Flying spot telecine and method of operating same
FIELD OF THE INVENTION 
     The present invention relates to photo-multiplier tube (PMT) tracking in telecines. 
     In a flying spot telecine, three or more PMTs are generally used to convert the light transmitted by cinematographic film running through the telecine into electrical signals which are processed to form a final video signal. It is a common practice to adjust the gain of the system as a whole by manipulating the voltages applied to the PMTs using potentiometers or the like. To avoid changes in the colour of the picture reproduced from the video signal output by the telecine, it is necessary to `track` the voltages applied to the individual PMTs so that the ratios between the gains of the PMTs remain as near constant as possible. 
     Related Prior Art 
     One known method of PMT tracking involves the use of two preset potentiometers for each PMT to adjust the voltage applied to the PMT at each end of the gain control range. This requires manual adjustment for each different film gauge used and for positive or negative operation. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided a method of photomultiplier tube (`PMT`) tracking in a flying spot telecine having cathode ray tube (`CRT`) means for generating a light spot for scanning a film strip, at least one PMT means for detecting light transmitted by the said film strip, and system gain control means for providing a control signal to the PMT means to control the voltage applied thereto; the method comprising: 
     (a) generating a MINIMUM GAIN REFERENCE value representing a minimum gain of the PMT means and storing said value in control processor means; 
     (b) generating a MAXIMUM GAIN REFERENCE value representing a maximum gain of the PMT means and storing said value in the control processor means; and, 
     (c) during subsequent operation of the telecine modifying the control signal provided to the PMT means in dependence on the stored MIMIMUM GAIN REFERENCE and MAXIMUM GAIN REFERENCE values. In a preferred embodiment of the method of the invention 
     (i) a control signal is provided to the PMT means; 
     (ii) the output of the PMT is compared to a reference level; and 
     (iii) the control signal is incremented or decremented according to whether the PMT output exceeds the reference level; 
     (iv) steps (i) to (iii) are repeated a predetermined plurality of times; and 
     (v) the final control signal value is stored as the MINIMUM or MAXIMUM GAIN REFERENCE value. 
     In a further aspect, the invention provides a flying spot telecine having cathode ray tube (`CRT`) means for generating a light spot for scanning a film strip, at least one PMT means for detecting light transmitted by the said film strip, and system gain control means for providing a control signal to the PMT means to control the voltage applied thereto; the telecine including control processor means operable to 
     (a) generate a MINIMUM GAIN REFERENCE value representing a minimum gain of the PMT means and to store said value 
     (b) generate a MAXIMUM GAIN REFERENCE value representing a maximum gain of the PMT means and to store said value; and, 
     (c) during subsequent operation of the telecine to modify the control signal provided to the PMT means in dependence on the stored MIMIMUM GAIN REFERENCE and MAXIMUM GAIN REFERENCE values. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A preferred method in accordance with the invention will now be described in detail, by way of example, with reference to the drawing which illustrates schematically telecine equipment in which the method of the invention is utilized. 
    
    
     DETAILED DESCRIPTION OF THE DRAWING 
     The telecine shown in the drawings includes cathode ray tube (`CRT`) means 12 for generating a light spot which scans a film strip 14 passing through the telecine. Light transmitted by the film strip 14 is collected by photomultiplier tubes (`PMTs`) 16 R , 16 G  and 16 B  which monitor the red, green and blue levels, respectively. The output signals from the PMTs 16 R , 16 G  and 16 B  are processed separately. 
     The master gain control 18 for the system as a whole is a trackerball system which provides a range of digital numbers to a control processor 20. The processor 20 then scales the numbers provided by the trackerball system according to reference values stored in memory associated with processor 20. The resulting values are converted into analogue voltages which are applied to the PMT supply controls to control the individual device gains of the PMTS. 
     Before being used for the first time, the PMT tracking must be aligned so as to avoid variations in colour in the digital video signal produced by the telecine 10 due to variations in the relationship between applied voltage and device gain as between the three PMTs. To align the system, the operator removes any film present from the light path. The CRT beam current is then set to its normal operating value and the PMT control number provided by the trackerball is set to provide the lowest operating voltage at the PMT. The digital video signal derived from the PMT output is then compared with the required peak level of the video signal over a scan of one whole frame. If at any point during the scan, the video signal level exceeds the required video level, then the PMT control number from the trackerball system is decremented and the comparison between the video signal level and the required peak level over a frame scan is repeated. 
     If the digital video signal level does not exceed the required peak level, the PMT control number is incremented and the comparison repeated. 
     The comparison over a frame scan is repeated 128 times incrementing or decrementing the PMT control number by 32 LSBs (least significant bits) and then 128 times more using increments of one LSB. The final number is then stored as MINIMUM GAIN REFERENCE in the memory associated with the control processor 20. 
     It will be noted that successive approximation techniques are not used because of the possibility of overloading the PMTs and the need for noise averaging. The comparisons are performed separately but simultaneously for all three PMTs 16 R , 16 G  and 16 B . 
     The whole comparison procedure outlined above is then repeated with the CRT beam current set to a value which corresponds to the maximum film density used, typically, at one tenth its normal value (this is equivalent to film of highlight density N.D.1.0.). The resulting number is then stored in the memory associated with the control processor 20 as the MAXIMUM GAIN REFERENCE. 
     Thus, for telecine equipment 10 utilizing three PMTs 16 R , 16 G  and 16 B , six values are stored in the control processor memory; MINIMUM GAIN REFERENCE and MAXIMUM GAIN REFERENCE for each of the three PMTs. It is envisaged that the comparison procedures described above will be carried out entirely automatically under the control of the processor 20. 
     During subsequent operation of the telecine, the control numbers provided by the trackerball system 18 are adjusted in scale and offset by the control processor 20 to correspond in range to the scale defined by the MINIMUM GAIN REFERENCE and MAXIMUM GAIN REFERENCE values stored for each PMT. The scale is assumed to be linear between these values but could, of course, also follow some predetermined curve. 
     Thus, the PMT voltages are tracked automatically as the master gain control is used to provide varying PMT control numbers. As a result variations in system control provide similar effects on all three colours regardless of individual light levels and PMT sensitivities. 
     Although the present invention has been described in connection with a preferred embodiment thereof, many other variations and modifications will now become apparent to those skilled in the art. It is preferred therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.