US2491804A - Synchronizing system - Google Patents

Description

1949 H. B. FLEMING ETAL 2,491,804

SYNCHRONIZ ING SYSTEM Filed Nov. 29, 1946 2 Shets-Sheet 1 Pi .I. 1 3

TELEVISION 6 RECE'VER LsvucHRomzme sYNcHRomzme VERTICAL SIGNAL PULSE DEFLECTION CLIPPER SEPARATOR '1 GENERATOR I5 HORIZONTAL DEFLECTION GENERATOR 35 1 H t2; LsAwTom H I: -1. l0 WAVE GENERATOR 22 2 Inventors: H ugh B.FIeming, George M.BTOWT|,

Their Attorney.

2 1949 H. B. FLEMING ET AL 2,491,804

SYNCHRONIZING SYSTEM Filed Nov. 29, 1946 2 Sheets-Sheet 2 F i2 3. l5 r' TO HORIZONTAL DEFLECTION GENERATOR SAWTOOTH a WAVE R GENERATOR l 34 55 FROM 9 SYNCHRONIZING l PULSE SEPARATDR Fig.4.

Inventors: Hugh 13. Fleming, George M. Bvown,

g m DM Their Attorney.

Pat ented Dec. 20, 1949 UNITED STATES PATENT OFFICE SYNCHRONIZING SYSTEM Application November 29, 1946, Serial No. 712,998

Claims. 1

Our invention relates to synchronization of a source of saw tooth wave energy with periodic energy pulses such as those from a television transmitter.

In present-day cathode ray television systems, periodic high energy pulses are transmitted as successive scanning cycles are commenced. These pulses are then used at the receiver to trigger the sweep circuits of the cathode ray tube used for image presentation, thereby causing the position of the ray to correspond with scanning operations at the television transmitter and the image on the tube screen to reproduce the visual program.

It is an object of our invention to provide improved means to synchronize a sawtooth wave generator in response to periodic energy pulses.

It is further an object of our invention to provide synchronization of a wave generator in response to applied energy pulses in a manner which prevents false operation from random noise pulses.

Still another object of our invention is to synchronize the sweep circuits of a cathode ray tube in a manner that permits a plurality of sweep cycles for each synchronizing pulse while at the same time providing uniform sweep cycles and preventing random noise pulses from triggering the sweep circuits.

A further object of our invention is to synchronize a sawtooth wave generator in a manner permitting it to be rapidly synchronized after it is started and at the same time to reduce to a minimum the possibility of triggering by random noise pulses.

Briefly, our invention resides in stabilizing the frequency of a multivibrator by two separate control signals: (1) a received control signal, which may for example be derived directly from a television receiver output, and (2) a control signal derived from a phase sensitive system. The former signal is fed directly to the multivibrator in a manner tending to cause it to change from one operating condition to the other, and the multivibrator voltage is utilized to trigger a sawtooth sweep generator each time a synchronizing pulse is received. The latter signal, which has a magnitude dependent on the phase relationship between the sawtooth sweep wave and the applied synchronizing pulses, is also applied to the multivibrator in a manner to vary the natural frequency at which that unit changes from one operating condition to the other. Under starting conditions, the control signal derived directly from the receiver output causes the multivibrator to come into synchronism rapidly while under steady state conditions the control signal from the phase sensitive system maintains the frequency and phase of the multivibrator at the correct value despite random noise pulses that may be present in the receiver output. The sawtooth generator output may then be used to cause synchronized scanning of the cathode ray tube beam, as is well known to the art.

The features of our invention which we believe to be novel are set forth with particularity in the appended claims. Our invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which Fig. 1 is a schematic diagram of a television receiving system incorporating synchronizing circuits employing the principles of our invention; Fig. 2 shows Wave forms illustrating the operation of the circuit of Fig. 1; Fig. 3 is a schematic diagram illustrating a second embodiment of our invention, particularly suitable for use where a plurality of sweep cycles are desired between successive synchronizing pulses; and Fig. 4 shows wave forms illustrating the operation of the circuit of Fig. 3.

