US2261762A

US2261762A - Interlaced scanning system

Info

Publication number: US2261762A
Application number: US134477A
Authority: US
Inventors: Hazeltine Alan
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1937-04-02
Filing date: 1937-04-02
Publication date: 1941-11-04
Anticipated expiration: 1958-11-04

Description

Nov. 4, 1941.

A. HAZELTINE INTERLAGED SGANNING SYSTEM Filed April 2, 1937 3' Sheets-Sheet l /3/ l ATTORNEY Nov. 4,

A. HAZELTIN INTERLACED S CANNING SYS TEM Filed April 2,

3 Sheets-Sheet 2 INVENTOR Al. A, HAzELTlN B ATTORNEY Patented Nov. 4, 1941 UNETED STATES f r orme Alan Hazeltine, Hoboken, N. J., assignor to Hazeltine Corporation, a corporation of Delaware Application April 2, 1937, Serial N0. 134,477

14 Claims.

This invention relates to television systems and, more particularly, to signal-generating apparatus for use in television transmitting systems* The invention is especially concerned with the provision of an improved method of, and means for, effecting scanning of the interlaced type.

Heretofore there have been devised various scanning systems for cathode-ray tubes utilized in television systems, systems of the so-called interlaced type having been found particularly advantageous. In this type of scanning, the electron beam traverses the target of the tube in a series of lines, usually horizontal, forming suc-cessive frames, each constituting a complete image, and these frames, or successive traversals of the image, are staggered; that is, the linesof one frame fall between the lines of a preceding frame. Due to persistence of vision, the optical effect produced by such an arrangement is as though each picture comprised twice the number of lines included in each frame.

Certain interlaced scanning systems of the prior art are of the so-called odd-line interlaced scanning type, the electron beam being deected to trace an odd number of lines for every two frames, and the phase of the line-scanning frequency with respect to the frame-scanning frequency is'such that the scanning of the odd-numbered frames' starts at one point, for example, the upper left-hand corner of the screen, while scanning of the even-numbered frames starts a distance of one-half line from the common starting point for the odd-numbered frames. In another type of interlaced scanning system, the socalled even-line type, the line frequency is an integral multiple of the frame frequency and, While the starting point of each frame is in the same vertical line, successive frames are vertically displaced so that their lines fall between each other.

In scanning systems such as are now generally favored, a single synchronizing wave is employed comprising both line-frequency and frame-frequency impulses and it is the initial part, or leading edge, of an impulse that ls the critical factor in effecting line-frequencysynchronizing, while the critical factor of a frame-frequency impulse is its duration. It is essential, therefore, that the line and frame impulses of a synchronizing signal be independent of each other, while occurring at the proper intervals substantially without interruption. Hence, it is desirable that each of the frame impulses be similarly related in phase to the line impulses, that is, initiate and terminate substantially at thev same relative 55 points of the cycle of vsimultaneously occurring line impulses. t

An interlaced scanningsystem of the prior art which' attains the desirable featurey just mentioned is the so-called serrated-wave system, which requires the introduction of additional short impulses midway between the line-frequency impulses fora portion of each frame, which,

however, tend to upset the line-frequency synchronism under certain conditions and require relatively complicated apparatus.

vIt is an object of the present invention, therefore, Vto provide a novel and improved type of interlaced scanning system for cathode-ray signal-generating and signal-reproducing tubes.

Itis a further object of the invention to provide an improved synchronizing-signal-generating apparatus for a scanning system of the character described. I

In accordance with the present invention, there is' provided an interlaced scanning system for a cathode-ray tube comprising means for deflecting the ray to scan the target in a first direction at a predetermined frequency and in a second direction normal'to the yfirst-mentioned direction at a lower frequency and in equal amounts on opposite sides of the axis of the tube. Means are further provided for initiating the deections in the second direction alternately after an even and an odd number of deflections in the first direction, so that successive frames or series of parallel lines are traced on the target and each line of one frame is traced between two successive lines of theneXt.

More particularly in accordance with the invention, there is provided synchronizing-signalgenerating apparatus which comprises means for developing recurring signal impulses of a predetermined iirst frequency, or a line-scanning frequency, and means for developing recurring impulses at a second lower frequency, or framescanning frequency. Means are further provided for initiating the frame-scanning impulses alternately after an even and an odd number of the line-scanning impulses. An arrangement is provided for combining the line-scanning and framescanning impulses to develop a single synchronizing signal including impulses of both the scanthereof, reference is had to the following description, taken in connection with the accom panying drawings, and its scope will be pointed out in the appended claims.

In the accompanying drawings, Fig. 1 is a diagram of a complete television transmitting system, partially schematic, including synchronizing-signal-generating apparatus embodying the present invention, while Figs. 2a-2t, inclusive, are curves representing the current or voltage wave forms developed at various parts of the system, to aid in the understanding of the invention.

