US2404173A - Television signal generator - Google Patents

Description

July 16, 1946. J. A. HANSEN TELEVISION SIGNAL GENERATOR Filed July 5, 1941 TELEVISION TRANSMITTING APPARATUS VIDEO- FREQUENCY AMPLIFIER VIVS\NA L I? 5 FIELD- SCANNING GENERATOR GENERATOR TIMING GENERATOR INVENTOR JOHN A. HANSEN ATTORNEY Patented July 16, 1946 '1" OFFICE TELEVISION SIGNAL GENERATOR John A. Hansen, Bayside, N. Y., 'assignor, by'mesn'e assignments, to Hazeltine Research, Inc., Chicago, 111., a corporation of Illinois Application July 5, 1941, Serial No.401, 080

8 Claims.

This invention relates to television signal generators and, particularly, to television signal gencraters of the type including a cathode-ray television signal-generating'tube.

The operating characteristics of television signal generators of the type which includes a cathode-ray signal-generating tube vary rather critically with the value of the tube beam current. Specifically, the shading efiect characteristic of tubes of this type and the sensitivity of such a, signal-generating tube vary with variations of the beam current of the tube. Many factors, such as a variation of the unidirectionalv supply potential, changes in temperature, etc., may cause the beam cur-'- rent of the tube to vary. In. a conventional television transmitter which includes a tube of the type under consideration; it is customary to provide a manual control for the beam current of the signal-generating tube and it is necessary for the operator to vary the adjustment from time-to-time to maintain a value of beam current which provides most satisfactory operation. Since the value of beam current for optimum sensitivity does not usually correspond to that giving minimum shading effect-sin practice, it is customary to adjust the beam. current to that value which provides a reasonable compromise between the shading efiectfln the signaioutput and maximum sensitivity. However; in some television signal transmitter installations it is necessary to leave the transmitters unattended. It is particularly desirable, therefore, to provide an arrangement for automatically maintaining a predetermined optimum'value of beam current in a cathode -ray television signal-generating tube independent of variations in operating conditions.

It is well known that the output signal of a television signal-generating tube of the type un der consideration includes spurious pulses which are generated due to any sudden changes in the beam current, which changes may be caused, for example, by spot-translation velocity changes or by the fact that the beamtravels from the pho'- tosensitive target to the nonphotosensitive margin or backing plate of the target, as weilas the desired video-frequency television signal components developed by light from the object't'o be televised and incident on the photosensitive elements of the mosaic. The spurious pulses in't'he output signal of the tube have an amplitude which is many times that of the desired video-frequency television signal components and frequencies corresponding to the line-scanning and field-scamning frequencies. It has been found-that the amsystem including a television signal generator in 2 plitude of. these spurious pulses varies with the magnitude of the beam current of the tube and that a suitable control effect can be derived therefrom to control the beam current in the desired manner mentioned above.

It is an object of the present invention, therefore, to provide an improved television signal generator of the type including a cathode-ray signal-generating tube which is not subject to one or more of the above-mentioned disadvantages of television signal generators of the prior art.

It is another object of the invention to provide an improved television signal generator including acathode-raysignal-generating tube in which the beam current of th'etube-is maintained substantially constant'during trace scannin periods of the signal generator.

In accordance with the invention, a television signal generator comprises a cathode-ray televisiorr signal-generating tube including a source of beam current and a beam-control electrode and means for deriving a signal output from the tube which includes video-frequency television signal components and spurious periodic pulses of larger amplitudethan the television signal components, which pulses vary in amplitude with variations of. the beam current. The signal generator of the invention also comprises means responsive primarily to the periodic pulsesfor deriving a unidirectional control effect and means for utilizing .the control effect to maintain the beam current of the signal-generating tubesubstantially constant during trace periods of the signal generator. I

For a better. understanding of the invention, together with other and further objects thereof,

reference is hadto the following description taken in connection with the accompanying drawing and. its scope will be pointed out in the appended claims.

' The single figure of the drawing is a schematic diagram of a complete television transmitting accordance with the invention.

