US2356141A - Synchronizing circuit - Google Patents

Description

Aug. 22, 1944. A. R. APPLEGARTH, JR

SYNCHRONIZING CIRCUIT Filed July 51, 1942 3 Sheets-Shea*I l rp Q Y NRS RGMQ Aug 22, 1944. A. R. APPLEGARTH, JR

SYNCHRONIZING CIRCUIT Filed July 31, 1942 3 Sheecs-SheerI 2 @www 5 Sheets-Sheet 3 A. R. APPLEGARTH, JR

SYNCHRONIZING CIRCUIT Filed July 5l, 1942 htm ms GPM@ Patented Aug. 22, 1944 SYNCHRONIZING CIRCUIT Alexander R. Applegarth, Jr., Dayton, Ohio, assignor to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Dela- Ware Application July 31, 1942, Serial No. 453,138

23 Claims.

'I'his invention relates to certain improvements in synchronizing circuits and more particularly synchronizing circuits of the type designed to be actuated by pulse signals, such for example as those employed in television receivers. As is well known, television receivers must be operated in synchronism with the transmitting equipment from which the received signal emanates. For this purpose it is customary at the transmitter periodically to reduce the video signal to a predetermined amplitude level and to superimpose during such intervals of reduction, in the opposite polarity from that of the video signal, pulse signals of substantially constant amplitude. The resulting signal may be caused to modulate a carrier wave of suitable frequency so that the signal may be radiated.

At. the receiver this signal may be detected and ampliiied by any of the well known means suitable for this purpose. It then becomes necessary to separate the synchronizing portion of the signal from the video portion in order that the former may be used to control the operation of deflecting circuits used in conjunction with cathode ray devices to reconstitute the visual image thus electrically transmitted. This separation must be performed with considerable care in order that good synchronizing may obtain. This raises many problems which must be taken into consideration in the design of equipment for eiecting this separation. For example it is essential that the selected portion should include only the synchronizing pulses to the exclusion of the video signal and to the substantial exclusion of noise pulses of large amplitude. This separation process is rendered diilicult by the fact that the strength of the received signal may not be uniform due to varying attenuation of the signal in transmission and also by the fact that,

for.` various reasons, the received synchronizing pulses may not all occur at the same amplitude level. It is further desirable that the pulses in the selected synchronizing signal should be of substantially constant amplitude.

The principal object of this invention is to provide means for overcoming these difficulties and for yielding a high quality synchronizing signal of substantially constant amplitude and substantially free from any video signal and large amplitude noise pulses. The details of the invention will be clearly understood from the following description with reference to the accompanying drawings, in which Fig. 1 is a diagrammatic illustration of one embodiment of the invention; I

(Cl. P18-7.5)

Fig. 2 is an explanatory diagram which will be referred to in explaining the operation of the invention; and

Fig. 3 is a diagrammatic illustration of another embodiment of the invention.

In Fig. 1 there is represented generally in block I those parts of the receiver which are conventional and unmodified by the invention. These circuits may include a radio frequency amplifier supplied with signal from antenna 2, a converter stage, and an intermediate frequency amplifier. As shown the iinal stage of the latter may be coupled by means of the transformer 3 to a detector circuit comprising the diode 6, resistor 5, condenser 6, and chokes 'I and 8. The detected signal develops across the load impedance comprising the resistor 5 and the peaking coil 8. Filtering to remove the carrier frequency is effected by the combination of the condenser 6 and the choke 1. The detected signal is applied to the grid of the amplifier tube 9 which may be a pentode. Suitable biasing for this tube is obtained across the voltage divider I0 by-passed by the condenser I I. The tube may be degenerated slightly by the inclusion in its cathode circuit of a small resistor I2 for the purpose of lengthening out the grid voltage-plate current characteristic so that the signal applied to the grid of the tube will not be limited by tube cutoil. Two separate output circuits may be provided for the tube 9, one in the anode circuit comprising the resistor I3 and the peaking coil I4, and another in the 'screen grid circuit comprising the resistor I5 and the peaking coil I6. Across a portion of the resistance of the potentiometer I1 shunting the anode load may be taken a signal which is fed to the video amplifier I8 and applied to the grid of the picture tube I9. Across the screen load may be taken the signal from which there is separated out the synchronizing component for controlling the oper-ation of the synchronizing circuits 20 which supply a deflecting signal to the deflecting coil 2l for defleeting the electron beam in the picture tube. For the sake of simplicity a single deflecting coil has been shown, though it is of course to be understood that the both horizontal and vertical deflecting means are employed in the usual televisionreceivers. The screen load may be chosen so as to peak at' a lower frequency than the plate load. This tends to improve the response in the video channel while giving a sumciently good response in the synchronizing channel which does not require as wide a band of frequencies as does the video channel. This feature is disclosed and claimed in U. S. Patent No. 2,289,291, granted July 7, 1942.

