US2284714A

US2284714A - Television system

Info

Publication number: US2284714A
Application number: US180493A
Authority: US
Inventors: Alda V Bedford
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1937-12-18
Filing date: 1937-12-18
Publication date: 1942-06-02
Anticipated expiration: 1959-06-02

Description

June 2, 1942. A. v. BEDFORD TELEVISION SYSTEM 7 sheets-sheet 1 Filed Dec, 18, 1937 :Snvmtor Bedford (Ittorneg June 2, 1942. A. v. BEDFORD TELEVISION SYSTEM Filed Dec. 18, 1957 7 Sheets-Sheet 2 nventor im@ 'uedfo nz Gttorneg June 2, 1942- A. v, BEDFORD n 2,284,714

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Delay and 'hafptng Circalls farlanhing 117 122 cagna/v7' +-l --l L '7 sheets-sheet e A. V. BEDFORD TELEVISION SYSTEM Filed DeG. 18, `193'7 112 -M1171 M- 10o l* 1a 294ai lua l l v 60N CIRCUITS F'OH .BLFNKNG' P l f l 14o 16o :Sunentor ford l l i l 147 F2619. l V l M 'Hwa-V531 www@ June 2, 1942. A. v. BEDFORD TELEVISION SYSTEM Filed Dec. 18, 1937 7 Sheets-Sheet 7 Patented June 2, 1942 'rELEvIsIoN SYSTEM Alda.v V. Bedford," Collingswood, N. J., assigner to `Radio Corporation oi America, a corporation oi Delaware Application December 18, 1937, Serial No. 180,493

' (ci. 17a-6.a)

g 8 Claims.

My invention relates to television systems and particularly to` thefeature of providing synchro- `msm between theV scanning at the television transmitter and the scanning at the television receiver.

In television systems it is the usual practice to `maintain synchronism betweenA the transmitter and the receiver by transmitting horizontal synchronizing impulses at the end of each scanning l line and vertical synchronizing impulses at the end of `each pictureframe, all the synchronizing impulses being `combined with the picture signals and `transmitted as a composite signal.

In orderfthat the synchronizingsignals may be separated from the picture signals at the re.4

ceiver, the synchronizing impulses aremade of f greater amplitude than picture signals of like polarity. Thus amplitude separation may be employed by means ofl a separating or clipping" tube.

After the picture signals have been removed it is the usual practice to separate the horizontal synchronizing impulses from the vertical synchronizing `impulses either by amplitude selection or by frequency selection.

When amplitude selection is employed it has been customary to transmit the vertical synchronizing impulses with a greater `amplitude than the horizontal synchronizing impulses or vicel versa.` While this method gives very exact synchronizing, it is undesirable because itis uneconomical of the transmitter power, too much power being utilized for ,synchronizing signals and not enough for picture signals.

Frequency `separation of horizontal synchronizing impulses and vertical synchronizing impulses is satisfactory from the transmitter power standpoint but it has been found that it does not give as exact synchronization as is desired, especially when the picture is being interlaced. This is due largely to the fact that, in order to obtain frequency` separation, 4the vertical synchronizing impulses are of much' longer duration than the horizontal synchronizing impulses and that the vertical synchronizing impulses are supplied to an integrating circuit at the receiver whereby the desired separation results. One way of improving the operation of.a system of this type is described and claimed in my United States Patent Noil 2,192,121, issued February 27, 194,0, entitled Television systems and the method of operation thereof," and assigned to the Radio Corporation of America.

An object of my invention is to provide Van improved method of and means for synchronizing scanning at the receiver.

the scanning at a picture `transmitter with the A further object of my invention is to provide a method'of and means for synchronizing the transmitter and receiver of `a television system which combine the advantages of the above-mentioned methods of synchronizing impulse vselection or separation without their disadvantages.

A further object of my invention is to provide a method of and means for obtaining very exact synchronizing in a television system without being wasteful of transmitter power.

A further object of my invention is to .provide4 a method of and means for obtaining complete isolation of horizontal synchronizing impulses and vertical synchronizing impulses.

A still further object of my invention is to provide improved'interlaced scanning for television systems.

A still further object of my invention is to provide an improved television transmitter.

A still further object of my invention is to `provide an improved television receiver.

A still further object of` my invention is to provide an improved means for and a method of producing recurring impulses each having a front side of different slope than its back side.

