US2308375A

US2308375A - Television synchronizing system

Info

Publication number: US2308375A
Application number: US326163A
Authority: US
Inventors: Arthur V Loughren
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1940-03-27
Filing date: 1940-03-27
Publication date: 1943-01-12
Anticipated expiration: 1960-01-12

Description

- Patented Jan. 12,4943

Arthur v. Loughren, Great new, N. 1., assignor to Haneltine Corporation. a corporation of Delaware Application mil 27, 1940. Serial No. 326,163

Claims. (c1. its-v.5)

This invention relates to television systems and is especially, concerned with theprovision of an improved means for effecting synchronization of television scanning paratus.

In accordance wi present television practice,

- there is developed and transmitted a signal which comprises a carrier wave modulated during successive intervals or trace periods by video components representative of the light and shade values of an image being transmitted. During retrace intervals between the trace periods, the carrier wave is modulated by synchronizing pulses or components which correspond to the initiation of successive lines andfields in the scanning of the image. At the receiver, a beam is so deflected as to scan'andilluminate atargetin aseries oi fields of parallel lines. The video components of the received signal are ,utilized to control the intensity of the beam. The line-scanning andfieldscanning components are separated from the video components and from each other and utilized to synchronize the operation of the receiver line-scanning and-field-scanning apparatus with the similar scanning apparatus utilized at the.

transmitter in developing the signal. The transmitted image is thereby reconstructed on the taret of the receiver.

I Various types of television scanning and synchronizing apparatus have heretofore ,been proposed. 'Ijhe typeoi sync-signal separating apparatus which is required ordinarily depends upon the type oiltelevision signal employed and the type of scanning utilized. For example. ininterlaced scanning systems certain of the field-synchronizing pulses generally occur with diiferent time relation to the line-synchronizing .pulses than others. In order that linesynchronizing and field-synchronizing pulses may be readily separated from the videocomponents and from each other for utilization, various types of synchronizing signals and separating apparatus have been proposed. For example, there have been proposed synchronizing signals in which the field-synchronizing impulses are of greater amplitude or of longer duration than the line-synchronizing impulses and separating ap-.- paratus comprising amplitude-discriminating or integrating circuits. Such arrangements, how- I .ever, have been found to be subiectto various objections, such as instability,'requiring an undesirably large portion of the total carrier amplitude and involving relatively complicated separating apparatus.

In copending application 8erial 'No. 196,885, filed March 19, 1193 8, applicant has disclosed an .improved method of, and means, for eifectim' scanning synchronization in a television system which overcomes the above-mentioned 'dlsadvantages of the prior art. In accordance with the invention of applicant's copending application, there is provided a television synchronizing system adapted to be energized by a composite synchronizing signal including line-synchronizing pulses and field-synchronizing pulses of the same polarity as, but of greater duration than, the

line-synchronizing pulses. The system comprises means responsive to the leading edges-of the line- 'synchronizing pulsesand to the duration and trailing edges of the field-synchronizing pulses for deriving line-synchronizing and field-synchronizing pulses of opposite polarities.' A linesynchronizing circuit is viduallyresponsive to'the derived line-synchronizing pulses and a field-synchronizing circuit is provided which is individually responsive to the derived oppositely poled field-synchronizing While the arrangement of applicant's copending application has been continuously used since the date of invention thereof and has been found generally satisfactory, experience hasindicated that it is desirable to provide a circuit which has all of the advantages of the arrangement of the above-mentioned copending application but whichis eflective to provide an improved p'erformance in oneparticular respect. In the arrangement of applicants copending application 4 and in other prior art arrangements, the fieldscanning generator is adapted to be synchronized at the trailing edge of the first broad pulse of the field-synchronizing sign-a1. Howevei', the succeeding broad pulses of the field-synchronizing signal specified by the Standards of the Radio Manufacturers Association may fire the fieldscanning generator in case it is not fired by the first broad pulse. Therefore, an improper operation of the synchronizing-signal separator or misadiustment of the field-scanning generator may cause the generator to trip at a later point in the field-synchronizing signal. Such operation would not be particularly objectionable if it were a, steady operating condition. In practice. h'oweven the operation is altered or modified by other factors, such as line-synchronizing signals, residual cross-talk from the line-scanning generator to the field-scanning generator, noise pulses, etc., so that proper interlace may not be obtained unless all fields of the frame trip at substantially the same point in the field-synchronizing signal. Under such conditions the reconstructed image provided which is indiline-scanninmgenerator i8 and a mm.-

generator II are also coupled to the output of the arrangements of the prior art.

