GB2215551A

GB2215551A - Drift correction in telecine

Info

Publication number: GB2215551A
Application number: GB8805219A
Authority: GB
Inventors: Terence William Mead
Original assignee: Rank Cintel Ltd
Current assignee: Rank Cintel Ltd
Priority date: 1988-03-04
Filing date: 1988-03-04
Publication date: 1989-09-20
Anticipated expiration: 2008-03-04
Also published as: GB8805219D0; JPH03504305A; WO1989008365A1; GB2215551B; EP0408620A1

Abstract

In a flying spot telecine including cathode ray tube scaning means for generating flying spot and a photodetector for receiving light transmitted by a film strip being scanned by the telecine, control means are provided for causing the CRT scanning means to scan an area (8, 18) corresponding to the position of one or more perforations (16) in the film strip (10) being scanned. The output from the photodetector during a scan of the area (18) provides a correction signal for adjusting the video signal level. Preferably, the scan of the area (18) is performed during line blanking or frame blanking periods.

Description

DRIFT CORRECTION IN TELECINE The present invention relates to the correction of video signal level errors arising from instability or progressive drift in the preliminary stages of telecine equipment. For example, such errors may be caused by variations in the intensity of the output of the cathode ray tube ("CRT") scanning means used in flying spot telecine equipment or by variations in photomultiplier tube sensitivity or amplifier gain. These variations are usually of a relatively long term nature. That is to say they cause the video signal level or the colour balance to drift over a period of minutes or longer.

Attempts have been made to reduce signal drift due to variations in the sensitivity of photomultiplier tubes by fitting heating elements adjacent the tubes to maintain them at a constant temperature. Whilst this does to an extent reduce signal drift, the elevated temperatures which result increase the noise contributions to the output signal from the photomultiplier tubes and adjacent amplifiers. Furthermore, the fitting of heating elements has no effect on drift due to factors other than temperature.

In accordance with the invention there is provided a telecine in which cathode ray tube ("CRt') scanning means used to generate the flying spot-is caused to scan an area corresponding to-the position of one or more perforations in a film strip being scanned by the telecine; the output from a photodetector receiving light transmitted during a scan of the said area providing a correction signal for adjusting the video signal level.

Thus, by collecting light which passes through the film perforations, equivalent to a 'no-film' condition, a direct measure can be obtained of the composite effect of all the errors outlined above. The video signal level can then be corrected by adjusting the gain of the amplifier or of the photomultiplier tube or by altering the brightness of the CRT scanning means.

Preferably, scanning of the film perforations to correct video signal drift is performed during line or frame blanking periods.

An embodiment of the invention will now be described, by way of example, with reference to the drawings, in which: Figure 1 is a fragmentary schematic view of the film skid plate region of flying spot telecine equipment; and Figure 2 is a block diagram of control circuitry in accordance with the invention.

In conventional flying spot telecine equipment, the film to be scanned passes through a film gate where it is scanned by a flying spot generated by CRT scanning means. Light transmitted by the film in its transparent and translucent areas is collected by one or more photomultiplier tubes after being split into three colour components and otherwise processed. The output signals from the photomultiplier tubes form the basis for the video signal. At the film gate the area of the film strip 10 actually scanned by the flying spot is defined by a viewing aperture 12 formed in a skid plate 14 over which the film strip 10 passes.

The film strip 10 is transported through the telecine film gate by means of a capstan or other drive means. Picture synchronisation is achieved by pulses from a suitable sprocket which engages in rows of perforations 16 formed along the longitudinal edges of the film strip 10. In the telecine equipment of the invention a second aperture 1S, the perforation aperture, is formed in the slcid plate 14 and the CRT scanning means is caused to perform an auxiliary scan of the film strip perforations 16 through the perforation aperture 18 during the frame blanking period. This is achieved by providing the scanning amplifiers with a suitable waveform which produces a small auxiliary scan at a location equivalent to a film perforation 16. The waveform includes a component to track the motion of the continuously moving film strip 10.

Light from the auxiliary scan passes through the film perforation 16 and the perforation aperture 18 of the skid plate 14 to be collected by the photomultiplier tubes in the usual way. The auxiliary scan thus gives rise to a video signal which corresponds to a 'no-film' condition, that is, a signal the level of which is dependent only on the conditions of the telecine equipment itself.

The signal resulting from the auxiliary scan is then compared by means of a comparator circuit 20, to an adjustable gain voltage and the result applied to a control circuit 22. The control circuit 22 increments or documents the photomultiplier control output which controls the voltage applied to the photomultiplier tubes and, hence, their gains. lVhen the photomultiplier tube gain is just sufficient to compensate for the error, the comparator circuit 20 will reach its threshhold and the photomultiplier control output be maintained constant until a further error occurs.

