US2231829A - Television synchronizing system - Google Patents

Description

Feb. 11, 19H41. M, Ewl`s v2,231,829

TELEVISION sYNcx-moNIzING SYSTEM Filed April 29, 19:58 2 sheets-sheet 1 INVENTOR H ROLD ,LLEWIS BY ATTORNEY Feb. l1, 1941. H, M Ew1s l 2,231,829 i TELEVISION SYNCHRONIZING SYSTEM Filed `April 29, 1938 2 Sheets-Sheet 2 FIG-.2.

Am LLILVH'Q'ALLUU- g I l H n T l l v T INVENTOR H ROLD M. LEWYIS ATTORNEY Patented Feb. 11, 194i TELEVISION SYNCHRONIZmG SYSTEM nai-01a M. Lewis, oms Neck, N. Y.. minor u Hazcltine Corporation, a corporation of Dela- 4ware Application April zo, 193s, serial No. 204.95m

12 Claims.

'Ihis invention relates to television systems and more particularly to synchronizing apparatus for such systems. The invention is especially concerned with the development of an improved syn- 6 chronizing signal.

In accordance with present television practice, there is developed and transmitted a signal which comprises a carrier wave modulated during successive intervals or trace periods by video-frequency components representative of the light and shade values of an image being transmitted. During retrace intervals between the trace periods, the carrier wave is modulated by synchronizing impulses or components which correspond to the initiations' of successive lines and ields in the scanning of the image. At the receiver, a beam is developed and so deflected as to scan and illuminate a target in a series of parallel lines or elds. The video-frequency components of the received 20 signal are utilized to control the intensity of the beam. The lineand held-scanning synchronizing components are separated from the videofrequency components and from each other and utilized to synchronize the operation of the re- 25 ceiver scanning apparatus with the similar scanning apparatus utilized at the transmitter in developing the signal. The transmitted image is thereby reconstructed on the target of the receiver.

In scanning of the interlacedjtype, the linescanning and field-scanning frequencies are so related that successive elds are staggered, the' lines of one'eld falling between or interlacing those of a preceding eld, and, due to persistence of vision, the optical eiect produced is as though each frame or group of fields comprised a multiple of the actual number of lines scanned per iield and the frame-scanning frequency equalled the eld-scanning frequency.

scanning, and synchronizing methods and apparatus heretofore have been proposed. The type of synchronizing signal required ordinarily depends in part upon the type of carrier signal employed and the type of scanning utilized. For example, in certain systems negative modulation iskemployed; that is, a decrease'in carrier amplitude corresponds to an increase in illumination; while in other systems positive modulation is utilized, in which an increase-in carrier amplitude corresponds to an increase in illumination. The synchronizing components of the signals used in these systems ordinarily have amplitude values outside of the range of the video-frequency components, being represented by increases in carrier Various types of carrier-signal modulation,

(ci. 11s-aa) v amplitude where negative modulation -is employed and by decreases in carrier amplitude where positive modulation is employed. v

In interlaced scanning systems, certain of the field-synchronizing impulses must be initiated be- 6 l tween successive line-synchronizing impulses. In4

order that the lineand field-synchronizing impulses may be successfully separated from the video-frequency components and from each other for utilization, various types of synchronizing siglo nals and separating apparatus have been proposed. yMore particularly, the line-synchronizing impulses have been given a certain critical wave form characteristic while the field-synchronizing impulses have been given a critical wave form 15 characteristic which corresponds to, but is different from, that of the line-synchronizing impulses.

Suitable means are provided at the receivers which are responsive to. these critical characteristics for separating the vfield-synchronizing impulses from 'the line-synchronizing impulses.

In various systems of the prior art, synchronizing signals have been utilized in -which the eldsynchronizing impulsesl have amplitudes, durations, .or shapes which are different from the corresponding characteristics, that is, lthe amplitudes, durations, or shapes, respectively, of the linesynchronizing impulses. In the ilrst case. amplimdc-discriminating circuits are employed as the separating apparatus at the receivers; in the second case, duration-responsive circuits, such as integrating or time constant networks, are utilized; while in the third case, diierentiating circuits are generally employed.

Thus, in order to utilize synchronizing signals from a given transmitter, a receiver must include the particular type of synchronizing apparatus responsive to the particular critical characteristic of the synchronizing signal utilized by such transmitter. That is, a given receiver has been capable of reproducing the programs of only such stations as transmit synchronizing signals of the particular type for which it is designed. Obviously, this materially limits the use of a receiver, since the various stations within a given range of reception may transmit synchronizing signals of different types.