Signal derived across the screen load impedance is supplied through a coupling condenser 22 and across grid-leak 23 to the grid of the tube 24. The latter is preferably a vbeam-power tube, though a pentode might be used. The tube is permitted to draw grid current on signal peaks whereby it acts in cooperation with the condenser 22 and the grid leak 2,3 to develop a varying ybias for leveling the peaks of the signal in the manner shown at 25. The cathode is maintained at the desired potential by connection to the voltage divider comprising resistors 26 and 21, the latter of which is by-passed by condenser 28. 'I'he tube is operated at low anode potential from the divider comprising resistors 29 and 30 whereby it is operative to pass only a portion between predetermined amplitude levels of the signal supplied to its input, the amplitude levels being determined respectively by plate current cut-ofi' and plate current saturation. As a result the tube operates on curves of anode current versus grid voltage as shown in Fig. 2'. Further it will be noted that the screen of tube 9 derives voltage from a divider comprising the resistors 3|, 32 and 33, of which 3| is connected between the screen of tube 24 and ground and suitably by-passed by the condenser 34 so that variations in the D. C. screen voltage of the tube 9 appear on the screen of tube 24. This variation in the screen voltage is effective to cause the tube 24 to select a portion of the signal intermediate between the synchronizing tips and the blanking level.

The operation of the circuit will now be considered in somewhat greater detail with reference to Figure l and 2. In view of the direct coupling between the detector and the grid of tube 9, any variations in signal strength will appear as variations in the level of the signal applied to the grid of that tube. For example, at 35 a weak signal is represented by the solid curve and a strong signal by the broken curve. Hence it will be seen that for weak signals the tube 9 draws a large screen and plate current thereby making its D. C. screen voltage low, while for strong signals the screen current will be reduced and the screen voltage will rise.

Considering now the operation of the tube 24, this tube is connected to operate as a gate capable of selecting a. portion between predetermined amplitude levels of the signal applied to its grid. Furthermore it is possible, by varying the screen voltage of this tube with respect to the anode and cathode voltages, to vary the level at which this portion is selected with reference to the signal applied to its input. This vnll be understood more clearly by reference to Fig. 2 which shows curves of plate current versus grid voltage for the tube 24 for various values of screen voltage when connected as shown in Fig. 1. 'Ihus for a relatively high screen voltage the tube may operate on the curve labeled E1 while for progressively lower screen voltages it may operate upon the curves E2 and Ea. Since the screen grid of tube 24 is connected so as to be subject to variations in the D. C. screen voltage of the tube 9, which is dependent upon signal strength,

it will appear that the characteristicupon which f the tube 24 operates will depend upon signal strength From Fig. 2 it will be apparent that the porc and d. Parallel tion of. the signal selected will depend upon the characteristic upon which the tube operates. Thus when operating on characteristic Es the tube will select that portion of the input signal corresponding to grid voltages between the values of a and 0, while, when operating on the c urve E2, that portion of the signal will be selected corresponding to grid voltage values between to the eg axis in Fig. 2 are shown the synchronizing tips and portions of the blanking pedestals of a weak and of a strong signal, designated f and g respectively. The portions of these signals selected by the tube 24 are shaded and the resultant plate current pulses are shown at f and g respectively. In the case of the weak signal f the voltage on the screen of tube 24 will be relatively low so that the tube may operate on the characteristic Es selecting that portion of the signal between the values a and 0 of grid voltage.v However, in the case of the strong signal 9' the screen voltage will be larger and Athe tube may operate on characteristic E2 selecting a portion between relatively lower values of grid voltage.

