US2384717A

US2384717A - Television scanning system

Info

Publication number: US2384717A
Application number: US400577A
Authority: US
Inventors: John C Wilson
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1941-07-01
Filing date: 1941-07-01
Publication date: 1945-09-11
Anticipated expiration: 1962-09-11
Also published as: GB553851A

Description

Sept.A 11, 1945. v J'. cfwlLsoN TI'II.EVISIC)N SCANNING SYSTEM Filed July 1, 1941 PatentedSept. 11, 1945 'UNITED STATES PATENT FFC 'rnLEvrsIoN scANNiNG SYSTEM John c. Wilson, Bayside', N. Y., assigner to Hazeltine Corporation, a corporation of Delaware Application July 1, 1941, Serial No. 400,577

12 Claims. This invention relates to television scanning systems and, particularly, to television scanning systems of the type provided for a cathode-ray tube and comprising an arrangement for controlling the aspect ratio of the scanning, that is,`

for controlling the relative amplitudes of the scanning deflections in two scanning directions.

A conventional television scanning system for acathode-ray tube comprises an arrangement for scanning a target of the tube in one direction at a predetermined or line-scanning frequency and for scanning the target of the tube in a second direction at a different or eld-scanning frequency. Some prior art television receiver scanning systems of the type underv consideration are intended to be adjusted at the factory to provide' a predetermined maximum scanning amplitude in each of the two scanning directions, thereby to provide a picture of a given length and of a given width, that is, of a denite aspect ratio. As used in this specification, the term aspect ratio is used in its customary sense to mean the ra/timof these two'dimensions. However, in. receivers of the type just mentioned, the relative maximum amplitudes of the scanning cieiiections` may vary with changes in operating conditions, such as changes in humidity, temperature, unidirectional operating voltagesl etc., which affect the scanning generators. However, unless the ratio of the picture dimensions or the aspect ratio is at all timesthe same, an objectionable picture distortion results. It has, therefore, been found to be desirable to provide controls in the receiver to permit adjustment of each of the picture dimensions and, furthermore, it is generally found to be necessary to adjust both picture dimensions inorder to effect a satisfactory operating condition. It is particularly desirable, however, to provide a television scanning system in whichonly a single control is provided for changing one of the picture dimensions and in which the aspect ratio of the reproduced picture is maintained substantially constant under all operating conditions. s

Furthermore, -in some cass it is desirable 'to control the relative incremental amplitudes of the scanning fields of a television receiver over their respective amplitude ranges, thereby to en- -sure a predetermined constant incremental aspect ratio as well as a predetermined over-all aspect ratio. `Moreover, in-some cases it is desirable to control the relative amplitudes of the scanning fields in accordance with the relative slopes or 'rates of change of-amplitude'of theI scanning elds over a scanning cycle or over a predetermined portion of such cycle.

It is an object of the present invention, therefore, to provide an improved television scanning system which is not subject to one or more of the above-mentioned disadvantages of prior art television scanning systems.

It is'another object of the invention to provide a television scanning system in which the aspect ratio is maintained substantially constant under,` all operating conditions.

It is still another object of the invention to provide a television scanning system in which control of the scanning amplitudes in two directions is effected by a single control means.

In accordance with the invention, a television system comprises a cathode-ray tube including a target to be scanned by the cathode-ray beam of the tube, a first means for developing and using a first scanning signal to scan the target with the beam in one direction at a predetermined frequency and with a first predetermined amplitude, and a second means for developing and using a second scanning signal to scan the `target with the beam in another direction at a different predetermined frequency and with an amplitude relative to said first amplitude which may tend to vary under operating conditions.

The scanning system Lalso comprises means responsive to the scanning signals and dependent tics which vary with amplitude and is specifically A intendedfto include the diiferential or rate of change of an .amplitude Value which varies with time.

For a better understanding of the invention,

.together with other and further objects thereof,

reference is had to the followingV description taken in connection with the accompanying drawing and its scope will be pointed' out in the appended claims.

Fig. 1 of the drawing is a circuit diagram, partially schematic, of a complete television receiver of the superheterodyne type embodying a scanning system in accordance with the. invention,

while Fig. 2 is a schematic diagram illustrating a modification of a portion of the scanning system of Fig. 1.