It is an object, therefore, of the present invention to provide a television system including a synchronizing-signal-generatng apparatus capable of developing and transmitting an improved synchronizing signal which is utilized by receivers including ,synchronizing-signal-separating apparatus designed to respond to synchronizing signals having critical wave form characteristics of various types.

In accordance with the invention, a television broadcasting system comprises a plurality of receivers, certain of which comprise synchronizingsignal separating apparatus designed to respond to synchronizing signals having individual critical wave form characteristics of different types, together with a television transmitter synchronizing-signal generator including means for developing line-synchronizing impulses, each having a plurality of predetermined critical wave form characteristics. Means are also provided in the generator for developing held-synchronizing impulses, each having a plurality of predetermined critical wave form characteristics, corresponding to, but different from, those of the line-synchronizing impulses, together with means for combining the developed line-synchronizing-and fieldsynchronizing impulses with like polarity to develop and transmit a composite synchronizing signal to synchronize all of said certain receivers.

'For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the accompanying drawings, Fig. 1- is a schematic circuit diagram of a complete television transmitting apparatus embodying the invention; and Fig. 2 is a group of curves illustrating the wave forms of various synchronizing signals to aid in the understanding of the invention.

Referring now more particularly to Fig. l of the drawings, there is illustrated a television transmitting system comprising a signal generator Il), which may be of a conventional design including the usual cathode-ray signal generating tube and scanning elements. For developing scanning voltages or currents for the generator I0, there are provided a line-frequency saw-tooth wave generator I I and a held-frequency saw-tooth wave generator I2, the output circuits of these generators being connected to the scanning elements of the signal generator I inv the conventional manner. In order to block out the cathode ray of the generator I0 during the retract scanning periods, there is provided a block-out wave generator I3 having its output circuit suitably connected to the signal generator III. For providing impulses to suppress undesirable signal impulses during retrace scanning periods and to ensure proper form of the modulation signal to be developed, there is provided a pedestal impulse generator I4 and, in order to vdevelop a scanning synchronizing signal in accordance with the present invention, there is provided synchronizingsignal-generating apparatus indicated generally at I5 and hereinafter more fully described.

For the purpose of synchronizing the generators I I-I4, inclusive, and the apparatus I5, there is provided a timing impulse generator I6 coupled to the input circuits of the generators II-I4, inclusive, and apparatus I5. The timing impulse generator I6 is preferably stabilized by means of `a connection Il to a suitable source of periodic video-frequency amplifiers I8, I9 and 20, a modulator 2| and associated coupled carrier-frequency oscillator 22, a power ampliiier 23, and an antenna system 24, 25, all according to conventional practice. The output circuit of the pedestal generator I4 ls coupled to the video-frequency amplifier I9, while the output circuit of-'the synchronizingsignal-generating apparatus I5 is coupled to the video-frequency amplifier 20.

Neglecting for' the moment the details of the synchronizing-signal-generatingA apparatus I5, the system just described comprises the elements of a television transmitting system of conventional design-and, the various parts thereof being oi.' any well-known construction, a detailed description of the general system and its operation is unnecessary. Brieily, however, the image of a scene to be transmitted is focused on the target of a cathoderay tube of the signal generator I0, in which tube a cathode ray is developed, focused and accelerated toward the target in the usual manner. Scanning or deflectlng currents or voltages developed by the generators II and I2 are applied to the scanning elements of the generator III to provide electric fields which serve to deiiect the cathode ray horizontally and vertically, thereby to scan successive series of parallel lines, or elds, upon the target. The deflecting currents or voltages, and hence the scanning fields, are of wellknown saw-tooth wave form, providing a relatively slow linear trace and rapid retrace. The number `oi lin'es per eld is determined by the relative beam during retrace portions of the scanning` cycles, while pedestal impulses developed by the generator I l are applied to the ampliiier I9 to 'modify the shape of the resultant video-frequency wave developed to aid in separating the line'- synchronizing and frame-synchronizing impulses at the receiver. The synchronizing-impulse signal developed by the apparatus I5 is applied to the modulation amplifier 20 while the timing impulses developed by the generator I6 are applied to the generators II-H, inclusive, and the apparatus I5 to lock these generators and apparatus in synchronism.