In a preferred embodiment of my invention l produce horizontal synchronizing impulses andl `vertical synchronizing impulses which differ from each other in that in one group of impulses each impulse has a steep front side and a gradually sloping back side while in the other group of impulses each impulse has a steep back side and a. gradually sloping front side. For example, each horizontal synchronizing impulse may have a steep front `while each vertical synchronizing impulse has 'a steep back side. Preferably, the two groups of impulses have the same amplitude whereby transmitter power is conserved just asin the case of frequency separation of synchronizing impulses. i

At the receiver, after removal oi picture'signals in a conventional manner, the horizontal and vertical synchronizing impulses are passed through a differentiating circuit whereby the steep front edge of a horizontal synchronizing impulse produces a sharp positive impulsewhile the steep back edge of .a vertical synchronizing chronizing impulses may then be separated from each other and impressed upon their respective deecting circuits for maintaining synchronism between transmitter and receiver. Or, as will be more fully explained hereinafter, after differentiation the two groups of synchronizing signals may be impressed upon the deilecting circuit oscillators without separation provided the polarity of the synchronizing signals is reversed for one deecting circuit. In other Words, certain oscillators, such as blocking oscillators, will synchronize only on positive impulses, the presence of negative impulses not being detrimental.

The invention will be better understood from the following description taken in connection with the accompanying drawings, in which Figure 1 is a block diagram of a television transmitter embodying my invention,

Figure 2 is a view of the` synchronizing and blanking impulse generating disc shown in Fig. 1,

Figure 3 is an enlarged view of the upper and lower sections of the disc shown in Fig. 2,

Figures 4 to l1 are curves showing the shapes of the synchronizing impulses and also showing how they are combined with the picture and blanking signals,

Figure 12 is a circuit diagram of a portion of the transmitter shown in Fig. 1,

Figures 13 and 14 are circuit diagrams oiy two television receivers constructed in accordance with different embodiments of my invention,

'Figure 15 is a circuit diagram showing one way of synchronizing a defiecting circuit in a system embodying my invention,

Figure 16 is a circuit diagram of an electronic generator for producing synchronizing signals in accordance with my invention,

Figures 17 and 18 are curves which are referred to in explaining a modification of the invention shown in Fig. 16, f

Figure 19 is a circuit diagram showing a modiiication of a portion of the circuit shown in Fig. 16, and

Figure 20 is a circuit diagram of another electronic generator for producing synchronizing signals in accordance with my invention.

Before describing the transmitter shown in Fig..l, it will be helpful to refer to Fig. 7 which shows the shape of the horizontal synchronizing impulses indicated at H and of the vertical synchronizing impulses indicated at V. It will be seen that each of theimpulses H has a steep front and a, gradually sloping back While the impulse V has a gradually sloping front and a steep back. It will be noted that the H and V impulses are of the same amplitude and that an H impulse has substantially the same duration and energy content as a V impulse. Yet, at the receiver the horizontal synchronizing impulses and the vertical synchronizing impulses may be separated if the signals shown in Fig. 7 are passed through a differentiating circuit. This produces signals of the character shown in Fig. 10. It is apparent that here the horizontal synchronizing impulses and the vertical synchronizing impulses are of opposite polarity and may be separated from each other by amplitude selection.

Referring now to Fig. 1, I have shown merely by way of example, a transmitter in which a disc I rotated by a motorv I I is employed for producing synchronizing impulses of the desired shape. Near the periphery of the disc I0 there are openings for producing the synchronizing impulses. The other row of openings is for producing blanking impulses. 'I'he shapes and spacings of the openings will be described later in connection with Figs. 2 and 3.

y'I'o produce the horizontal synchronizing impulses, a source of light I2, a mask I3 having a slit therein, and a suitable optical system form a vertical slit of light on the row of synchronizing openings. As an opening rotates past this slit of light an amount of light dependent upon the shape of the opening passes therethrough and impinges upon a photoelectric cell I4 to produce voltage impulses of the desired shape at the input of an amplifier I6. i

Blanking impulses are produced in a similar manner by means of a light source I1, a slotted mask I8 and a suitable optical system which form a vertical slit of light on the row of blanking openings. Thus an amount of light dependent upon the shape of a blanking opening reaches a photoelectric cell I9 whereby voltage impulses of the desired shape and duration are supplied to the input of an amplifier 2|.

The device for generating the picture signal is illustrated as a cathode ray transmitter tube of well-known design. It comprises a highly evacuated envelope 22 having therein a cathode 23, a grid 24, a first anode 26 and a second anode 21 for producing an electron beam. Envelope 22 also contains a mosaic 28 of lphotoelectric elements which are insulated from each` other and from a metallic back plate 29.

An optical image of the subject to be transmitted is projected upon the mosaic 28 whereby an electrostatic image is formed thereon. Thus, when the mosaic is scanned by the cathode' ray, picture signals are produced which appear across an output resistor 3| connected between the back plate 29 and the second anode 21. These pictures are then amplied by a picture amplifier 30 and supplied to a radio transmitter 31.

'I'he cathode ray may be caused to scan the mosaic 28 by means of horizontal and vertical deiiecting coils 32 and 33, respectively, which are supplied with saw-tooth current from their

respective deiiecting circuits

34 and 36.

