US2797378A - Electric signal synchronising apparatus - Google Patents

Description

June 2 5," 1957" K. c. JOHNSON 2,797,378

" ELECTRIC SIGNAL SYNCHRONISINGAPPARATUS Filed March 24, 1953 5 Sheet s-Sheet 1 c 2% Ms GONTROL UNIT "CL COMPUTING MACHINE v i I Mel/w:

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7 ELECTRIC SIGNAL SYNCHRONISING APPARATU S I Filed March 24, 1955 s Sheets-Sheet s FIG 4 [NYE/V T R June 25, 1957 K. c. JOHNSON ELECTRIC SIGNAL. SYNCHRONISING APPARATIJS Filed March 24, 1953 5 Sheets-Sl ieet 4 I/v VE/V TOR Mud-W 3 g 1 m9 W% 8:

QM Qw 5 @w 3 MG 2 h .W n 3 Q 8 H 5 mm B Q M? Q. a H ww June 25, 1957 K. c. JOHNSON ELECTRIC SIGNAL SYNCHRONISING APPARATUS Filed March 24, 1953 5 Sheets-Sheet 5 w m8 8N$ A AA A A v I l/VIEA/ T R m6 mmw ATsa we a ELECTRIC SIGNAL SYNCHRONISING APPARATUS Kenneth C. Johnson, Manchester, England, assignor to National Research Development Corporation, London, England, a British corporation Application March 24, 1953, Serial No. 344,366

Claims priority, application Great Britain March 26, 1952 19 Claims. (Cl. 318-302) This invention relates to apparatus for synchronising electric signals having pulsed waveforms and in particular to apparatus for synchronising each of the pulses of a first series of pulses whose periodicity is controllable with related pulses in a second series of cylically repeated control pulses. A particular but by no means exclusive application of the invention is to synchronise the pulses of a continuous train of slave pulses to each nth pulse of a cyclically repeated train of pulses.

Such apparatus has useful application, inter alia, to binary digital computers employing both a cathode-raytube data store (as described in British patent specification No. 645,691 and in Proc. I. E. E., part III, March 1949, pages 81 to 100 and part II, February 1951, pages 13 to 28) and also employing a magnetic data store in the form of a rotating drum (as described in copending patent application Serial No. 146,446, filed February 27, 1950, by Frederic C. Williams, Patent No. 2,652,554 and in Pros I. E. E., part II, pages 29 to 34 and part II, April 1952, pages 94 to 106).

In such last mentioned particular application it is convenient to synchronise the speed of the rotating drum with the set rhythmic operation, i. e. the line scanning or beat period, of the cathode-ray-tube store which is continuously determined by a master or clock oscillator. In apparatus hitherto used for efiecting this, there is generated a signal of square waveform in synchronism with each nth pulse of the train of digit pulses occurring during each line scan period of the cathode-ray-tube. This square wave signal has a period time equal to the time interval between successive nth pulses, i. e. the beat interval of the machine, and is symmetrical about the wave axis, that is to say the duration of its positive-going period is equal to the duration of its negative-going period. From the rotating drum there is derived, by suitable electromagnetic pick-up means, a train of slave pulses having a frequency which is proportional to the speed of rotation of the drum. The phase of these slave pulse signals is then compared in a phase discriminator with the phase of the aforesaid square wave and an output voltage from the phase discriminator, indicative of phase divergence, is applied to servo apparatus for controlling the speed of rotation of the drum whereby the drum is constrained to rotate in synchronism with the remainder of the computing machine. This phase discriminator is designed to give Zero output when the centre point of each slave pulse coincides in timing with that edge (hereinafter called the reference edge) of a cycle of the square wave signal which occurs in synchronism with the corresponding nth pulse of the train of digit pulses and to supply an output of one polarity or the other polarity whenever the slave pulses slip away from the reference edge due to phase lag or phase advance as the case may be. The resultant output of appropriate polarity delivered by the phase discriminator thus so controls the speed of rotation of the drum as to adjust both the repetition frequency and the timing of the slave pulses relative to such reference edges of the square waveform in a sense appropriate nited States Patent to arrest the slip and restore phase coincidence and thereby to maintain the drum rotating at the correct speed and at the correct instantaneous angular positional relationship with respect to the operating rhythm of the machine whereby signals derived from electromagnet pickup means associated with other recording tracks on the drum occur in correct time relationship with the similar signals in the remainder of the machine.