'I'he photosensitive elements of the target-in the cathode-ray tube of the generator I4 being electrically affected to an extent dependent upon the varying values of light and shade at the corresponding incremental areas of the image focused thereon, as the cathode ray scans the target, a vdeo-frequency voltage of correspondingly varying amplitude is developed in the output circuit of the generator I0 and applied to the videofrequency amplifier I8, wherein this voltage is amplified and from which it is translated to the amplifier I9. Here the video-frequency voltage is further amplified and mixed with pedestal impulses supplied from the generator I4. The ampliiied and mixed voltages in the output circuit of the amplifier I9 are thereupon applied to the amplifier 20, wherein they are further amplified and mixed with the synchronizing-impulse signal supplied from the apparatus I5. The modulation voltages are thereupon supplied to the modulator 2I wherein they are so impressed upon the carrier wave generated by the oscillator 22 as to develop a modulated carrier, preferably a positively modulated signal such as described above. The resultant modulated-carrier, signal isv delivered to the power amplifier 23 for amplification and is thereafter impressed upon the'antenna system 24,

25 tobe broadcast.

Referring now more particularly to the apparatus I5 embodying the present Invention, for the purpose of developing line-synchronizing impulses of the proper frequency and wave-form characteristics, there is provided a' generator 25 which may comprise apparatus of any suitable type adapted to develop periodic impulses of linescanning frequency, for example, 13,230cycles per second, and-of rectangular-peak -impulse wave form, as indicated by the Jcurve immediately above the generator 25. The input circuit of thisy generator is coupled to the timing generator I5 for synchronization, while its output circuit is coupled to a limiter and amplier 21 which may comprise a suitable conventional circuitarrangement including, for example, vacuum tubes so biased as to cut oi! predetermined portions of the applied waves, for example, the portion shownA above and below the broken lines on the curve above the generator 25.

For the purpose of developing alternate periodic impulses of the same frequency and wave form as the line-synchronizing impulses but displaced 180 degrees with respect thereto, that is, occurring between the line-synchronizing impulses, a generator 28 and a limiter and amplier 25 are provided which may be of the lsame design as the stages 26 and 21, respectively. The generator 28 is also coupled to the timing impulse generator I6 an oscillator circuit is described in detail inI Patent 2,226,706 issued Dec. 31, 1940 to Madison Cawein forV Television scanning system to which reference may be had for details of construction. ,50 The input circuit of vthis generator is coupled to the timing generator I5 for synchronization thereby at the required frequency. It will be noted that the'impulses of the last-mentioned wave initiate at the same time as the line-synchronizing impulses and alternate line impulses developed by the generators I9 and 20 and. if desired, the several generators may be suitably locked together to maintain these relations. These relations are correct for the purpose of developing the nal composite synchronizing wave, as will be apparent from the description of the general operation of the system hereinafter set forth.

A combining network 3l is included in the generator I5 and comprises a line-synchronizing impulse amplifier 32, an alternate-impulse amplier 33 and a field-synchronizing impulse amplifier 34, having their input circuits connected to the output circuits of the limiters and amplifiers 21 and 25 and the generator 30, respectively, as shown. 'Ihe output circuits of the ampliilers 32, 33 and 34 are connected in parallel and coupled to a final limiter 35, whose output circuit is -in turn connected to the video-frequency amplifier 20 for supplying the composite synchronizing signal developed by the generator I5.

For the purpose of controlling the operation of the combining network 3| there are provided delay and duration control circults, including field-frequency generators 35 and 31 designed to develop periodic impulse waves of rectangular wave form, 5 such as are illustrated immediately above the generators 35 and 31, and of iield frequency, for example, 60 cycles. The timing impulse generator I5 is designed to develop in one of its output circuits a periodic voltage wave of this 60-cycle frel0 quency which may be of the wave form illustrated at the left of thev generators 36 and 31 and this-output circuit is connected tow the input circuits of the generators 35 and 31 for synchronizing the same. 15

The timing impulses from timer I6 occur at the initiation of the ileld retrace scanning periods. The rectangular impulses developed by the generators 35 and 31 are of predetermined durations which are the determining factors in the delays 20 to` be obtained. Differentiating circuits 35 and 39 are connected to the output circuit of the generators 35 and 31, respectively, and may comprise any suitable conventional voltage-differentiating circuit arrangements for diilerentiating the rec- 25 tangular-impulse waves developedv by the generators 35 and 31 to derivev therefrom double-impulse waves, such as shown immediately above the eleas shown above, and of tbe same frequency as the wave derived from the timing generator. The circuits 40 and 4I are so coupled to the circuits 38 40 and 39 as to be synchronized by the negative, or second, peaks of each of their respective double impulses; that is, the impulses of the outputs of units 40 and 4I are initiated upon the occurrence of the second or negative peaks of their respective 45 double impulses. The periodic impulse outputs of umts 40 and 4l, therefore, are delayed in phase with respect to the impulses developed by the timing generator I5 by an amount equal to the duration of the rectangular impulses developed by the 50 generators 35 and 31 and are of different predetermined durations, as determined by the duration control circuits 40 and 4I. For a detailed description of circuits suitable for the stages 35, 35, 40 and 31, 39, 4I, reference may be had to the 55 above-mentioned Patent 2,226,706, which discloses the details of a similar circuit arrangement.