The horizontal deflection and the vertical defiection of the cathode ray of the transmitter tube are made to occur in the proper time relation by means of the synchronizing impulses supplied from amplifier I6 over a conductor 38 to a differentiating circuit 39 in the output of which appear signals of the character shown in Fig. 10.

The deflecting

circuits

34 and 36 may be of any suitable design, each deiiecting circuit including an oscillator such as a blocking oscillator. A de-v iiecting circuit of this character is described in Tolson Reissue Patent 20,338.

Since a blocking oscillator is pulled into synchronism by impulses of positive polarity, the output of the differentiating circuit may be irnpressed directly upon the horizontal defiecting circuit whereby the positive horizontal synchronizing impulses control the horizontal deflection.

By passing the output of circuit 39 through a polarity inverter 4I, such as a single amplifier tube, the vertical synchronizing impulses are given a positive polarity whereby they, when impressed directly upon the vertical deecting circuit, control the vertical deflection.

Suitable differentiating circuits will be described in detail hereinafter.

Blanking signals from the output of amplifier 2I are supplied over a conductor 42 to the grid 24 of the cathode ray transmitter tube for blocking the cathode ray during .the lhorizontal return period and during the vertical return period.

Since the synchronizing and blankingfsignals are to be transmitted together with the picture signals to a receiverfthe blanking signals 4are `supplied through an amplifier 43 to a suitable ampliiier stage in the picture amplifier 30 for reture signals and for producing pedestals upon which the synchronizing impulses are'` tov be located.

Thev synchronizing impulses through an amplifier 44` toa later stage in the picture amplifier` 30 where they are added to the picture'and blanking signals.

vThe way in which picture, blanking and synchronizing signals `are added is illustrated in Figs.,4'to 8. 'I'he picture signal appearing across resistor 3| is shownin Fig. 4 at 46. It is mixed with the blanking signals shown` in Fig. 5 whereby undesired transients lli'l are removed from the region of the picture signals. These transients are next clipped off to produce the signal shown are supplied moving undesired transient signals from the pica horizontal synchronizlngfrequency of 13,230 impulses per second. On the drawings,` no .attempt has been made to draw the disc openings in the proper scale to include all 441 openings H1 v on a disc of the size shown.

Referring ,now to the openings for producing the blanking impulses of the character shown in Fig. 5,

openings

52 and 63 are opposite openings l openings are located slightly ahead of thesynin Fig. 6 consisting of picture signals 46 and pedi estals 48 and 49 which, at the receiver, provide horizontal return lineand vertical return line blanking, respectively.

The synchronizing signals shown in Fig. 7 are then added tothe signal shown in Fig. 6 to produce `the complete signal shown in Fig. 8this complete signal being used to modulate the radio transmitter carrier.

The above-described method of combiningl picture signals, pedestals and synchronizing impulses is described and claimed in my abovementioned United States Patent 2,192,121.

Now referring more in detail tc` the impulse generating disc I0, the shapes of the synchronizing and blanking openings and their relative locations are shown in Fig. 2 and in theiragmen- `tary views in Fig. 3. Referring to Figs. 2 and 3,

the two diametrically opposedopeningsVi and Vn for producing the vertical synchronizing irnpulses V shownin Figs. 8 and 9 have asteep front side and a gradually sloping back side, it being noted that the disc lll is rotated in the direction of the arrow. "I'he slit of light which is projected'upon the row of synchronizing openings is indicated at 5i.

'I'he openings H1 which produce the horizontal synchronizing impulses H shown in Figs. 8 and 9 chronizing openings to produce a shelf at the front of the synchronizing impulses, as indicated at 51 and 58 in Fig. 8. 1

While my invention is being described in connection with an interlaced scanning system and is especially `usei'ul in such a system for obtaining exact interlacing, it should be understood that the invention is not limited t0 scanning of this type.

In Fig. 9 there is shown the complete signal as it occurs 1/60 second after the occurrence of the signal shown in Fig. 8, the steep fronts of the imhave steep front sides and gradually sloping back sides.

The relation between thecpenings V1, Vz-and H1 is such as to produce interlaced scanning in the manner described and claimed in United StatesPatent No. 2,152,234, issued March 28, 1939, in the name of R. C. Ballard, entitled "Television systems and assigned to the Radio Corporation of America. In my disc shown in Fig. 2, the steep edge of an opening corresponds to the center of an opening in the disc shown in this Ballard patent.

As taught by Ballard, the interlacing is obtained by having the horizontal synchronizing* "second are transmitted. To produce a 44lline picture, 441 openings H1 are required, this giving pulses H in Fig. 9 for the odd lines falling half way between the steep fronts of the impulses H in Fig. 8 for the even lines.

shall now describe how the synchronizing impulses supplied irom amplifier i6 inFig, l are utilized for synchronizing purposes.

character shown in Fig. 7 and of positive polarity are supplied 'to the differentiating circuit 39. In the specific embodiment shown in Fig. l2, circuit 39 comprises a vacuum tube 6i which/is biased during operation to function as an amplifier. This bias may be grid leak bias maintained by the periodically recurring horizontal synchronizing impulses which are of positive polarity at the input of tube 6i. Or the control grid of tube iii may be properly biased by means of a biasing battery or other suitable biasing source.

