GB Patent 784856 - Improvements in recording television pictures on cinematograpic film

Description

PATENT SPECIFICATION

Date of Application and filing Complete Specification: Feb 12, 1954.

Application made in France on Feb 21, 1953.

Application made in Franceon Mor 16, 1953.

Complete Specification Published: Oct 16, 1957.

784,856 No 4249/54.

Index at Acceptance:-Class 40 ( 3), F( 2 81 I: 2 F 2: 5 B: 6 K).

International Classificatior n:-HO 4 n:

COMPLETE SPECIFICATION

Improvements in Recording Television Pictures on Cinematograpic Film.

We, SOCITE NOUVELLE DE L'OUTILLAGE R.B V ET DE LA RADIO-INDUSTRIE, 43-45 Avenue Klber, Paris 16 eme, France, a Body Corporate organised according to the laws of France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: -

The invention concerns an improved apparatus for recording television pictures displayed on a cathode ray tube, on a cinematographic film.

The main problem in this recording results from the fact that the duration of the shutter closure in a film camera is much longer than the vertical fly-back period in any television standard In standard film cameras the time during which the shutter is closed (intermittent drive) is equal to the duration of aperture, that is to say to half the picture recording cycle In television, on the contrary, the cathode ray picture blanking (or picture return time) is very short compared with the scanning period; for instance, according to the French Standard, this period is about 10 % of the scanning period The use of standard cinematographic equipment, inoperative as far as recording is concerned for 50 %' of the time, causes the loss of nearly 50 % of the light energy emitted by the cathode ray tube screen and consequently the density of the picture on the recorded film is very weak.

Besides, it becomes very difficult to provide "trick pictures" as is usual in television or cinematographic techniques Many solutions have been suggested to solve this problem, all of them requiring an alteration of the mechanical devices of the cameras or other unwinding devices.

It is an object of the invention to provide entirely electronic means to perform a correct film recording of television interlaced pictures using standard cinematographic equipment It has been suggested (British Patent No 640,186) to display the television picture on a picture tube with long afterglow so that the parts of the picture scanned 50 during the time when the shutter is closed should still be present during the exposure time together with the part of the picture displayed during said perod of exposure.

Each of said parts is one frame of a tele ss vision picture However, due to phosphorescence decay, both frames of the picture would not show the same mean brilliancy.

It is proposed in this patent that the gain of the television receiver be so pulsed that 60 the lines of one frame are initially brighter than the lines of the second frame.

However, as will be shown below, such a pulsing introduces spurious signals in the picture conveying signal which it is not pos 65 sible to suppress According to said Patent, with cameras having a faster pull through time than 50 %, it is necessary to use fixed or mechanically synchronised neutral filters to adjust the relative brightness, since in 70 that case, the amount of information recorded as afterglow on the fluorescent screen is reduced, the amount of brightness correction being accordingly less This means is not practical with up to date high defini 75 tion television.

According to the present invention, we provide an apparatus for recording television pictures each consisting of two interlaced frames on intermittently moving cine 80 matographic film, comprising a video channel and a display cathode ray tube on which the pictures are produced, wherein the video channel comprises at least one video amplifier whose gain is controlled by a voltage 85 generated by a gain control generator operating through a bi-directional switching means, said switching means being controlled by switching pulses occurring during line retrace periods 90 (Price 3/6) 784,856 According to a feature of the invention he video channel comprises two gain controlled video amplifiers operated independently from two separate generators through two bi-dire'ctional switching mrneans, said switching means being controlled by switchng pulses occurring during line retrace periods.

The switching pulses may be made coincident with the line synchronising pulses.

The invention will be better understood by reference to the accompanying drawings in which:

Fig I is a diagram showing two function1 Sing cycles of operation of the recording device; Fig 2 shows a curve representing the variation of the brilliancy of a point with respect to time; Fig 3 is a block diagram of the system according to the invention; Fig 4 shows a few curves helping in the understanding of the working of the system of Fig 3 at different stages Figs 5 and 8 show two embodiments of the present invention; Fig 6 shows the control generator 4 of Fig 3; Fig 7 shows the characteristic curves i,.

v, of the cathode ray tube.

In Fig I the diagram shows two functioning cycles of operation of thle recording device operating under the conditions under which the invention works, that is to say when the camera device is operated in svnchronism with the frame or traversal succession on the ricture tube This is easily obtained since both the television scann ns and the hvltter operation are synchronised bv the mains sup Dlv It is sunnosed, as is usual in standardised television systems.

that each picture is made ulp of two interlaced frames, which means that each complete picture is obtained by successive reproduction of the even lines followed by the reproduction of the odd lines of the picture.