To aid in understanding our improved synchronizing system it is shown in Fig. 1 in its application to a television receiving system, the conventional portions of which are shown only in simplified schematic form, since their details form no part of the invention. Thus, television carrier signals received by antenna l are supplied -to television receiver 2 which may conventionally comprise the high frequency amplifier, oscillator, intermediate frequency amplifier, demodulator and video amplifier circuits of a wide-band superheterodyne receiver. The detected video signals are supplied over conductor 3 in well-known manner to the intensity control element of a cathode ray tube 4 and also to the usual synchronizing signal clipper 5 and synchronizing pulse separator 6 from which the horizontal and vertical synchronizing pulses are derived. These pulses are supplied to synchronize the horizontal and vertical deflection generators l and 8 respectively, the outputs of which are applied to the corresponding deflecting elements of tube 3 to produce the picture scanning action as is wellunderstood in the art.

Apparatus embodying our invention, now to be described, is shown diagrammaticall in Fig. 1 between the separator B and the deflection generator l in the horizontal synchronizing circuits. The horizontal synchronizing pulses from separator 6 are coupled through capacitor 9 to a multivibrator I0. This multivibrator may be any one of the well-known types having a free-runhing or natural frequency of oscillation and capable of being triggered by applied synchronizing pulses. The particular circuit shown in Fig. 1 utilizes the two triode sections 1 la and Nb of electron discharge device II. The anodes of device H are connected through resistors l2 and I3 respectively and conductor M to a suitable source of anode potential, indicated conventionally by B-{. The control grids are connected to a suitable source of bias potential, derived from a potentiometer l6 across the anode supply, through resistors I 8 and I9 respectively. The potentiometer I6 is bypassed for the operating frequency of multivibrator ID by a capacitor IT. The control grid of each tube section I la and I lb is cross-connected with the anode of the opposite section through capacitors 20 and 2| respectively. The grids are also connected to ground through resistors 22 and 23 and a common capacitor 2 3. In addition to the direct current paths to ground from the grids through resistors l8 and I9 and bias potentiometer l5, paths are also provided through resistors 22 and 23 and thence through a common conductor 25 and the anode space path of a discharge device 26, for a purpose presently to be described.

In the multivibrator I0, capacitors 20 and 2| are alternatively charged through anode resistors 18 and I9 and discharged through the grid network just described. As is well-understood in the art, multivibrator l successively passes between two conditions of operation, one corresponding to conduction through first section Ila and the other corresponding to conduction through second section llb. Hence, approximately square- Wave voltages are developed across the anode resistors I2 and 13. The natural, or free-running, frequency of operation of multivibrator H] can be adjusted by the bias potentiometer [6. It is also dependent upon the resistance in th paths from the grids through discharge device 26. Device 23 therefore exercises a control action on the multivibrator frequency, as will become more fully apparent as the description proceeds.

Square wave potentials appearing at the anode of section llb of multivibrator H] are supplied through capacitor 32 to synchronize the operation of a sawtooth wave generator 33. Since many suitable circuits for this purpose are wellknown to the art, generator 33 has been indicated Only in block form to simplify the drawing.

The sawtooth waves from generator 33 are supplied over conductor 34 to a balanced peak detector 35 shown as comprising two duo- diodes 33 and 3? connected in a bridge network. Across one diagonal of the bridge, between points a and b, are connected the sawtooth wave generator output in series with an output impedance comprising potentiometer 38 and capacitor 39 in parallel. Across the other diagonal of the bridge, between points 0 and d, is connected an impedance network comprising a resistor All and capacitor M in parallel, both in series with the secondary winding 42b of a transformer 42. P0- tentials appearing on potentiometer 38 are supplied to the control grid of discharge device 26 through adjustable tap 43.

Synchronizing pulses from separator 6 are also supplied over conductor 50 to the control grid of a discharge device connected in a cathode follower circuit. Pulses appearing across the out put resistor 52 are supplied to the primary winding 32a of transformer 42 through a coupling capacitor 53 for a purpose now to be described.

