495,724

PATENT SPECIFICATION

Application Date: June 7, 1939. No. 15729/37

Complete Specification Left: May 21, 1938.

Complete Specification Accepted: Nov. 18, 1938.

-------------------------

PROVISIONAL SPECIFICATION

Improvements in or relating to Television Receiving Systems

I, ALAN DOWER BLUMLEIN, a British subject, of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:

The invention relates to television receiving systems operating from a transmission system in which the object for transmission is scanned in two successive scanning operations, the lines scanned being interlaced at the receiver for the production of a composite picture.

In the usual type of interlaced transmitting systems, the scanning spot, whether the system employs mechanical scanning, such as a mirror drum or apertured disc or a concentrated beam of cathode rays, is arranged to be of such dimensions that the lines of the picture frame scanned during one scanning operation are spaced apart, and that for the successive scanning operation the spacing between such lines in the successive frame are scanned the picture being thus only completely scanned in two frames. The scanning spot at the receiver is likewise so dimensioned so that the complete picture is reconstituted in two successive traversals of the screen.

If a picture reproduced from such interlaced scanning is examined closely it is possible, if the observer directs his vision vertically of the picture at a suitable rate to obtain the impression that the lines constituting the picture are halved in number and are moving steadily upwards. Furthermore, if the reproduced lines are made particularly bright and the observer’s vision maintained at one point on the reproduced picture, an effect which can perhaps be best expressed colloquially as ‘dithering’ is produced due to the alternate illumination of the two interlaced sets of lines. These undesirable effects may of course be removed by scanning the object or reproducing the picture without interlaced scanning, but then a reduction in vertical definition of horizontal edges results for the same total number of lines and the same resultant flicker frequency.

It is therefore the chief object of the present invention to provide an improved receiving system with a view to reducing or avoiding the above-mentioned defects due to interlaced scanning but without undue loss of definition such as occurs without interlacing.

According to the invention a television receiving system is provided operating from an interlaced transmitting system, in which the size of the scanning spot or spots and the line-scanning frequency are so co-related that one frame of the transmitted object is reproduced as a substantially complete picture during one scanning operation and the succeeding frame of the object is likewise reproduced as a substantially complete picture in the next scanning operation and interlaced with the preceding frame but the axes of the lines reproducing in the succeeding frame are arranged to lie intermediate the axes of the lines reproduced in the preceding frame.

For example, in a television receiver operating with the television transmitting system now in use in which interlaced scanning is employed, the same line and frame frequencies may be used but the size of the scanning spot for reproducing two frames to be interlaced is so dimensioned at right angles to the scanning lines, that for a frame composed of n lines interlaced with another frame also of n lines giving a resultant picture composed of 2n lines the vertical height of the spot corresponds to the 1/n of the height of the picture at right angles to the scanning lines.

The invention is particularly applicable to the reproduction of large pictures where many of the audience may be sitting quite close to the screen. The invention may be employed both in systems employing cathode ray scanning and mechanical scanning but is particularly applicable to the latter type of scanning where the height of the spot in a direction at right angles to the reproduced lines can be correctly controlled.

For example, assuming that a reproducing screen is used which is approximately 200 inches high, and which is scanned in two frames each of 100 lines and normally interlaced to give a resultant picture of 200 lines, then the usual method of scanning in such an arrangement would be to arrange that the scanning spot is made approximately one inch high and if not particularly intense and the spot is accurate in height the picture will be effectively reproduced in two successive scans. At a single scan, however, only half the picture will be reproduced so that if an observer directs his vision upwardly at the rate of one inch for each frame, he will observe what appears to be a picture composed of 100 lines moving steadily upwards. If, however, the spot height be increased to two inches in accordance with the invention so that adjacent lines of a single frame just touch it would be difficult for an observer to observe the lines of any one frame since there will be no gap between them. The size of the spot will now correspond to a picture reproduced from a system without interlaced scanning and in which one frame is completely scanned in 100 lines but providing that the subsequent frame is interlaced with the preceding frame in accordance with the invention then the lines of the succeeding frame in accordance with the invention then the lines of the succeeding frame will overlap the lines of the preceding frame and a better vertical resolution will be afforded than is possible with scanning a single frame without interlaced in 100 lines. For example, assuming that a picture is being reproduced composed of 50 black and 50 intermediate white horizontal lines, then if the black lines and the white lines coincide exactly with the scanning lines at the transmitter with 100 line scanning without interlacing then perfect reproduction can be obtained. On the other hand, if the black and white lines at the transmitter fall out of step with the scanning lines the resultant picture will be grey. With the interlaced receiving system in accordance with the invention employing a scanning spot corresponding to 100 lines at the receiver and scanned as though interlaced to a total of 200 lines, then there will be no position of the black and white lines described above which will be completely lost, that is to say, although the reproduction of these lines will not necessarily be as good in all cases as that obtained with interlaced scanning using the usual spot height, the reproduction will be more consistent than that possible with a 100 line system without interlacing.

