US2210995A - Television and like system - Google Patents

Description

lg- 1940- E. c. WHITE 2,

TELEVISION AND LIKE SYSTEM v Filed July 2, 1936 Del 04 Neil var)? INVENTOR ERIC L. C. WHITE BY 7% 5M ATTORNEY Patented Aug. 13, 1940 UNITED STATES PAT NT OFFICE 2,210,995 TELEVISION AND LIKE- sY's'rEi'vlr Eric Lawrence Gasling White, Middlesex, En

land, assignor to Electric & Musical Industries Limited, a. British company V Application July 2, 1936, Serial No. 88,528

In this specification, for convenience, the term- D. 0. component will be applied to any or all of these low frequency components, and it should be borne in mind that the D. C. component need not of necessity include a component of direct current.

It is known that when signals, such as television signals, which contain a- D. C. component are amplified in an A. C. amplifier, or are passed through any other channel incapable of transmitting the D. C. component, this component is lost, and in. the output signal, variations of signal amplitude take place about what is known as the electrical zero; the electrical zero is such that the areas enclosed by the graph representing variations of signal amplitude with time, above and below the electrical zero respectively, are equal. Means which have been proposed to compensate for this loss of the D. 0. component are described in the specifications of British Patent No. 422,906 (U. S. Serial No. 720,205). and U. S. application Serial No. 40,532. r

In addition to causing loss of the D. 0. component, A. C. amplifiers and like channels having a low-frequency cut-off may also cause a further form of distortion; the manner in which this distortion manifests itself can best be appreciated of the object from a consideration of the base of a television signal, and for this purpose reference will now be made to Fig. 1 of the accompanying drawing in which the signal voltage is plotted as ordinate against time as abscissa,

In Fig. 1, form of the television signal, the pulses a being line synchronizing pulses and the parts b representing the picture signals corresponding to lines being transmitted, and the horizontal dotted line drepresents the average position of the datum, that is, the electrical zero line of the whole signal over a period of time substantially longer than that shown.

A portion of the signal is shown after passing through an amplifier in which the 110. component is lost, and the figure shows the wave form in the neighborhood of the point e where a relatively large change in average picture brightness from uniform dark grey to uniform white has taken place. On the left of the point e, the signal the full line-represents the wave.

Britain July .12, 11935 I (oi; risk-'6) is symmetrical about the electrical zero, and on the right of point c it is tending to take up a position 'in which it is again'symmetrical about the electrical zero.

Itswill be seen that on den change in average brightness, the signal does not at once take up a symmetrical position about the electrical zero; the signal rises to a relatively large amplitude, relative to the electrical zero, and

thereafter the amplitude falls until the steady l symmetrical :state is reached; The dotted line e indicates approximately how the signal amplitude the occurrence of a sud decays overseveral line periods immediately after a sudden change in average brightness, will be noted that the envelope of the signal through the peaks of the synchronizing pulses follows "a curve similar to'the curve e.

Now it will be observed that the picture signals on the left of point 0, in Fig. 1, and those on the extreme right of this figure, although'representing uniform grey and uniform white respectively, have a slight tilt from left to right towards the datum. I This tilt, which is exaggerated. in the drawing for purposes of illustration, is introduced during the passage of the-signals through theA; C. amplifier and is due to the fact that the amplifier has an imperfect response to low-frequency signal components such as occur when the average picture britghness changes and remains at or near. the new value, for a time which is comparable with the time constants of the couplings in the amplifier which determine the lowfrequency cut-off. Due to the decay indicated by dotted line e, the picture signals of each line, insteadfof sloping or left to right as do those tot-he left of point 0, are tilted much more steeply, the amount of tilt decreasing towards the right until finally it is of the same order of magnitude as that of the signals to the left of point c. The present invention is concerned with the distortion of the type represented by the extra tilt introduced, in the manner discussed above, when there is asudden change in average brightness. The invention is, of. course, not limited to television systems, and is applicablein other systems where distortion of the same type arises.

Known methods of re-inserting the D. C. component leave the line signals with the additional of the present inventionto provide a method of tilting only slightly from and it and means for removingor reducing this distor- The present invention accordingly provides a method of reducing or eliminating distortion of the type referred to, for use in television and like systems in which electrical signals are passed through a channel having a low-frequency cutoff, which method comprises the steps of utilizing the distorted signals, during predetermined spaced periods of time, to cause a reactance element to assume a charge dependent upon the peak amplitude of said signals during said periods, 1

generating compensating electrical variations under the control of changes in the charge held by said reactance element, and mixing said compensating variations with said distorted signals in such sense and at such an amplitude as to reduce or eliminate said distortion.

Further features of the invention will appear from the following description and appended claims.

An embodiment of the invention will now be described with reference to Fig. 2 of the accompanying drawing, in which there is shown apparatus for generating compensating variations which can be added to a television signal before the re-establishment of D. C. to compensate for the additional tilt introduced, in the manner described above, when the signal changes suddenly in average brightness. The television signal, which is without its D. C. component, and is of the nature of the signal shown in Fig, 1, is applied from terminal I between the control grid of valve 2 and earth and, through condenser 3, between the control grid of valve 4 and earth, it being arranged that the synchronizing signals drive the grids of valves 2 and 4 in the positive direction.