Referring now more particularly to Fig. 1 of the drawings, the television transmitting system there illustrated comprises a cathode-ray signal-y generating tube and camera devicerof Aconventional design, indicated generally at I andincluding a signal-generating tube-II having the usual electron gun, photosensitive target, and

rapid retrace. The number of lines per frame are determined by the relative frame and line frequencies. The fields are so developed as to be properly centered at all times, that is, so that the cathode ray is deflected equally from the horizontal and vertical center lines of the target,

that is, in equal amounts on opposite sides of the axis of the tube. Pedestal impulses developed by the generators I4 and I 5 are applied to a control electrode of the tube II to suppress or block-out the beam during the retrace portions of the scanning cycles and are applied to the amplifier I8 to suppress undesirable, impulses 'scanning elements, as indicated. For developing -scanning currents for the signal-generating tube,

a line-frequency generator I2 and a frame-frequency generator I3 are provided, their output circuits being connected with the scanning elements of the cathode-ray tube II'.' In order to provide pedestal impulses for blocking out or suppressing the cathode ray during retrace portions of the scanning cycles, for suppressing undesirable impulses and for insuring the proper form of the signal -to be transmitted, there are provided line-pedestal and frame-pedestal impulse generators I 4 and I5, respectively, their output circuits being connected with the control grid of the signal-generating tube II. In order to synchronize the frequencies developed by the generators I2, I3, I4, and I'5, as well as the corresponding apparatus at the receivers, there is provided synchronizing-frequency-generating apparatus, indicated generally at I6, and embodying the present invention, as hereinafter described in detail. The output circuits of the apparatus I6 areconnected directly to the pedestal generators I4 and I5 and, by way of a separator I'I for separating the lineand frame-synchronizing impulses, to the lineand frame-frequency generators I2 and I3.

A modulation-frequency amplifier I8 is coupled to output circuits of the signal-generating tube II, the pedestal generators I'4 and I5, and the generating apparatus I6. Connected in cascade to the output circuit of the amplifier I8, in the order named, are a modulator I9, which is coupled also to an oscillator20, as shown, a power amplifier 2l, and an antenna system 22, 23, all according to conventional practice.

Neglecting for the moment the details of construction and operation of the synchronizingfrequency-generating apparatus I6, the system just described constitutes a television transmitting system of conventional design, those parts of the system illustrated schematically being of any well-known construction so that a detailed description of the system and its operation is unnecessary herein. Briefly, however, the image of a scene to be transmitted is focused on the target of the tube II in which a cathode ray or beam is developed, focused, 'and accelerated toward the target. Scanning or deflecting currents developed by the generators I2 and I3 are applied to the scanning coils of the generating tube II to provide magnetic fields which serve to deflect the cathode ray horizontally and vertically, thereby to scan successive series of parallel lines, or frames, on the target;

The defiecting currents and,v hence, the magnetic fields are vof well-known saw-tooth Wave .form providing a linear relatively .slow trace and developed in the system and to aid in obtaining the required wave form of the transmitted synchronizing signal. The photosensitive elements of the target being electrically affected to an extent dependent upon the varying values of light and shade at the corresponding incremental areas of vthe image focused thereon, as the cathode ray passes over the target, a voltage of correspondingly varying amplitude is developed in the output circuit of the signal-generating tube I I' and applied to the amplifier I8. Lineand frame-frequency synchronizing impulses developed by the generating apparatus I6 are applied, by way of the separator I'I to the generators I2 and I3 and directly to the pedestal generators I4 and I5 and to the amplier I8, for the purpose ofmaintaining the generators and the corresponding apparatus at the receiver in synchronism. The various modulation signal components applied to the amplifier I8 are properly combined and amplified therein and, in turn, are supplied to the modulator I9 wherein they are impressed upon the carrier wave generated bythe oscillator 20. The resultant modulated-carrier signal is delivered to the power amplifier 2I for amplification and is thereupon impressed upon the antenna system 22, 23 to be broadcast.

Coming now to that part of the system embodying the present invention, the synchronizing-frequency generating apparatus I6 comprises a primary generator or master oscillator 24 for developing impulses at the line-scanning frequency, and double this frequency, and five separate relaxation oscillators or impulse generators. The relaxation oscillators comprise a chain of

generators

25, 25, 21, and4 28 for successively dividing the frequency of the output of the primary generator 24 to derive impulses of the frame-scanning frequency, and a generatorl y29 for developing frame-scanning impulses of proper phase and Wave form. An output tube 30 is coupled to the primary generator 24 and to the generator 29 for developing the final composite synchronizing signal from the outputs of these two generators.

The main oscillator 24 is of the push-pull type comprising a pair of vacuum tubes 3| and 32 having included in their respective anode circuits frequency-determining circuits comprising a winding 31, a condenser 38,.and a resistor 39, and a winding 31a, a condenser 38a, and aresistor 39a. Grid-exciting windings 33 and-33a connected in series with blocking resistors 34 and 34a, respectively, are included-in the gridcathode circuits of the tubes and a suitable leak resistor 35 and'by-pass condenser 36 are provided in a common branch of the grid-cathode circuits; as. shown. .All of the. windings are closely coupled, as by the iron core shown schematically, to each other and to a winding 42 across which is` connected an adjustable condenser. 43. Thefrequency is determined jointly by the three resonant circuits including condensers 38, 38a, and 43 with their associated windings 31, 31a, and 42, respectively, and is adjusted to the desired line-scanning frequency, for example, 10,290 cycles. Resistors 54 and' 54a `are included in the plate circuits of the tubes 3l and 32, respectively. In order to couple the generator 24 to the tube 30, the lower end of the resistor 54a is connected directly to the screen of this tube and by way of a condenser 5S to the grid of the tube. In order to couple the generator 24 to the generator 25, a resistor 50 is included in the common cathode circuit of the tubes 3| and 32 and the voltage developed thereacross is applied to the generator 25 by way of a coupling condenser 5I and resistor 52.

