US2229964A - Television synchronizing system - Google Patents

Description

a 1941- R. B. DOME ETAL 2,

TELEVIS ION SYNCHRONI Z INCT SYSTEM Filed 001;. 26, 1958 Inventors: Robert B. Dome,

Robert E. M e, y JV a) T-he 'r Attorneg.

tented Jan. 28, 1941 .1 lTE STATES mater PATEN OFFW TELEVISION SYNCHRONIZING SYSTEM Application October 26, 1938, Serial No. 237,025

13 Claims.

Our invention relates to an improved synchronizing arrangement for a television system and more particularly to a circuit arrangement for separating in such systems the vertical from the horizontal synchronizing impulses.

In television systems involving operations of incremental picture area traversal at the transmitter and receiver, it becomes necessary by whatever method such operations are performed to insure that the periodicity of a complete traversal is the same at both the transmitter and the receiver. The prevalent use of the cathode ray type of tube for the purpose of converting the image to be transmitted into electrical impulses and for reconverting the electrical impulses into a visible image at the receiver has led to the practical transmitting on the carrier wave the picture or so-called video signal component and, in addition, a synchronizing component which after separation from the picture component may be used to synchronize the timed scanning movement of the electron stream in the cathode ray tube of the receiver with that of the cathode ray tube of the transmitter. Since the horizontal and vertical synchronizing impulses are transmitted on the same carrier wave, it becomes necessary to separate them in the television receiver in order that each may be television system a greatly simplified and im proved arrangement for separating the vertical from the horizontal synchronizing impulses.

A further object of our invention is to provide a separating circuit for the synchronizing impulses which operates to give a materially improved accuracy in the synchronized operation of the vertical oscillator employed in the television receiver. I

Another object of our invention is to provide a separating circuit for the synchronizing impulses which does not require the use of any equalizing pulses and one which, therefore, does not depend for the accuracy of its separation upon the transmission of any synchronizing impulses preceding the vertical synchronizing impulses.

An additional object of our invention is to provide a synchronizing impulse separating circuit which operates to accomplish in a complete and clean-cut manner the entire separation of the vertical synchronizing impulses from the horizontal synchronizing impulses.

The novel features which we believe to be characteristic of our invention are set forth with particularity in the appended claims. Our invention itself, however, both as to its organization and method of pperation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Figure 1 represents an embodiment of our invention, and Figure 2 illustrates graphically the potential vrelationships at selected points in a system embodying our in-- vention.

Referring more particularly to Figure 1 of the .drawing, we have illustrated our invention as embodied in an arrangement for separating in a television receiver the synchronizing impulses from the picture or video signal and for thereafter, in accordance with our invention, separating the vertical from the horizontal synchronizing impulses. The television signal, after being received and amplified by circuit elements, not shown, is supplied through the circuit l0. H to a diode rectifier l2 and series connected diode load resistors l3, 14. The resistor I3 is by-passed by a condenser IS, the time constant of the resistor 13 and condenser l5 being large in comparison with GO-cycles which is the rate of occurrence of the vertical synchronizing impulses. The potential drop appearing across the resistors l3 and I4 is supplied through a ripple filter, comprised by a resistor l6 and condenser H, to an automatic volume control conductor 3 which may be connected to control the gain of preceding amplifier stages, not shown, in a manner well known in the art.

The potential appearing across the resistor 54 is supplied through a condenser H! to the control grid 20 of the first one of a pair of triode type electron discharge devices included within the envelope 2|. This triode has, in addition to the control grid 20, a cathode element ZZ-and an anode element 23. The cathode is connected directly to ground and through a resistor 26 to the grid 20 while the anode is connected through a resistor 24 and a conductor 25 to the positive terminal of a source of anode potential, not shown. A condenser 21 maintains the conductor 25 at ground potential for alternating currents having the frequency of those which flow through the resistor 24.

The alternating current potentials appearing between ground and the upper terminal of the resistor 24 are supplied through a'condenser 28 to the control grid 29 of the second triode elec tron discharge device included within the tube envelope 2|. The circuit connections of the second triode are similar to those of the first triode,

the cathode 30 being directly connected to ground, the anode 3| being connected through a resistor 32 to the conductor 25, and a resistor 33 being used to connect the grid 29 with the oathode 30.