In European standards, standard equipments employ effectively 24 film picture frames per second, but it is usually and universally adopted when used in connection with television reproducing tubes, to svnchronise the shutter operation with the nmains as it is done in television scanning.

Duration of each frame is 1/50 of a second, the complete picture being thus reproduced in 1/25 of a second As is usual in standard cinematographic equinments the shutter is closed (pull through time) for 50 % 5 of the total time, a complete working cycle 60.(obturation and exposure) taking 1/25 of a second For instance, the shutter is closed during the first period of 1/50 of a second, open during the second period, closed during the third, and so on As it appears, if one uses a cathode ray tube with a phosphorescence time short with respect to 1150 of a second, the printing of the film will correspond only to one frame for each picture It has been proposed to use a picture tube, the screen of which shows an after-70 glow period such that the frame displayed on the tube when the shutter is closed be still brilliant during the succeeding exposure.

However, the brilliancy of any point on the screen decays with time according to an 75 exponential law, as shown on the curve of Fig 2 Therefore, the frame displayed during the closure of the shutter, although still apparent during the exposure time, will be considerably attenuated with respect to 80 g the frame displayed on the screen during the exposure period.

Besides, each frame is reproduced in I 50 of a second, which, (taking the extreme cases, means, in the case of a standard scan 85 ning (from left to right and from top to bottom), that the light emitted from the picture elements located at the left upper corner of the frame acts on the film during nearly the whole exposure time, while the 90 light from the picture elements located at the right lower corners falls on the film only during a very short fraction of this period The attenuation of brilliancy (decay) of the picture element, on the screen 95 with respect to time tends to produce a partial compensation of this operating characteristic but dees not suffice The compensation due to the attenuation is not sufficient to provide a uniform printing of the film 100 The object of this invention is to provide this necessarys, compensation.

Fic q Thchsts: record Inc:im-a-t c ^prising a gain controlled video amplifier in a schematical form It is well understood 105 that this example is in no way limitative and that one could provide a device for film recording from a set of several cathode ray tubes, each working simultaneously or alternately 110 A wide band amplifier 1 receives the video signal to be displayed on the screen of cathode ray tube 6 The very low frequency components and the D C component of the video signal are not transmitted 115 by the video amplifying stages but are restored at end of the channel by means of a D C restoring circuit 2 of known type.

The video signal is then applied to a variable gain D C amplifier 3 The gain of 120 this amplifier 3 is controlled by means of a gain control generator 4 so as to compensate for the light decay of the screen of cathode ray tube 6 The signals generated by means of the gain control generator 4 125 are shown by curve B on Fig 4 Curve A represents a saw-tooth voltage waveform representing the vertical frame scanning voltage During the frame corresnonding to the non-exposure of the film the even 130 784,856 frame in the instance chosen above, the gain of amplifier 3 is high, and reproduces the exponential curve representing the decay of phosphorescence as shown in Fig 2 During the frame corresponding to the exposure of the film, the gain of the amplifier remains always less than its gain during the preceding frame; and may be either maintained constant as is shown by the curve B or follow a more complex law of variation, as is shown by the curve H The law of variation during the exposure frame results from the combination of the exponential curve (Fig 2) with a linear increase intended to compensate the difference of time of pring:

with the point location due to the fact that the frame is actually scanned during the exposure period.

The output from amplifier 3 is then applied to the picture tube 6 by means of a filter network 5 designed to suppress the spurious signals resulting from the gain control of the video amplifier 3 from the video signal The optical system 7 focuses the picture from the screen of cathode ray tube 6 onto the objective of the cinematographic recording equipment 8 Owing to the exponential law of decay of phosphorescence, it has been found difficult to design such a filter as 5 to give good results since the video signal and the gain controll Ing signals occupy the same frequency band.

The video signal includes components the frequency of which ranges from zero (D C) up to several Mc/s The frequency components in signal B range from zero (D C level) up to several times the frame frequency ( 50 cus) It has not been found possible to separate the video signal from the spurious one introduced by the gain variation of amplifier 3 Spurious D C components have a very bad effect on the picture since they tend to introduce variations in the black level.

4 $ According to one feature of the invention, the gain variation of amplifier 3 is controlled in a non-continuous way, the controlling voltage being operative during intervals when the video signal has a reference level.

Such intervals exist, for instance, during the line and frame blanking pulses The signal is usually provided with line synchronising pulses or frame synchronising pulses superposed on such blanking signals In most of the standardised signals used for television a pedestal is provided on both sides of a synchronising pulse The level of the signal during the pedestal is related or equal to the black level Reference parts are therefore provided in the video signal which must normally keep a constant level through a whole transmission By restoring said constant level as will be explained later a complete elimination of the spurious D C.

and low frequency component may be achieved.

Fig 5 shows a circuit diagram of part of Fig 3 in greater detail, those parts of Fig.