Each of the generators 25-29, inclusive, is of the same general construction and operation and comprises essentially a multi-vibrator or regenerative relaxation oscillator. Each generator includes two vacuum tubes 44 and 45, which are shown as being of the pentode type, but insofar as the generators, per se, are concerned, the tubes of the

generators

26, 21, 2B, and 29 and the tube 45 of the generator 25 are connected to operate In the case of each of the generators, -29, inclusive, the input voltage is applied to the control grid of the tube 44, while the local output voltage of this tube is developed across a load resistor 44a and applied to the control grid of the tube by way of a coupling condenser 45 and leak resistor 41. The screen electrode of each tube 45, serving as a local output electrode, is connected to the control electrode of the tube 44, this connection being a direct one to the second grid in the case of the generator 25 and by way of a coupling condenser 48 and leak

resistors

49, 52a to the innermost grid in the case of each of the other generators. As inentioned above, the output voltage of each of the generators 25-29, inclusive, is taken from the tube 45 and, in the case of the generator 29, it is 'also taken from the tube 44. Operating potentials are applied to the various electrodes of the tubes 44 and 45 from a suitable source, indicated at +B, by way of suitable resistors and by-pass condensers, as shown.

The control grid of the tube 44 of the first generator 25 is excited with the voltage developed across the resistor of generator 24 and applied by way of the coupling condenser 5l and grid-leak resistor 52. A load resistor 52a is included in the plate circuit of the tube 45 of each of the generators 25-28, inclusive, and the voltage thereacross is applied to the input circuit of the tube 44 of the succeeding generator. In order to couple the frame-frequency output of the final wave-forming generator 29 to the combining amplifier 30, the plate circuit of the tube 44 of this generator includes the load resistor 56, which is connected to the control grid of the tube 35 by way of grid-leak resistor 58. Resistor 51 by- 4passed by condenser tl provides the proper plate potential for tube 30, relative to its cathode; and by-pass condenser 55 maintains the cathode at a fixed potential. Load resistor 60 is included in the anode circuit of the tube. Each ofthe elements included in the input and output circuits of the tube A30 will be further described in connection with the operation of the system.

The operation of the synchronizing-frequencygenerating apparatus I0, described above, may be' best described with reference to'Figs. 2li-2t, inclusive, which illustrate voltage and current wave forms at various parts of the apparatus. The master oscillator 24 is adjusted to the linescaning frequency, for example, 10,290 cycles per second. The grid windings 33, 33a and the anode windings 31, 31a may be so proportioned relative to the effective resistances of the oscillation circuits that the peak voltages of coils 33, 33a are about twice the cutoff gridvoltages of the tubes 31 and 32. The resistors 34, 34a, and 35 should be .proportioned so that the voltage drop in the resistor 35 is about equal to this cutoff voltage. The potential at the upper end of the kresistor 34 lis then Yrepresented approximately by the sine wave of Fig. 2a, the positive dotted portion nearly al1 appearing as voltage drop in resistor 34; and the actual grid potential of tube 3| is shown by the full line, the portion of the wave between cutoi'l` and zero being about 30 degrees of the cycle (since sin30 degrees=1/2). This interval determines ultimately the duration of the linescann-ing pulses, 30 degrees corresponding to a pulse of about 0.1 of a cycle; if a shorter pulse is desired, the peak voltages of coils 33, 33a should be increased. Thus, the anode current of tube 3l rises roughly linearly for about 30 degrees, remains nearly constant for about degrees, falls roughly linearly for about 30 degrees, and is zero for the remaining half cycle, as represented in Fig. 2b. The same is true of the screen current and, hence, of the total cathode current. The grid potential and the anode and cathode currents of tube 32 are like those of tube 3| but displaced degrees in phase, as represented in Figs. 2c and 2d, respectively. The sum of the two cathode currents, represented in Fig. 2e, iiows through resistor 50 and produces a voltage drop having the same Wave form. It is to be noted that this voltage has twice the frequency of the master oscillator 24.

Referring now particularly to the operation of the generator 25, the tube 44 may be considered as initially passing current and tube 45 as having a grid potential below cutoi. The condenser 5I and resistor 52 are chosen to have a time constant of the order of 30 degrees of the cycle of the master oscillator 24. Hence, the voltage across the `resistor 50, Fig. 2e, impressed on the condenser 5l and resistor 52 in series produces across the resistor 52 a voltage of the general form represented in Fig. 2f. This voltage is impressed on the grid of tube 44 of generator 25. When it becomes negative, the anode current falls, which raises the anode potential and-hence, through condenser 45, the grid potential of tube 45, which, as stated above, is initially below cutoil. Current quickly starts to flow in the screen (anode) circuit of tube 45, lowering the potential of the screen of this tube. The screens of the tubes 44 and 45 being connected together, the potential of the screen of tube 44 is likewise lowered and the current of this tube is quickly interrupted by this cumulative action, the rapidity being limited only by inherent capacitances, which should be kept small. The potential of the grid of tube 45 thus rises 'rapidly to a positive value and remains positive until current is again establishedin tube 44 by the succeeding positive pulse in the grid potential,

as illustrated in Fig. 2f. At this instant, the anode of tube 44 and, hence, the grid of tube 45 quickly fall in potential, cutting oif the current in tube 45. The grid potential of tube 45 now rises toward cutoff at a ratedetermined by the time constant of condenser 46 and its associated resistances, this time constant not being critical but being of the order of the period of the master oscillator 24. This action is then re-initiated bythe next double impulse of Fig. 2f and rey sults in a grid-potential wave for tube 45 of the form shown in Fig. 2g.