The alternating current potential appearing between ground and the upper terminal of the resistor 32 is now supplied through a condenser 34 to a synchronizing impulse separator arrangement constructed in accordance with our invention. Thisarrangement includes a first triode type of electron discharge device enclosed within the tube envelope 46, whose control grid 35 and cathode 36 are connected to have impressed thereon the alternating current potential supplied through the condenser 34. The anode 31 of this triode is connected to groundthrough a pair of paths, the first of which is comprised by a condenser 38 and a resistor 39, while the second is comprised by a resistor 49, a conductor 4|,

and a source of anode potential, not shown. The

triode, as thus connected, operates as will be explained in detail hereinafter to change the synchronizing impulses into abrupt transient potential impulses across the resistor 39.

These transient potential impulses are supplied through a series circuit comprised by a parallel connected condenser 42 and resistance 43 and through a diode rectifier 44 tothe control grid 45 of a second triode type of electron discharge device included within the tube envelope 46. The second triode has a cathode ele-- ment 41 connected to ground and an anode element 48 which is connected through a resistor 49 to'the conductor 4| .and therethrough to a source of anode potential, not shown. A resistor 50 is connected between thegrid 45' and the cathode 41 of the second triode. The amplified transient potential impulses which appear between ground and the upper terminal of the resistor 49 are supplied through a condenser 5| to a resistor 52 and to a circuit 53, 54 so connected as to control the timed oscillation of a vertical oscillator, not shown, thereby to control the timed movement of the receiver cathoderay as it moves from the upper to the lower edge of the receiver cathode ray tube screen, also not shown.

The synchronizing pulses appearing in the output circuit of the tube 2| are also supplied through the condenser 34 and through a conductor 55 to the control grid 56 of an electron discharge device 51. The device 51 is connected as an amplifier with a biasing resistor 58 and parallel connected condenser 59 included in its cathode circuit and with its anode 50 connected through a resistor BI and the conductor 4| to a source of anode potential, not shown. A small condenser 62 is connected to supply only the relatively high frequency horizontal synchronizing pulses which appear in the anode circuit of the discharge device 51 to a resistor 63 and to the control circuit conductors 64, 65. The horizontal pulses arethereafter used to control the timed oscillation of a horizontal oscillator, not shown, which may be connected in a manner well known in the art to control in the cathode ray tube, not shown, the movement of the cathode ray as it moves horizontally across the cathode ray tube screen.

The operation of our invention will now be explained by the aid of the curves shown in tion the use of any equalizing pulses.

Figure 2 of the drawing. The television carrier wave is received and amplified by apparatus, not shown, after which it is supplied to the input circuit I0, I] of Fig. 1. The wave at this point is symmetrical about a reference axis and its general form is as illustrated in Figure 2a. where only the lower half of the carrier wave envelope (lying below the reference axis 0, O) is shown for purposes of convenience. The carrier wave consists, as is well known in the art, of a picture or video signal portion 1 which controls the instantaneous intensity of the cathode ray as it sweeps across the cathode ray tube screen to form the television picture. Between the video signals ,f are the so-called blanking pedestals g which serve to extinguish the cathode ray in order that the cathode ray tube screen will not be illuminated during the return movement of the cathode ray across the screen at the end of each horizontal traversal. The wave shown in Figure 2a is of the negative transmission type in which the amplitude of the pedestal g from the reference axis 0, 0 corresponds to the black portion of the picture whereas a point on the television signal j close to the line 0, 0 corresponds to white in the picture. Superimposed on the pedestals g are the horizontal synchronizing impulses h which, being of greater amplitude than the pedestals 9, lie in the relatively black region known as blacker-than-black. The synchronizing impulses It therefore cannot be seen in the reproduced image, since no point on the image can be blacker than that corresponding to the amplitude of a pedestal g. These impulses may be separated from the composite television picture in a. manner presently to be described to control the horizontal oscillator of the receiver. That portion of the television signal lying to the right of the last picture or video signal I and extending to the next video signal, not shown, (but which would appear off of the right-hand edge of the drawing) is the portion of the signal during which the cathode ray is extinguished to allow the ray to return from the lower edge of the picture to the upper edge of the picture to begina new picture frame. During this interval, the conventional carrier wave includes a number of so-called equalizing pulses i, of double the frequency and one-half the durasynchronizing pulses i. The wave shown in Fig.