Referring now lmore particularly to the drawing, the system illustrated comprises a receiver of the superheterodyne type including an antenna system I0, I I connected to a radio-frequency am? plifier I2 of one or more stages to which are connected, in thev order named, a frequency changer or oscillator-modulator I3, an intermediate-frequency amplifier I4 og one or more stages, a detector I5, a video-frequency amplifier I8 of one or more stages, and a cathode-ray image-reproducing device Il. A line-scanning generator I9 and a held-scanning generator I9 are coupled to an output circuit of detector I through a syninclusive, and -24, inclusive, may all be of conventional well known construction so that detailed illustration and description thereof are deemed unnecessary herein.

Referring brieily. however, to the operation of the system described above, television signals intercepted by antenna circuit I0, II are selected and amplified in radio-frequency amplier I 2 and applied to the oscillator-modulator I3 wherein they are converted into intermediate-frequency signals which, in turn, are selectively amplied inthe intermediate-frequency ampliner I4 and delivered to the detector I5. The modulation components of theisignal are derived by the detector I5 and are supplied to the video-frequency amplifier IB wherein they are amplified and from which they are supplied in the usual manner to the brilliancy-control electrode of image-reproducing device I1. Modulation components are also supplied from detector I5 tothe synchronizng-control elements of generators I8 and I9 through the synchronizing-signal separator 20. The intensity of the scanning ray of the device il is thus modulated or controlled in accordance with the video-frequency voltages impressed upon the control electrode in the usual manner. Scanning waves are generated in the line-scanning and eld-scanning generators I8, I9, which are controlled by synchronizing voltage pulses supplied fromdetector I5 through synchronizing-signal separator 20, and applied to the

scanning windings

2l, 22 and 23, 24, respectively, of the imagereproducing device I1 to produce electric scanning nelds, thereby to deiiect the scanning ray in two directions normal to each other so as to trace a rectilinear scanning pattern on the screen o1' tube Il and thereby reconstruct the transmitted image. An automatic amplication control bias derived from detector I5 and applied to one or more tubes of stages I2, I3, and I4 serves to maintain the amplitude of the signal input tounit 9 and to detector I5 within relatively narrow limits for a wide range of received signal amplitudes. Sound signals accompanying the received televis- -ion signals are reproduced in unit 9 in a conventional manner.A i

Referring now more particularly to the portion of the system of Fig. 1 embodying the invention,

v/ Y l scanning generator I8, which is a scanning oscillator of the inductance-resistance type, comprises a vacuum tube 21 having an input circuit in which is included a cathode-biasing resistor 28 bypassed by a condenser 29 in order to provide a suitable operating bias for the tube. An vinductance 30 is also included in the input circuit of the tube and is inductively coupled to an inductance 3| in the output circuit of the tube, the inductance 3l being connected to a source of suitable operating potential +B through a resistor 33. In order to maintain the unidirectional operating potential applied tothe anode of tube 2l substantially constant, a smoothing condenser 34 is connected between the common junction of inductance 3l and resistor 33 and ground. The output scanning current of the oscillator is derived from .The circuit of ,eld-scanning' generator I9 is generally similar to that of line-scanning generator I8 and circuit elements which are similar have identical reference `numerals with the addition of a prime Resistor 33' of field-scanning generator I9 is an adjustable resistor, being provided for the purpose of adjusting the maximum output amplitude of the generator, thereby to adjust the maximum amplitude of the scanning in the fieldscanning direction, 'that' is, to adjust one dimension of the picture repreduced on the fluorescent screen of tube I1.

In order to develop a :control voltage for automatically controlling the ratio of the output amplitudes of generators I8 and I9 upon any variation Of either of them, as, for example, upon adjustment of resistance 33', there is provided in series with line-scanning windings 2 I, 22 and

eldscanning windings

23, 24

resistors

25 and 26, respectively, across which are developed voltages, the'wave forms of which correspond with those of the scanning currents applied to the scanning windings and thus with those of the scanning ilelds. There is also provided an arrangement for i deriving a control effect from the voltages developed across

resistors

25 and 26 and for utilizing this control eiect to control the amplitude of the output of line-scanning generator I8.` For this purpose, the voltage across resistor 25 is applied through a diierentiating circuit 40 and a diode rectier 4I to a condenser 42 While the voltage across resistor 26 is applied through a diierentiating and polarity-reversing circuit 43 and a diode rectler 44 to a condenser 42';

In order to utilize the control eiects developed across condenser 42 and condenser 42' to control the amplitude of the output of line-scanning gen'- erator I8, there isy provided a con-trol tube 43 which is of the double-triode type, having its single anode-cathode circuit electively coupled across resistor 33 and the source of unidirectional operating potential +B for tube 2l and having one input circuit connected to condenser 42 through a time-constant circuit or filter including a series resistor 41 and a shunt condenser 48 and its other input circuit connected to condenser 42' through a time-constant circuit or lllter including a series resistor 41 and a shunt condenser 48. Grid-leak resistors 46 and 46' are provided for the tube 45.