Referring now to the operation of the generator 26, when the grid of tube 45 of generator 25 becomes positive, its anode resistance becomes low. This lowers the normally positive grid potential oftube 44 of the generator 26, by means of the connection to the

resistors

49 and 52a, of which the resistor 49` has the lower resistance and may be omitted if the internal grid resistance is sufficiently high. The anode current of tube 44 then falls and the anode potential rises, this rise in potential being impressed on the grid of tube 45 through condenser 46. The grid potential of tube 45 being initially only slightly below cutoff,as represented at the lefthand portion of the curve of Fig. 2h, it then rises above cutoif The consequent flow of screen current lowers the screen potential, this fall in potential being impressed back on the grid of tube 44 through condenser 48. The result of this cumulative action is a quick interruption of the current of the tub-e 44, the rapidity again being limited only by the small inherent capacitances. The grid potential of tube 44 now rises toward cutoff at a rate determined by the time constant of condenser 48 and its associated resistances, which rate, while not critical, may be such that cutoff-is reached after a time of the order of twice the duration of the pulse from generator 25. During this time, current ows through resistor 44a., condenser 46, and the grid of tube 45 of generator 26, charging the condenser 46 and maintaining this grid positive. But as soon as the grid potential of tube 44 rises above cutoff, the consequent flow of anode current lowers its anode potential and, hence, the grid potential and the screen current of tube 45. This, in turn, raises the screen potential of tube 45 and the grid potential of tube 44, the action being cumulative. The grid potential of tube 45 is thus rapidly forced far below cutoff, subsequently rising at a rate determined by the time constant of condenser 46 and its associated resistances. If it is desired that generator 26 have one-seventh the frequency of generator 25, for example, this time constant is chosen so that the next six positive pulses of grid potential received from generator 25 by way of tube 44 are not sufficient to bring the tube 45 above cutoff, but the seventh is, as represented in Fig. 2h. With this seventh pulse, the action described above is repeated. The timeconstant is most conveniently adjusted by varying the resistance 41, indicated as variable in Fig. 1, for Variation of the capacitance 46 varies both its rate of charge and its rate of discharge, the two effects partly counterbalancing each other.

Generators

21 and 28 are provided further to subdivide the frequency and operate essentially like generator 26.1 For example, if the inaster oscillator has the frequency 10,290 cycles per secondgeneratorY 25 has twice this frequency,

or'20,580 cycles per second. Then generator 26 may have one-seventh this frequency, or 2940 cycles per second; generator 21 again oneseventh, or 420 cycles per second; and generator 28 again one-seventh, or 60 cycles per second, which is usually the frequency desired, being equal' to the most common power frequency. In

-any case, the successive frequencies are divided by odd integers. Thus, there is derived from the line-'scanning impulses signal impulses of a second frequency which is an odd sub-multiple of twice the line-scanning frequency. The wave forms of grid potential of tube 45 of

generators

21 and 28 are like that for generator 26 (Fig. 2h) except in time scale and are represented in part in Figs. 2i and 2y, respectively. In each case, the time constant of condenser 45 and its associated resistances is adjusted by means of variable resistor 41 to give the desired frequency. The two grid resistors for the tube 45 in generator 28 have the purpose of providing a grid bias intermediate the potential of the cathode and that supplied to the screen. A suitable value of this bias gives the lowest possible value of the external direct current grid resistance consistent kwith a given emission in tube 44 and with a high rate of change of grid potential just before it swings positive. In generator 21, the time constant of condenser 48 and its associated resistances is not critical, but in generator 28, this time constant' should be such that the grid of tube 44 remains below cutoff for approximately 0.8 cycle of the master oscillator 24, causing the grid of tube 45 to remain positive for the same interval, as represented in Fig. 27' and for a purpose to be explained.

In the generator29, the grid of the tube 44 receives control pulses from two sources, that is, from the plate of tube 45V in the preceding generator through

resistors

49 and 52a, as in the generators 26-28, inclusive, and from the anode circuit of tube 3| through the condenser 53a. This grid is normally biased so positively that it becomes negative only when negative pulses occur simultaneously from both sources and it is only under this condition that the screen current of this tube 44 falls vsufliciently to bring the grid of tube 45 above cutoff. The negative pulses from the plate of tube 45 of generator 28 correspond to the positive grid pulses shown in Fig. 27. The anode currentof tube 3|, as shown in Fig. 2b, produces a voltage'drop of the same wave form in resistor 54 and this subtracted from the +B voltage gives the potential at the upper terminal of the condenser 53a, as represented in Fig. 2k. This potential, impressed upon condenser 53a, which may be given a value a few times the total interelectrode grid capacitance of tube 44, produces a potential of a wave-form, in the absence of other voltages; at the grid of tube 44, such as representedin Fig. 2l.

