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R&D Report 1958-22

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R&D Report 1958-22 RESEARCH OEPARfMfMT TELEVISION SIGNAL STORAGE USING IMAGE ICONOSCOPE Report Mo, T~070 ( 1958/22) GcF, "ewell, A.I'o1,LEEo PcHoCc Legate Thi. Report i. the propert1 of the Briti.h BroadcastiDg CorporatioD aDd aa1 Dot be reproduoed in an1 form without the writteD perai •• ioD of the Corporation. Report No. T-070 TELEVISION SIGNAL STORAGE USING IMAGE ICONOSCOPE Section Title Page SUMMARY 0 0 • • • • • 0 0 • 0 C • • " • 0 0 • • 0 • 0 • , • co, 0 •• 1 1 1 INTRODUCTION .. • '" f) 0 ,., .. .. • • ., • c .. '" .. '" " () • C '" ~ r <i" .. • ,. .. 2 DESCRIPTION OF THE SYSTEM 1 1 2.1- General. 0 0 2.2. Production of the Charge Pattern 2 2 2.3. "Reading off" the Stored Signal 0 3 2.4. Inherent Advantages of the System 3 3 PRACTICAL IMPERFECTIONS IN THE SYSTEM. 0 ••• 0 , • 0 • 0 0 •• 3.1. The Non-uniform Effect of the Potential Gradient between the Final Anode and the Mosaic Elements 3 3.2. "Whi te-crushing" • • • • • • • • • • • 3 4 3.3. Spurious Charge Patterns • e. • 3.4. Shadow Effects due to "Over-Writing" 4 4 3.5. "Black-crushing" . 0 • • • • • , 5 3.6. Possible Remedy for these Defects 3.7. Possible Damage to Photocathode • 5 5 4 DESCRIPTION OF EXPERIMENTAL SYSTEM • • 0 • • • • • • • • • • • • • • • I 8 I 5 RESULTS OF INVESTIGATION .. .. " Cl " " 0 0 ,~ • '" e .. '" e @ '" • .. ., .. " .. ( J 8 6 CONCLUSIONS 0 , • • • 0 , 0 • • 0 , • 0 0 9 7 REFERENCES () <, " 0 ::) 0 " " .. (., [} 0 f) .. " 0 " .. .. .. G " Cl " .. 0 0 0 0 " Report No. T-070 August 1958 (1958/22) TELEVISION SIGNAL STORAGE USING IMAGE ICONOSCOPE SUMMARY This report describes an investigation of a system of television storage using the image section of an image-iconoscope camera tube. The results show that the inherent structural limitations of the image iconoscope prevent the system from having operational value. A system of this kind would probably be capable of further development, however, if a special storage tube were produced. 1. INTRODUCTION The ability to delay a television signal by the precise time duration of one field without appreciable deterioration would greatly facilitate operations such as standards conversion, telerecording and bandwidth compression. A system of storage, utilizing the afterglow of a fluorescent phosphor', 1,2 has been successfully developed for use in telerecording. Another system, which makes use of a charge pattern to control the passage of an electron beam through a fine mesh grid,3 has been developed for the storage of television pictures for periods up to many hours. Both these systems have particular applications, but neither is convenient for delaying a television waveform for one field period. This report describes an investigation into a system4 of field storage, devised by A.V. Lord, which uses the mosaic of an image iconoscope in order to store a charge pattern. 2. DESCRIPTION OF TEE SYSTEM 2.1. General The method consists of scanning the mosaic of an image-iconoscope tube with an electron beam the intensity of which is proportional to the instantaneous magnitude of the video signal to be stored. By this means, a single field is reproduced as a charge pattern on the mosaic. This pattern is reconverted to a voltage waveform by exploring it with an electron beam of constant current and taking the output voltage at the signal plate of the tube. If it is required to delay the complete video signal, it is necessary to operate two such channels, so that one is storing a field while the other is reproducing the previous field. In the particular system investigated, no use is made of the conventional electron gun of the image iconoscope; the electron beam used for "writing on" the charge pattern and, later, for "reading" it off, is obtained from the photocathode. 2 Alternate fields, say the even fields, of the picture to be stored are applied to produce a picture on the screen of a conventional cathode-ray scanning tube. An optical system is used to produce an image of the picture on the photocathode of the image iconoscope, the image section of which functions in the normal manner so that an electrical charge pattern corresponding with signal to be stored is formed on the mosaic. During the period of the odd fields, the picture is "read off" by repeating the process but with a plain "white" raster in place of picture. The mosaic is thus scanned by an electron beam of uniform current which is sufficient to stabilize the mosaic, element by element, up to the potential of the final anode. The current necessary to achieve this stabilization flows in the signal-~late circuit and con­ stitutes the output signal. 