3 which are shown in Fig 5 having the same reference numbers The grid of the variable 70 gain amplifier 3 is fed with video signals which have no D C component The gain control signal is fed by means of a low impedance cathode coupled stage 9 to an intermediate point 10 of a rectifier bridge 75 circuit, which constitutes a bi-directional switching network 11 At the diagonally opposite points 12 and 13 of the bridge network 11 are applied two oppositely polarised switching pulses occurring at line 80 frequency and occurring during line synchronising signals called clamping pulses in the art Switching network 11 connects the grid of tube 3 to point 10 during the clamping pulses without delay, and network 11 as 85 such works as a well known type of clamping circuit The gain of amplifier 3 therefore remains constant for the duration of one line, but varies from one line to the next, according to the law of variation of 90 the signal B of Fig 4 delivered by stage 9.

The curve representative of the gain variations of amplifier 3 is a stepped curve following signal B; the different steps are of equal duration (one line) The output sig 95 nal from the amplifier 3 is therefore a video signal the A C components of which show the correct values in order to obtain a correct display of the two frames of the picture taking into account the phosphores 100 cence decay However, as explained above, the black level (D C component) follows the gain variations of amplifier 3 In order to obtain a high quality picture, it is necessary to bring the black level back to a fixed 105 value, which should remain constant with respect to time This is obtained by filtering the output from the amplifier 3 through the wide band pass filter 5 which is constituted by a wide band amplifying stage 14 11 U The anode of the amplifier 3 is connected to the grid of stage 14 and to a D C restoring network 15 constituting a bi-directional switching network, via a coupling condenser 16 The bi-directional switch 15 15 is similar to the bidirectional switch 11.

The presence of the coupling condenser 16 prevents the transmission of the D C component of the video signal The output from amplifier 14 is fed directly to the grid 120 of the cathode ray tube 6.

Fig 6 shows the circuit diagram of the gain control generator 4 whose output is shown by curve B of Fig 4 A switching stage 17 is fed with negative pulses, shown 125 by curve C of Fig 4, which are synchronous with the frame blanking pulses of the video signal The purpose of these pulses is to cut-off tube 17 so that a positive pulse appears across the terminals of resistor 18 130 784,856 This positive pulse is transmitted to the anode of a diode 19 which becomes conductive and quickly charges condenser C.

The load impedance of stage 17 is low so as to charge more rapidly condenser C 2.

After the end of pulse C, tube 17 becomes again conductive, the diode 19 is cut-off, and the condenser C discharges through the high resistor R> The voltage appearing at the terminals of the circuit RC, is represented by the curve D of Fig 4 and follows an exponential law with respect to time.

The shape of the exponential curve is controlled by the time constant of circuit R C 2 and can be made equal to that of the curve of Fig 2 showing the brilliancy decay of phosphorescence of the screen of tube 6.

To obtain an accurate reproduction of the decay curve, it may be necessary to substitute a more elaborate network made up of several circuits having different time constants, discharging themrnselves, either simultaneously, or successively, instead of the condenser-resistor RC network The delayed discharge of condensers may be obtained by connecting the resistor-condenser circuits by means of polarised unidirectional conducting devices.

The gain control generator 4 also comprises a bistable trigger circuit 20 which is also fed with the negative pulses C This bistable trigger circuit is designed so as to pass from one stable state to the other one each time it is actuated by the pulse C.

The output from the trigger stage 20 is a square signal which is shown by curve E of Fig 4 The signals D and E are then added by means of two tubes 21 and 22 having a common load impedance 23 The voltage developed across the load 23 is the summation of curves D and E and is shown by curve F on Fig 4.

It has been explained that there were two possibilities for the setting of the gain of amplifier 3 The first one was to leave the gain of this amplifier constant during the exposure period; whilst the second possibility was to have the gain of amplifier 3 follow a more complex law The law of variation of the gain during the exposure period must compensate for the difference in luminous energy which is radiated by the different picture elements of the screen owing to the fact that one frame is being written on the screen during said period.

Using normal vertical frame scanning the upper left hand corner of the picture is inscribed first, and as a result the corresponding part of the screen will radiate luminous energy during almost all of the exposure period, whilst the lower right hand corner of the picture will be scanned at the end of the exposure period and therefore will radiate a very small quantity of energy To provide for this time compensation the gain of the amplifier should increase linearly sc that the brilliancy of one white element or the picture should be inversely proportiona.

to the time interval during which it emits light on the film Indeed, the film density 70 is proportional to the luminous energy whici:

is received during the exposure, that is te the product -brilliancy times exposure time" However, owing to the phosphorescence decay of the screen, the brilliancy 75 of a particular picture element on the screen is not constant with respect to time There is an exponential decay as has been explained at the beginning of the specification.