In considering the operation of the television scanning system of Fig. 1, it will be seen that the system includes the cathode-ray -tube I1 having a target which is to be scanned by the beam of the tube, a rst scanning means comprising line-scanning generator I8 for developing and using a first scanning signal to scan the target with the beam in one direction at a -predetermined frequency and with a predetermined amplitude, and a second scanning means comprising field-scanning generator I9 for developing and using a second scanning signal to scan the target with the beam in another direction at a different predetermined frequency and with an amplitude relative to the signal amplitude of the first oscillator which may tend to vary, for example, due to an adjustment of resistor 33.

In considering the effect of an adjustment of given aspect ratio is maintained. If, however, one

resistor 33', it will be seen that generator I9 is of a conventional design effective to provide a scanning current of saw-tooth wave form. Specically, neglecting for the moment the effector the synchronizing signals on the generator and assuming that a proper bias is maintained across resistor 28 so that. tube 21 is normally conductive, the current through the tube rises in accordance with the exponential law for aicircuit comprising resistance, resistor 33' and the internal resistance of tube 21', and inductance,A [the inductance of transformer winding 3|', in

series with a source of steady potential. As the current rises, a voltage proportional to its rate of change is induced in the grid winding 30' in such a sense as to increase the potential of the grid of the tube positively. This action regenerates the output current and the current continues to rise in the anode circuit until, according to the exponential law, its rate of rise begins. materially to diminish. However, the induced voltage in the .grid circuit, which tends to keep the grid positive, is then reduced and the grid potential decreases and this action is highly regenerative. At a predetermined point in the cycle, the rtube is operated to cutoff and the anode current then falls extremely rapidly to zero, the grid at the same time being driven strongly negative by the induced voltage due tothe fall of anode current.

The action during this return stroke or retrace interval is also highly regenerative. When the anode current has ceased and the induced poten- `tial collapses, [the cycle begins anew. Adjustment of resistor 33' in series with winding3I- has .the effect of changing the rate of rise of current therethrough and, therefore, of changing the amplitude of the output of rthe oscillator. The effect of the synchronizing pulses which are applied negatively to the grid of tube 21' is to initiate the retrace interval of-fthe cycle shortly before it would otherwise take place.

If the function of tube 45 is omitted, it is seen that an adjustment of resistor 33'y in the circuit of Fig. 1 has the effect ofchanging the fieldscanning amplitude, whilethe line-scanningamplitudev is not changed and, therefore, has the eect of changing' the aspeot ratio of the reproduced picture... Itis for thepurpose of preventing such af' change that/the control tube 45 and its controlling circuits'are provided. Thus, under normal operation of th'e circuit, the voltage developed across resistor 25 is of saw-tooth wave form and is differentiated in circuit 40 "to provide at the output circuit thereof a voltage or the Iwave form illustrated adjacent terminal C. This voltage. is applied during line-trace intervals through the rectifier 4I to condenser 42 and tends to charge the condenser positively, thereby to increase the internal conductance of tube 45. Simof the scanning amplitudes tends to vary, for example, if the field-scanning amplitude is increased in response to an adjustment of resistor 33', it is necessary simultaneously to increase the amplitude of the output of line-scanning generator I8 in order to maintain the same aspect ratio.A Under the conditions assumed, the increased amplitude of the output of field-scanning generator I9 results in a voltage of greater amplitude across resistor 26, which, in turn, has the effect of producing, through differentiating and polarity-reversing circuit 43 and diode 44., a greater negative potential across condenser 42'. This potential has the effect of decreasing the conductance of tube 45, thereby effectively applying a higher anode voltage to tube 21 andincreasing the amplitude of the saw-tooth current output thereof to such a point that a condition of equilibrium is again established in the system, that is, to such a point that the aspect ratio is restored to its normal value.

The charging circuits for condensers 42 and 42', therefore, comprise means responsive to the linescanning and field scanning signals and dependent jointly upon an amplitude characteristic of the line-scanning signal and upon an amplitude characteristic of the field-scanning signal for developing a control eilect which is utilized to control the ratio of the amplitudes of the scanning fields. If the time constants of resistor 41 and condenser 4B and of resistor 41 and condenser 48' are very small or zero, the control of the arrangement just described is very fast-acting and depends jointly upon the instantaneous-amplitude characteristics of the two scanning signals and serves to control the ratio of the instantaneous-amplitude values of the scanning fields which, in turn, determinesl the incremental aspect ratio of the reproduced picture over the scanning amplitude ranges. However, it will be understood that these time constants ca n be made to be appreclable and ln such case the control of tube 45 is not fast-acting but serves to maintain a constant over-all aspect ratio averaged over an 'appreciable interval.