"It is another object of the invention to provide an improved means for effecting scanning synchronization in a television system such that the 1 field-scanning generator is fired at a predetermined point in the field synchronizing signal or is not fired at all by the received field-synchronizing signal during that field-blockout interval.

In accordance with the present invention, a 20 television synchronizing system adapted to be energized by a synchronizing signal including a group of field-synchronizing pulses of the same polarity during each field-blockout interval of overcome the above-mentioned disadvantages of tion so that detailed illustrations and descriptions thereof are unnecessary herein.

Referring briefly to the general operation of the system just described, television, signals in- 5 tercepted by the antenna circuit III, II are se-' lected and amplified in the radio-frequency amplifier I2 and supplied to the oscillator-modulator l3 wherein they are converted to intermediate-frequency signals which, in turn, are

selectively amplified in the intermediate-ire quency amplifier I4 and delivered to the detector ii. The modulation components of the signal are derived by the detector i5 and are supplied to the video-frequency amplifier i6 wherein they are amplified and'jron'i which they are supplied to a brilliancy-control electrode of image-reproducing device ll. The intensity of the scanning beam of image-reproducing device I! is thus modulated or controlled in accordance with the video-frequency voltages impressed upon its control electrode in the usual manner. The modulation components are also supplied to the apparatus wherein synchronizing components are separated from the video-frequency the sy e comprises me ns respo sive t the components and the line-synchronizing and above-mentioned groups of pulses for deriving leld-synchronizing pulses individually correrponding to the pulses of the groups. and means .or rendering the above-mentioned means eifec ively unresponsive to all pulses of the groups ther than the first pulse of eachgroup. A fieldcanning circuit is provided which is responsive ;o the derived field-synchronizing pulses.

In accordance with a preferred embodiment of ;he invention, the means for rendering the system unresponsive to anyexcept the first pulse of each group is responsive to the first pulse ofeach group and, further in accordance with this embodiment of the invention, this means is responsive to the duration and mum; edge of the first pulse of each group.

For a better understanding of the present invention, together with other and further objects thereof, reference is had. tothe following description taken in connection with the accomnanyins drawing, and its scope will-be pointed out in the appended claims. o In the accompanying drawing. Fig. 1 is a circuit diagram, partly schematic, ofla complete television receiving-apparatus embodying the ino vention: while Figs. 2-9, inclusive. are graphs illustrating the wave forms of periodic current or voltage wavesdeveloped at different points of the system of 1 to aid in the understanding I of the invention. I

Referring now more particularly t Fi 1, the system there illustrated comprises a reoeiver of the superheterodyne type including an antenna system II, II connected to a radio-frequency amplifier l2, to which there are connectedincascade,intheordernamed,anoscillator-modulator ll, an'intermediate-frequency amplifier N, a. detector II, a video-frequency amplifier II, and 'an image-reproducing device l'l, preferably of the cathode-raytube t I detector II by way-of suitable synchronizingsignal separatin paratus indicated generally -at2landtothescannlngelements ofimageunit II. The stages or units ll-ll, inclusive, may all be of conventional well-known construcsynchronizing pulses b t not apm bl I mutual interference.

field-synchronizing pulses are effectively separated from each other, as further explained hereinafter, and applied to the generators l8 and i8, respectively. Saw-tooth current or voltage scanning waves are generated by'the linescanning and field-scanning generators i8 and I! which are controlled by the synchronizing pulses supplied from the apparatus 20 and these scanning waves are applied to the scanning elements of image-reproducing device H to produce scanning fields, thereby to deflect the scanning beam in two directions normal to each other so as to trace successive interlaced fields of'parailel lines on the screen of the image-reproducing device to reconstruct the transmitted image.

Unit 20 comprises the synchronizing-signal separating apparatus of applicant's copending application for the purpose of effectively separating the line-synchronizing and field-syn:- chronizing pulses from the video components and from each other so that they may be separately applied to" generators l8 and I8 free from n Thus. the apparatus 20 comprises a tetrode vacuum tube 2| having its control grid coupled to the output'of the detector l5 by way'of a suitable coupling condenser 22 and grid-leak resistor 23; The con-Q denser 22 and the resistor 23 are such as to have atime constant relatively long compared to the period of the positive line-synchronizing pulses translated from the detector l5 and the value of resistor 23 is high compared to the gridcathode resistance'of the tube II when the grid is positive. A load resistor -24 :is included in the anode circuit of the tube 2i and operating poggngig? trims to gig anode of the tube 0 resis r an ectly to the screen from suitable sources indicated as +3 and +80, respectively. The screenpotential is rela- A 65 'tively low so that the video components of the aiinal received from the detector II are well below the grid potential corresponding to anode current cutoif.