` 'I'he impulse voltage outputs of the generators 45 and 4I are applied to the amplifiers 33 and 34, respectively, of the combining network to con- 60 trol the operation of these amplifiers. Preferably, the amplifiers 33 and 34 yare normally biased beyond cutoff so that the signals applied thereto from the ampliers 29 and 30 are not passed thereby, and the control impulses are applied positively from the circuits 40 and 4I so as to unblock the amplifiers 33 and 34, respectively, for periods of predetermined duration, delayed with respect to the original 60-cycle r-0 impulses developed by the timing generator I5, by predetermined amounts, and occurring at the field frequency.

'The general operation of the system may best be explained with reference to the curves of 75 :wave forms of four diil'erent composite lineand field-synchronizing impulse signals, such as have, heretofore been individually employed in various systems oi the prior art. In each case, the line-synchronizing impulses have a single predetermined critical wave form characteristic while the field-synchronizing impulses have a single predetermined critical `wave form characteristic which corresponds to, but is diil'erent from, that of the line-synchronizing impulses. tue of this difference in wave form characteristic that synchronizing-separating apparatus of a receiver designed to respond to the particular type of synchronizing signal is able to separate the field-synchronizing impulses from the linesynchronizing impulses prior to utilizing them for synchronizing its scanning Wave-generating apparatus. In other words, such synchronizingsignal-separating apparatus is generally designed to be responsive only to the particular distinguishing wave form characteristic of a particular type of synchronizing signal and, hence, such a wave form characteristic is referred to herein and in the appended claims as critical.

In curve A, the critical wave form characteristic of the synchronizing impulses is their amplitude, the eld-synchronizing impulses, as indicated at f, being of substantially greater, amplitude than the line-synchronizing impulses indicated at l. The separating apparatus for such a synchronizing signal need comprise only a hunting or amplituderesponsive device, for example, a diode to which the combined signal is applied negatively, and the anode of which is initially so biased as to render the tube conductive only during the occurrence of the field-synchronizing impulses. The result of such arrangement is that only the eld-synchronizing impulses are repeated in the output circuit of the device. Such an arrangement is described in detail in copending application Serial No. 177,663, illed December 2, 1937 by Harold M. Lewis,V for Television scanning systems, to which reference may be had for details of construction.

The critical wave form characteristic of the which develops a signal having impulses with amplitude proportional to the durations of the impulses of the composite synchronizing signal.

vThe impulses of different amplitudes may then be readily separated by a suitable limiting device, as explained above. For details of construction of a suitable duration-responsive device, reference may also be had to copending application Serial No. 177,679, led December 2, 1937 by A. V. Loughren, for Television scanning systems.

Curve C is somewhat similar to curve B in that the field-synchronizing impulses f differ from the line-synchronizing impulses l by being of greater duration. The effective held-synchronizing impulses of this signal may be considered as having a duration such as indicated at g. 'This impulse is of the serrated type however; that is, portions of the impulse are in e'ect cut away to permit the uninterrupted recurrence of the line-synchronizing impulses. The separation of the eldsynchronizing impulses from the line-synchronizing impulses in this case is 'generally accomplished by time-constant circuits oi' the integrat- It is by vlrpulses.

ing type. In order that such circuits shall not be undesirably` aiected by the fact that the phase oi succeeding field-synchronizing impulses relative to their respective preceding line-synchronizing impulses is diiIerent in interlaced scanning systems, for which such signals are generally ernployed, a special separating arrangement must be resorted to. More particularly, for predetermined periods indicated at h, initiating prior to the occurrence of the serrated held-synchronizing impulses, the line-frequency impulses agre doubled, as by the addition of alternate impulses inserted in the composite signal. Since this impulse doubling is initiated just prior to the occurrence of each field-synchronizing impulse, the integrating apparatus is always in substantially the same condition at the commencement oi' all field-synchronizing impulses, regardless of the variations in the phase displacement of alternate held-synchronizing impulses from their respective preceding line-synchronizing impulses which are inherent in interlaced scanning systems. For details of construction oi' a suitable integrating device for separating the tleldfrom the linesynchronizing impulses of this type of signal, reference may be had to British Patent No. 423,685, dated November 3, 1933. Y