The actual grid potential applied to the grid of tube 44 of generator 29 is, therefore, represented in Fig. 2m and remains positive after a pulse is received from generator 28 until the first following negative pulse of Fig. 2l is received. It then becomes negative and causes the anode current of tube 44 to fall, which, in turn, raises the screen potential of tube 44 and the grid potential of tube 45, causing the latter to rise above cutoff, thus cumulatively lowering the grid potential of tube 44 by the consequent fall in screen potential of tube 45, this action being essentially the same as in the preceding generators. Thus, the grid potential of tube 44 rapidly falls below cutoff, as shown at the left of Fig. 2m, after which it rises at a rate depending on the time constant of condenser 43 and its associated resistances. This time constant is so chosen that at some desired positive pulse from the generator 24, as, for example, the second following one (Fig. 2l), the grid potential of tube 44 rises above cutoff. The consequent ow of screen current in tube 44 then rapidly sends the grid potential of tube 45 below cutoff by the same cumulative a-ction previously described. The time constant of condenser 48 and its associated resistances is not critical, but is so chosen that the grid potential of tube 45 rises above cutoff during the following pulse from generator 28, that is, approximately 1&0 second later, if the frequency of generator 28 is 60 cycles per second. During this interval the grid of tube 44 remains positive, as indicated in Fig. 2m. Thereafter, the action described is repeated.

It is to be particularly noted that, while the negative pulses applied to the grid of the tube 44 of generator 29 from generator 28 all occur at equal intervals, as 1,@ second, as shown by the positive pulses of Fig. 27, there are no negative pulses applied to this grid from the generator 24 having this interval (see Fig. 2l) for this interval is an odd multiple of half the period of the master oscillator 24; that is, in the example chosen, 1GO second is an odd multiple of 1/20,580 second, which is half of 1/10,290 second and is, in fact, 7 7 '7/2=171.5 cycles of the master oscillator, corresponding to 171.5 scanning lines. Hence, on alternate negative pulses from generator 23, the negative pulses of Fig. 2l arrive a half-cycle later and cause the grid of tube 44 of generator 29 to swing negative a half-cycle later, as represented at the right of Fig. 2m. The duration of the negative pulse from generator 2B is chosen as about 0.8 cycle of the line frequency in order to insure that it always includes one negative pulse of Fig. 2l. The result of the arrangement described is that the negative swings of grid potential of tube 44 are always separated by a whole number of scanning lines, but this number is alternately larger and smaller by one line, being alternately I`I2 and I'Il lines in the example chosen.

Across the resistor 54a, in the anode circuit of the tube 32 of generator 24, is produced a voltage drop of the same wave form -as the current, this being as shown `in Fig. 2d, causing the potential at the lower end of resistor 54a to be as repref' sented in Fig` 2n. This potential is impressed on the screen of tube 30. The space current of tube 33 passes through resistors 56, 45'I to theanode of tube 44 of generator 29. These resistors have such resistances that the cathode potentialof tube 3Q, which isl maintained constant by the most positive Hence, current iiows through tube Sil only when the screen and the grid both receive positive pulses of potential. The grid receives positive pulses from two sources:` from the anode of tube 44 of the generator 29 Vwhen the grid of this tube becomes4 negative (Fig. 2m)

and so interrupts the anode current; and from the lower end of resistor 54a (Fig. 2n) through condenser 5S, these pulses being shown in Fig.'2p and being similar to those of Fig, 2l, which are 5 produced in a like manner. It will be seen that a pulse through condenser 50 (see Fig. 2p) immediately precedes each pulse from generator 2i), these two pulses coalescing. The resulting grid potential of tube 3l) is thus as represented l0 in Fig.v 2q and the plate current is as represented in Fig, 2r. This current flowing through resistor 6G builds up a plate potential of like wave form, except that it is inverted. This plate p0- tential is then impressed on separator Il and l5 on the modulation-frequency amplier I8.

The separator Il is arranged in a well-known manner to produce two potentials: one depending almost solely on the rate of decrease of the impressed potential (rate of rise of current, Fig.

2) and so consists of equally spaced pulses, liig. 2s, which are only slightly larger at the beginning of the long pulses of Fig, 2r than of the short pulses and which provide the linescanning impulses for the generator I2; the other potential depends on the integrated value of the impressed potential and is cut off at low values, with the result that only the vlong pulses of Fig. 2r are effective and these provide pulses which wholly or partially coalesce, Fig. 2t, and which provide the frame-scanning impulses for the generator I3.

VThe output potential of tube 30, after being amplified in the amplifier I8, modulates the carrier wave in modulator I9, giving it an envelope corresponding to Fig. 2r (in addition, of course,

to the modulation eifected by the pedestal gen-AA erators I4 and I5 and by the video signal between pulses). A separator at the receiver, corresponding to Il, .there produces pulses corre- 450 spending to Figs. 2s and 2t comprising the linescanning and frame-scanning impulses.