As stated above, the invention is particularly applicable to mechanical scanners where the height of the spot at right angles to the scanning lines can be correctly controlled, as for example, by the provision of the usual aperture. The invention is also applicable to systems where more than one spot is scanned simultaneously, such as a system which employs a number of light-control elements operating in delayed relationship. With a cathode ray tube scanner it is found that the size of the scanning spot tends to vary with the brightness of the picture, but in such cases it is possible to control the height of the spot by constructing a tube having noncircular apertures in the electrodes of the electron gun so as to produce a spot which has a relatively small dimension along the length of the line but a dimension in a direction at right angles to the lines which is sufficiently accurate for the purpose in view.

The invention is also particularly applicable to cathode ray picture reconstituting devices of the kind described in the Specification of British Patent No. 432,485 in which several spots on the fluorescent screen are scanned simultaneous. The image of the cathode in this case with the control grids is projected onto the fluorescent screen through a suitable electron optical system. When employing the present invention the height of the aperture associated with the cathode may be made to correspond to two scanning lines.

With either cathode ray or mechanical scanning if the focus of the spot is not sufficiently sharp to cause reproduction of the picture without visible gaps between the lines in any one frame, the height of the scanning lines that is to say the height of the spot at right angles to the scanning line is adjusted to give most uniform illumination (absence of apparent scanning lines in for example a steady white picture) for a single scan. Thus in some cases the lines of a single frame may overlap slightly where the intensity decreases from the maximum value.

Dated this 7th day of June, 1937

F. W. Cackett

Chartered Patent Agent

Complete Specification

Improvements in or relating to Television Receiving Systems

I, ALAN DOWER BLUMLEIN, a British subject, of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

The invention relates to television receiving systems operating from a transmission system in which the object for transmission is scanned in two successive scanning operations, the lines in each frame being interlaced at the receiver for the production of a composite picture.

In the usual type of interlaced transmitting systems, the scanning spot, whether the system employs mechanical scanning, such as a mirror drum or apertured disc or a concentrated beam of cathode rays, is arranged to be of such dimensions that the lines of the picture frame scanned during one scanning operation are spaced apart, and that for the successive scanning operation the spacing between such lines in the successive frame are scanned the picture being thus only completely scanned in two frames.

The scanning spot at the receiver whether the receiving system employs mechanical scanning, such as a mirror drum or apertured disc or concentrated beam of cathode rays, is likewise so dimensioned so that the complete picture is reconstituted in two successive traversals of the reproducing screen.

If a picture reproduced from such interlaced scanning is examined closely it is possible, if the observer directs his vision vertically of the picture at a suitable rate to obtain the impression that the lines constituting the picture are halved in number and are moving steadily upwards or downwards. Furthermore, if the reproduced lines are made particularly bright and the observer’s vision maintained at one point on the reproduced picture, an effect which can perhaps be best expressed colloquially as ‘dithering’ is produced due to the alternate illumination of the two interlaced sets of lines. These undesirable effects may of course be removed by scanning the object or reproducing the picture without interlaced scanning, but then a reduction in vertical definition of horizontal edges results for the same number of lines per second and the same resultant flicker frequency.

It is therefore the chief object of the present invention to provide an improved receiving system with a view to reducing or avoiding the above-mentioned defects due to interlaced scanning but without undue loss of definition such as would occur without interlacing.

According to the invention a television receiver is provided designed to operate from an interlaced transmitting system, and in which the size of the scanning spot or spots and the line-scanning frequency are so co-related that one frame of the transmitted object is reproduced as a substantially complete picture during one scanning operation and the succeeding frame of the object is likewise reproduced as a substantially complete picture in the next scanning operation and interlaced with the preceding frame with the axes of the lines reproducing in the succeeding frame lying substantially intermediate the axes of the lines reproduced in the preceding frame.

For example, in a television receiver operating with the television transmitting system now in use in Great Britain in which interlaced scanning is employed, each complete picture being scanned in 405 lines, with 50 frames per second, i.e.. 25 complete pictures per second, the same line and frame frequencies may be used but the size of the scanning spot for reproducing may be so dimensioned at right angles to the scanning lines that for a frame composed of n lines interlaced with another frame also of n lines giving a resultant picture composed of 2n lines the vertical height of the spot corresponds to the 1/n of the height of the picture at right angles to the scanning lines.

