the EPF 10K family of logic as for the embodiment of Figure 1.

In the embodiment of Figure 5, it will be appreciated that a separate burn cell could be provided in addition to the afterglow cell 21. The burn signals measured by the burn cell could then be corrected for afterglow using the afterglow correction coefficients Al to A5.

In summary, according to preferred features of the invention the following processing steps are provided.

- The sampling from the burn cell(s) of many blanked picture elements after the end of many scan lines in a picture frame; - The sampling of the burn cell(s) of many lines of picture after the last line of a picture frame - The calculation of the afterglow decay at several instances of time from the above data. For example, knowing the pixel interval, and the line rate, the proportion of light can be determined which is still being emitted from the CRT after known periods of time.

- The "averaging" of the sets of determined afterglow values to form one set of afterglow correction data for the whole frame.

- The application of these coefficients to the afterglow circuitry on a "frame by frame,, basis, either to the next frame to be processed, or on a "one frame delayed" copy of the same frame as was used to determine the coefficients.

24

Claims

1. A method of correcting for afterglow of a cathode ray tube in a flying spot scanner, wherein the light output from the cathode ray tube is measured during a scanning operation of the flying spot scanner at a time during the scanning operation when image information is not being recorded, and an afterglow correction value is derived from the measured light output.

2. A method as claimed in claim 1, wherein the derived afterglow correction value is used to correct digital image data obtained by scanning one or more frames of a film in the scanner.

3. A method as claimed in claim 1 or 2, wherein the light output of the cathode ray tube is measured in the line blanking and/or frame blanking period of the scanning operation.

4. A method as claimed in claim 1, 2 or 3, wherein an afterglow correction value is derived for each frame of a film being scanned by the scanner.

5. A method as claimed in any preceding claim, wherein the light output from which the afterglow correction value is derived is measured by a light measuring means which is additional to the light measuring means through which image information is recorded.

6. A method as claimed in claim 5, wherein the additional light measuring means is arranged to receive light from the face of the cathode ray tube which has not been modulated by film in the scanner.

7. A method as claimed in 5 or 6, wherein the additional light measuring means is arranged as a burn detector.

8. A method as claimed in claim 7, wherein when image information is being recorded, the light measured by the burn detector is used to compensate the image information for variations in the light level produced by the cathode ray tube.

9. A method as claimed in any preceding claim, wherein an afterglow correction value is obtained for each of the red, green and blue components of the measured light output.

10. A method as claimed in any preceding claim, wherein an afterglow correction value is determined for a first frame of film in the scanner and the value is used to correct digital image data obtained by scanning a subsequent frame of the film.

11. A method as claimed in any preceding claim, wherein digital image data obtained by scanning film in the scanner is temporarily stored to allow a corresponding afterglow correction value to be determined, and applied to the image data.

12. A method as claimed in claim 11, wherein the digital image data is stored in a frame store for the time taken to scan one frame of the film, to allow an afterglow correction value to be determined for one frame of the film, and to be applied to the digital image data obtained for that frame of film.

13. A method as claimed in claim 11, wherein one or more lines of digital image data obtained by the scanner are held in a buffer so that an afterglow correction value corresponding to the said lines of digital image data is applied to that data.

14. A method as claimed in any preceding claim, wherein a feedback system is used to ensure that the afterglow correction value is correctly determined to compensate the image information for afterglow.

15. A method as claimed in claim 14, wherein the afterglow correction value is derived by applying an initial afterglow correction value to a signal representative of the light output to generate a corrected light output signal and adjusting the initial afterglow correction value by reference to the corrected light output signal.

16. A method as claimed in any preceding claim, wherein a first afterglow correction value is derived by reference to a measurement of the light output from the CRT over a first period of time, and a second afterglow correction value is derived by reference to a measurement of the light output from the CRT over a second period of time, the second period being longer than the first period.

17. A method as claimed in claim 16, wherein the second afterglow correction value for a first frame is adjusted to provide a second afterglow correction value for a second frame by reference to a comparison of the first afterglow correction value for the first frame and the first afterglow correction value for the second frame.

18. A method as claimed in claim 16, wherein the first afterglow correction values determined from a currently scanned film frame are used in combination with the second afterglow correction value determined at the end of the previous frame to apply afterglow correction to the current frame.

19. A method as claimed in claim 18, wherein the image information is temporarily stored in a frame store so that the first and second correction values determined during scanning of the current frame are applied to the current frame.

20. A method as claimed in any of claims 16 to 19, wherein the measurement of light output in the second period of time is taken in the frame blanking period.

21. A method as claimed in any of claims 16 to 19, wherein the measurement of light output in the first period of time is taken in the line blanking period.

22. A method as claimed in any preceding claim, wherein the scanning operation is conducted at a data rate higher than the data rate at which image information is output from the scanner, and the scanning operation is interrupted for a period of time during which the light output from the CRT is measured in order to determine the afterglow characteristics of the CRT.

23. A method as claimed in any preceding claim, wherein afterglow correction values are averaged over a plurality of frames or a plurality of measurements.

24. A method as claimed in any preceding claim, wherein afterglow correction values are processed to characterise an afterglow function for the CRT.