The operation of the balanced peak detector 35 is known to the art and has been described in detail in an articl beginning on page 7 of the Proceedings of the I. R. E., January 1943, to which reference may be made. Briefly, the four diode elements are so poled that current may flow in either direction between the input and output circuits (i. e. between points a and b) when they are in conducting condition. The sawtooth waves applied to point a from conductor 34 therefore tend to produce equal symmetrical waves in the output and no resultant direct current component at point b. However, auxiliary pulses, corresponding in frequency and phase to the synchronizing pulses, are impressed from transformer secondary 42b across the opposite diagonal cd of the diode bridge in push-pull relation. Current flow due to these pulses is in such direction as to build up a unidirectional potential across resistor 40 and capacitor 4| tending to bias the diodes to nonconducting condition. The Values of resistor 40 and capacitor 4| are chosen to effect peak rectification of the applied pulses, preventing conduction through the diodes and. between points a and 1) except when the synchronizing pulses occur. During the synchronizing interval, capacitor 39 is charged in a sense depending upon the relative phase between the sawtooth and synchronizing waves so as to hold point I) at a direct potential level nearly equal to that of point 0. The potentials applied from transformer secondary 421) do not appear across capacitor 39 because the charge built up across capacitor 3| substantially cancels them from the standpoint of current flow between points a and b\ The balanced peak rectifier 35 may be regarded as a means to develop a unidirectional bias potential at point D dependent upon the phase relationship between the synchronizing pulses and the sawtooth wave. The operation of the circuit of Fig. 1 will be better understood by reference to the wave shapes shown in Fig. 2 on a common time scale. Curve A represents the horizontal synchronizing pulses supplied from the output of pulse separator 6. Curve B represents the corresponding output wave from multivibrator I0 when perfect frequency and phase synchronization exist. Curve C illustrates the shape and phase of the sawtooth wave supplied to point a from generator 33 under such conditions. It will be observed that the retrace portion 44 of curve C is midway between its maximum and minimum values at the time the leading edges of the synchronizing and multivibrator pulses occur. Inasmuch as it is this voltage that determines the voltage of point b, that point has a corresponding voltage under these conditions. If, however, the operation of multivibrator I0 is delayed with respect to the synchronizing pulses of curve A, each positive portion of wave B will occur at a later instant of time than is shown in curve B. Curve D illustrates such a condition. In this case the output wave from generator 33 is also shifted slightly in phase as shown in curve E, this voltage now having peak magnitude at the instant each pulse of wave A is applied. Hence, the potential at point I) is greater than in the condition or normal phase synchronism. Should the operation of multivibrator 10 lead in phase oped across them is substantially constant between successive synchronizing pulses.

A selectable portion of the control potential acrossresistor 43 is supplied to the control grid of discharge device 26. As shown in Fig. 1, device 26 is connected as a direct current amplifier to provide a controllable resistance between point e and ground. As more positive bias is applied to the grid of amplifier 26, its effective resistance between point e and ground is decreased. This efiectively decreases the time constant of the discharge network for capacitors 20 and 2| in multivibrator l0, increasing its free-running frequency and thereby counteracting the lagging phase condition which produced the increased voltage across capacitor 39. Reverse action takes place upon reduction in bias due to a leading phase condition. Hence, the sawtooth wave generator 33, the balanced detector 35 and the direct-current amplifier 26 comprise a closed, phase-control system tending to maintain synchronism between multivibrator l and the applied synchronizing pulses.

The pulses from multivibrator I0 are supplied over conductor [5 to synchronize the horizontal deflection generator I in well-known manner.

It will be observed that the phase-control system just described could be utilized to effect horizontal synchronization if capacitor 9 were omitted and synchronizing pulses were supplied over conductor 50 to the phase-control circuit alone. However, the synchronizing action of the phase-control circuits alone is relatively Weak when the system is far from synchronism, for successive synchronizing pulses appear at greatly difierent points on the sawtooth wave. Ordinarily, this would render the entire system very slow in achieving synchronism on starting, thereby causing an undesirable delay while the oscillator is slowly pulled into step. In accordance with our invention this difficulty is avoided by the use of a supplementary synchronizing system which gains control during the starting period but is not as efiective as the phase control circuits under normal operation. In the circuit shown in Fig. 1, we achieve this supplementary synchronization by the use of the coupling condenser 9 which also applies a small direct synchronizing pulse to multivibrator l0. Inasmuch as this pulse exerts a substantial synchronizing effect even when the unit is first started, normal operation is quickly obtained and the delay otherwise associated with the phase-control network is avoided. Since the direct synchronizing voltage applied through condenser 9 is made relatively small, random noise voltages from separator 6 do not cause multivibrator l0 to make a false change. Hence, freedom from nois triggering, which is characteristic of the slow acting phase synchronizing system, is obtained without excessive synchronizing time requirements.