The invention is particularly applicable to the reproduction of large pictures where many of the audience may be sitting quite close to the screen. The invention may be employed both in systems employing cathode ray scanning and mechanical scanning but is particularly applicable to the latter type of scanning where the height of the spot in a direction at right angles to the reproduced lines can be correctly controlled. Scanning at the transmission will of course be unaltered, that is to say, each frame scan will generate signals representative of only half of the complete picture.

In order that the said invention may be clearly understood and readily carried into effect it will now be more fully described with reference to the accompanying explanatory drawings in which:

Figure 1 is a diagram illustrating the lines traced out by a television receiver of known form,

Figure 2 is a diagram similar to Figure 1 but indicating the lines traced out by a television receiver operating in accordance with the present invention,

Figure 3 illustrates diagrammatically a mechanical television receiver in accordance with the invention,

Figure 4 illustrates diagrammatically a cathode ray television receiver according to the invention, and

Figure 5 illustrates a detailed view of an electrode of the cathode ray tube of the receiver shown in Figure 4.

The rectangle shown in Figure 1 illustrates diagrammatically the reproducing screen of a television receiver and the lines 1, 2 and 3, and A, B an C, indicate the axes of the lines traced out by the scanning spot. With an interlaced scanning system the lines 1, 2 and 3 indicate the lines traced out in one frame scan at the receiver and the lines A, B and C indicate the lines traced out in the next frame scan at the receiver, the complete picture being reproduced at the receiver in two successive frame scans. The width of the scanning spot at the receiver is such that during each frame scan one-half of the complete picture is traced out by the spot. The width of the lines scanned is indicated for the lines 1, 2 and 3 by the dotted line shading and the width of the lines A, B and C by the full-line shading.

It will be seen from Figure 1 that the extent of the dotted line shading occupies one-half of the rectangle representing the screen whilst the full line shading occupies the other half.

Figure 2 of the drawings indicates a screen of similar size to that shown in Figure 1 and in which the height of the scanning spot is increased in accordance with the invention so that during each frame scan the complete area of the screen is traced out by the scanning spot and in the next frame scan the complete area of the screen is again traced out by the scanning spot, the axes of the lines traced out in one frame scan being interlaced with the axes of the lines traced out in the next frame scan.

In Figure 2 the same notation for the axes of the lines scanned are employed and with the spot increased in size it will be seen that the area traced out by the line in Figure 2 is twice the area traced out by the line Figure 1 and so on. The same shading is adopted in Figure 2 and it will be readily seen that the dotted line shading occupies the whole area of the screen and likewise the full-line shading. The lines A, B and C interlace with the lines 1, 2 and 3, as in Figure 1. Figs. 1 and 2 are not intended to represent the true arrangement of scanning lines but are merely diagrammatic.

In the case of a reproducing screen of approximately 200 inches high and which is scanned in two frames each of 100 lines and normally interlaced to give a resultant picture of 200 lines, then the usual method of scanning in such an arrangement would be to arrange that the scanning spot is made approximately one inch high and if not particularly intense and the spot is accurate in height the picture will be effectively reproduced in two successive scans. At a single scan, however, only half the picture will be reproduced so that if an observer directs his vision upwardly at the rate of one inch for each frame, he will observe what appears to be a picture composed of 100 lines moving steadily upwards. If, however, the spot height be increased to two inches in accordance with the invention so that adjacent lines of a single frame just touch it would be difficult for an observer to observe the lines of any one frame since there will be not gap between them. The size of the spot will now correspond to a picture reproduced from a system without interlaced scanning and in which one frame is completely scanned in 100 lines but providing that the subsequent frame is interlaced with the preceding frame in accordance with the invention then the lines of the succeeding frame in accordance with the invention then the lines of the succeeding frame will overlap the lines of the preceding frame and a better vertical resolution will be afforded than is possible with scanning a single frame without interlaced in 100 lines. For example, assuming that a picture is being reproduced composed of 50 black and 50 intermediate white horizontal lines, then if the black lines and the white lines coincide exactly with the scanning lines at the transmitter with 100 line scanning without interlacing then perfect reproduction can be obtained. On the other hand, if the black and white lines at the transmitter fall out of step with the scanning lines the resultant picture will be grey. With the interlaced receiving system in accordance with the invention employing a scanning spot corresponding to 100 lines at the receiver and scanned as though interlaced to a total of 200 lines, then there will be no position of the black and white lines described above which will be completely lost, that is to say, although the reproduction of these lines will not necessarily be as good in all cases as that obtained with interlaced scanning using the usual spot height, the reproduction will be more consistent than that possible with a 100 line system without interlacing. To obtain the best advantage of the invention the scanning spot at the transmitter will be small i.e. will only completely scan the picture in two frames although even if the spot size at the transmitter is slightly varied good results can also be obtained in a receiver constructed in accordance with the invention.