The grid of tube of generator 29, as in the preceding generators, is positive during the short interval that the grid ofV tube 44 is negative (as 45A. represented by the negative portions of Fig. 2m) During this interval, the anode resistance of tube 45 is low, this lowering of resistance being-employed to control the frame pedestal generator I5, which may be of the same type as generators 5,0;` 26-28 in the same Way thatthese generators are each controlled by the lowered plate resistance of tube 45 of the preceding generator. The framepedestal pulse, therefore, starts simultaneously with the frame-scanning pulse derived from gen-v E@ erator 29 through the plate of tube 44, and may be given the desired duration by suitable choice of the time' constant of a condenser-resistor circuit, just as in the preceding generators.

` The line-pedestal generator I4 may also be of 'v the same type as the preceding generators, its* control circuit including a small condenser, corresponding to the control of generatorlZS through condenser 53a. The control potential for generator I4, like that of tube' t, is derived from 6,5). resistor 54a so that the line-pedestalpulse starts simultaneously with the line-scanning pulse and may be given the desired'duration by suitable choice of the time constant of a condenser-resistor circuit, as in the preceding generators. As

7 0 in tube 30, a line-pedestal pulse from generator I4 immediately precedes leach frame-pedestal pulse from generator I5 and coalesces with it in the vcommon output circuit.

, The wave forms of Figs, 2a-2t have been drawnV 5 on the assumption that the effects ofV inherent capacitance are negligilbe. Actually, inherent capacitance causes some distortion and must be minimized by taking certain precautions. In the rst place, the vacuum tubes should be of a type having low interelectrode capacitances and should also have a sharp cutoff. In the second place, leads Whose potentials; change suddenly should be made as short and direct as possible. This applies particularly to the leads from the lower terminals of resistors 54, 54a and to the leads connecting the anode of tube 44 of generator 29 to the grid of tube 30 through

resistors

51, 58, some of which have been drawn with relatively long paths in Fig, l for clarity of representation.

In the master oscillator 24, the inherent capacitance primarily to be minimized is the capacitance to ground of the anodes of the tubes and their connected apparatus including windings 3'I, 31a, condensers 38, 38a, resistors 39, 39a, 54, 54a, and the leads from the lower terminals of the resistors 54 and 54a to condensers 53a and 59 and that in the input of generator I4 (not shown). Such inherent capacitance causes undesired currents to return through resistors 54, 54a and so to distort the voltages of Figs. 2k and 2n. Coils 3'I, 31a should, therefore, be as compact as possible, being of the multi-layer type with fine Wire and a minimum of internal insulation and being Well spaced from each other, from coils 33, 33a and 42, and from the iron core. This spacing of coils 3l, 31a necessarily somewhat lowers their magnetic coupling, which tends to give high self-induced voltages when their currents change rapidly. To minimize the consequent voltage ripples, the condensers 38, 38`a, which are connected across these coils, are made so large as to constitute the greater part of the tuning capacitance. Resistors 39, 39a have been found helpful in further reducing such ripples and may be large enough to constitute a considerable portion of the load on the oscillator. Inherent capacitance in the grid windings 33, 33a is not as important, since their voltages to ground are sinusoidal; these coils should be very closely coupledmagnetically so as to force the two gridsY to vary exactly oppositelyin potential, thus preventing parasitic oscillations. j Winding 42 also has a sinusoidal voltage to ground and, in addition'to its connection to the small adjustable tuning condenser 43, may be connected to an automatic tuning system for synchronizing the E-cycle voltage of generator. 28 with a GII-cycle power line.

In generators 25-29, inherent capacitance tends to delay the beginnings of the pulses, the effect being cumulative in the successive generators. `However, a margin of safety has been provided in that the pulses transmitted from generator 28 to generator 29 normally start a short interval ahead of the pulses through condenser 53a, as shown at the extreme left of Fig. 2m. 'I'he total delay should, therefore, be kept less than this interval.

In the output circuit of generator 29, which is connected to tube 30, the resistor 5T is shunted by the condenser 6I for the purpose of maintaining the voltage across resistor 51 nearly constant While the space current of tube 44 is interrupted. The change in anode potential of tube 44 is thus relatively small (being only the voltage drop in resistor 56) and so produces a relatively small capacitance current, which does not appreciably delay the voltage pulse delivered to the grid of tube 3U.

In each of the generators 25--29, the -l-B voltage is first lowered and filtered by a suitable series resistor and a large shunting condenser, as shown, to provide the highest direct voltage needed, which is that of the anodes of each of tubes 44. In each of generators 26-29, a second cornbination of series resistors and shunt condensers provides a lower direct voltage for the screen of tube 45 and for biasing one'or both of the grids. None of the values involved is critical; the only design requirement that may be severe is that, in generator 28, a relatively high anode current is required in tube 44, so that when this is interrupted a correspondingly high current Will charge condenser 46, requiring this condenser to be relatively large and so requiring lower values of grid leak resistance in tube 45 for the required time constant on discharge.