This mode of operation will be seen to be desirable for several reasons. For weak signals the tube selects substantially the entire synchronizing pulse between its tip and the blanking level, but for stronger signals only a fraction of the pulse is required. It then becomes desirable to take this portion intermediate between the usual level of synchronizing tips and the blanking level. The advantage in doing this obtains when, as is frequently the case, the synchronizing tips do not occur at a constant level. Thus, in the case of the pulse shown at h, if the tube were operating on the characteristic E3, the grid would not be swung sutiiciently far in the positive direction to give the desired output, the resulting pulse being shown 'by the cross-hatched area at h. However, when operating on the characteristic Ez, the swing, though -not as great as in the case of the pulses f and g, is suflicient to give the desired output. Likewise it follows 'that this level of selection cannot be maintained for weak signals Without the inclusion, in the selected portion, of a portion of the video component which would be very undesirable.y

It will be observed from the characteristics in Fig. 2 that the gate operates to select a constant amplitude portion of the signal supplied to its input, regardless of the level at which the selec"- tion is made. Such operation is conducive t'o' good synchronization and tends..to reduce -the amplitude of any noise pulses which may be present in the signal. However it is to be noted that this is but a ypreferred mode of operation to which the invention is not restricted. It is of course conceivable that the tube might be used in a circuit such that each of the characteristics differs in slope and in'remoteness of cut-oil from the others. In such a case. the gate would not only select a portion of the signal at a varying level but would also select a portion of varying magnitude, which might prove satisfactory in certain applications. This may also depend upon the type of tube used, for, although applicant has found a beam-rpower tube suited to his requirements, it is obvious that tubes of other types such as pentodes and those having even a greater number o1' electrodes might also be used.

'I'he following circuit element values may be taken as exemplary in the embodiment of Fig. 1

but they are not to be regarded as restricting the scope of the invention in any way:

Tube 9=type 1232 Tube 24=type 6Y6G Resistor I2:56 ohms Resistor I 3:4700 ohms Resistor I5:10,000 ohms Resistor 23:1 megohm Resistor 26:100,000 ohms Resistor 21:1000 ohms Resistor 29:1(),000 ohms Resistor 30:1 megohm Resistor 3|:l2,000 ohms Resistor 32:330,000 ohms Resistor 33=4'7,000 ohms Inductor I4=70 microhenries Inductor I6:1000 microhenries Condenser 22:.05 microfarad Condenser 23:4 microfarads Condenser l34:10 microfarads Referring now to Fig. 3 there is shown another embodiment of the invention wherein the control applied to the screen grid of the gate tube is supplemented by the application to the anode and cathode of the tube, of a control voltage varying in the opposite sense (i. e., becomes more positive with decreasing signal strength). Such a voltage may conveniently be obtained across an impedance in the cathode circuit of the amplifier tube following the detector. It will of course occur to those skilled in the art that it is possible to derive this last-mentioned control voltage, as well as the voltage varying in the opposite sense, in various ways and at various points in the system, and it is not intended to limit the invention to the specic means and methods herein set forth.

In the circuit of Fig. 3 those circuits included in the block designated by the reference numeral I as well as the video and synchronzing channels may be the same as in the circuit of Fig. l. However the functions of the detector and the succeeding am-plier are here performed by a single diode-pentode 36. The output circuit for this tube may include, in additionto the resistor 31, the peaking coils 38 and 39. Bothvideo and synchronizing outputs are taken across the impedance consisting of the resistor 31 and the inductor 39. The synchronizing component is supplied through an isolating resistor 40 and a coupling condenser 4| across a grid leak 42 to the grid of tube 43 which is operated at loW plate voltage'obtained from the voltage divider comprising resistors 44, 45 and 46. As in the circuit of Fig. 1, the tube 36 derives screen voltage from a divider including the resistors 41, 48 and 49, of which 41 is connected between the screen of tube 43 and ground and suitably by-passed by condenser 50 for the purpose already explained of varying the operating level of the gate.