A differentiating circuit a condenser 2" in series with a resistor 28 is also included in the anode circuit of tube 2| in parallel with the resistor 24. The elements 2! and 2! are propor tioned to have a time constant which is apprea ciably greater than the duration of the lineand preferably less than, the duration of.the

current and the voltage across the resistor 26 decrease exponentially. At the trailing edge of the impressed voltage pulse, the condenser again 1 passes this voltage transient, which appears en- 'tirely across resistor 26 and the condenser -26 again begins to charge exponentially to its steady-state value. Due to the fact that condenser 25 has discharged only slightly during the short pulse, the transient across resistor 26 comsynchronized by negative and positivesynchronizing pulses, respectively.

Neglecting for the moment the operation of the unit 36, the operation and results obtained by the systemiust described may be best understood by reference to the curves of Figs. 2-6, inclusive, of

the drawing. Assuming a modulated-carrier wave to have been detected by the detector IS,

the modulation signal developed in its output' grid of this tube which, in the absence of the sig-.

rial is provided with zero bias, that the signal is stabilized, ensuring that the peaks of the synchronizing impulses are at a substantially fixed level as impressed on the control grid. Where the I connection between the grid oftube 2| and the detector I5 is substantially a direct one, so that theunidirectional component, of the signal is not last, the signal may be sufiiciently stabilized without employing the grid leak 23 and condenser 22. Where. a nonconductive coupling, suchas condenser 22, intervenes between the detector and the tube 2|, however, the signal tends to center itself about its alternating current zero axis and thus-to appear as illustrated by the curve of Fig. 3. In .this case, therefore, the stabilizing .circuit is required. Y

The grid-bias circuit of the tube 2| and its characteristics are also preferably such as to cause the video-frequency portion of the signal tov be cut off or limited so that the wave form of the current in the anode circuit ofthe tube 2| is as illustrated by the curve of Fig. 4. Due to the high resistance of the tube 2|, the voltage developed across the resistor 24 is of substantially the same waveform as the anode currentbut of reversed polarity, as represented by the curve of Fig. 5, ineluding negatively -poled synchronizing pulses. These pulses are applied'through condenser 21 to the synchronizing circuit of theline scannin'g generator it, which, as stated above; is responsive to negatively-poled synchronizing pulsesfand are efiective to synchronize the operation of this generator with corresponding apparatus at the transmitter in the conventional anner. i

The diflerentiatingfcircui including the condenser 26 and resistor-26, is responsive to the duration as well as the trailing edges of the pulses impressed thereacross. The alternating component of the voltage'across this circuitis the same as that developed across the resistor 24, as shown in Fig.5. During the intervals. between the pulses of the impressed voltage, the condenser 25 becomes charged to the steady value of this voltage, "its charging current falling to zero so transient is passed by the condenser 25 and appears entirely across the resistor 26. For the duration of this pulse, the condenser discharges but at a decreasing rate so that the discharge prising the trailing edge of the 'pulse reverses polarity but the amplitude of the reverse value is-very small. As the condenser 25 recharges exponentially to its steady-state value, the voltage across the resistor 26 again decreases exponentially to zero. Upon the occurrence of a relatively long duration field pulse, the condenser 25 discharges substantiallyand the voltage pulse across resistor 26 of. reverse polarity, developed at the trailing .edge of the field pulse, is of substantial amplitude. The voltage appearing across the resistor 26, therefore, is of the wave form illustrated hy the curve of Fig. 6 including positively-poled field-synchronizing pulses initiating at the trailing edges of the corresponding pulses of the wave impressed upon the inputcircuit of the tube 2| and of an amplitude dependent upon the duration of such'pulses. It will be appreciated that exact synchronization is unnecessary. With conventional linescanning systems of the type employing 441 lines for each'coinplete picture of two fields and with a GO-cycle field-scanning frequency, the permissible error is of the'order of 0,05 per cent of the respective scanning cycle. The field-synchronizing pulses developed as just described are within this limit and entirely satisfactory.

Thepositive pulses developed in the manner 2 just described'are translated to and utilized by the synchronizing circuit of the field-scanning generator I9 ,to synchronize the operation thereof with the scanning apparatus at the transmitter, these pulses being applied to fieldscanning generator I9 through apparatus 30 in accordance with the invention and in a manner to be hereinafter fully described. 4

It isfnoted, however, that the trailing edge of each of the group of field-synchronizing pulses derived pulse otherthan he first one. In order to prevent this type of o eration, applicant has provided unit 36 in accordance with the present invention. The'unit 30 includes means responsive to the groups of pulses for deriving fields'ynchronizin pulses individually corresponding to the pulses of the groups, which-means comprises a vacuum tube 3| having an input circuit' coupled across resistor, 26 and an output circuit including a resistor 32 through which a unidirectional operating potential +B is supplied. The anode of tube 3| is grounded by condenser 33 while there is included in the cathode circuit of a tube 3| a load resistor 36 to which is coupled a second source-of unidirectional operating potential +,C for tube 3| and across which is coupled the input circuit of field-scanning generator 19.