In Figs. Zm-Zt, conditions have been shown where the frame-scanning pulse has been produced by two successive impulses of the voltage of Fig. 211, but, if desired, any number of successive impulses may be used., such as one or three, a larger number somewhat simplifying the design of the separator to reduce its cost, which is particularly important in receivers. The number of pulses is determined wholly by the time constant of condenser 43 in generator 29, with its associated resistances. This is because the time of charge of condenser 46 in generator 28 has been chosen sufficiently short (0.8 of a linescanning cycle) to accompany even a single impulse of Fig. 2n. However, if more than a single impulse is used, it is permissible correspondingly to lengthen the time of charge of condenser 46 in generator` 28, thus reducing the severity of the requirements in the design of this generator, as mentioned in the preceding paragraph.

In each of the generators 25-23, the anode of tube 45 merely initiates the pulse of the following generator; for this anode, which is positive at the beginning of the pulse, is carried negative with the grid of tube 44 in the following generator and then acts as an open circuit. It is, therefore, merely necessary that the pulse in one generator be no longer than that of the following generator in order that its duration shall not affect the following generator, which relation is exhibited in Figs. 2g-27`.

The arrangement of the anodes of tubes 45 in generators 25-29, so that they do not take part in the relaxation oscillation, prevents the oscillation ofone generator from affecting that of the preceding generator. This freedom from return effects is made substantially complete by the use of the suppressor grid in each of these tubes to screen the plate. In generator 25 a like effect is attained by arranging the control gride of tube 44 so that it does not take part in the relaxation oscillation, except to initiate it, so that this oscillation does not return to the master oscillator 24. Such an arrangement of a control grid may also be employed in the other tubes 44, as an alternative to the use of isolated plates in tubes 45. In summary, therefore, it is seen that the circuit of Fig. 1 comprises an interlaced scanning system for cathode-ray tube apparatus I0 comprising means including line-frequency generator I2' for deflecting the ray to scan the target of the tube'in a first direction at a predetermined frequency and means including frame-frequency generator I3 for deecting the ray to scan the target in a second direction normal to the first direction and in equal amounts on opposite sides of the axis of the tube. Furthermore, the system aaemcaY includes unit I6 for initiating deflections` inthe second or Yframe-scanning direction alternately after an even and an odd number of deections in the first orline-scanning direction, whereby successive frames of parallel lines are scanned on the target of tube Il and each line of one frame is traced between two successive lines of the next. Specically, it is seen` that the unit I6 is comprised in an interlaced-scanning Vsystem for the cathode-ray tube apparatus l and includes a synchronizing-signal generating apparatus comprising means for developing signal impulses of a predetemined line frequency at the output circuit of generator 24, means for developing recurring frame-scanning impulses at the output circuit of generator 29 and means including the generators 24-29, inclusive, for initiating the frame-scanning impulses alternately after an Ieven and an odd number of the line-scanning impulses.

While there has been described what is at present considered the preferred embodiment 'of the invention, it will be obvious to those skilled in Ythe 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 and modiiications as fall within the'true spirit and scope of the invention.

What is claimed is:

l. An interlaced scanning system for cathoderay tube apparatus comprising means for deflecting the ray to scan the target of the tube in a rst direction at a predetermined' frequency, means for deflecting the ray to scan the target in a second direction normal to said first direction and in equal amounts on opposite sides of the axis of the tube, and means forinitiating deections in said second direction alternately after an even and an odd number of deflections in said rst direction, whereby successive frames of parallel lines are traced on said target and each line` of one frame is traced between two successive lines of the next.

2. An interlaced scanning systemfor cathoderay tube apparatus comprising means for deflecting the ray to scan the target of the tube in a vertical direction at a predetermined frequency, means for deflecting the ray to scan the target in a horizontal direction normal to said rst direction and in equal amounts on opposite sides of the axis of the tube, and means for initiating the vertical deflections substantially simultaneously with the initiation of horizontal deections and alternately after an even and an odd number of horizontal deflections, whereby successive frames of parallel lines are traced on said target and each line of one frame is traced between two successive lines of the next.

3. In an interlaced scanning system for cathode-ray tube apparatus, a synchronizing-signalgenerating apparatus comprising means for developing recurring signal impulses of a predetermined line-scanning frequency, means for developing recurring frame-scanning impulses, and means for initiating said frame-scanning irnpulses alternately after an even and an odd number of said line-scanning impulses.

4. In an interlaced scanning system for cathode-ray tube apparatus, a synchronizing-Signalgenerating apparatus comprising means for developing recurring signal impulses of a predetermined line-scanning frequency, means for developing recurring frame-scanning signal impulses, means for initiating said frame-scanning impulses alternately after an even and an odd number of said line-scanning impulses, and

mean for so combining said line-scanning and.

frame-scanning impulses as to provide a single synchronizing signal including impulses of both types with each of said frame-scanning impulses similarly related to saidvline-scanning impulses.

5. In an interlaced scanning system for cathode-ray tube apparatus, a synchronizing-signalgeneratingapparatus comprising means for developing recurring signal impulses of a predetermined line-scanning frequency, means for deriving from said line-scanning impulses signal impulses of a second frequency which is an odd sub-multiple of twice the line-scanning frequency, and means for deriving from said impulses'of said second frequency signal impulses of a frame-scanning frequency and initiating alternately after an even and an odd number of said line-scanning impulses.