In certain cases the amount of control thus afforded may be insuiiicient. Another method of obtaining equivalent control is by the application of a control voltage varying in the opposite sense to the cathode and anode of the gate tube. For this purpose the resistor 46 forming a, part of the anode voltage divider of tube 43 is included in the cathode circuit of tube 36 suitably by-passed by condensers 5| and 52. By reason of this connection the cathode and anode of tube 43 are caused to assume a more positive potential as the signal decreases and vice versa.

obtains when the cathode and anode potentials are fixed and the screen voltage caused to vary in the opposite sense. .These two controlling effects cooperate to yield the desired variation in the orperating level of the gate with reference to the input signal. 'i

The following circuit element values are exemplary of those which may be used in the circuit of Fig. '3 but are not to be regarded as restrictive of scope:

Although the invention has been described with reference to certain specific embodiments, it will be understood that it is capable of physical expression in numerous other forms without departing from its scope which is subject only to the limitations implosed by the appended claims.

I claim:

l. Inan electrical system; a source of a signal of varying strength; a space discharge device having its input coupled to said source for selecting a portion between predetermined amplitude levels of the signal supplied to its input, said device being operated at low anode potential thereby to determine the magnitude of the se- The eilect of this upon the operation of the gate g lected portion, and having an auxiliary control element, the potential of which determines the level at which said portion is selected; means for deriving a control voltage which varies with said signal strength; and means for applying said voltage to said control element to control the level of selection..

2. In an electrical system; a source of a signal of varying strength; a space discharge device having its input coupled to said source for selecting a portion between predetermined amplitude levels of the signal supplied to its input, said device being operated at low anode potential thereby to determine the magnitude of the selected portion, and having an auxiliary control element, the potential of which determines the level at which said portion is selected; means for deriving a con-trol voltage which varies with said signal strength; and means for applying said voltage to said control element to con-I trol the level of selection, said voltage varying in such sense that said level of selection becomes more remote from the peaks of said signal with increasing signal strength.

3. In a carrier wave receiving system; a source of a modulated carrier wave signal; means coupled to said source for demodulating said carrier wave signal; a space discharge device having at lease an anode, a cathode, a control grid, and an auxiliary accelerating electrode interposed between said cathode and said anode, said device having its control grid direct coupled to said demodulating means; means having aninput cir- 8. In an electrical system; a source of a signal cuit coupled to said source for selecting a portion between predetermined amplitude levels of the signal applied to its input, said selecting means being controllable so as to vary the level at which said portion is selected; and means for utilizing the D. C. potential of said accelerating electrode to control the level at which said selecting means operates.

4. In an electrical system; a source of a signal of varying strength; means having an input circuit coupled to said source for selecting a portion between predetermined amplitude levels of the signal supplied to its input, said means being controllable sov as to vary the level at which said portion is selected; space discharge means having a cathode, a control grid, and a plurality of output electrodes, said control grid being coupled to said source, and the D. C. potential of at least one of said output electrodes varying with said signal strength; and means for utilizing the D. C. potential of the said output electrode to control the level at which said selecting means operates.

5. In an electrical system; a source of a signal of varying strength; aspace discharge device having a cathode, a control grid, and a plurality of output electrodes, said control grid being coupled to said source, and the D. C. potential oi' at least one of said output electrodes varying with said signal strength; a plurality of load impedances associated with said output electrodes; means having an input circuit connected across one of said load impedances for selecting a portion between predetermined amplitude levels of the signal developed across said input, said means being controllable so as to vary the level at which said portion is selected; and means for utilizing the D. C. potential of said one output electrode to control the level at which said selecting means operates. l

6. In an electrical system; a source of a signal of varying strength; aspace discharge device having at least a cathode, a control grid and an anode, said control grid being coupled to said source; a load impedance in the anode circuit of said space discharge device; means having an input circuit connected across said load impedance for selecting a portionvbetween predetermined amplitude levels of the signal developed across said input, said means4 being controllable so as to vary the level at which said portion is selected; an impedance in the cathode circuit of said space discharge device for developing a D.y C. voltage varying with signal strength; and means for utilizing said voltage to control the level at which said selecting means operates.