Plate voltage may b e supplied to the plate of tube 6I through a plate resistor 62.

The synchronizing impulses appearing across resistor 62, which are as shown in Fig. 7 but ampliiled vand inverted in polarity, are impressed across a diierentiating circuit consisting of a `1 condenser 63 of small capacity and a resistor 64 of comparatively low resistance. The resistance of plate` resistor 62 should be low compared with the resistance of resistor` 64 whereby the current flow through resistor 64 is determined mainly by the impedance of condenser 63. The resistance of 62 may be Vgn of the resistance of 64, for example. The time constant of the circuit 62- 63--64 should be short compared with the duration of each horizontal synchronizing impulse supplied `over conductor 38. For example, this time constant may be 1/5 of the duration of such an impulse. It will beunderstood that the values are not critical and that exact or complete diierentiating is not required. Substantial differentiation is all that is required.

The resulting signal appearing across the resistor 64 is asV shown in Fig. `10. The action of the differentiating circuit will `be seen by comparing Figs. 7 and l0 which are in the same time Referring to Figs. l and l2, synchronizing impulses of the relation where Fig. 7 shows the signal which is being impressed upon the input circuit of a tube having a diierentiating circuit in its output circuit. It will be seen that the steep front of impulse H causes a negative impulse 66 of substantial amplitude while the gradually sloping back of impulse H causes a positive impulse 61 of small amplitude.

On the other hand, the steep back of impulse V causes a positive impulse 68 of substantial amplitude while the gradually sloping front of impulse V causes a negative impulse 69 of small amplitude.

Where interlacing is employed, the impulses occurring lfm second later are in the time relation shown in Fig. 9 and these impulses produce l across resistor 64 the impulses shown in Fig. 11. Since, as indicated by the legend in Fig. 12, the vertical impulses 68 have been made to appear with a positive polarity at the high potential end of resistor 64, they may be applied directly to the blocking oscillator (not shown) in the deilecting circuit 36, the negative horizontal impulses having no effect on the oscillator.

As indicated at 4| an amplifier tube 69 reverses the polarity of the diiierentiated'signals whereby the horizontal impulses 66 are applied with a positive polarity to the blocking oscillator (not shown) in the deiiecting circuit 34, the negative vertical impulses having no effect upon the oscillator.

In Fig. 13 there is shownv a receiver designed for the reception of the signals radiated by the transmitter shown in Fig. l. It comprises a suitable radio receiver 1| which demodulates the receved carrier wave and supplies the composite signal shown in Figs. 8 and 9 to a cathode rayA receiver tube 12 through

ampliiier tubes

13 and 14.

From the output circuit of tube 13, where the synchronizing impulses are of positive polarity, the composite signal is fed over a conductor 16 to a separating tube 11 which clips the signal at the level indicated by the dotted line 18 inFigs.

8 and 9 to pass only the synchronizing impulses.

Separating tube 11 preferably is grid leak ,biased as described and claimed in United States Patent No. 2,207,839, issued July 16, 1940, in the name of W. A. Tolson, and assigned to the Radio Corporation of America.

In this embodiment of the invention the plate circuit, of tube 'l1 includes an inductance coil 19 which, at the highest frequency components of the synchronizing impulses, has an impedance which is low compared with the plate impedance of tube 11. With this impedance relation of coil 19 and tubes 11, the current ow through coil 19 is substantially independent of the coil impedance whereby the current impulses through coil 19 have the same wave shape as the voltage synchronizing impulses on the control grid of tube 11. y f

Since the voltage appearing across coil 19 is equal to di Ld: f

where L is the coil inductance and where i is the current flowing through the coil, it is apparent that the voltage across coil 19 is the derivativeA of the voltage impressed upon the control grid of tube 11. Y

Thus, impulses like those shown in Figs. 10 and 11 are supplied to the deflectlng circuits. In the vertical `deilecting circuit the positive vertical impulses 68 (Figs. l0 and 11) are impressed directly upon the grid circuit of a blocking oscillator 8| which is part of the vertical detlecting circuit.

The oscillator 8|, which is adjusted to oscillate uncontrolled at a slightly lower frequency than the rate at which impulses 68 occur, is pulled in step by impulses 68 whereby sharp negative impulses of large amplitude are supplied to a suitable saw-tooth current generator 82 which makes up the remainder of the vertical deiiecting circuit for causing a flow of saw-tooth curernt through vertical deecting coils 83.