As stated above, the invention is particularly applicable to mechanical scanners where the height of the spot at right angles to the scanning lines can be correctly controlled, as for example, by the provision of the usual aperture.

Figure 1 of the drawings illustrates a mechanical scanner comprising a radio receiver 6 which supplies signals to a light modulating device 7, the light from which is projected through the apertures of a scanning disc 8, driven by a motor 9, the picture being reconstituted on a screen 10. The disc 8 is designed for interlaced scanning, but the apertures in the disc, indicated by the dotted lines, instead of being their normal height are increased to twice their normal value in accordance with the invention. The invention may of course be applied to any other form of mechanical scanning system.

Figure 4 illustrates diagrammatically a cathode ray tube television receiver, comprising a radio receiver 11, a cathode ray tube 12, the cathode ray tube having the usual electrodes associated therewith, comprising a cathode 13, a modulating electrode 14, a first anode 15 and a second anode 16. Signals from the radio receiver are applied to the modulating electrode 14 and line and frame synchronising signals are also applied to line and frame oscillation generators 17 and 18 which, in turn, supply the required current to scanning coils 19 and 20, in known manner. The necessary source of potential for applying appropriate operating potentials to the electrodes of the cathode ray tube is omitted from the drawings. In accordance with the invention the scanning spot which scans the fluorescent screen of the tube 12 is increased to twice its normal height in accordance with the invention.

With the cathode ray tube scanner shown in Figure 4 it may be found that the size of the scanning spot tends to vary with the brightness of the picture, but in such cases it is possible to control the height of the spot by constructing a tube in which the scanning spot is non-circular, so as to produce a spot which has a relatively small dimension along the length of the line but a dimension in a direction at right angles to the lines which is sufficiently accurate for the purpose in view. For example, the electrode 15 instead of being provided with the usual circular aperture, is provided with an oval aperture 21, as shown in Figure 5, the vertical dimension of the aperture being substantially twice the diameter of the usual circular aperture.

If desired, the usual cathode shield, not shown in Figure 4, may also be made of oval form as shown in Figure 5, or in some cases only the aperture in the cathode shield may be of such oval form, the aperture in the first anode 15 being in this case of the usual circular form. Alternatively, in order to obtain a noncircular scanning spot an assymetric focussing field may be employed which deforms the scanning spot into a suitable noncircular form. As an alternative, in order to obtain a noncircular spot, two opposing magnetic fields may be used, generated, for example, by coils disposed on opposite sides of the cathode ray tube, and serving to defocus the spot to a noncircular form, and a main magnetic or electrostatic field is also provided so that the noncircular spot is refocussed along its main axis.

Other means may of course be employed to provide noncircular spots.

The invention is also particularly applicable to cathode ray picture reconstituting devices of the kind described in the Specification of British Patent No. 432,485, in which several spots on the fluorescent screen are scanned simultaneously. The image of the cathode in this case with the control grids is projected onto the fluorescent screen through a suitable electron optical system. When employing the present invention the height of the aperture associated with the cathode ray may be made to correspond to two scanning lines.

With either cathode ray or mechanical scanning if the focus of the spot is not sufficiently sharp to cause reproduction of the picture without visible gaps between the lines in any one frame, the height of the scanning lines, that is to say the height of the spot at right angles to the scanning line is adjusted to give most uniform illumination (absence of apparent scanning lines in, for example, a steady white picture) for a single scan. Thus, in some cases the lines of a signle frame may overlap slightly where the intensity decreases from the maximum value.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-

  1. A television receiver designed to operate from an interlaced transmitting system in which the object for transmission is only substantially completely scanned in two successive frame scanning operations, wherein the size of the scanning spot or spots at the receiver and the line scanning frequency are so co-related that one frame of the transmitted object is reproduced as a substantially complete picture during one frame scanning operation and the succeeding frame of the object is likewise reproduced as a substantially complete picture in the next frame scanning operation and interlaced with the preceding frame with the axes of the lines reproduced in the succeeding frame lying substantially intermediate the axes of the lines reproduced in the preceding frame.
  2. A television receiver according to claim 1 and employing a cathode ray reproducing device wherein the scanning spot is made of noncircular form so as to produce a spot which has a relatively small dimension along the length of the line and a larger dimension at right angles to the scanning lines.
  3. A television receiver substantially as described with reference to Figures 2 and 3 or 2, 4 and 5 of the accompanying drawings.

Dated this 20th day of May 1938

F. W. Cackett

Chartered Patent Agent