Curve D illustrates a composite synchronizing signal in which the critical characteristic of the impulses is their shape. More particularly, this wave includes line-synchronizing impulses which have leading edges of steeper slope than their trailing edges and held-synchronizing impulses which have trailing edges of relatively steeper slope than their leading edges. In order to separate the held-synchronizing impulses from the line-synchronizing impulses of this type of signal, the synchronizing apparatus at the receiver comprises devices ywhich are responsive to the shape or, more particularly, to the slope of the leading and trailing edges of the synchronizing impulses. For example, devices may be employed which diierentiate the composite signal to derive a synchronizing signal in which linesynchronizlng and eld-synchronizing impulses are poled in opposite senses. Hence,'by applying this derived vsignal with the proper polarity to the scanning wave generator independently of the field-synchronizing impulses will eiect synchronization of the field-scanning wave generator independently of the line-synchronizing im- In copending application Serial No. 177,663, above referred to, apparatus suitable for eil'ecting'separation of the lineand fieldsynchronizing impulses of this type of signal is described in detail and reference may be had thereto for details of construction.

It will be understood that, in each of the cases just described, Agenerally the composite wave is differentiated and the line-scanning generator is then excited by the resultant `dierentiated linesynchronizing components so thatthe ileld components do not interfere with the normal operation of the line-scanning apparatus. In certain cases, however, such as that represented by curve D, distinctive derived line-synchronizing impulses are developed and they may be used for synchronization.

The purpose and general results obtained by using the system of the present invention may be more clearly explained by reference to the other curves of Fig. 2.

In Fig. 2, curve E illustrates the wave form of the modulation signal developed by the transmitting system of Fig. 1 including the synchrolby decreases in carrier amplitude.

nizing signal-generating apparatus of the pres-v ent invention. The portion of the curve indicated at represents a field retrace interval. AThe letters w, b, and s represent the respective signal levels corresponding to white, black, and the peak amplitude ofthe synchronizing impulses inthe black direction. If this signal, as shown, is applied to the carrier'wave with proper polarity as modulation, it will be apparent that increases in amplitude of the carrier correspond to increases in illumination of the image, whileV the synchronizing impulses are all represented Such a signal is generally termed a positively modulated signal.

In the operation of the generating apparatus I5, the generators 26 and 2B, synchronized by the timing generator I6, serve to develop periodic rectangular or peaked-impulse waves of linescanning frequency. but with the impulses developed by the generator 28 occurring 180 degrees out of phase with respect to, or midway between, the impulses developed by the generator 26. These waves are delivered to the limiters and :amplifiers 21 and 29, respectively, wherein, by virtue of the limiting and amplifying action, as

has been explained, there are developed signals comprising periodic impulses having leading edges of steeper slope than their trailing edges.

The latter signals are thereupon impressed upon the input circuits of the amplifiers 32 and 3,3 of

' the combining network 3l. The line-synchronizing and alternate impulses areindicated at l and a in the composite signal illustrated by curve E of Fig. 2.

The generator 30 serves to develop a periodic wave comprising impulses of substantially rectangular Wave form, occurring at double the line frequency and initiating with the alternate and line-synchronizing impulses developed in the output circuits of the limiters and amplifiers 21 and 29. This rectangular-impulse wave is impressed upon the input circuit of the amplifier 34. Certain of these rectangular impulses, which serve as the field-synchronizing impulses, are indicated at f on curve E.

The timing impulse generator I6 also develops in one of its output circuits a periodic wave cornprising impulses at the field frequency, cycles in the present case, which occur at the initiation of the field retrace periods, one of which is indicated at a: in the curve E of Fig. 2. The generators 36 and 31, synchronized by these el'd- A and of durations corresponding to the delay periods di and d2, respectively, indicated in curve E; that is, delayed for predetermining periods with respect to the initiation of the field retrace periods.

These rectangular-impulse waves are differentiated by the circuits 38 and 39 to derive the dou ble-impulse waves having the second pulse of each double impulse occurring at the trailing edges of the last-mentioned rectangular impulses. The second pulses of the double impulses are utilized to synchronize the duration control circuits 40 and 4I so that periodic waves are developed therein having impulses of rectangular wave form, initiating with the occurrence of the second pulses of the double impulses and of predetermined diiferent durations. The durations of the impulses developed by the circuit 40 correspond to the period y indicated on curve E while the duration of the impulses developed by the circuit 4I corresponds to the similarly indicated period z.