I'he negative synchronizing impulses 66 (Figs. 10 and 11) are reversed in polarity by ampliiier tube 61 whereby positive horizontal synchronizing impulses are supplied to the blocking oscillator 84 of the horizontal deflecting circuit. Oscillator 84 impresses sharp negative impulses upon the saw-tooth current generator 86 to cause a ilow of saw-tooth current through the horizontal deflecting coils 81.

The above-described vertical and horizontal deflecting circuits may be designed in accordance with theteachings of Tolson Reissue Patent 20,- 338 and/or United States Patent No. 2,101,520, issued December 7, 1937, as a joint invention of W. A. Tolson and J. R. Duncan, and assigned to the Radio Corporation of America.

It Willbe understood that my invention is not limited to any particular type of deflecting circuit. For example, a multivibrator or a gas tube oscillator may replace the blocking oscillator, or the synchronizing impulses may be amplified sufciently to be impressed directly upon the sawtooth generator without the use of an oscillator. Obviously electrostatic deflection may be utilized inr place of electromagnetic deflection.

In some cases it may be desirable to isolate the horizontal synchronizing impulses 66 and the vertical synchronizing impulses 68 from each other before they are supplied to their respective deflecting circuits. This may be done as shown in Fig. 14, Where parts corresponding to those in Fig, 13 are indicated by the same reference numerals;

In this circuit the separating tube operates the same as the separating tubes shown in Figs. 12

and 13 while the differentiating circuit is the 'Y same as shown in Fig. 12, comprising the small capacity condenser 88 and the resistor 89.

The differentiated signals, having the polarity indicated by the legend, are suppliedy to a negatively biased clipping tube 9i in the vertical deflecting channel whereby only the positive vertical synchronizing impulses appear in the clipping tube output circuit. These impulses are then reversed in polarity or inverted by a tube 92 whereby they are supplied with a positive polarity -to the deecting circuit 8 l-82.

In the horizontal deflecting circuit, the differentiated signals are inverted by a tube 93 whereby positive horizontal synchronizating impulses are supplied to a clipping tube 94. Only horizontal synchronizing impulses appear in the output of tube 94. As they are of negative polarity, they are inverted by a tube 96 before being impressed upon the deflecting circuit 84-86.

It will be understood that the diiferentiated synchronizing impulses may be applied in various ways to an oscillator for pullingit intol synchronism. For example, as shown in Fig. 15, a blocking oscillator of the type described in the above-mentioned Tolson patent may comprise a screen grid tube 91 having its cathode control1 grid' and p1ate` `connected "in 'the `usual way to `form av blocking oscillator.' The screen 'n'grid 98 is utilized forsynchronizing purposes, the

diierentiated'synchronizing impulses being imsigned to the Radio Corporation of America. As"

described in the said patent, a main oscillator |00, operating at double the horizontal scanning frequency, supplies voltage impulses to a chain of frequency dividers |02 and |03 to produce.`

rectangular impulses occurring at the picture frame frequency of 60 per second as indicated at |04.

The main oscillator |00 also supplies voltage impulses to an oscillator |06 which supplies rectangular voltage impulses, indicated at |01, occurring at one half the main oscillator `frequency. In the generator illustrated, the horizontal and vertical synchronizing impulses occur at the proper frequencies to produce a 44l-line interlaced picture.

The several oscillators may beof any suitable type such as multivibrators or blocking oscillators. y

The desired horizontal synchronizing impulses having steep front and `gradually sloping backs are produced as follows:

The impulses |01 are passed through a differentiating circuit comprising a small capacity condenser |08 and a resistor `|09 in series," the resistor |09 being connectedas the grid leakhof a vacuum tube The resulting voltage wave appearing on the grid of tube is shown at ||2. l i

The tube is so biased and the amplitude of the input signal ||2 is such that only the positive polarity impulses occurring at the rate of 13,230 per second appear in the plate circuit of tube their polarity being inverted as indicated at I I3. l

Impulses` I3 are impressed upon the grid of Va vacuum tube |I4 to produce a saw-tooth voltage comparatively large capacity connected across the plate circuit. The saw-tooth voltage appearing across condenser H6 is indicated at ||1. It will be seen that the voltage wave ||1 is produced by the` condenser H6 being charged through the plate resistor H8` while vthe tube ||4 is driven to cut-olf `byimpulses H3, and

being discharged through tube m `during the remaining period. It will be understood that the.v 60 saw-tooth voltage I|1 may be produced inother well-known ways.

In accordance with my invention the `voltage wave ||1 is supplied to the input of a vacuum tube ||9 which is properly biased to pass only thev peaks ofthe wave ||1. That is, the wave |11 is cupped at the level indicated by the dotted line |2I, to produce in the plate circuit of tube 9 the voltage impulses indicated at |22.` It will be seen that they are recurring impulses ing steep fronts and sloping backs.

The tube H9, like the other clipping tubes in the circuit, may be suitably biased by a battery the' ieaerfmeesef 'Basins 'Being employed in, the

electronic generator4 illustrated. k

Preferably. the peaks ofithe impulses |22 Vare clppedoff bygmeansY of another clipping `tube |23 to produce the final horiz` ntal synchronizv ing,` impulses indicated at H.