7. In a television system; a source of a composite signal comprising video and "blanking components originally established so as to have apposite polarities with reference to a blanking level; means having an input circuit coupled to said source for selecting a portion between predetermined amplitude levels of the signal supplied to its input, said means being controllable so as to vary the level at which said portion is selected; means for deriving a control voltage which varies with said signal strength; and means for utilizing said voltage to control the level at which said selecting means operates so that said means selects a portion of said input signal intermediate between said blanking level and the peaks of said blanking signal for signal strengths within a given range.

of varying strength; a space discharge device having an anode, a cathode and a plurality of output electrodes, the D. C. potential of at least one o1' said output electrodes varying with signal strength; load impedances associated with each of` said output electrodes; a second space discharge device having' its input connected across one of said load impedances for selecting a portion between predetermined amplitude levels of the signal supplied to its input, said second device being operated at low anode potential thereby to determine the magnitude of ythe selected portion, and said second device having an auxiliary control element, the potential of which determines the level at which said portion is selected; a source of voltage and a voltage divider p connected thereacross; a connection from a point on said divider to the said electrode whose D. C. potential varies; a by-pass condenser shunting a portion of said divider and means for applying the substantially D. C. potential developed across said portion to said auxiliary' element to determine the level at which said portion oi' said signal is selected.

9. In an electrical system; a source of a signal of varying strength; a space discharge device having at least an anode, a cathode and a control grid, said control grid being coupled to said source; a load impedance in the anode circuit of said space discharge device; a second space discharge device having at least an anode, a cathode, a control grid, and an auxiliary electrode, the signal developed across said load impedance being supplied to the control grid of said second device and the said device being operated at low anode potential, thereby to determine the magnitude of the signal portion selected by said second device; a source of voltage and a voltage divider vshunting said source, a portion of said divider being included in the cathode circuit of said rst device and being suitably bypassed by means of a condenser; and connections from points on s aid voltage divider to the anode and cathode of said second space discharge device whereby the potentials of said electrodes are varied in accordance with the current in the cathode circuit of said first space discharge device thereby to determine the level at which said portion of said signal is selected.

10. In an electrical system, a source of a signal of varying strength, an electron discharge tube coupled to said source so as to receive signals therefrom, means for operating said tube so that it transmits only a portion of an applied signal deined by lower and upper output limits, means for deriving from said system a control signal which'varies with signal strength, and means for applying said control signal to said tube so as to shift the eilective range of operation of the tube without substantially varying the output limits of the tube.

11. In an electrical system, a source of a signal of varying strength, an electron discharge tube coupled to said source so as to receive signals therefrom, means for establishing substantially constant output limits of said tube, whereby said tube transmits only a portion of an applied signal dened by said limits, means for deriving from said system a control signal which varies with signal strength, and means for applying said control signal to said tube so as to shift the eiective range of operation of the tube without substantially varying the output limits of the tube.

12. In an electrical system, a source of a signal of varying strength, an electron discharge tube coupled to said source so as to receive signals therefrom, said tube having a cathode, a control grid, an anode, and at least one auxiliary electrode, means for applying potentials to said grid and anode such that the effective operation of said tube is established between substantially constant lower and upper limits of space current now, whereby said tube transmits only a portion of an applied signal defined by said limits, means for deriving from said system a control signal which varies with signal strength, and means for applying said control signal to said auxiliary electrode so as to shift the effective range of operation of the tube without substantially vary-l uration level, whereby said tube transmits a limited portion of said signal, and means for effectively shifting the said operating characteristic of said tube according to the strength of said signal.

14. In an electrical system, a source of a signal of varying strength, a rst electron discharge tube coupled to said source so as to receive signals therefrom, a second electron discharge tube coupled to said rst tube so as to receive signals therefrom, means for establishing substantially constant limits of output of said second tube, whereby the said tube transmits only a portion of the signal supplied thereto, means for deriving from said rst tube a control voltage which varies with the signal strength, and means for applying said control signal to said second tube so as to Vary the effective operating range of the tube without substantially varying its established output limits.

15. In an electrical system, a source of a sig" nal of varying strength, a rst electron dischargevv limits are established therefor, whereby the said tube transmits only a portion of the signal supplied thereto, means for causing the potential of the auxiliary electrode of said first tube to varyk with signal strength, and means for causing the potential of the auxiliary electrode of said second tube to vary as a function of the aforesaid potential variation, whereby to vary the effective operating range of said second tube without substantially varying its established output limits.

16. In an electrical system, a source of a signal of varying strength, a rst electron discharge tube coupled to said source so as to .receive signals therefrom, a second electron discharge tube coupled to said first tube so as to receive signals therefrom, each of said tubes having at least triode elements and an auxiliary electrode, means for applying such operating potentials to said second tube that substantially constant output with signal strength, means for causing the potential of the auxiliary electrode of said second tube to vary as a function of the aforesaid potential variation, whereby to vary the effective operating range of said second tube without substantially varying its established output limits, meansfor derivingv a variable control voltage from one of the triode elements of said first tube, and means for applying said control voltage to a triodeelement of said second tube so as to further control the operation of the latter.

