US2378547A - Television system - Google Patents

Description

June 19. 194.5. 7 GRAHAM: 2,378,547

TELEVISION SYSTEM V V 7 Filed D80. 31, 1.942 2 Sheets-Sheet 2 rocusso 01v :AME 50 OBJECT H-R SI! FILTER 1/ TELEVISION SIGNAL '3, spscmuu V E r 5. U l Q I FREQUENCY n 01 (f=1./-E FREQUENCY) INVEN TOR R. E. GRAHAM A TTORNEV Patented June 19, 1945 UNITED STATES PATENT OFFICE TELEVISION SYSTEM Robert a. Graham, New York, N. Y., aesig'nor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 31, 1942, Serial No. 470,792 1Q Claims. lit-7.3)

This invention relates to electric circuit arrangements and more specifically to means .for

, removing spurious electricalvariations from the output currents of electron camera tubes.

all; is anobject of this-invention to provide 5 novel electric circuit means for removing unwanted or spurious components fromelectric currents.

The storage type of electron camera tube, such as the well-known Iconoscope," normally proare focused on the same object. The main camduces an output current which contains distortera tube preferably comprisesa tube of the storing or spurious variations. These variations are ase type such as, for example, the well-known believed to. be caused .by secondary electrons, Iconoscope." The auxiliary'tube is preferably emitted from the scanned elemental area, being a tube of the "Dissector" type but is modified to attracted to more positive elemental areas. Cerid have a long slit scanning aperture instead of tain of these spurious variations produce the the usual square ne. th ns dim nsion of the cloud or dark spot effect which manifests s it ei g in t e line scannins eoti Some itself in the form of one or more large extraneous of the output energy from the main camera tube dark areas in the reconstituted picture. This (comprising both video and spurious comeil'ect is essentially a low frequency distortion. to ponents) is removed and passed to a side circuit- The variation producing the "cloud eifect are w e n the h sh i eque ey components of the. usually quite appreciable in amplitude in comy-d m e sy re fil red out and the resultparison with the amplitude of the desired picant low frequency components are amplified in ture signal, and have been compensated to a l mm l i r to whi h 8180 p certain extent in practice by the introduction in p per p ase o oppose the output of the filter.

into the picture signal of manually controlled electrical shading variations or signals. Where the pictorial subject-matter is continually changing, this technique requires continual manual tube which produces a signal which is substantially free from low frequency spurious signals. The output currents of the two tubes are combined to eliminate the low frequency spurious variations appearing in the main camera tube output current.

In accordance withone embodiment of the invention, two electron camera tubes are provided at the transmitting station both of which the amplified output wave from the auxiliary camera tube. The output of this differential amplifier, which contains only the low frequency spurious components as the signal components manipulation of the dials controlling the inve balanced out lh he fi rentlal amplisertion of these shading signals. The present invention relates to the provision of means for the automatic compensation of these spurious electrical variations.

fier, is then applied to a second differential amplifier to which is also applied, in proper phase so that the spurious variations are balanced out. that portion of the energy from the main cam- It is another object of this invention to proe ere b which was not w to h e r ui vide novel apparatus for automatically remov ing or compensating for the spurious variations produced by certain forms of electron camera tubes andwhich when present with the video The output current of this latter amplifier contains the full range of video frequencies without the low frequency spurious variations. This out- 7 put current may be transmitted to the receiving signal, produce cloud or "dark s ot? ff t; in 40 station by well-known means.

the reconstituted picture. A further object is to provide a novel repeater eedback arrangement.

These and other objects will be apparent from the followingaescription of various specific em- "bodim'ents of the invention hereinafter fully dc acribed. In each of these specific embodiments there is utilized an auxiliary electron camera orpick-up tube which produces signals c orrespending only to the low frequency components (ujpto several times the line frequency) of the complete picture signal produced by the'main electron camera tube, which preferably is of the storage type. This auxiliary tube is preferably a "Dissector" tube or some other form of pick-up In a'second embodiment, a similar result is of the spurious variations.

. In a third embodiment, the desired result is obtained by passing the video signals from the main camera tube through ahigh-pass filter, the

output of which is, after amplification, combined U with the amplified output of the auxiliary cam-z gram form, of a circuit arrangement for compensating for spurious variations produced in electron camera tubes of the storage type, which spurious variations will, if not compensated, produce a cloud" effect in the received picture;

Fig. 2 is another circuit for producing a result similar to that obtained by the circuit of 18. 1;

Fig. 3 is still another circuit for producing a result of this typ and Fig. 4 is a diagram to assist in the explanation of the invention.