Therefore, in order to obtain a fine control 80 the law of variation of the gain of the video amplifier during the exposure periods shoulcd be the sum of the increasing linear variatio.

and the exponential decreasing variation used to compensate the luminous decay 85 during the traversals corresponding to the closure of the shutter.

Referring again to Fij 6 ales 2 1 a 2 22 sum up the waveforms D and E in ordeto obtain the waveform F which is then 90 fed to a valve 50 _ in order to compensate for a phase reversal due to valves 21 and 2:.

The output of valve 50 is fed to an electronic switch 54 (shown schematically ir Fig 6) which is designed to feed the output 95 from valve 50 alternately to two paralle' circuits switching from one to the other aframe frequency When the shutter of the camera is closed the output from valve 56 is fed to the anode of a clipping diode 24 100 whose cathode is biased at a potential suck that diode 24 cuts off any negative transients which may be produced during the switching of electronic switch 54 The output from diode 24 is shown by the wave 105 form B, this diode fixing the D C level of waveform B When the shutter of the camera is open the output from the valve is fed to a limiter 53 Also fed to th:

limiter is a saw tooth voltage waveform G 110 generated by a generator 51, and added te curve F by means of valve _'52 The wav form G is provided to compensate for the difference in duration of light emission of the successive lines of the frame scanned 115 during the exposure period The limiter 5is designed to suppress any positive transients which may be produced durinswitching, thus fixing the D C level of waveform H, and giving the correct gain 120 control voltage during the exposure time open as shown by curve H of Fig 4 The output from diode 24 and limiter 53 is fed to a stage 55 where the curves B and H are combined to form a waveform J, which is 125 fed to the control grid of video amplifier ? via the bidirectional switching network 11.

As a simplified form of the invention, an increasing (saw-tooth) correcting signal may be used during the odd fields 130

784,856 Up to now, the video signal D C level has been controlled in order to compensate for the phosphorescence decay in the cathode ray tube There is another cause of Spoor picture rendition, which lies in the non-linear characteristic of such a tube.

Fig 7 shows the i, -V characteristic of the cathode ray tube.

The following explanations will point out 10the bad effect-of said lack of linearity on the picture rendition, especially when the mean level of the A C Components of the video signal varies in such a way as has just been described If one considers the lotransmissions of an element (or dot) of the object with known brilliancy, a white dot, for instance, the amplitude of the corresponding signals Nm, and Nm, are read on the negative part of axis Vg N being the crosspoint of the curve representing the variation of ip in terms of Vg, and m, and m, respectively the cross points of the curve with the values of the video signal corresponding to a white dot situated at the beginning and end of an even frame, that is to say in the upper left corner and lower right corner of the frame due to the gain control Let i, and i 2 be the corresponding beam intensities (supposed proportional to 30the brilliancies of the dots on the fluorescent screen); the ratio of the brilliancies is not equal to the ratio of the signal amplitudes As a matter of fact, the slope of the characteristic curve decreases as the signal intensity decreases The condition for a perfect recording is that, in the absence of the video signal, the luminous energy radiated by any dot of the raster should be constant and independent of the position of this dot on the screen When the amplitude of the video signal is controlled to compensate for the phosphorescence decay, it is still necessary to take into account the characteristic of the picture tube or gamma of this tube Said correction is achieved by using, in connection with said gain controlled video amplifier, a second amplifier with a non-linear amplification characteristic fed by a negatively polarised video signal, the gain of which is controlled in accordance with the control signal of said first video amplifier By negatively polarised is meant a video signal such that a white dot corresponds to a grid voltage more negative than a black dot The characteristic of said second amplifier is chosen as identical as possible with the characteristic of cathode ray tube 6 The fact that a negatively polarised signal feeds said non-linear amplifier provides for a pre-distortion of said signal which will be compensated by the distortion introduced by the cathode ray tube as well known per se.

Fig 8 shows an embodiment of the invention incorporating the gamma correcting means, and in order to make the circuit easier to understand, the reference numerals used are the same as those for Fig 5 An adjustable gain video amplifier 29 is connected in front of amplifier 3 The charac 70 teristic (anode current to grid voltage) of 29 is not linear and is approximately repre sented by the characteristic shown in-Fig 7.

The amplifier 29-is fed with the negatively polarised video signal The -gamma correc 75 tion is generated by means of a generator 30, and is fed to the amplifier 29 by means of the bridge network 31, which is identical with the bidirectional switching network 11 The generator 30 delivers an output 80 signal identical with that generated in the signal generator 4 and used to control the gain of the video amplifier 3 D C restoration is achieved on video amplifier stage 14, which feeds directly the grid of cathode ray 85 tube 6, by means of the bi-directional clamp circuit 15.