2a is the wave which has been adopted as standard for the United States by the Radio Manufacturers Association and tentatively adopted by the Federal Communications Commission. It will become apparent as the detailed operation of our invention is described that our invention does not require for its proper opera- The time required for the equalizing pulses may with our invention be devoted to more useful purposes.

The composite television signal shown graphiing a magnitude as indicated by the broken line m, m of Figure 2a, and this potentialbiases the anode of diode l2 negatively with respect to the cathode by an amount corresponding to the distance between lines 0 and m of Fig. 2a. The peak of each synchronizing pulse causes the diode anode to become positive with respect to its cathode and thereby produces a current flow through the resistor l4, the current having a wave shape corresponding to that of the synchronizing pulse. The picture or video signal I having a smaller amplitude, is not effective to produce current in rectifier l2 and is thus eflectively separated from the synchronizing impulses h, z, and k.

The potential appearing across the resistor l4, and which has the form indicated by the graph illustrated directly above resistor M in Fig. 1, is supplied through the condenser I9 to a limiter circuit comprised by the several electron discharge devices included within the envelope 2| and their associated circuit. The limiter circuit operates in a manner well known in the art both to amplify the synchronizing impulses, thereby to increase their amplitude, and to shape to a square wave form the top and bottom of each.

The wave form of the potential appearing acrossthe resistance 32 in the output circuit of the limiter is shown in Figure 2b.

The potential appearing between ground and the upper terminal of the resistor 32 is supplied through a condenser 34 to the grid 35 of the first triode included in the tube envelope 46. Each synchronizing impulse p, q, and 1' has a polarity such that it biases the first triode to cut-01f so that no current flows from the anode 31 to the cathode 36. .The condenser 38 thereupon starts charging through the resistors 39 and 40. As soon as a respective synchronizing impulse p, q, or r is completed, the negative bias is removed from the grid 35, the anode-cathode impedance of this triode abruptly decreases, and the condenser 38 thereupon discharges through the anode to cathode path and through the resistor 39. The vertical synchronizing pulses r are of longer duration than the horizontal synchronizing pulses p and therefore provide a longer time in which the charge may accumulate in the condenser 38. The potential appearing across the terminals of the condenser 38 during its charging interval may be represented by the curve of Figure '20. It will be evident from this curve that the maximum amplitude of the potential s appearing across the condenser 38 corresponding to the horizontal synchronizing pulses p is smaller than that of the potential t corresponding to the vertical synchronizing pulses 1.

Both the charging and the discharging currents of the condenser 38 flow, as previously stated, through the resistor 39. The potential appearing across this resistor may be represented by the curve of Fig. 201 where the flat topped loops correspond to the charging currents and the vertical lines correspond to the abrupt transient potential impulses produced by the discharge currents. The discharge currents corresponding to the potentials s of Figure 2c are 4 smaller than the discharge currents corresponding to the relatively larger potentials t and therefore produce across the resistor 39 transient potentials u smaller in magnitude than the transient potiential pulses 1;. It is important to note that the transient potential pulses u and 1) occur on the lagging edge of the synchronizing impulses. It has been found that this produces a very materially improved synchronization of the television receiver under adverse atmospheric receiving conditions.

The potential pulses appearing across the resistor 39 are supplied through the resistance 43 and parallel connected condenser 42 and through the diode rectifier 44 to the grid of the sec- 0nd triode device included within the tube envelope 46. The time constant of the condenser 42 andresistor 43 is so chosen that the first few synchronizing pulses of each transmission charge the condenser 42 to a potential whosemagnitude may be represented by the broken line w, w of Figure 2d. The polarity of this charge is such that the anode element of the diode rectifier 44 is biased negatively with respect to the diode cathode. It will thus be evident that the potential pulses appearing across the resistor 39 are ineffective to' produce a flow of current through the resistor 43, the diode rectifier 44, and the resistor until the potential pulses have a magnitude greater than that represented by the broken line 10, w. Since only the potential pulses 1) which result from the vertical synchronizing impulses have the required amplitude necessary to produce such fiow of current, the potential pulses appearing across the resistor 59 may be represented graphically by the curve of Figure 2e'. Each of the potential pulses of Fig. 2e has an amplitude proportional to that value of the potential pulse 1; in excess of the potential represented in Figure 2d by the broken line 10, w.