An alternate embodiment of our invention is shown in Fig. 3, in which elements corresponding to those to Fig. l have been given corresponding reference numerals. The conventional portions of the television receiver, shown in block form in Fig. 1, have been omitted from Fig. 3 and only the features essential to a clear understanding of this form of synchronizing system have been shown. This embodiment differs from that of Fig. 1 in that the multivibrator I0 is designed to operate at a frequency which is a multiple of the synchronizing pulse frequency, and a gear multivibrator 60 is added. The multivibrator 60 is conventional. It is similar to multivibrator I0 in circuits and operation but is notprovided with the frequency controls. Its circuit constants are adjusted to make its natural frequency of operation nearly equal to the frequency of the synchronizing pulses from separator 6. Pulses from multivibrator I0 are supplied through capacitor 6| to cause multivibrator 6D to look into step with multivibrator I0.

Figure 4 shows wave shapes on a common time scale illustrating the operation of the circuit shown in Fig. 3. Curve A, Fig. 4, shows the synchronizing pulses from pulse separator 6, as before, and curve B shows the voltage wave developed by multivibrator I0. In this case, multivibrator lll is shown as making two cycles for each synchronizing pulse. Other ratios may of course be used. Curve C shows the output of multivibrator 60, this voltage corresponding in frequency to the applied synchronizing pulses. Curve D illustrates the sawtooth wave output from generator 33 under conditions of phase balance. Inasmuch as the latter wave corresponds in frequency to the synchronizing pulses, it may be used to develop a frequency stabilizing voltage when applied to the balanced peak rectifier, just as in the case of Fig. 1.

The embodiment of our invention shown in Fig. 3 has a number of advantages. In the first place, the number of synchronizing pulses is reduced by the frequency step-up used in multivibrator IU. This reduces proportionally the radio-frequency power requirements for these pulses, thereby reducing the required transmitting capacity. In addition, the interval between successive synchronizing pulses is increased over that which would otherwise be required. This further reduces the possibility that noise and undesired random transients will trigger the system, thereby providing a more satisfactory image under adverse conditions of noise.

A further advantage of the circuit of Fig. 3 over other synchronizing circuits resides in the fact that the pulses from multivibrator I0 following each synchronizing pulse are of uniform frequency. This results from the fact that under steady conditions the phase control system exercises primary control over the multivibrator frequency and changes that frequency until synchronization is achieved. If only the direct synchronization through capacitor 9 were provided, the synchronizing pulses would be effective only to trigger the multivibrator and subsequent oscillations would take place at the free running frequency of that unit until occurrence of the next synchronizing pulse. Hence correspondingly poor synchronization would be obtained.

It is possible to operate the system of Fig. 3 without the gear multivibrator 60, but in this case the system is somewhat less stable than that shown in the figure.

In the circuits of Figs. 1 and 3, tube 5| is connected in a cathode follower circuit. This connection permits matching the impedance of transformer 42 to the synchronizing pulse in ut, which is important to provide optimum operation of the system and minimum distortion of the pulse wave applied to the balanced rectifier 35.

In both Figs. 1 and 3, the amplifier 26 is shown as shunted by an adjustable resistor 21 and a capacitor 28 in series. The purpose of resistor 21 and capacitor .28 paralleling the feedback loop in Figs. 1 and 3 is to provide a damping circuit to prevent hunting of the system. Capacitor 24, in parallel with this damping circuit further reduces the effect of noise components and stabilizes operation of the system. Proper selection of these capacitors and adjustment of resistor 21 provides a system that is highly stable in operation.