The positive impulses |32 are impressed upon the grid of a vacuum tube |33 `having a condenser |34 of comparatively large capacity connected across its plate circuit. As a result, the saw-tooth voltage indicated at appears across the condenser I34. It will be noted that this sawtooth voltage has a sloping front and a steep back," just the opposite of the saw-,tooth ||1 produced in the other branch of the circuit.

The saw-tooth voltage |36 is produced by the condenser |34 being charged through the plate 30 resistor |31 between the positive impulses |32,

during which timetube |33 is biased substantially to cut-ofi' by grid leak biasing, and then being discharged through tube |33 for the dura` tion of a positive `impulse |32. Of course,` a

suitable saw-tooth voltage may be produced in any well-known manner.

The saw-tooth wave |36 is impressed upon a tube |30which is biased to clip the wave at the level indicated by the dottedline |39, thus 40 passing only the positive peak of each sawtooth. `The resulting-I impulses which appear in the plate circuit of tube |38 are indicated at |4I. It will be noted that their steep side is on the back of the wave rather than on the front of the Wave, as in the case of the horizontal impulses |22.

As in the case of the horizontal impulses, the peaks of the vertical synchronizing impulses |4| are preferably clipped off by a clipping tube |42 to produce the ilnal vertical synchronizing ine-its plate circuit due to a condenser ||6 of impulses indicated at V.

It maybe notedthat the synchronizing impulses H and V are not drawn to the same scale and that, because of this, the impulses V appear much closer together as compared with the spacing of the impulses H than they actually are.

The impulses H and'V are added or combined by providing the tubes |23 and |42 with a common plate resistor |43. The resulting signal is indicated at |44 where the combined impulses H and V are shown in substantially their correct time relation. It will be understood that an amount, if necessary, as taught by United States Patent No. 2,132,655, issued `October 11, 1938, in the naine of J. P. Smith, and assigned to the Radio Corporation of America. having a time interval between them and hav- Since in the above-described method of producing synchronizing impulses only a very small (not shown) or byemeans of grid leak biasing, through the clipping tube. This. is especially The vertical synchronizing impulses are 'produced as follows:

true in the case of the vertical impulses where only about lzoo of a saw-tooth is utilized.

Less accuracy in clipping the horizontal sawtooth voltage is required if the coupling oondenser |46 between tubes ||4 and ||9 is made of such small capacity that the low frequency components of the saw-tooth are greatly attenuated. 'This causes a voltage of the wave A shape shown in Fig. 18 to be supplied to clipping tube ||9 which clips the wave at the level indicated by the dotted line |41.

In producing the vertical synchronizing impulses, the saw-tooth may b e distorted as shown in Fig. 17 by accentuating the low frequency components of the saw-tooth.` This may be done by inserting between tubes |33 and |38 at the point indicated by the dotted line |30 the circuit shown in Fig. 19. Referring to Fig. 19, the sawtooth wave appearing across condenser 134 (Fig. 16) is impressed upon an ampliiler tube |40 which has in its plate circuit a illter comprising a resistor |48 and a condenser |49. The amplifier tube |50 inverts the saw-tooth wave whereby the tube |30 (Fig. 16) clips'it at the level indicated by the dotted line in Fig. 17. The low fre-` quency components may be accentuated the desired amount by making condenser |49 of the proper capacity.

'I'he desired blanking signals for the television system may be provided by suitable delay and shaping circuits represented by the blocks |52 and |53. Such circuits are described in the above-identied Smith patent. Y

Another suitable electronic generator for producing synchronizing signals in accordance with my invention is shown in Fig. 20. This specific generator is described and claimed in United States Patent No. 2,166,688, issued July 18, 1939, in the name of R. D. Kell, entitled Television apparatus, and assigned to the Radio Corporation of America.

Referring to Fig. 20, there is shown another generator for a television system which produces a 441line interlaced picture. It will be understood that the apparatus shown in Fig. 20, like the generator shown in Fig. 16, is located at the television transmitter and supplies horizontal and vertical synchronizing impulses to the defleeting circuits of both the cathode ray transmitter tube and the cathode ray tube at the receiver.

'Ihe horizontal and vertical synchronizing impulses are derived from a common source such as a multivibrator or other suitable oscillator 20|. A second multivibrator 202 divides the frequency of the oscillator 20| by two to produce impulses at the line frequency, 13,230 cycles per second in the specific system'being described.