As explained above, the control impulses developed by the circuits 4U and 4I are applied positively to the amplifiers 33 and 34, which amplifiers are normally biased beyond cutofl', so as to unblock these amplifiers and permit the alternate impulses occurring between the line-synchronizing impulses to be combined with the line-synchronizing impulses during the periods y occuring at the field-scanning frequency and the rectangular field-synchronizing impulses to be combined with like polarity with the line-synchronizing and alternate impulses during the periods z occurring Iduring, but of lesser duration than, the periods y, the composite signal, including the three diiferent types of impulses, being dcveloped in the output circuit of the combining network and applied to the limiter 35. This limiter serves to remove peak pulses corresponding to line-synchronizing and alternate impulses which are superimposed on the rectangular impulses in the signal developed in the output circuit of the network 3I, as shown in dotted lines on curve E, so that the composite synchronizing signal developed in the output circuit of the limiter 31 is of the form illustrated. by the solid line portion x of curve E of Fig. 2. Y It will be seen'that the line-synchronizing impulses and the' fieldsynchronizing impulses of the composite synchronizing signal of Curve E have the different distinguishing wave form characteristics of those of Curves A-D inclusive; that is,they are of different amplitudes, different durations, and have leading and trailing edges of different slopes so that the impulses are of different shapes. This complete synchronizing signal, as explained above, is translated to the amplier 20 wherein it is amplified and mixed with the video-modulav.

Referring now to 4curves F-I, these curves A serve to illustrate how the synchronizing signal of the present invention, as illustrated by curve E, is adapted for use by receivers having the various different types of synchronizing apparatus briefly described above; that is, adapted for utilizing a particular o ne of synchronizing-signal components illustrated by the curves A-D, inclusive.

Consider i'lrst a receiver having synchronizingsignal-separating apparatus designed for a signal such as illustrated by curve A, that is, apparatus responsive to a signal in which the difference in amplitude is the critical wave form characteristic. It will be clear that the line-synchronizing and field-synchronizing impulses of the composite synchronizing signal of the present invention have this characteristic so thatl if this signal is received by the receiver under consideration, its limiting device will operate as described' above in connection with the curve A to derive from the composite synchronizing signal a field-synchronizing signal such as illustrated by curve F.

Second, consider a receiver having synchronizing-signal-separating apparatus designed to utilize a signal such as illustrated by curve B,

that is, responsive to a signal in which the difference in duration is the critical wave form characteristic. Since the signal of the present invention also has this characteristic, if such signal is received by the second receiver, its synchronizing-signal-separating apparatus will operate, as described above in connection With curve B, in response to the diferences in durations of the line-synchronizing and field-synchronizing impulses, to develop a signal such as illustrated by curve G and this derived signal may then, if desired, be applied to a limiting device to remove the line-synchronizing impulses.`

Again, consider a third receiver having synchronizing-signal-separating apparatus designed to utilize a signal such as illustrated by curve C, having field-synchronizing impulses of greater duration than the line-synchronizing impulses, with alternate impulses inserted during part of the field retrace period. Here again, the composite synchronizing signal of the present invention includes the same critical characteristic as the signal C. It includes eld-synchronizing impulses of relatively long duration as well as additional alternate impulses. When received by the third receiver, therefore, the signal of the present invention will be applied to integrating circuits of its synchronizing-signal separator, as described above in connection with curve C, and a field-synchronizing signal such as illustrated by curve H is developed thereby. This derived signal may then be utilized in the conventional manner to synchronize the held-scanning apparatus independently of the line-synchronizing impulses, by virtue of the greater amplitude of the portion of the wave of curve H corresponding to the held-synchronizing impulses of the signal of curve E..

Finally, consider a fourth receiver which is designed to utilize a synchronizing signal such as illustrated by curve D; that is, one in which the critical wave form characteristic is the shape, specifically the relative steepness, of the leading and trailing edges of the lineand field-synchronizing impulses. Here again, it will be apparent that the synchronizing signal of the present invention includes the particular critical wave `form characteristics in question. That is, the leading edges of the line-synchronizing impulses are relatively steep compared to their trailing edges, while both the trailing and leading edges of the field-synchronizing impulses are relatively steep compared to trailing edges of the linesynchronizing impulses. In this case, therefore, when the composite synchronizing signal of the present invention is received by the fourth receiver, it is applied to the differentiating device of the synchronizing-signal-separating apparatus and a signal is derived thereby having a wave form such as is illustrated by curve I. As explained above in connection with curve D, by applying the derived synchronizing signal with proper polarity, the field-scanning generator may be synchronized independently of the linesynchronizing impulses.