6. In an interlaced scanning system for cathode-ray tube apparatus, a synchronizing-ignalgenerating apparatus comprising means for developingrecurring signal impulses of a predetermined line-scanning frequency, means for deriving from said line-scanning frequency impulses signal impulses of la second frequency which is an odd submultiple of twice the line-scanning frequency, means for deriving from said impulses of said second frequency signal impulses of a framescanning v frequency and initiating alternately afteran even and an odd number of said linescanning impulses, and means for combining said line-scanning and frame-scanning impulses to provide a single synchronizing signal including impulses of both said scanning frequencies with each of said frame-'scanning impulses Similarly related to said line-scanning impulses.

7. In an interlaced scanning system for cathode-ray tube apparatusa synchronizing-signalgenerating apparatus comprising a first generator for developing signal impulses of a predetermined line-scanning frequency, a second generator for developing signal impulses at a second frequency which is an odd sub-multiple of twice the linescanning frequency, means interconnectingsaid first and second generators to synchronize the operation thereof, a third impulse generator, andy means for applying said line-scanning impulses and impulses of said second frequency to said third generator, whereby signalimpulses are developed in its output circuit at a frame-scanning frequency initiating alternately after an even and an odd number of said line-scanning impulses.

8. In an interlaced scanning system for cathode-ray tube apparatus, a synchronizing-signalgenerating apparatus comprising a first generator for developing signal impulses of a predetermined line-scanning frequency, a second generator for developing signal impulses at a second frequency which is an odd sub-multiple of twice the line-scanning frequency, means interconnecting said rst and second generators to synchronize the operation thereof, a third impulse generator, means for applying said line scanning impulses and impulses of said second frequency to said third generator whereby signal impulses are developed in its output circuit at a framescanning frequency, said impulses initiating alternately after an even and an odd number of said line-scanning impulses, and means for com- -bining said line-scanning and frame-scanning pulses similarly related to said line-scanning impulses.

9. In an interlaced scanning system for cathode-ray tube apparatus, a synchronizing-signalgenerating apparatus comprising means for developing recurring signal impulses of a predetermined line-scanning frequency, means for deriving from said line-scanning impulses signal impulses at a second frequency which is an odd sub-multiple of twice the line-scanning frequency, means for deriving from said impulses at said second frequency signal impulses of a frame-scanning frequency and initiating alternately after an even and an odd number of said line-scanning impulses, a combining vacuumtube repeater, and means for so applying both said line-scanning and frame-scanning impulses to control electrodes of said tube as to develop in the output circuit thereof a: single synchronizing signal including impulses of both said scanning frequencies with each of said frame-scanning impulses similarly related to said line-scanning impulses.

l0. In an interlaced scanning system for cathode-ray tube apparatus, a synchronizing-signalgenerating apparatus comprising means for developing recurring signal impulses of a predetermined line-scanning frequency, means for deriving from said line-scanning impulses signal impulses at a second frequency which is an odd sub-multiple of twice the line-scanning frequency, means for deriving from said impulses at said second frequency signal impulses of a frame-scanning frequency and initiating alternately after an even and an odd number of said line-scanning impulses, a combining vacuumpentode tube repeater, means for applying said line-scanning and said frame-scanning impulses to the control grid of said tube, and means for applying said line-scanning impulses also to the screen of said tube, said tube being normally biased below cutoff, whereby a single synchronizing signal is developed in the plate circuit of said tube including impulses of both said scanning frequencies with each of said frame-scanning impulses similarly related to said line-scanning impulses.

11. In an interlaced scanning system for cathode-ray tube apparatus, the method of scanning the target of the tubewhich comprises deecting the ray to scan the Atarget in a firstdirection atA a predetermined frequency, deflecting the ray to scan the target in a second directionnormal to said first direction and in equal amounts ,on op.- posite sides of the axis of the tube, and initiating the deflections in said vsecond direction alternatelyafter an even and an odd number of deflections in said first direction so that successive frames of parallel lines are traced on said target and each line of one frame is tracedbetween two successive lines of the next. 12.In an interlaced scanning system for cathode-ray tube apparatus, the method ofdeveloping synchronizing signals which comprises developing recurring signal impulses of a predetermined line-scanning frequency, ldeveloping successive frame-scanning impulses and initiating said frame-scanning impulses alternately after an evenv and an odd number of saidlinescanningimpulses.

13. In an interlaced scanning system for cathode-ray tube apparatus, the method of developing synchronizing signals which comprises developing recurring signal impulses of a predetermined vline-scanning frequency, developing successive frame-scanning impulses, initiating said frame-scanning impulses alternately after an even and an odd number of said line-scanning impulses, and combining said impulses to produce a single synchronizing signal including impulses of both said frequencies with each of said frame-scanning impulses similarly related to said line-scanning impulses.

14. In an interlaced scanning system for cathode-ray tube apparatus, the method of developing synchronizing signals which comprisesdeveloping .recurring signal impulses of a predetermined line-scanning frequency, deriving from said line-scanning impulses signal impulses at a secondfrequency which is an odd sub-multiple of twice .the line-scanning frequency, and developing from said impulses at said second frequency signal impulses at a frame-scanning frequency and initiating alternately after an even and an odd number of said line-scanningiimpulses.

ALAN HAZELTINE.