A chain of multivibrators 203,204 and 206 divide the' frequency of oscillator 20| to produce impulses occurringat the picture frame frequency of 60 per second. Itwill be understood that the 60-cycle and 13,230-cycle impulses having the desired odd-line relationship for interlacing may be obtained in any other suitable way as by means of a rotating disc having suitable openings therein, as dscribed and claimed in Patent 2,152,234, issued March 28, 1939, inthe name oi' R. C. Ballard, entitled Television systems, and assigned to the Radio Corporation of America, or by means of blocking oscillators as described and claimed in Patent 2,178,218, issued October 31 1939, in the name of A. V. Bedford, entitled Television systems, and assigned tothe Radio Corporation of America.

The portion of the circuit which produces the horizontal synchronizing impulses will first be described. The saw-tooth voltage wave, which is to be combined with a rectangular impulse, is produced in this speciilc example by supplying a rectangular voltage impulse indicated at 205 from the oscillator 202 over a conductor 201 and through a resistor 203 to a condenser 209. 'I'he condenser 209 is connected across the input electrodes of a screen grid tube e and is shunted by a resistor 2| the resistor 2|| having a high resistance, such as one megohm, as compared with the resistance of resistor 208. In practice, the resistor 208 may be the plate impedance of one of the multivibrator tubes.

Condenser 209 is charged suddenly through resistor 200 and discharged gradually through resistor 2| whereby a saw-tooth voltage wave indicated at 2|2 is impressed upon the control grid of tube e. Stated diierently, the

circuit

209, 2|| integrates the impulses 205 to produce the sawtooth wave 2|2.

The rectangular voltage which is to be combined with the saw-tooth wave is produced, in this specic example, by supplying rectangular voltage impulses indicated at 2|0 from oscillator 202 to a vacuum tube c through a diierentiating circuit comprising a condenser 2| 3 of small capacity and a resistor 2|4 of comparatively low resistance, the resistor 2 4 being connected across the input electrodes of the tube c. It will be understood that impulses 2 |0 occur at the rate of 13,230 per second. A differentiating circuit of this character and its action is described and claimed in the above-mentioned Smith Patent No. 2,132,655.

'I'he derivative of impulse 2| 0 appears on the grid of tube c as indicated at 2|6.

Since tube c is adjusted to act as a clipping tube, only the upper impulse portion of wave 2||` passes through the tube. This portion appears inverted in the plate circuit of tube c, as indicated at 2|1, and is impressed upon a clipping tube d whereby a square-topped voltage wave appears in the output of tube d as indicated at 2|0. It will be noted that, by differentiating and clipping, the impulse 205 has been narrowed but not delayed.

Impulses 2|8 are combined with the saw-tooth waves 2|2 after impulses 2|8 have been suitably delayed in a delay network 2|9 to make the two groups of signals combine in the correct time relation. The desired addition of signals is obtained by impressing the delayed impulses 2|8 upon the screen grid of tube e. 'I'he combined signal, after being inverted by a tube f, is of the character indicated at 22|.

Signal 22| is supplied to a clipping tube g, which clips signal 22| at the level indicated by the dotted line to produce the desired impulse, indicated at 222, having a steep front side and a gradually sloping back side.

'The delay network 2|9 may be of any suitable design such as a conventional capacity-inductance network of recurrent sections, or it may be a network comprising vacuum tubes and wave shaping networks oi' the character described in the above-mentioned Smith United States Patent No. 2,132,655. In certain cases it will be preferable to insert the delay network 2|9 ahead of tube c in order that the delay network will not injure the wave shape obtained by the clipping action of tubes c and d.

Referring now to that portion of the circuit which produces the vertical synchronizing impulscs having a steep back side and a sloping front side, rectangular voltage impulses occurring at the framing frequency of 60 cyclesper` second and indicated at 225 `are supplied from oscillator 206 over a conductor 226, through -a resistor 221 to a `condenser' 228 which is connectedacross the input electrodes of vacuum tube e. Condenser 228 is shunted by a resistor 229having comparativelyhigh resistance. Thus, asaw-tooth voltage wave is suppliedto the grid of tube e as indicated at 23l.- It will be seen that tube ef corresponds to tube e in the horizontal synchronizing circuit.

e Similarly, the vacuum tubes c. d', `f and g" now to be described correspond to tubes c, d, f and g in the. horizontal synchronizing circuit. e

The rectangular voltage impulse to be added to the saw-tooth 23| is obtained by taking from oscillator 206 a. rectangular voltage impulse indicated at 232,whichoccurs at the frame frequency i of 60 per second, `and supplying it to a differenafter further clipping by tube `d appears in the plate circuit of tube d' as a narrowed rectangular impulse indicated at 231.

i Impulses 231 are delayed by a delay network 238 which may be of the same type as delay net- 'work 2|9. The delayed impulses 231 are added that impulses 231 are delayed is such as to make them add on the front side of the saw-tooth wave rather than on the back side as is the case in the horizontal synchronizing circuit. Thus. after being inverted by tube f', the combined signal appears in the output circuit of tube j', as indicated at 239. This signal is then clipped bythe tube g', as indicated by the dotted line, to produce the desired signal having a steep back side and a sloping front sideyas shown at 24|.