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

What is claimed is:

1. A television broadcasting system comprising, a plurality of receivers, certain of which comprise ,synchronizing-signal separating apparatus designed to respond to synchronizing signals having individual critical wave .form characteristics of different types, a television transmitter synchronizing-signal generator including means for developing line-synchronizing impulses each having a plurality of predetermined critical wave form characteristics, means for developing field-synchronizing impulses each having a plurality of predetermined critical wave form characteristics corresponding .to but different from those of said line-synchronizing impulses, and means for combining said line-synchronizing and held-synchronizing impulses with like polarity to develop and transmit a composite synchronizing signal -to synchronize vall of said certain receivers.

2. A television broadcasting system comprising, a plurality of receivers, certain of which comprise synchronizing-signal separating apparatus designed to respond to synchronizing signals having individual critical wave form characteristics of different types, a television .transmitter synchronizing-signal generator including means for developing line-synchronizing impulses each having a predeterminedv amplitude and duration, means for developing field-synchronizing impulses each having a predetermined ampliltudev and duration different from the corresponding characteristics of said line-synchronizing impulses, and means for combining said line-synchronizing and field-synchronizing impulses with like polarity yto develop and transmit a composite synchronizing signal to synchronize all of said certain receivers.

3. A .television broadcasting system comprising, a plurality of receivers, certain of which comprise synchronizing-signal separating apparatus designed to respond to synchronizing signals having individual critical Wave form characteristics of diierent types, a television .transmitter synchronizing-signal generator including means for developing line-synchronizing impulses of a iirst predetermined frequency, each having a predetermined amplitude and duration, means for developing other impulses of double said predetermined frequency, each having a predetermined amplitude and duration different from the corresponding characteristics of said line-synchronizing impulses, means for developing alternate impulses at said predetermined frequency and occurring between said line-synchronizing impulses, means for combining said line-synchronizing and said alternate impulses during predetermined periods occurring at a. predetermining held-scanning frequency, and means for combining said other impulses with said linesynchronizing and alternate impulses during predetermined periods occurring during, but of less duration than, said rst named periods and for transmitting the resultant signal to synchronize said certain receivers.

4. A television broadcasting system comprising, a plurality of receivers, certain of which comprise synchronizing-signal separating apparatus designed to respond to synchronizing signals having individual critical Wave form characteristics of different types, a television transmitter synchronizing-signal generator including means for developing line-synchronizing impulses, each having a predetermined amplitude nizing impulses, each having a. predetermined amplitude and shape diilerent from the corresponding characteristics of said line-synchronizing impulses, and means for combining said linesynchronizing and eld-synchronizing impulses with like polarity to develop and transmit a composite synchronizing signal to synchronize all of said certain receivers.

5. A television broadcasting system comprising, a plurality of receivers', certain of which comprise Asynchronizing-signal separating apparatus designed to respond to synchronizing signalshaving individual critical wave form characteristics of different types, a television .transmitter synchronizing-signal generator including means for developing line-synchronizing impulses each having a predetermined amplitude, shape and duration, means for developing eldsynchronizing impulses, each having a predetermined amplitude, shape and 4duration different from the corresponding characteristics of said line-synchronizing impulses. and means for combining said line-synchronizing and ield-synchronizing impulses with like polarity to develop and transmit a composite synchronizing signal to synchronize all of said certain receivers.

6. A television broadcasting system ycomprising, a plurality of receivers, certain of which comprise synchronizing-signal separating apparatus designed to respond to synchronizing signals having individual critical wave form characteristics of different types, a television .transmitter synchronizing-signal generator including means for developing line-synchronizing impulses of a first predetermined frequency, each having a predetermined shape, amplitude and duration, means for developing field-synchronizing impulses of double said predetermined frequency, each having a predetermined amplitude, shape and duration diilerent from those of said line-synchronizingimpulses, means for ldeveloping alternate impulses at said predetermined frequency occurring between said line-synchronizing impulses, means vfor combining said line-synchronizing and said alternate impulses during predetermined periods occurring at a predetermined fleld-scanning frequency, and means for combining said eld-synchronizing impulses with said line-synchronizing and alternate impulses during predetermined periods occurring during, but of less duration than, said rst-named periods and for transmitting the resultant signal to synchronize said certain receivers.