The voltage +C is, so chosen that tube 3| is operated beyond cutofli in the absence of derived .input field-synchronizing pulses from resistor 26,

as represented by the curve of Fig. 6. The unidirectional anode-cathode resistance R31 of tube 3| is so chosen that the discharge time constant Ru C's: of tube II and condenser 33 is small and preferably less than the duration T1 of one of the derived positive field-synchronizing pulses of the curve of Fig. 6. In the system described, Ti is about microseconds and R31 Ca: should not exceed more than about half this value if I succeeding pulses are to be adequately suppressed.

,In a preferred embodiment of the invention, R31

is about 30 kilohms and C33 is about 90 micrornicrofarads. Ra: must be made large to make, the charging time constant Ra: Caz, when-tube Ii is nonconductive, long compared to the interval between pulses; in the embodiment of the invention specifically described, 'Rszpreferably has a value of 5 megohmswhich results in a; charging time constantof 450 microseconds or about.6 lines. As will be described more fully hereinafter, this proportioning of the'time constants of resistor 32 and-condenser 33-provides means for rendering tube 3i effectively unresponsive to all pulses of the groups other than the first pulse of each group; 'Rsi must be made sufllciently high to permit a. proper synchronizing-pulsevoltage to be developed thereacross but should not be sufllciently large to affect the time constants of the system. a

out interval, the derived pulses' th first derived pulsewill beer the sameamplitum and will all be of insuillcient amplitude to fin .the field-scanning generator It By this ar- 5 rangement, therefore, the field-scanning genl. A television synchronizing system adapted .to be energized by a synchronizing signal includ inga group of field-synchronizinspulses of the same polarity during each field-blockout interval of the system comprising, means responsive to said groups of pulses for deriving field-synchro- Reference is made to the curves of Figs. 6-9,

inclusive, for a description of the operation of the circuit of unit 30. The solid curve of Fig. 6 thus represents the voltage applied to the grid of tube 3! while the top dotted line of Fig. 6 represents the cutofi point of this tube so that only pulses which extend above this line are efi'ective to fire tube 3|. The anode current of tube ii in response to the applied voltage represented by the curve of Fig. 6 is illustrated by the curve of Fig. 7 which results in an anode voltage as represented by the curve of Fig. 8; that'is, upon the occurrence of the first derived synchronizing pulse of Fig. 6, condenser 33 is rapidly and substantially discharged and, in one embodiment, may be substantially completely discharged, and, due to the long charging time constant Ra: C33, it does not charge up to an appreciable potential during the succeeding fieldsynchroniz'ing pulses so that they cannot result in succeeding discharge pulses of appreciable amplitude. The voltage across resistor 34 in response to this excitation is illustrated by the curve of Fig. 9. Itwill be understood that, if field-synchronizing pulses of opposite polarity of those of Fig. 9 are acceptable, resistor 84 may be included in the anode circuit of tube 3i in series with condenser 33. It is seen that a positive impulse .is applied to field-scanning generator H at a time coincident with the trailing edge of the first field-synchronizing pulse of the group of field-synchronizing pulses illustrated 'by the curve of Fig. 4 and that all other fieldesynchronizing pulses of the group are eifectively suppressed by being'sub tude.

a The sensitivity curve during the field-blackout derived fieldsynchronizing pulses is represented by thedotted curve of Fig; 7. It is seen that stantially reduced in amplinizing pulses individually corresponding to the pulses of said groups, means for rendering said first-mentioned means effectively unresponsive to all pulses of said groups other than the first pulse of each group, and a field-scanning circuit responsive to said derived field-synchronizing pulses.

2. A television synchronizing system adapted to be energized bya synchronizing signal including a group of field-synchronizing pulses of the same polarity during each field-blackout interval of the system comprising, means responsive to said groups of pulses for deriving field-synchronizing pulses individually corresponding to the pulses of said groups, means responsive to the first pulse of each of said groups of pulses for rendering said first-mentioned means effectively unresponsive to all pulses of said groups other than .the first pulse of each group, and a fieldscanning circuit responsive to said derived fieldsynchronizing pulses.