The potential at any instant on the grid 45 of the second triode is determined by the voltage drop across the resistor 50. It will thus be ap-' parent that since the horizontal potential synch'ronizing pulses appearing across the resistor 39 produce no current flow in the resistor 50 the} have no control over the potential of the grid 45 and, therefore, are effectively separated from the vertical synchronizing pulses. The potential pulses appearing across the resistor 50 are amplified by' the second triode discharge device included in the envelope 46 and are thereafter supplied through the control conductors 53, 54 to control the timed oscillation of a vertical oscillator, not shown, included in the television receiver.

Both the vertical and horizontal synchronizing impulses appearing between ground and the right-hand terminal of the condenser 34 are supplied to the control grid 56 of the electron discharge device 51. The amplified impulses are supplied to an output circuit comprised by the condenser 62 and resistor 63. The condenser 62 has a relatively small capacity and therefore presents a relatively high reactance to the low frequency vertical synchronizing impulses but a relatively low reactance to the much higher frequency horizontal synchronizing impulses. Only the horizontal impulses are therefore supplied with appreciable magnitude through the capacitor 62 to the control circuit 64, 65 to control the timed oscillation of the horizontal oscillator, not shown included in the television receiver.

It will now be evident that a synchronizing impulse separator arrangement constructed in accordance with our invention operateswith a minimum of apparatus to accomplish the complete separation of the vertical from the horizontal synchronizing impulses which appear in the synchronizing circuit of a television receiver. It will further be evident to one skilled in the art that our invention accomplishes in a simplified and improved manner a more perfectly timed contional pulses to said generator, said means beingtrol of the vertical oscillator of the television receiver since the triggering of the oscillator occurs on the lagging edge of the synchronizing impulse rather than according to present practice, on the leading edge where the triggering may be so easily upset by extraneous noise impulses received with the television signal. Our invention has the additional important advantage that it does not depend for its operation upon the transmission of any equalizing pulses and the impairment of the reproduced image, which may be caused by improper synchronization resulting from-the failure to receive the proper number of equalizing pulses entirely free of noise disturbances and of the required amplitude, is thereby avoided. By omitting the equalizing pulses, a greater portion of the available transmitting time may be devoted'to the transmission of the picture signals thereby to provide a larger picture area.

While we have shown a particular'embodiment of our invention, it will, of course, be understood that we do not wish to be limited thereto since diflerent modifications may be made both in the circuit elements and their arrangements. We, of course, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

What we desire to claim by United States Letters Patent is: i

1. In a television system, the combination of a source of synchronizing impulses to be separated, said impulses being of different duration, means responsive to each of said impulses to produce a corresponding potential pulse having an intensity dependent upon the duration of the respective synchronizing impulse, and means responsive to termination of those of said corresponding potential pulses having an intensity greater than the intensity of the weaker of said corresponding potential pulses to supply addiunresponsive to said weaker of said potential pulses to supply such additional pulses, whereby the longer of said impulses of different duration are separated from the shorter thereof.

2. In a television system, the combination ,of a source of synchronizing impulses to be separated, said impulses being of different duration, means responsive to each of said impulses to produce a corresponding potential pulse having an intensity dependent upon the duration of the respective synchronizing impulse, an impulse generator to be synchronized, and meansrespom.

sive to termination of each of said impulses for producing a transient voltage dependent in intensity upon the intensity of the respective potential pulse, and amplitude selecting means responsive only to the more intense of said transient voltages to supply voltage impulses to said oscillator.

3. In a television system, the combination of a source of synchronizing impulses to be separated, said impulses having differing duration, a. condenser, means for charging said condenser to an extent dependent upon the duration of each of said synchronizing impulses and for discharging said condenser at the cessation of each of said impulses, means responsive to each discharge current of said condenser for producing a potential pulse whose magnitude varies in proportion to the magnitude of said discharge current, a scanning device, and means for transmitting a pulse to said scanning device in response to each of said potential pulses having said means being unresponsive to said weaker pulses.

4. In a television system in which synchronizing impulses of different duration are to be separated, the combination of means for converting said impulses of different duration to impulses of different intensity, a unidirectional current conducing path, means to supply a bias voltage to said path polarized to render it noneconductive, means to supply said impulses of difierent. intensity to said path polarized in the conducting direction of said device, the stronger of said impulses being of such magnitude as to overcome said bias and produce a current impulse through said path and the weaker of said impulses of different intensity being insuflicient to overcome said bias whereby said impulses of different duration are separated.