The rate of incrementing or decrementing the photomultiplier control output must be slow enough to avoid the possibility of translating video noise into frame-by-frame flicker which would be visible to the viewer. However, in telecines using programmable colour correction techniques it may be necessary to change the system gain very quickly. This can be achieved by temporarily increasing the speed of the gain control circuitry whenever a change to the gain control voltage is required.

Where dense film or negative film is used, one or more colour signals may need a greater system gain than is usual; during the scan of the perforations 16 the whole of the light beam produced by the CRT scanning means is transmitted to the photomultiplier tube.

With the greater system gain, there is a risk that overloading of the video signal during the scan of the perforations may occur which would prevent correct operation.

To overcome this difficulty, an attenuating filter may be placed over the perforation aperture 18 in the skid plates 14.

This results in an overall reduction in the video signal level, leading to a lower auxiliary scan signal level during normal operation but avoiding overload problems when a higher gain is required. Alternatively, the output of the CRT scanning means may be reduced by turning the beam current down during the auxiliary scan of the film perforations 16.

The above method can be applied using either digital or analogue techniques as appropriate to the particular telecine equipment being used. It will be appreciated that, although the method described above applies a correction signal to the photomultiplier gain, it might equally well be applied to the CRT scanning means to control the brightness or to the amplifier to adjust the gain.

Preferably, the method is applied to each colour independently so as to maintain the correct colour balance as well as the overall video signal level.

Claims

1. A method for correcting drift in the level of a video signal provided by a flying spot telecine in which cathode ray tube ("CRT") scanning means used to generate the flying spot is caused to scan an area corresponding to the position of one or more perforations in a film strip being scanned by the telecine; the output from a photodetector receiving light transmitted during a scan of the said area providing a correction signal for adjusting the video signal level.

2. A method according to claim 1 in which the correction signal is obtained by comparing the output from the photodetector with a reference signal to provide a signal representing the difference therebetween, the difference signal being applied as the correction signal.

3. A method according to claim 1 or 2 in which the CRT scanning means is caused to scan the area corresponding to perforations in the film strip during line blanking or frame blanking periods.

4. A method according to any preceding claim in which the correction signal is applied to the CRT scanning means to adjust the brightness of the flying spot.

5. A method according to any of claims 1 to 3 in which the correction signal is applied to an amplifier used to amplify the photodetector output to adjust the gain of the amplifier.

6. A method according to any of claims 1 to 3 in which the correction signal is applied to the photodetector to adjust the gain thereof.

7. A method according to any preceding claim in which the light transmitted by the film strip is split into a plurality of beams containing different wavelength ranges respectively, there being provided a plurality of photodetectors, each arranged to receive a respective one of the said beams so that each photodetector provides a correction signal for one of the wavelength ranges.

8. A method according to any preceding claim in which an attenuating filter is provided over said area corresponding to the position of one or more perforations in the film strip.

9. A method for correcting drift in the level of a video signal provided by a flying spot telecine, the method being substantially as hereinbefore described with reference to the drawings.

10. A flying spot telecine including cathode ray tube ("CRT") scanning means for generating the flying spot, a photodetector for receiving light transmitted by a film strip being scanned by the telecine and control means for causing the CRT scanning means to scan an area corresponding to the position of one or more perforations in a film strip being scanned, the output from the photodetector during a scan of the said area providing a correction signal for adjusting the video signal level.

11. Apparatus according to claim 10 including a comparator for comparing the output of the photodetector with a reference signal t3 provide a signal representing the difference therebetween, the difference signal being applied as the correction signal.

12. Apparatus according to claim 10 or 11 in which the CRT scanning means is caused to scan the area corresponding to perforations in the film strip during a line blanking or frame blanking period.

13. Apparatus according to any of claims 10 to 12 in which the correction signal is applied to the CRT scanning means to adjust the brightness of the flying spot.

14. Apparatus according to any of claims 10 to 12 including an amplifier used to amplify the photodetector output, the correction signal being applied to the amplifier to adjust the gain thereof.

15. Apparatus according to any of claims 10 to 19 in which the correction signal is applied to the photodetector to adjust the gain thereof.

16. Apparatus according to any of claims 10 to 15 including means for splitting light transmitted by the film strip into a plurality of beams containing different wavelength ranges respectively, there being provided a plurality of photodetectors, each arranged to receive a respective one of the said beams so that each photodetector provides a correction signal for one of the wavelength ranges.

17. Apparatus according to any preceding claim in which an attenuating filter overlies the said area corresponding to the position of one or more perforations in the film strip.

18. A flying spot telecine substantially as hereinbefore described with reference to the drawings.