While we have shown and described our invention with reference to particular embodiments thereof, it will be obvious to those skilled in the art that changes and modifications may be made without departing from our invention in its broader aspects. For instance, the particular form of balanced rectifier 35 shown in the drawings may be replaced by other known circuits providing similar performance. In some cases it may be desirable to omit the coupling amplifiers 26 and While our invention has been shown in its application to horizontal synchronizing circuits of a television receiver, it obviously may also be applied to the vertical synchronizing circuits or to other forms of pulse apparatus. We aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. A synchronizing system comprising, in combination, a source of periodic synchronizing pulses, a square wave generator having a freerunning frequency adapted to be synchronized with said pulses, a sawtooth wave generator adapted to be synchronized by said square wave, means for supplying said square Wave to synchronize said sawtooth generator, means comprising a phase detector responsive to both said synchronizing pulses and said sawtooth waves for developing a unidirectional control potential dependent upon their relative phase, means for altering said free-running frequency of said square wave generator in response to said potential in a sense tending to maintain said potential constant, and means for additionally synchronizing said square wave generator directly from said synchronizing pulses.

2. A synchronizing system comprising, in combination, a source of synchronizing pulses of predetermined frequency, a multivibrator for generating control pulses and having a free-running frequency adapted to be synchronized with said predetermined frequency, a generator of sawtooth waves operating at substantially said predetermined frequency, means for synchronizing said sawtooth wave generator from said control pulses, means comprising a phase detector responsive to the time phase between said synchronizing pulses and said sawtooth waves for controlling the free-running frequency of said multivibrator, means for additionally synchronizing said multivibrator directly from said synchronizing pulses, and output means utilizing said control pulses.

3. In combination, a source of periodic voltage pulses, a multivibrator having a first operating condition and a second operating condition and a natural frequency of change between said operating conditions, said natural frequency being of value to cause said multivibrator to tend to operate through a plurality of cycles for each of said pulses, means responsive to said pulses tending to cause said multivibrator to change from said first operating condition to said second operating condition when said pulses take place, means to generate a sawtooth voltage wave: each time said multivibrator changes from said first operating condition to said second operating condition, a phase detector, means for impressing both said periodic voltage pulses and said sawtooth wave on said phase detector, means comprising said phase detector to generate a unidirectional voltage dependent on the voltage of said sawtooth wave at the instant said pulses occur, and means responsive to said last voltage to alter the natural frequency of said multivibrator in a sense to cause said sawtooth wave to occur in synchronism with said pulses.

4. In combination, a source of periodic voltage pulses, two multivibrators each having a first operating condition and a second operating condition and a natural frequency of change between said operating conditions, the natural frequency of one of said multivibrators being approximately equal to an integral multiple of the frequency of said pulses and the natural frequency of the other of said multivibrators being approximately equal to the frequency of said pulses, means tending to cause said first multivibrator to change from said first operating condition to said second operating condition when said periodic voltage pulses take place, means tending to cause said second multivibrator to change from its first operating condition to its second operating condition when said first multivibrator changes from its first operating condition to its second operating condition, means to generate a sawtooth voltage wave each time said second multivibrator changes from its first operating condition to its second operating condition, a phase detector, means for impressing both said periodic voltage pulses and said sawtooth wave on said phase detector, means comprising said phase detector to generate a unidirectional voltage dependent on the voltage of said sawtooth wave at the instant said pulses occur, and means responsive to said last voltage to alter the natural frequency of said first multivibrator in such sense as to cause said sawtooth wave to occur in synchronism with said pulses.

5. In a synchronizing system responsive to a synchronizing pulse wave of predetermined frequency, the combination of a square wave generator adapted to be synchronized by said pulse wave and having a higher normal operating frequency equal to an integral multiple of said predetermined frequency, means for producing a periodic voltage wave normally recurring at said predetermined frequency and synchronized rigidly with said square wave, a phase detector, means for impressing both said synchronizing pulse wave and said periodic voltage wave on said detector, means comprising said phase detector for producing control potentials dependent upon the time phase between said synchronizing pulse wave and said periodic voltage wave, means utilizing said potentials for varying the normal operating frequency of said square wave generator in such sense as to maintain it in synchronism with said periodic voltage wave, means for additionally impressing said synchronizing pulse wave on said square wave generator directly to assist in maintaining said synchronism, and utilization means responsive to said control pulse wave.

HUGH B. FLEMING. GEORGE M. BROWN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,141,343 Campbell Dec. 2'7, 1938 2,277,000 Bingley Mar. 17, 1942

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