Referring more specifically to the drawings,

Figs. 1, 2 and 3 show in block diagram form circuit arrangements for compensating for spurious variations in the output currents of certain types of electron camera tubes, such as, for example, the storage type of camera tube known to the art by the term Iconoscope." Such a camera tube is described in an article by V. K. Zworykin in the Proceedings of the Institute of Radio Engineers for January 1934, starting at page 16. These tubes have a fairly high signal-to-noise ratio, and it is therefor desirable in many cases to use them. While tubes of the Dissector" type (described on pages 230 to 233, inclusive of the book "Television" by V. K. Zworykin and G. A. Morton) are substantially free of the abovementioned spurious variations, they have a low signal-to-noise ratio. In the present invention, certain advantages of both are combined to improve the appearance of the received picture. Stated differently, a tube, of the "Dissector" type, modified so that the scanning aperture is a long slit (instead of a small square or a rectangle the two dimensions of which are nearly equal), is utilized to produce low frequency video signals representative of the scanned object (which signals are substantially free of the spurious variations found in the Iconoscope" output current). While high frequency signals are also produced, they are of such small amplitude they can be neglected. The Iconoscope" or similar tube is utilized as the main camera tube and while it produces a full range of signals, that is, low frequency signals and high frequency signals, there are also present'the aforementioned spurious variations of relatively low frequency. These spurious variations are substantially eliminated in the novel circuits of this invention. The output currents of the two tubes are combined in any one of these circuits in such a way that the resultant output current is free of these spurious variations and at the same time (because the high frequency portion of'the resultant signal current is supplied by the IconoscopeYL a relatively high signal-to-noise ratio exists for the high frequency signals (the most important part of the band as far as the signal-to-noise ratio is concerned because it includes much the greater portion of the band width).

In the arrangement of Fig. 1, the Iconoscope H) serves as the main electron camera tube,

while the auxiliary tube Ii is of the "Dissector type modified to have a slit as described above. The length of the slit is preferably of the order ofone-fourth to one-third of a scanning line-but may be as small as one-tenth. The long dimension of the slit is, as pointed out above, in the focused on the same object (not shown) by suitable lens systems l2 and i3, respectively, and are arranged so that the images reflected on the targets of the tubes l0 and II are substantially the same. While for exactness of result this can be accomplished, if desired, by any well-known optical means, such as by mirrors or prisms, generally two lenses, side by side, will sufllce for objects sufficiently far away so that substantially the same view is picked up by each tube.

The output current of the electron camera tube in which contains high frequency components and low frequency components (collectively identified as the signal S) and, in addition, spurious variations which are identified in the drawings by the reference character d are applied by means of the usual signal resistor is to an amplifier It, the output signal of which is divided between two paths, a portion of the signal being appliedto the amplifier it while the rest is applied to the low-pass filter H which filters out the high frequency components and passes a current identified in the drawings as Sir-Hi (or in other words the low frequency portion of the signal plus the spurious variations or disturbances). The output of this filter ll is I applied to the differential amplifier H, to which is also applied in proper phase so that the signal components balance out, the amplified output from the camera tube Ii. This output current which is applied to the amplifier is by means of the signal resistor 20 contains the entire frequency range of the output current of the camera tube H but, due to the use of the long slit instead of the square aperture, the output wave does not have any components of appreciable amplitude of frequencies higher than those in Bar. If desired, a low-pass filter (not shown) may be inserted before or after the amplifier I! to ensure that no high frequency components are present in the line 80. The filteril is chosen so that the cut-oil at the low frequency end generally coincides with the upper limits of the freq quency 'band of the signal current formed by camera tube ii. The output of the differential amplifier l8, which contains only the spurious variations or disturbances d, is applied to a second differential amplifier 2| to which is also applied the unbypassed portion of the output of the amplifier II which has been amplified by the amplifier it. The gain of the amplifier i8 is so chosen that the amplitudes of the spurious components in its output circuit are substantially the same as those in the output of the differential amplifier It. The output of the amplifier 2| contains, as indicated in the drawings, only the signal S which comprises both high and low frequency components. This signal current may be transmitted to a receiving station by any wellknown means,

Fig. 2 shows a modification of the arrangement of Fig. 1. In the arrangement of Fig. 2 a similar result is obtained by a degenerative feedback arrangement. A portion of the energy from the main camera tube I0 is, after amplification by the amplifier equal to amplifier II and the amplifier it, passed through a low-pass filter II and then combined in proper phase in the differential amplifier 32 with the amplified output from the auxiliary camera tube II to produce signals which are fed back into the amplifier I0. Representing the output of the tube i as 8+1! and ignoring (for simplicity of description) the gain produced by the amplifier II, the current in the output of the low-pass filter ll may be represented by the expression which, when diflerentially combined with the current from the auxiliary camera tube (81.!) forms in the output of the differential amplifier a' current represented by I which, when fed back into the input of the amplifier 30, produces a current in the output of the d v v -ufl) It will thus be seen that the degree of suppres sion of distortion is represented by the fraction same as before. A high-pass filter 40 is inserted in the output circuit of the tube I0 to remove the portion of the output signal falling in the same frequency region as that occupied by the output current of the tube II. The remaining signalfrom tube II which may be represented by the symbol Baris then, after amplification by the amplifier Ii, applied to the mixer amplifier 42 in such phase that it is added to the amplified lowfrequency signal from the camera tube ii. The output ofthe amplifier 42 is then expressed by: S=Su+Sna Thus, the complete television signal is free of spurious components d.