`In somecases it is sumcient to provide an as peet-ratio control arrangement which is responsive only to the peak4 amplitudes 'of the two scanning flelds and which serves to control the ratio vof the peak amplitudes of these fields. Forthis purpose the peak rectlflers of Fig. 2 can be connected into the circuit of Fig. l by omitting the elements 4I) and 43 and tubes 4I and 44 and connecting the terminals of the elements of Fig. 2 to thecorrespondingly designated terminals of Fig. 1.

In considering the operation of this modiiication of the invention, it will be seen that each'of Athe peak rectiilers is eilectiveto derive a voltage which `is averaged over an appreciable period. These voltages are applied to condensers 42 and 42', respectively. in a manner similar to that deto reduce the conscribed above in connection with Fig. .1 `and serve to provide a control eect for tube 4I which serves to control the aspect ratio as before. However, in this case, due to the time constant of the circuits of peak rectiflers 50 and 5I, the control effect does notvary instantaneously in accordance with the rate of change of the amplitudes of the scanning fields, but varies only in accordance with their relative peak amplitudes averaged over an appreciable period. Therefore, the arrangement of Fig. 2 is effective to control the ratio of these amplitudes averaged over an appreciable' period.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be 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 al1 such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A television scanningvsystem comprising, a cathode-ray tube including a target to be scanned by the cathode-ray beam of the tube, a first means for developing and using a flrst scanning signal to scan said target with said beam in one direction-at a predetermined frequency and with a first predetermined amplitude, a second means for developing and using a second scanning signai to scan said target with said beam in another direction at a different predetermined frequency and with an amplitude relative to said first amplitude which may tend to vary under operating conditions, means responsive to said signals and dependent jointly upon an amplitude characteristic of said first signal and upon an amplitude characteristic of said second signal for deriving a control effect, and means responsive to said control efect for controlling the ratio of said amplitudes.

2. A television scanning system comprisinga cathode-ray tube including a target to lie-scanned by the cathode-ray beam of the tube, a first means for developing and using a first scanning signal to scan said target with said beam in one direction at a predetermined frequency and with a first predetermined amplitude, a second means for developing and using a second scanning signal to scan said target with said beam in another direction at a different frequency and with an amplitude relative to said first amplitude which may tend to vary under operating conditions, means responsive to said signals and dependent jointly upon an instantaneous-amplitude characteristic of said first signal and upon an instantasignal to scan said target with said beam in one direction at a predetermined frequency and with afirst predetermined amplitude, a second means for developing and using a second scanning signal to scan said target with said beam in another direction at a predetermined frequency and with an amplitude relative to Said first amplitude which may tend to vary under operating conditions, means responsive to said signals and de- 'pendent jointly upon the instantaneous slope of la. first predetermined peak amplitude, a second means for developing and using a second scanning signal to scan said target with said beam in another direction at a differentv predetermined frequency and with a peak amplitude relative to said first peak amplitude which may tend to vary under operating conditions, means responsive to said signals and dependent jointly upon the peak amplitude of said first signal and upon the peak amplitude of the scanning wave of said second signal for deriving a control effect, and means responsive to said control effect for controlling the ratio of said peakv amplitudes.

5.'A television scanning system `comprising, a cathode-ray tube including a target to be scanned by the cathode-ray beam of the tube, a first means for developing and using a first scanning signal to scan said target with said beam in one direction at a predetermined frequency and with a :first predetermined amplitude, a second means for developing and using a second scanning signal to scan said beam in another direction at a different predetermined frequency and with an amplitude relative to said rst amplitude which may tend to vary under operating conditions, means responsive to said signals and dependent jointly upon an amplitude characteristic of said first signal averaged over an appreciable period and upon an amplitude characteristic of said second signal averaged over an appreciable period for deriving a control eiect, and means respon? to said control effect for controlling the raf. said amplitudes averaged over period.

6. A television scanning system comprising, a cathode-ray tube including a. target to be scanned by the cathode-ray beam of the tube, a first means for developing and using a first scanning signal to scan said target with said beam in one direction at a predetermined frequency and with a first predetermined amplitude, a; second means for developing and using a second scanning signal to scan said beam in another direction at a predetermined frequency and with an amplitude relative to said rst amplitude which may tend to vary under operating conditions, means responsive to said signals and dependent jointly upon the average slope of said first signal and upon the average slope of said second signal forderiving a control effect, and means responsive to said control effect for controlling the ratio of the amplitudes ofthe first and second signals.