Referring now more particularly to the drawing, the television transmitting system represented includes a conventional camera or projector 1'0" and a cathode-ray tube II. This tube comprises an elongated envelope l2 including a cylindrical end portion or neck and an enlarged end portion. In the neck of the tube l I there are disposed a source of beam current such as a cathode IS, a beam-control electrode I 6, a first anode H, a second anode! and a collector electrode 3a. in the order named, these electrodes c0nmay be constructed in a conventional manner and include athin sheet .of dielectric rnaterial, such. i as mica, having atransparent metallic coatin'g' or' signal :plate facing the camera Ill, and a photosensitive mosaic front surface of suitable ma-fi l-terial, such as a silver-caesium-oxygen mixture Q or compound, thereby providing, in effect, a mul 2,404,173 l 1 J- Ti antenna system 32, 33 in a conventional manner.

Line-synchronizing pulses and field-synchronizing pulses developed in timinggenerator 23 and applied to transmitting apparatus 3| are also modulated upon the. carrier signal for translation with the video-frequency signal generated in tube i ,Co'ming now'tq-the portion of the system constitutin'g thepresent invention-, in order to derive a normal operating bias 'for signal-generating tube H, there is provided a duplex rectifier 39 titude. of minute"ondensers,having the signal plate as a common electrode. Suitable operating of tube H from a source of-unidirectional operating potential, such' as a battery '20;

There are also provided'i'n the transmitter. of

the drawing a line-scanning generator 2| and a field-scanning generator 22 which are timed by l a timing generator 23 and whichhave their output circuits connected to deflecting plates 25 and 25 r tube II, respectively. The output circuit of tube include'sa load resistor28 connected to. the signal plate of target .|9 through'a coupling condenser 29 and to a conventional videofrequency amplifier 3B of one or more stages,

which, in turn, is coupled to a conventional television transmitting apparatus, indicated generally at 3|, andterminated in an antenna systern33, 33. Line-synchronizing and'field-synchronizing signals are also applied from timing enerator 237 to the television transmitting ,ap a

ratus 3| for transmission with the video-fre quency :signalgenerated intube Y The apparatus just describedjn general, represents a conventional television transmitting system and, the construction and operation of such apparatus, being well known in the .art, a

detailed description thereof is unnecessary here:

in., In brief, however, in the operation, of. the

apparatus described,.imagesjof ,theisceneto'be;

transmittedare projected by the camera It on. the mosaic surface of target |9 effectingphotoj-z emission by the various mosaic elements in 3.01,

cordanoe. withthe light intensity at. theVcorreelectron emission byv the elementary mosaic elements of a value varying in accordance .withits. potential as determined byits photoemission during the preceding scanning. cycle. Potentials of saw-tooth wave form, generated by the.genera-.

tors 2| and 22, are applied to the scanning plates 25 and 26, respectively, thereby producing electro-. static fields which deflect the beam in two directions'normal to each other, causing it to ,scan

fields or series of parallel lines on'the targetQin the usual manner. The secondary. electrons which are emitted from the mosaic elements are collected by the collector electrode |Ba. Voltages are thereby developed at the backing plate of target 19 which, in turn, cause to be developed in'the output circuit of tube H, which includes resistor 28, a video-frequency voltage which varies.

with variations in light and shade in the succes-g sive incremental areas of the scene to be trans:

mitted. This video-frequency voltage is ampli fiedin the video-frequency amplifier 3ii and is. utilized in the transmitting apparatus 3| tomodate, a erw fv ieh is b c t ir n; h

1.5" potentials are supplied to the several electrodes including cathode 40 and anode 4| for rectifying a'signal output from line-scanning generator 2|. The signal input for diode 4|], 4| derived from line-scanning generator 2| is' applied thereto through condenser lt and load resistors 43 and --44 connected in series. Resistor 44 is by-passed by a condenser 45 while resistors 43 and 44 in series are by-passed by a condenser 38. The direct current path for diode 4|), 4| is completed by means of a resistor 46 connected between cathode 40 and ground. The voltage developed across resistor 44 is applied as an operating bias to the control electrode I6 of tube I! through a resistor Hand .a resistor 48 in series.