In the arrangement of Fig. 3 there appears the problem of relating the high-pass cut-off of the current from the system including the tube II and the low-pass cut-oi! of the current from the system including tube It in such a manner as to obtain an over-all level response characteristic at frequencies where both systems make appreciable contribution. A similar problem exists, of course, in connection with the arrangements of Figs. 1 and 2. One way of minimizing this difgficulty is to place the frequency region where the two signals overlap half waybetween two successive multiples of the line frequency where .(as is well known), there is little energy concenq tration. This is illustrated in Fig. 4. The load impedance it mayserve as a portion of the highpass filter, if desired.

In any of the arrangements described above.

if the auxiliary tube]! is of the type containing.

a direct current component in its output, it can,

,of course, be used to provide direct current inser-.

tion for the television system by injecting a very .low frequency portioninto the main signal pathat a desired point.

It should be noted that the auxiliary camera tube Ii need have little refinement of 'any sort With a long slit aperture extending from one-j tenth up to one-fourth or one-third of the length of a line, the signal obtained from such a tube contains, for practical purposes, only low frequency components as the amplitude of the high frequency components is so low that it can be neglected.

Various modifications may be made in the embodiments disclosed above without departing from the spirit of the invention, the scope of which is indicated in the appended claims.

What is claimed is:

1. In combination, electron camera means for producing a video signal having a relatively wide band of frequency components, said signal containing some unwanted or spurious amplitude variations of certain frequency components at least, a second electron camera means for producing a video signal having a smaller band of frequency components, said smaller band overlapping, at least in the part containing said certain frequencies, said wide frequency band but being substantialy free of the unwanted or spurious variations .of said certain frequencies in said wide band, and means for combining signals from said two bands to produce a resultant video sigmill which is substantially free of said unwanted or spurious variations of said certain frequencies.

2. In combination, means for producing a signal having a relatively wide band of frequency components, said signal containing some upwanted or spurious amplitude variations of certain frequency components at least, means for substantially free of said unwanted or spurious variations of said certain frequencies, said lastmentioned means including a negative feedback circuit for feeding back and combining with the variations of the first band a portion of the energy of Said first band.

.. 3. In combination, means for producing; in signal having a relatively wideband of frequencycomponents, said signal containing some unwanted or spurious amplitude variations of certain frequency components at least, means for producing a signal having a smaller band of frequency components, said smaller band overlapping, at least in the part containing said certain frequencies, said wide frequency band but being substantially free of the. unwanted or spurious variations of said certain frequencies in said wide band, and means for combining signalsirom said band to produce a resultant signal which is substantially free of said unwanted or. spurious variations of said certain frequencieasaid lastticned means including a negative feedback circuit for feeding back, and combining with the variations of the first band a portion of the energy of said first band, and-means for feeding energy of said second band into said feedback circuit.

- 45A signal transmission system comprising a transmission path including an amplifier, a source able electromotive force, and means for applying to said amplifying device energy from said second source to modify the variations impressed upon the input of said amplifier by said amplifying device in accordance with the variations in the electromotive force from said second source.

5. A signal transmission system comprising a transmission path including an amplifier, a source of signals containing undesired components at one end of said path and a load circuit at the other end thereoiia negative feedback connection, including a repeating device, between the output and the input of said amplifier to balance out at least a portion of said undesired compcnents, a second source of signals, and means nents, a second source of signals, and means for applying to said repeating device signals from said second source in such phase that at least a portion of the signals in said feedback path taken from the output of said amplifier is balanced out, the'second source being substantially free from said undesired components.

7, A signal transmission system comprising a on path including an amplifier, a source of signals containing undesired components at one end of said path and a load circuit at the other end thereof, a negative feedback connection, including a repeating device, between the output and the input of said amplifier to balance out at least a portion of said undesired components, a second source of signals, and means for applying to said repeating device signals from said second source in such phase that at least a portion of the signals in said feedback path taken from the output of said amplifier is balanced out,

said second source producing signals having a much smaller frequency band width than said first source.