7. A television scanning system comprising, a cathode-ray tube including a target to be scanned e by the cathode-ray beam of the tube, a first means for developing and using a rst scanning signal to scan said target with said beam in one direction at a predetermined frequency and with a first predetermined amplitude, a second means for developing and using a second scanning signal to scan said target with said beam in another direction at a predetermined different frequency and with a predetermined amplitude, manual an appreciable dllmeans for adjusting one of said amplitudes, whereby the ratio of said amplitudes tends to vary under operating conditions, means responsive to said signals and dependent jointly upon an amplitude characteristic of said first signal and upon an amplitudecharacteristic of said second signaifor derivingl a control effect, and means responsive to said control eect for so controlling the ratio of said amplitudes that the aspect ratio of said scanning is maintained substantially constant upon a manual adjustment of said one of said amplitudes.

8. A television scanning system comprising, a cathode-ray tube including a target to be scanned by the cathode-ray beam of the tube, a first means for developing and using a iirst scanning signal to scan said target with said beam in a line-scanning direction at a predetermined frequency and with a first predetermined amplitude, a second means for developing and using a second scanning signal to scan said target with said beam in a field-scanning direction at a pre- -determined different frequency and with a predetermined amplitude, manual means for adjusting said held-scanning amplitude, wherebyl the ratio of said amplitudes tends to vary under operating conditions, means responsive to said signals and dependent jointly upon an amplitude characteristic f said first signal and upon an amplitude characteristic of saidv second signal for'deriving a control effect, and means responsive to said control eiect for so controlling the ratio of .said amplitudes that the aspect ratio of said scanning is maintained substantially constant upon a manual adjustment of the scanning amplitude in said held-scanning direction.

9. A television scanning system comprising, a cathode-ray tube including a target to be scanned by the cathode-ray beam of the tube, a rst means for developing and using a first scanning signal to scan said target with said beam in one direction at a predetermined frequency and with a rst predetermined amplitude, a second means for developing and using a second scanning signal to scan said target with said beam inv another direction at a diiierent predetermined frequency and with an amplitude relativeto said first amplitude which may tend to Vary under operating conditions, means. kincluding a condenser adapted to be charged in accordance with an amplitude characteristic of said first signal and a condenser adapted to be charged in accordance with an amplitude characteristic of said second signal for deriving control eiect, and means responsive to said control effect for controlling the ratio of said amplitudes.

10. A television scanning system comprising, a cathode-ray tube including a target to be scanned by the cathode-ray beam of the tube, a iirst means for developing and using a rst scanning signal to scan said target with said beam in one direction at a predetermined frequency and with a rst predetermined amplitude, a second means for developing and using a second scantiating circuits with a given polarity and for applying the other of said` derived voltages to the other of said condensers through the other of said differentiating circuits with opposite polarity to derive a control eiect, and means responsive l to said control effect for controlling the ratio of said amplitudes. l

11. A television scanning system comprising, a cathode-ray tube including a target to be scanned by the cathode-ray beam of the tube, a rst means for developing and using a iirst scanning signal to scan said target with said beam in one direction at a predetermined frequency and with a first predetermined amplitude, a second means for developing and using a second scanning signal to scan said target with said beam in another direction at a different predetermined frequency and with an amplitude relative to said first amplitude which may tend to vary under operatingk conditions, means for deriving a first voltage the wave form of which corresponds to that of said nrst signal, means for deriving a second voltage the wave form of which corresponds to that of said secondv signal, two condensers, means for peak-rectifying each of said derived voltages, means for applying said peak-rectied voltages respectively to said condensers with opposite polarities to derive a control effect, and means responsive to said control effect for controlling the ratio of said amplitudes.

12. A television scanning system comprising, a cathode-ray tube including a target to be scanned by the cathode-ray beam of the tube, a first means for developing and using a first scanning signal to scan said target with said beam in one 'direction at a predetermined frequency and with a r'st predetermined amplitude, a second means for developing and using a second scanning signal to scan said target with said beam in another direction at a different predetermined frequency and with an amplitude relative to said first ampntude which may tend to vary under operating conditions; means responsive to said signals and dependent jointly upon an amplitude characteristic of said rst signal and upon an amplitude characteristic of said second signal for deriving a control effect, and means responsive to said control eiiect for Varying oneof said predetermined amplitudes, thereby to control theratio of said amplitudes.

JOHN C. WILSON.