5. In a synchronizing system for television, the combination of a source of synchronizing impulses of differing duration, an electron discharge device having a control element, means for supplying synchronizing impulses from said source to said control element thereby to control the conductivity of said device, a condenser, means controlled by said device for charging said condenser throughout the duration of each of said synchronizing impulses thereby to produce a charge therein dependent in magnitudeupon the duration of the respective synchronizing impulses and for abruptly discharging said condenser at the cessation of each'of said impulses, means responsive to each discharge current of said condenser for producing a potential pulse whose magnitude in dependent upon the magnitude of said discharge current, a translating device, and means for transmitting to said translating device the portion of any of said potential pulses in excess of a predetermined magnitude.

6. In a television system, the combination of a source of synchronizing impulses of differing duration, an electron discharge device having a control element, means for supplying synchronizing impulsesfrom said source to said control element thereby to control the conductivity of said device, a: condenser, means for charging said condenser throughout the duration of each ofi said synchronizing impulses and for discharging rent of said condenser for producing a potential pulse whose magnitude varies in proportion to the magnitude ofsaid discharge current, a unidirectional current conducting path, means including said path for distinguishing between potential pulses having an amplitude smaller or greater than a predetermined amplitude, a translating device, and means for supplying to said translating device the portion of any of said potential pulses in excess of said predetermined amplitude.

7. In a synchronizing system for a television receiver, the combination of an electron discharge device having a control electrode, an anode and a cathode, a source of horizontal and vertical synchronizing impulses, said vertical impulses being of longer duration than said horizontal impulses, means for supplying said impulses to said control electrode in a manner to bias saidelectrode more negatively thereby abruptly to increase the anode to cathode impedance of said device whereby said anode to cathode impedance abruptly decreases upon ter- Ill accuses rnination oi any of said impulses, a condenser, a source at unidirectional potential, means responsive to the impedance of said device for charging said condenser irom said last named source to an errtent dependent upon the duration of said impulse supplied to said control electrode and abruptly to discharge said condenser upon ter rnination of said impulse, a translating device, and means for reflecting control of said translatinp device by discharge currents of said con denser in excess oi a predetermined value.

c, in a synchronizing irnpulse separator for television system, the combination of a source at horinontal and vertical synchronising impulses to he separated, said vertical synchronisrint, inipulses diderine in duration from said horizontal svnchronisine impulses, means responsive to the cessation of each of said synchronising impulses tor producing a potential pulse, the nuaenitude at said pulses varying suhstantially in direct proportion to the duration of each initiating impulse, and means responsive to the magnitude of said potential pulse for distinguishing those po-'- tcntiai pulses which correspond either to said horinontal or to said vertical synchronizing lrn-' pulses,

h. in a television receiver in which scanning impulses oi diuerent duration are received and separated and supplied to separate channels, the method which includes storing each of said impulses to produce a potential having a value dependent upou the duration oi? the respective stored impulse, and transmitting to one or said channels only said impulses which produce potentials which exceed a predetermined value in crease of the potential produced by storing the shorter of said impulses.

it, in a television receiver in which scanning impulses are received and separated for transmission. to separate channels, a condenser, means to store each of said impulses in said condenser to produce a charge thereon dependent upon the duration of the respective impulses, means to mined value, and means to transmit to the other channel impulses which, when stored, produce potential of less than said value. p

ii. In a television receiver in which scanning impulses are received and separated for transmission to separate channels, a condenser, means to store each of said impulses in said condenser to produce a charge thereon dependent upon the duration of the respective impulses, means to discharge said condenser through an impedance in one of said channels after termination oi each impulse, and means connected across said innpedance to pass current only in response to po tentials on said impedance in excess oi the potential produced hp discharge at said condenser alter storing of a shorter oi said impulses.

12. Ina television svstern in which synchronizing impulses of diiler'ent duration are to he separated, the combination oi means to convert the till synchronizing impulses oi different duration to transient pulse at the end of each of said impulses of difierent amplitude, amplitude selecting said transient pulses of greater magnitude from said transient pulses of smaller magnitude. and frequency selecting said synchronizing pulses of o lesser duration from those of greater duration.

ROBERT B. DOME.

ROBERT E. MOE.

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