17. In a television receiving system of the type employing a composite signal having video and synchronizing components established on opposite sides of a blanking level, a signal channel for receiving said composite signal, a video signal channel coupled tosaid rst channel, a synchronizing signal channel coupled to said rst channel, an electron discharge tube in said last-named channel, means for causing said tube to have a predetermined operating characteristic with a substantially iixed saturation level, whereby the tube transmits a limited portion of said composite signal dened by the cut-off and saturation points of said operating characteristic, and means for shifting the operating characteristic according to the strength of said composite signal thereby to select a substantially constant portion of each synchronizing component to the substantial exclusion of said video component.

18. In a television receiver, a source of modulated carrier Wave television signals, a signal detector circuit, a synchronizing signal pick-ofi circuit coupled to said signal detector, said pickf oil" circuit being constructed and arranged to select only a portion of the available amplitude of the detected synchronizing signals, said portion being dened by both lower and upper cutoil limits, means for deriving from said receiver a control voltage` which varies with signal strength, yand means for applying said control voltage to said pick-o circuit so as to shift said limits in accordance with the strength of the received signal, without substantially varying the output of said pick-olf circuit. 4

. 19. AIn a television receiver, a detector for the modulated carrier Wave television signals received thereby, a pick-offdevice consisting of a space discharge device having its input coupled to said detector and` being operated at low plate voltage whereby it is caused to select a portion of the detected signal ybetween predetermined amplitude levels for synchronizing purposes, said devicehaving an auxiliary control element adapted, in accordance with its potential, to determine the level at which said portion is selected, means for developing a control voltage which varies with limits are established therefor. whereby the said the strength of said detected signals, and a vconnection between said means and said control element whereby the level at which said pick-oit device operates is controlled in accordance with said signal strength.

2.0. In a television receiver, a detector for the modulated carrier Wave signals received thereby, a space discharge device having a, cathode, a control grid coupled to said detector, and a plurality of output electrodes, a plurality of load impedances associated with said output electrodes, a pick-olf device coupled to one of said load irnpedances and adapted to select a portion between predetermined amplitude levels of the signal thereacross', said device being controllable so as to vary the level at which said portion is selected, and a D. C. connection between one of said load impedances and said pick-oil device whereby the level at which said pick-oil' device operates is controlled in accordance with the D. C. potential developed across said-last-named load impedance.

21. In a television receiver, a. detector forthe modulated carrier wave television signal received thereby, said signal comprising a carrier modulated by a composite signal including video and synchronizingcomponents originally established so as to have opposite polarities with reference to a blanking level, a pick-oil device coupled to said detector and adapted to select a portion of the detected signal between predetermined amplitude levels for synchronizing purposes, said device being controllable so as to vary the level at which said portion is selected, means for developing a control voltage which varies with the strength of said detected signal, and a connection between said means and said pick-off deviceA whereby said pick-oir device operates to select a portion of said signal intermediate between said blanking level and the peaks of said synchronizing signal component.

22. In a television receiver, a detector for the modulated carrier wave television signal received thereby, an electron discharge tube coupled to said detector so as to receive signals therefrom. means for operating said tube so that it transmits only a portion of an applied signal defined by lower and upper output limits, said transmitted portion being employed for synchronizing purposes. means for deriving from said receiver a control voltage' which varies with signal strength, and means for applying said control voltage to said tube so as to shift the e'ective range oi' operation oi' the tube without substantially varying the output limits of the tube.

23. In a television receiver adapted to receive a signal of varying strength, a synchronizing signal pick-oil' device comprising an electron discharge tube coupled to said receiver so as to be supplied with said received signal, said tube having a predetermined input-output operating characteristic and a substantially fixed saturation level, whereby said tube transmits a. limited portion of said signal to be used for synchronizing purposes, and means responsive to the strength of the received signal for effectively shifting the operating characteristic of said tube to thereby vary the level at which said portion is selected.

R. APPLEGAR'I'H, J R.

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