The vertical synchronizing signals 24| and the horizontal synchronizing signals 222 are com bined` in the common plate circuit of tubes g and g', the combined signals then have `their sharp peaks/clipped oil by aclipping tube h whereby there appears in the output circuit 'of tube h the desiredsignal of the character shown at 242.

From the foregoing description, it will be apparent that various modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and are set forth in the appended claims.

I claim as my invention:

1.` In a picture` transmitting system, the method of synchronizing scanning at the receiver with scanning at the transmitter which comprises transmitting one group of synchronizing impulses each having a steep front side and having a gradually sloping back side and transmitting another group of synchronizing impulses each having a steep back side and a gradually sloping front side; intercepting said impulses at the receiver and substantially i diierentiating them, and utilizing the differentiated signals for synchronizing the scanning at the receiver.

2. The method of `synchronizing the scanning of a view to be transmitted with the scanning at a receiver for receiving said view which compulses occurring at a relatively high frequency and, transmitting a grcupof synchronizing impulses occurringat a relatively low frequency, the

impulses'in `one `of said groups each having a -1 steep front side and `a Ygradually sloping back side,`the impulses in `the other of said groups each having a steep lback side and a gradually sloping front side, intercepting both groups of said impulses at said receiver and substantially differentiating them whereby the steep sides of said impulses produce impulses of greater amplitude than the gradually sloping sides of said impulses and whereby the said impulses of greater amplitude produced by one of said groups of impulses are of opposite polarity to the said impulses of greater amplitude producedby the other of said groups of impulses, andutilizing the said impulses of greater amplitude for synchronizing the scanning atgsaid receiver.

3. In a picture transmitting system, a picture transmitter including means `for transmitting picture signals, means for generating and transmitting a group of recurring synchronizing impulses each having. av front side which is'` steep as compared withits backside, means for simultaneously generating and transmitting another group of recurring synchronizing impulses each having a back side which is steep as compared with its front side, a picture receiver for receiving said picture signals andsaid groups of impulses, said receiver including means for differentiating said impulses whereby impulses of maximum amplitude and of a certainV polarity are derived from one of said groups and wherebyimpulses of maximum amplitude and of the opposite polarity to said certain polarity are derived from the other of said groups, and means for utilizing said im- 4 pulses of maximum amplitude for synchronizing the' scanning at the receiver with the scanning at the transmitter.

4. A generator of synchronizing signals comprising means for producing a group of saw-tooth Waves occurring at a comparatively high frequency, each of said waves having a front slope which differs irom its back slope, means for clipping said waves near their peaks and passing said peaks to an output circuit, means for producing another group of saw-tooth waves occurring at a comparatively low frequency. each of said lastmentioned waves having a front slope which differs from its back slope, the ratio of the front slope to theback slope of the waves in one of said groups being greater than unity and the ratio` ofthe front slope to the back slope of the waves in the other of said groups beingless than unity, and means for clipping said low frequency waves near their peaks and passing said peaks to said output circuit. i

5. In a television transmitting system, means for developing picture signals, means for developing line synchronizing signals having a leading edge of predetermined slope and a trailing edge of a different slope, means for developing a. frame synchronizing signal having leading and trailing edges'of different slopes, the slope of the leading and trailing edges "of the frame synchronizing signal being different from the leading and trailing edges of the line synchronizing signal, and

prises transmitting a group of. synchronizing `iment slope, means for developing a frame synchronizingsignal having leading and trailing edges of diil'erent slopes which are diierent from the slopes of the leading and trailing edges of the line synchronizing signal, means for combining the picture, line and frame synchronizing signal, means for transmitting the combined wave, comprising means for developing and transmitting said carrier wave comprising means for generating a video-frequency wave of one polarity, means for developing a composite synchronizing signal of the other polarity including line-synchronizing and eld-synchronizing impulses having ieading and trailing edges of a predetermined difference in slope, the corresponding diierences in slope of the line-synchronizing and held-synchronizing impulses being opposite in sense. means for modulating said carrier with said video-frequency wave and said composite synchronizing signal, means for receiving and utilizing said carrier comprising amplitudelimiting means for separating said synchronizing impulses from said composite signal, and means for deriving from the leading edges of one type of synchronizing impulses characteristic\synchronizing pulses and for deriving from the vtrailing edges of the other type of synchronizing vimpulses different characteristic synchronizing pulses.

8. A television synchronizing system, comprising means for developing synchronizing impulses of a first predetermined frequency and having leading edges of steeper slope than their trailing edges, means for developing synchronizing impulses of a second predetermined frequency and having trailing edges of steeper slope than their leading edges, one of said predetermined frequencies beinga fractional multiple of the other, and means for combining said synchronizing impulses of both of said frequencies to develop a composite synchronizing signal.

' ALDA V. BEDFORD.