8. In combination, means including an electron camera tube for producing, when radiations from an object are applied thereto and by a scanning operation, a video signal and, mixed there'- with, some low frequency spurious'signals, means including an auxiliary scanning device for producing, when radiations from the same object are applied thereto and by a scanning operation at the same rate of scanning as said first scanning operation, a video signal corresponding to the same object or field of view as the video signal of said electron camera tube, but which signal has a relatively small band width compared with the video signal of said camera tube and is relatively free of said low'frequency spurious signals, and

' means for utilizing the two video signals 'to produce a resultant video signal which has a band width comparable to that produced by said electron camera tube but which is substantially free of said low frequency spurious signals.

9. The combination as in claim 8 in which said auxiliary scanning device is a camera tube of the type having a scanning aperturewhich scans an electron image, said ap rture being rectangular with sides of unequal length, the long dimension being in the direction of line scanning.

10. The combination as in claim 8 in which said auxiliary scanning device is a camera tube of the [type having a scanning aperture which scans an electron image, said aperture being rectangular with sides of unequal length, the long dimension being in the direction of line scanning, said long dimension being from one-third to one-tenth the length of a scanning line.

ii. In combination, a main camera tube, an auxiliary camera tube, means including said main camera tube for producing a band of high frequency video signals representative of a scanned object or field of view and mixed therewith some low frequency spurious signals, means including said auxiliary camera tube for producing a band of low frequency video signals representative of the same object or field of view scanned at the same rate, and means for combining the two bands to produce a resultant video signal current in which the low frequency spurious signals are substantially absent.

12. The combination of elements as in claim 11 in which the upper boundary of the low frequency band substantially coincides with the lower boundary of the high frequency band.

13. In combination, a main camera tube, an auxiliary camera tube, means including said main camera tube for producing a band of high frequency video signals representative of a scanned object or field of view and mixed therewith some low frequency spurious signals. means including said auxiliary camera tube for producing a band of low frequency video signals representative oi the same object or field of view scanned at the same rate, said video signals being characterised in that they have in their frequency spectrums high energy regions and low energy regions, and means for combining the two bands to produce a resultant video signal current in which the low frequency spurious signals are substantially absent, the effective upper boundary of the low fre- .quency band and the effective lower boundary of the high frequency band each being within the same low energy region of the combined spectrum.

, 14. In combination, means including a first electron camera tube for producing a video signal and, mixed therewith, some low frequency spurious variations, means including a second camera tube for producing a video signal corresponding to the same object or field of view which controls the formation at the samescanning frequency as the video signal of the first tube but which signal has a relatively small band width compared with the video signal produced by the first tube and is relatively free of said low frequency spurious variations, means for diverting some of the output energy of said first tube, means for filtering out the'frequencies in the diverted energy which are higher than the upper boundary frequency of the signal produced by the means including the second camera tube, means for combining the energy passed by said filtering means with the video signal produced by the means including the second camera tube in such phase that only the spurious variations remain,

and means for combining the spurious variations with theundiverted of said first tube in such phase and amplitude portion of the output energy tube.

16. The combination of elements as in claim 11 in which said means for producing a band of high frequency video signals comprises an electron camera tube of the charge storage type, and a filter which passes only those frequencies in the output current of said storage type tube which are above the highest frequencies of appreciable amplitudes produced by said auxiliary camera 1'1. In combination, a scanning device for setting up signal variations representative of 'a subject scanned by it, which device introduces spurious low frequency variations into the signal current produced by it, asecond scamiing device of a type which does not produce said spurious low frequency variations and which produces a signal frequency spectrum limited in range to the lower portion of that which said first scanning device produces when the two scan the same subject in the same direction and at the same rate, means forcausing both said devices simultaneously to scan the same subject in the same manner and storage elements and said second scanning device so is an electron tube in which an electron image of the subject scanned is moved progressively over a stationaryaperture which is long enough in the direction in which it scans said image to make the effective upper limit of the resulting signal frequency spectrum much lower than that of the spectrum produced by said first scanning device.

19. In combination, a scanning device for setting up signal variations representative of a subject scanned by it which device introduces spurious low frequency variations into the signal current produced by it, a second scanning device of a type which does not produce said spurious low frequency variations and which produces a signal frequency spectrum limited in range to the lower portion of that which said first scanning device produces when the two similarly scan the same subject, means for causing both said devicles simultaneously to scan the same subject in the same direction and at the same rate. means for selecting those frequencies in the signal current produced by said first scanning device which are higher than the effective upper boundary of the frequency spectrum produced by said second scanning device. said selection eliminating the spurious low frequency variations, and means for combining the selected frequencies from said first device and the signal from said second scanning device to produce a resultant signal in which the low frequency spurious signals are substantially absent.

nonna'r E. GRAHAM.

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