25. A method of correcting for afterglow in a burn signal which is used during scanning of film to correct detected image information for variations in the light level emitted by a cathode ray tube, wherein the burn signal is corrected using an afterglow correction value derived by the method of any preceding claim.

26. A method of correcting for afterglow in a burn signal as claimed in claim 25, wherein the afterglow correction value determined in the line blanking period at the end of each line of image data is used to correct the image information from that line (or an adjacent line), without performing an averaging operation.

27. A method as claimed in claim 26, wherein interpolation is carried out between the neighbouring afterglow correction values that bound each line of image data.

28. A method of correcting for afterglow of a cathode ray tube in a flying spot scanner, wherein the light output from the cathode ray tube is measured by an image information recording means after it has passed through a film in the scanner and by an additional light recording means before it has passed through the film, and an afterglow correction value is derived from the light output measured by the additional light recording means.

29. A method as claimed in claim 28, wherein an afterglow correction value is derived for each frame of a film being scanned by the scanner.

30. A method as claimed in claim 28 or 29, wherein an afterglow correction value is determined for a first frame of film in the scanner and the value is used to correct digital image data obtained by scanning a subsequent frame of the film.

31. A method as claimed in any of claims 28 to 30, wherein digital image data obtained by scanning film in the scanner is temporarily stored to allow a corresponding afterglow correction value to be determined, and applied to the image data.

32. A method as claimed in claim 31, wherein the digital image data is stored in a frame store for the time taken to scan one frame of the film, to allow an afterglow correction value to be determined for one frame of the film, and to be applied to the digital image data obtained for that frame of film.

33. A method as claimed in any of claims 28 to 32, wherein a feedback system is used to ensure that the afterglow correction value is correctly determined to compensate the image information for afterglow.

34. A method as claimed in claim 33, wherein the afterglow correction value is derived by applying an initial afterglow correction value to a signal representative of the light output to generate a corrected light output signal and adjusting the initial afterglow correction value by reference to the corrected light output signal.

35. A method as claimed in any of claims 28 to 34, wherein the afterglow correction value is derived from a part of the light output from the cathode ray tube.

36. A method as claimed in claim 35, wherein the light output from one quarter of the area of a scan patch formed on the face of the cathode ray tube is used to determine the afterglow correction value.

37. An apparatus for correcting for afterglow of a cathode ray tube in a flying spot scanner, comprising means for measuring the light output from the cathode ray tube during a scanning operation of the flying spot scanner at a time during the scanning operation when image information is not being recorded, and means for deriving an afterglow correction value from the measured light output.

38. An apparatus as claimed in claim 37, the apparatus further comprising menas for correcting digital image data obtained by scanning one or more frames of a film in the scanner using the derived afterglow correction value.

39. An apparatus as claimed in claim 37 or 38, wherein the means which measures the light output from which the afterglow correction value is derived is additional to the light measuring means through which image information is recorded.

40. An apparatus as claimed in claim 39, wherein the additional light measuring means is arranged to receive light from the face of the cathode ray tube which has not been modulated by film in the scanner.

41. An apparatus as claimed in 39 or 40, wherein the additional light measuring means is a burn detector.

42. An apparatus as claimed in any of claims 37 to 41, the apparatus further comprising a store for temporarily storing digital image data obtained by scanning film in the scanner to allow a corresponding afterglow correction value to be determined, and applied to the image data.

43. An apparatus as claimed in any of claims 37 to 42, the apparatus further comprising a scanner for holding one or more lines of digital image data obtained by the scanner so that an afterglow correction value corresponding to the said lines of digital image data is applied to that data.

44. A apparatus as claimed in any of claims 37 to 43, the apparatus further comprising a feedback system for ensuring that the afterglow correction value is correctly determined to compensate the image information for afterglow.

45. An apparatus for correcting for afterglow of a cathode ray tube in a flying spot scanner, comprising image information recording means for measuring the light output from the cathode ray tube after it has passed through a film in the scanner, an additional light recording means for measuring the light output from the cathode ray tube before it has passed through the film, and means for deriving an afterglow correction value from the light output measured by the additional light recording means.

46. An apparatus as claimed in claim 45, the apparatus further comprising means for imaging only a part of the light output from the cathode ray tube onto the additional light recording means.

47. An apparatus as claimed in claim 46, wherein the imaging means comprises a lens.

48. An apparatus as claimed in claim 46 or 47, the apparatus further comprising a matt black light sink onto which the other part of the light output from the cathode ray tube is imaged.

49. A method of correcting for afterglow of a cathode ray tube in a flying spot scanner substantially as herein described and with reference to Figures 1 and 2 of the accompanying drawings.

50. A method of correcting for afterglow in a burn signal which is used during scanning of film to correct detected image information for variations in the light level emitted by a cathode ray tube substantially as herein described and with reference to Figures 1 and 2 of the drawings.

51. A method of correcting for afterglow of a cathode ray tube in a flying spot scanner substantially as herein described and with reference to Figures 3 to 5 of the accompanying drawings.

52. An apparatus for correcting for afterglow of a cathode ray tube in a flying spot scanner substantially as herein described and with reference to Figures 1 and 2 of the accompanying drawings.

53. An apparatus for correcting for afterglow of a cathode ray tube in a flying spot scanner substantially as herein described and with reference to Figures 3 to 5 of the accompanying drawings.