The control grid circuit of valve 4 contains a leak resistance l1, and the flow of grid current in valve 4 when synchronizing pulses are present on its grid, causes condenser 3 to become charged to such a negative potential that no current flows in valve 4 until a synchronizing pulse arrives on its grid. By suitably determining the grid bias of valve 4, the synchronizing pulses can thus be freed from picture signals, the valve 4 being caused to pass current, that is, in other words, being switched on by each synchronizing pulse. The anode of valve 2 is connected to the positive terminal of a source (not shown) of anode current, the negative terminal of which is earthed, the cathode of valve 2 is connected to the anode of valve 4 and the cathode of'the latter valve is earthed. Means (not shown) are provided for biasing the control grid of valve 2 to a negative potential relative to its cathode; valve 2 acts as a so-called cathode follower valve, the valve 4 serving as its cathode circuit impedance, and the arrangement being such that the potential of the cathode of valve 2 substantially follows variations in the potential of its grid.

In operation, when a synchronizing pulse is present and valves 2 and 4 are conducting, the cathode of valve 2 takes up a potential slightly negative relative to the potential of the grid thereof; when there is a change in average brightness and the level of the tips of the synchronizing pulses changes, a charge is given to condenser 5; the change in the level of the tips of the pulses is, as has been explained, a measure of the amount of additional tilt for which it is desired to compensate. The charge on condenser 5 maintains the cathode potential of valve 2 between pulses.

The potential set up across condenser 5 due to changes in the level of the tips of the synchronizing pulses, is applied to the grid of a cathode follower valve 6, the cathode of which is connected through resistance 7 to one pole of a condenser 8, the other pole of which is earthed.

Any change in the level of the tips of the synchronizing pulses, which change indicates the presence of additional tilt in the line signals, thus causes charging or discharging of the condenser 8; there are thus set up across the condenser 8 potential variations which can be employed to compensate for the addition tilt in the line signals.

The'signals from terminal I. are fed to valve I0, which is biased by means of resistance II to operate as an amplifier, and the potential variations set up across condenser 8 are reversed by valve 9 and fed to an amplifying valve l2 biased by resistance I3. Valves l0 and I2 have a common anode resistance I 4, and these valves accordingly serve to mix the television signals with the compensating anti-tilt variations. The mixed signals set up at terminals l5 are then fed to means, which may be of the kind described in the specification of co-pending application Serial No. 40,532, for re-inserting the D. C. component.

Since the anti-tilt variations may only commence to be generated in practice after an interval of the order of one line period from the point at which the average brightness changes, the television signals fed to valve I0 may be delayed by about one line period by means of a delay network I6.

The invention is not limited to the embodiment described and many modifications within the scope of the appended claims will occur to those versed in the art.

What is claimed is: V a

1. In television transmission wherein trains of video signals are developed, each train representing a single line of a scanned object, and synchronizing signals are interposed between the trains of video signals, an

trains of signals are passed through channels having a low frequency cut-off value and are distorted due to the abrupt large amplitude changes of said video signals,'the method of reducing said distortion which comprises the steps of developing electrical energy of a value dependent upon the peak amplitude of said signals, utilizing changes in said developed electrical energy value to control the development of compensating signals, and mixing said compensating signals with said distorted signals in proper phase and amplitude so as to reduce said distortion,

2. In television transmission wherein trains of video signals are developed, each train representing a single line of a scanned object, and synchronizing signals of a value outside the range of said video signals are interposed between the trains of video signals, and wherein abrupt changes in amplitude of the video signals occur due to abrupt changes in the shade of the object along a linear portion thereof and in which said trains of signals are passed through channels having a low frequency cut-off value and are distorted due to the abrupt large amplitude changes of said video signals, the method of reducing said distortion which comprises the steps of developing electrical energy of a value dependent upon the peak amplitude of each successive synchronizing pulse, maintaining said developed electrical energy constant in value during the development of video signals, utilizing changes in said developed electrical energy value to control the development of compensating signals, and mixing'said compensating signals with the distorted video Signals in proper phase and amplitude so as to reduce said distortion. 3. Television transmission apparatus comprising means for developing video signals having large amplitude changes due to abrupt changes in shade of the scanned object along a linear scanned portion thereof, means for developing synchronizing signals, amplifying means for said video and synchronizing signals having a low frequency cut-ofi value whereby said signals are distorted due to the action of the amplifier means on the large abrupt amplitude changes in the video signals, means for developing electrical energy of a value dependent upon the peak amplitude of said video and synchronizing signals, means for passing said signals from said low frequency channels to said electrical energy development means, means for generating compensating potentials, means for controlling said compensating potential generator in accordance with changes in the developed electrical energy, and means for mixing said compensating potentials with said video and synchronizing signals whereby the distortion due to the action of the low frequency cut-01f amplifier acting on the abruptly changing video signals is substantially compensated.

4. Television transmission apparatus comprising means for developing video signals, means for developing synchronizing signals having large amplitude changes due to abrupt changes in shade of the scanned object along a linear scanned portion thereof, amplifying means for said video and synchronizing signals having a low frequency cut-off value whereby said signals are distorted due to the action of the amplifier means on'the large abrupt amplitude changes in the video signals, means for developing electrical energy of a value dependent upon the peak amplitude of said video and synchronizing signals, means for passing said signals from said low frequency channels to said electrical energy development means, means for generating compensating potentials, means for controlling said compensating potentialgenerator in accordance with changes in the developed electrical energy, a time delay circuit interposed between said distorted signal generator and said compensating potential generator, and means for mixing said compensating potentials with said video and synchronizing signals whereby the distortion due to the action of the low frequency cut-off amplifier acting on the abruptlychanging video signals is substantially compensated.

ERIC LAWRENCE CASLING WHITE.

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