7. A television broadcasting system comprising, a plurality of receivers, certain of which comprise synchronizing-signal separating apparatus designed to respond to synchronizing signalsv having individual critical wave form characteristics of different types, a television .transmitter synchronizing-signal generator including means for developing line-synchronizing impulses each having a predetermined duration and shape, means for developing held-synchronizing impulses, each having a 'predetermined duration and shape different from the corresponding characteristics of said line-synchronizing impulses, and means for combining said line-synchronizing and field-synchronizing impulses to develop and transmit a composite synchronizing signal to synchronize all of said certain receivers.

8. A television broadcasting system comprising, a plurality of receivers, certain of which comprise synchronizing-signal separating apparatus designed to respond to synchronizing signals having individual critical wave form characteristics of dlierent types, a television transmitter synchronizing-signal generator including means for developing line-synchronizing impulses of afirst predetermined frequency, each having a predetermined shape and duration, means for developing eld-synchronizing impulses of double said predetermined frequency, each having a predetermined shape and duration diiierent from the corresponding characteristics of said line-synchronizing impulses, means for developing alternate impulses at said prede- .termining frequency occurring between said linesynchronizing impulses, and means for combining said line-synchronizingand said alternate impulses during predetermined periods occurring at a predetermined eld-scanning frequency, and means for combining said field-synchronizing impulses with said line-synchronizing and alternate impulses during predetermining periods occurring during,` but of less duration than, said firstnamed periods andfor transmitting the resultant signal to synchronize said certain receivers.

9. A television broadcasting system comprising, a pluralityv of receivers, certain of which comprise synchronizing-signal separating apparatus designed to respond to synchronizing signals having individual critical Wave form characteristics of different types, a television'transmitter synchronizing-signal generator including means for developing line-synchronizing impulses of predetermined duration, each having an edge of a predetermined slope, means for developing eld-synchronizing, impulses, veach havl ing a different duration from said line-synchronizing impulses `and having an edge corresponding to said edges of said line-synchronizing impulses of a predetermined different slope, and

Ameans for combining said line-synchronizing and' field-synchronizing impulses with like polarity to develop and transmit a composite synchronizing signal to synchronize all of said certain receivers.

10. 'A television broadcasting system comprising, a plurality of receivers, certain of which comprise synchronizing-signal separating apparatus designed to respond to synchronizing signals having individual criticalwave form characteristics of different types, a television .transmitter synchronizing-signal generator including means for developing line-synchronizing irnpulses of predetermined amplitude, each having an edge ofV a predetermined slope, means for developing field-synchronizing impulses each having a different amplitude from said line-synchronizing impulses and having an edge corresponding to said edges of said line-synchronizing impulses of a predetermined diierent slope, and means for combining said line-synchronizing and field-synchronizing impulses with like polarity to develop and transmit a composite synchronizing signal to synchronize all of said certain receivers.

11. A television broadcasting system comprising, a plurality of receivers, certain of which comprise synchronizing-signal separating appar-atus designed to respond to synchronizing signals having individual critical wave form characteristics of diierent types, a television .transmitter synchronizing-signal generator including means for developing line-synchronizing impulses of predetermined duration and amplitude, each having an edge of a predetermined slope, means for developing field-synchronizing 'im-- pulses, each having a different duration and amplitude from said line-synchronizing impulses and having an edge corresponding to said edges of said line-synchronizing impulses of a predetermined different slope, and means for combining said line-synchronizing and ileld-synchronizing impulses with like polarity to develop and nals having individual critical wave form char,V

acteristics of diilerent types, a television transmitter synchronizing-signal generator including means for developing line-synchronizing impulses of a rst predetermined frequency and duration, each having an edge of a predetermined slope, means for developing field-synchronizing impulses of double said predetermined frequency and of durations different from those of said line-synchronizing impulses, each having an edge corresponding to said edges of said line-synchronizing impulses and of a predetermined different slope, means for developing .alternate impulses at said predetermined frequency similar in duration to said line-synchronizing impulses, means for combining said alternate impulses and saidlinesynchronizing impulses duringpredetermined periods occurring at a predetermined field-canning frequency, and means for combining said l0 eld-synchronizlng impulses with said line-synchronizing and alternate impulses during. predetermining periods occurring during, but of lless duration than, said tlrst periods and for t nsmitting the resultant signal to synchronize said 715 certain receivers.

HAROLD M. LEWIS.

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