3. A television synchronizing system adapted to be energized by a synchronizing signal including agroup of field-synchromzing pulses of the same polarity during each field-blackout intera group of field-synchronizing pulses'of the same interval of the field-scanning generator iii to the )polaflty during each fleld'blockout Interval-bf the r system comprising, avacuum-tube repeater having an input circuit adaptedto be energized by said synchronizing signal and having an output circuit including a high resistance element connected. directly in parallel with a capacitance element and low resistance element in series for deriving field-synchronizing pulses individually corresponding to the pulses of said groups, means for rendering said field-synchronizing pulse-deriving means effectively unresponsive to all pulses of said groups other than the first pulse of each derived field-synchronizing group, and a field-scanning circuit responsiveto said derived field-synchronizing pulses.

5. A television synchronizing system adapted .inresponse to the first pulse of each of said groups of pulses and. a time oonstant circuit so proportioned that its charge is substantially changed in the other sense between succeeding ones of said first pulses for rendering said first-mentioned means efiectively unresponsive to all pulses of said groups other than the first pulse of each group, and a field-scanning circuit responsive to said derived field-synchronizing pulses.

6. A television synchronizing system adapted to be energized by a synchronizing signal including a group of field-synchronizing pulses of the same polarity during each field-blockout interval of the system comprising, means including a V vacuum tube responsive to said groups ofpulses for deriving field-synchronizing pulses individually corresponding to the pulses of said groups, means for rendering said vacuum tube effectively unresponsive to all pulses of saidgroups other than the first pulse of each group, and a fieldscanning circuit responsive to said derived fieldsynchronizing pulses.

7. A television synchronizing system adapted to be energized by a synchronizing signal 'including a group of field-synchronizing pulses of the same polarity during each field-blackout interval of the system comprising, means including a vacuum tube having a time-constant anode circuit including a series-connected resistor and condenser for deriving field-synchronizing pulses individually corresponding to the pulses of said groups, means for rendering said vacuumtube effectively unresponsive to all pulses of said groups other than the first pulse of each .group, and a field-scanning circuit responsive to said derived field-synchronizing pulses.

of field-synchronizing pulses of the same polarity during each field-blackout interval of the system comprising, means for eiIectively separating said field-synchronizing pulses from said line-synchronizing pulses, means coupled to said separating means and responsive to said groups of pulses Y for deriving field-synchronizing pulses individually corresponding to the pulses of said groups, means for rendering said last-mentioned means efiectively unresponsive to all pulses of said groups other than the first pulse of each group, and a field-scanning circuit responsive to said derived field-synchronizingpulses.

9'. A television synchronizing system adapted to be energized by a composite synchronizing signal including line-synchronizing pulses and a group of field-synchronizing pulses of the same polarity as said line-synchronizing pulses occurring during each field-blackout interval of the system comprising, means responsive respectively to said line-synchronizing pulses and to the dura-' tion and trailing edges of said groups or pulses for deriving line-synchronizing pulses of one polarity and. field-synchronizing pulses individually corresponding to the pulses of said group and of opposite polarity, means for rendering said field-synchronizing pulse-deriving means unresponsive to all pulses of said groups other than the first pulse of each group, and line-scanning and field-scanning circuits individually responsive to said oppositely-poled line-synchronizing and field-synchromzing derived pulses.

10. A television synchronizing system adapted to be energized by a field-synchronizing signal including a group of field-synchronizing pulses of the same polarity during each field-blackout interval of the system comprising, means responsive to said groups of pulses for deriving fieldsynchronizing, pulses corresponding to said group,

a'fleld-scanning-circuit having a synchronizingsensitivity which decreases during the field-blockout interval of the system, meansfor causing said derived field-synchronizing pulses to decrease at a faster rate than said sensitivity of said fieldf i scanning circuit only by the first of said derived 8. A television synchronizing system adapted to be energized by a composite synchronizing signal including line-synchronizing pulses and a group pulses during a given field-blockout interval.

' ARTHUR v. LOUGHREN,

, CERTIFICATE QF CORRECTLON. Patent No. 2,508,575. a January 12, 19B,

' ARTHUR v. LOUGHREN;

It is hereby certified that error appears. in the firinted specification of the above numbered patent requiring correction as follows Page 5, second column, line 27, claim 9, after "means" insert --effecti vely; line 59', claim 10, for "corresponding to said group read "individually corresponding to the pulses of said gronps-'; and that the said Letters fPatam; should be'read with this correction therein that the sane may conform to the record oi the case in the Patent Office.

signed and sealed, this 6th day of April, A; D. 1914.5.

Henry Van Arsdaie, gSeal) Acting Commissioner of Patents.