In order to, derive a control efiect which varies primarily with the spurious periodic pulses generated inv tube ||,Itube 39 includes a diode 50, 5| coupled to the output circuit of video-frequency amplifier 3|! through av coupling condenser 52, a

condenser 53 coupled in parallel with resistor 41,.

and a condenser 54., A direct current path is provided fo diode 50, 5| which includes resistor 41 and a resistor 55; the. inclusion of resistor 41. in

, the. load circuit of diode 50, 5|.provides a means for utilizing the control effect derived from diode so, 5| to maintain the beam currentof'tube substantially constant during trace periods of the signal-generating tube ll.

In ordertocutoff the beam current of tube H, during field-retrace scanning periods, periodic,

pulses are derived from field-scanning generator 2.2 and. applied through a condenser 51 to beamcontrol-electrode |6. I

.In consideringthe operation of the circuit just described, it wilLbeseen. that an output from line scanning.generatorll is rectified by diode 4.8, 4| to provide across the load circuit thereof includingresistors 43, '44, a sourceof unidirectional voltage which is negative. with respect to ground. The :portion of this voltage across resistor.44.is applied through resistor 41 and isolating resistor48, tothebeam-control electrode l6 oftube. Lto provide a. suitable normal negative operating bias for the. beam-control electrode l6 of the tube. However, this bias voltage is substantially, constant and, unless other precautions were taken, thebeam current of tube would. vary with variations. in operating conditions, as described above, thereby necessitating a manual adjustment ..of1the beamcurrent to an optimum valuesuch that satisfactory sensitivity and minimum. shading characteristics areimparted to the tube,.||. L

- Inasmuch as .the signal output from tube I which is. amplified by video-frequency amplifier of the beam current of the tube, these spurious pulses can beutilized to control the beam current of tube |l to; maintain it at any preselectedoptirnum operating value. The. circuit 4]., 5.3. of

rectifier 50, 5| is given such a time constant that it operates as a peak rectifier for developing ineluded in series in the'grid-cathode path of tube II, the portion of this voltage developed across resistor 4'! serves to control th beam current of tube H. stants, this control voltage may be made effective to maintain the beam current substantially constant during trace periods of tube l l. lJhe time constant of the circuit is preferably not apprea ciably shorter than the field-scanning period of the systemand may be, for example, only slightly greater than the field period;

' The pulses derived from field-scanning generator-2,2 and applied through condenser 5'! to the beam-control electrode l6 serve to cut off the beam current during the field-retrace periods, in a manner which is well understood by those skilled in the, art, to avoid the appearance of spurious lines in the reproduced image. If the tube beam current is not cut oil during fieldretrace periods the charge pat-tern on the mosaic is disturbed in such fashion that a record of the spot path during retrace remains on the mosaic for some portion of the ensuing traceperiod. During the ensuing trace period this disturbed condition of the mosaic is translated into a normal television signal which is then reproduced by the picture of the system. It will be understood that this feature of the circuit can be omitted, if desired.

While applicant doesnot intend to be limited to any particular circuit design, there follows a tabulation of design information which has been found to be particularly useful in practicing the invention:

Tube ll Type 1847 Tube 39 Type 6H6 Resistor l3 megohms 10 Resistor 28 do 2.2 Resistor 43 kilohms 100 Resistor 44 megohms 1.5 Resistor 46 kilohms 250 Resistor 41 megohm 1 Resistor 48 do 1 Resistor 55 kilohms" 220 Condenser 29 micro-microfarads 250 Condenser 38 microfarad 0.01 Condenser 42 micro-microfarads 500 Condenser 45 microfarad 0.1 Condenser 52 do 0.002 Condenser 53 do 0.1 Condenser 54 do 0.25 Condenser 51 micro-microfarads 500 Line-scanning frequency 2000 Field-scanning frequency 10 It will be understood that an average rectifier can be used in place of the peak rectifier 50, 5| and that, if a peak rectifier is used, it can be caused to be responsive to the peaks of the signal of either polarity although the rectifier is preferably made to be responsive to peaks of the signals which correspond to light values which are blacker than black.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will b obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes By proper selection of circuit con- ,6 and modifications as fall within-the true spirit and scope of the invention.