2.2. Production of the Charge Pattern Before the charge pattern is written on to the mosaic, it is necessary to ensure that the majority of secondary electrons emitted by the mosaic are collected by the final anode. For this purpose the mosaic is established at a potential which is negative with respect to that of the final anode. This is achieved in the following manner. In the field blanking interval preceding the field to be stored, the photo:­ cathode is uniformly illuminated by means of a cathode-ray flash tube and an optical system for a period of up to one millisecond. Simultaneously, the final anode is rendered negative by some twenty or thirty volts. The mosaic is flooded by electrons from the illuminated photocathode and, being unable to lose secondary electrons, acquires a uniform negative charge. Eventually this charge would equal the negative potential on the final anode and stabilization would occur. In practice, however, the magnitude of the electron beam from the photocathode and the capacity of the mosaic are such that in the one millisecond duration of the pulse and flash the mosaic potential changes by only some three to six volts. When the pulse ceases and the final anode returns to zero potential, the mosaic is left charged negatively with respect to it. Following the establishment of this potential difference the picture­ modulated scan commences. The optical image on the photocathode gives rise to a beam of electrons modulated in intensity in accordance with the picture information and this electron image is, in turn, focused on the mosaic. The primary photo-electrons constituting the scanning beam cause secondary electrons to be emitted by the mosaic, the number depending upon the secondary emission ratio for the mosaic surface and the energy of the primary electrons; for the con­ di tions under which the image iconoscope is used the ratio is approximately five to one. Following the light flash and final anode pulse, already mentioned, the mosaic is left negatively charged. The effect of bombardment by the intensity­ modulated electron beam is to cause each element of the mosaic to lose electrons by secondary emission in proportion to the instantaneous intensity of the electron beam. If the secondary electrons are all collected by the final anode, the charge pattern so formed on the mosaic is exactly representative of the picture to be stored. 2.3. "Reading off" the Stored Signal Following the writing-on process, the optical scan using the cathode-ray 3 tube is repeated with no intensity modulil,tion. 'rhe raster is focused on the photo- cathode as before, and gives rise to a primary electron beam of constant intensity (except for line blanking pulses), and this, in turn, is focused upon the mosaic. As the scanning beam passes over each picture element, secondary electrons are again released and are collected by the final anode. The process continues during bombardment of each element until sufficient electrons have been lost for the element to reach final-anode potential. At this point, stabilization occurs and any additional secondary electrons which are released return to the element being bombarded or to other more positive areas of the mosaic. The number of electrons leaving each element depends upon the charge on that element, so that the number of electrons leaving the mosaic varies in accordance with the picture signal. This causes a corresponding current to flow in the signal plate and load resistor, thus producing the output voltage waveform. 2.4. Inherent Advantages of the System This system has the inherent advantage that the same scanning waveform is used for both writing on and reading off the stored pattern; thus, non-linearity of the scan waveform will not cause any geometric distortion of the stored picture. Similarly, any geometric distortion produced by either the optical or electronic lens systems,5 e.g. pincushion distortion, will not cause distortion of the stored picture. The collection of secondary electrons by the final anode will, if complete, prevent the spurious charge pattern defects which normally occur with image-iconoscope camera tubes. Theile and Townsend6 have shown that some improvement in this respect has been obtained in a similar application of the image iconoscope. 3. PRACTICAL IMPERFECTIONS IN THE SYSTEM 3.1. The Non-uniform Effect of the Potential Gradient between the Final Anode and the Mosaic Elements The foregoing describes the ideal behaviour of the image iconoscope used for signal storage; in practice this is not fully achi.eved due to unsuitable features of the tube structure. The final anode consists baSically of a cylinder co-axial with, and normal to, the mosaic. Thus the potential gradient between any element of the mosaic and the final anode is a function of the distance between the element and the mosaic centre. One result of this non-uniformity is that secondary electrons emitted from the mosaic near to its periphery are more likely to be collected by the final anode than are secondaries emitted from areas near the mosaic centre and which may pass close to positively charged areas of the mosaic in the course of their journey towards the anode.
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