What is claimed is:

1. Atelevision signalgenerator comprising, a

cathode-ray television signal-generating tube in cluding a source of beam current and a beamv control electrode, means for deriving a signal output from saidv tube including video-frequency television signal components and. spurious periodic pulses of larger amplitude than said television signal components which pulses vary in amplitude with variations of said beam current, means responsive primarily to said periodic pulses for deriving a unidirectional control effect, and means for utilizing said control effect to maintainr'sa'id beam current substantially constant during' trace periods of said signal generator.

2. A television signal generator comprising, a cathode-ray television signal-generating tube including a source of beam current and a beamcontrol electrode, means for deriving a signal output from said tube including video-frequency television signal components and spurious periodic pulses of larger amplitude than said television signal components which pulses vary in amplitude with variations of said beam current, a rectifier responsive primarily to said periodic pulses for deriving a control potential, and means for utilizing said control potential to maintain said beam current substantially constant. during trace periods of said signal generator.

3. A television signal generator comprising, a

cathode-ray television signal-generating tube including a source of beam current and a beamcontrol electrode, means for deriving a signal output from said tube including video-frequency television signal components and spurious periodic pulses of larger amplitude than said television signal components which pulses vary in amplitude with variations of said beam current, a peak rectifier responsive primarily to said periodic pulses for deriving a control potential, and means for utilizing said control potential to maintain said beam current substantially constant during trace periods of said signal generator.

4. A television signal generator comprising, a cathode-ray television signal-generating tube including a source of beam current and a beamcontrol electrode, means for deriving a signal output from said tube including video-frequency television signal components and blacker-thanblack spurious periodic pulses of larger amplitude than said television signal components which pulses vary in amplitude with variations of said beam current, a peak rectifier responsive primarily to said periodic pulses for deriving a unidirectional control potential, and means for "utilizing said control potential to maintain said beam current substantially constant during trace periods of said signal generator. 4

5. A television signal generator comprising, a cathode-ray television signal-generating tube including a source of beam current and a beamcontrol electrode, a field-scanning means for said signal-generating tube, means for deriving a signal output from said tube including videofrequency television signal components and spurious periodic pulses of larger amplitude than said television signal components which pulses vary in amplitude with variations of said beam current, means responsive primarily to said periodic pulses for deriving a unidirectional control effect and including means for eliminating variations in said control effect at the frequency of said field-scanning means, and means for utilizing said control efiect to maintain said beam current substantially constant during trace 1 periods of said signal generator.

6. A television signal generator comprising, a

cathode-ray television signal-generating tube including a source of beam current and a beamcontrol electrode, a field-scanning means for said 3 signal-generating tube; means for deriving a signal output from said tube including videofrequency television signal ''componentsand spurious periodic pulses of larger amplitude than 3 said television signal components which pulses vary in amplitude with variations of said beam current, means responsive primarily to said periodic pulses for deriving a unidirectional control efiect and including a rectifier and a load circuit having a time constant not appreciably 1 shorter than the period of said field-scanning trode, means for applying a unidirectional operating bias potential to said electrode, means for deriving a signal output from said tube including video-frequency television signal components and spurious periodic pulses of larger amplitudethan said television signal components which pulses vary in amplitude with variations of said beam current; means responsive primarily to said periodic pulses for deriving a unidirectional control potentiaLand means for applying said control potential to said electrode in series with said bias potential to maintain said beam current substantially constant during trace periods of said signal generator.

8. A television signal generator comprising, a cathode-ray television signal-generating tube including a source of beam current and a beam-, control electrode, a field-scanning generator for said tube, means for deriving a signal output from said tube including video-frequency television signal components and spurious periodic pulsesof larger amplitude than said television signal components which pulses vary in amplitud with variations of said beam current, means responsive primarily to said periodic pulses for deriving a unidirectional control efiect, means for utilizing said control effect to maintain said beam current substantially constant during trace periods of said signal generator, and means for deriving blocking pulses from said field-scanning generator and applying them to said beam-control electrode to cut off the beam current of said tube during field retrace periods of said scanning generator.

JOHN A. HANSEN.

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