Such an arrangement works satisfactorily when the slip is slow but whenever the slip is rapid, i. e. whenever the frequency of the slave pulses is widely different from the frequency of the control pulses and the derived master square wave signal, there is necessarily applied to the output smoothing circuit of the phase discriminator, voltages which alternate rapidly between positive and negative polarity as the slave pulses slip past the respective half cycles of the master square waveform and as this latter waveform is symmetrical about the wave axis, the smoothed output voltage from the phase discriminator over several slip cycles is zero and no control voltage is derived. Such a failure of control, is of course, well-known where the inputs to a phase discriminator differ widely in frequency.

The object of the present invention is to provide apparatus of the type stated which is effective, despite wide differences between the repetition frequencies of the master and slave pulse waveforms.

In accordance with the broadest aspect of the invention, apparatus for synchronising electric signals having pulsed waveforms comprises an electric square waveform generator which is adapted to be triggered by each of a series of control pulses to generate a master signal of square waveform having a period time equal to the time interval between successive master control pulses, a phase discriminator for comparing the phases of said master square wave signal and of input slave pulses which are to be synchronised therewith and for deriving an output voltage which is of one or the other polarity according as said slave pulses are predominantly in phase with the positive or negative half cycles, as the case may be, of the master square waveform signal, servo means adapted to be actuated by the output of said phase discriminator and when so actuated to adjust the periodicity of said slave pulses in a sense dependent upon the polarity of said output and frequency discriminating means which is adapted to be actuated by the phase discriminator output when the frequencies of said master square wave signal and said slave pulse signals are unequal for rendering the square waveform of said master signal asymmetrical with respect to the wave axis whereby the mean value of said phase-discriminator output over a plurality of slip cycles is not zero but is of a polarity appropriate to correct the frequency of said slave pulses.

In a particular embodiment of the invention, applied to the synchronisation of each slave pulse with each nth pulse of a cyclically repeated train of pulses, said master square-waveform generator comprises a two stable state trigger circuit, e. g. of the so-called Eccles-Iordan type, which is adapted to be triggered from a first stable state to its second stable state by each of said nth pulses and to be restored from such second stable state to its first stable state by either the ensuing (n+m)th pulse or the (n-m)th pulse as determined by the polarity of the output voltage from said phase discriminator.

In the above and hereinafter the frequency when used with respect to the slave pulse means the slave pulse repetition frequency, while the term predominantly in phase with a half cycle of the master square wave is intended to mean with more than half the length of any one the slave pulse being considered is in phase with a part of that half cycle.

In order that the nature of the invention may be more readily understood one embodiment thereof as applied to an electronic digital computing machine will now be described with reference to the accompanying drawings in which:

Fig. 1 is a block schematic diagram illustrating certain elements of an electronic digital computing machine and an associated magnetic drum store provided with a drum synchronising arrangement according to the present invention.

Fig. 2 illustrates in somewhat greater detail certain of the basic waveform generating means of the computing machine which are concerned with the drum synchonising operations. V V

Figs. 3 and 4 each comprise a series of explanatory waveform diagrams. I

Fig. 5 isa detailed circuit diagram showing the parti'cular arrangements of the master square-wave generator, the phase discriminator, the switch valve and visual indicator and the coarse speed servo control while Fig. 6 is a detailed circuit diagram showing the arrangement of the shaping or stabilising valve and the final output power amplifier which supplies the brake control current. I 7

Referring first to Fig. 1 MS indicates the main data store of the computing machine comprising a device of the cathode ray tube type as described inthe aforesaid references. As both the manner of operation and the construction of such devices are now well-known in the art and as adequate information thereon is obtainable from the references quoted it will not be described further. Such a store forms part of digital computing machines operating in the series mode with pulse trains representing both numbers and instructions and this again has been described in detail in the quoted'references.

Such machines operate in a series of major cycles or bars during each of which one computation step out of a programme of computation steps is performed. Each major cycle or bar comprises a plurality of minor or subcycles known as beats during which the required next instruction is first selected and transferred to a control unit CL where it, in turn controls the selection of and the setting up of the appropriate transfer route for (together with any accompanying arithmetical operation) a required number which is also held within the main data store. The operating rhythm of the machine is determined by a series of electric waveforms which are generated within the unit WGU which will be described in greater detail later. Both numbers and instructions are represented within the machine, in their dynamic form, as electric pulse signal trains in which the presence of a pulse at a predetermined time instant within any beat period indicates the binary value 1 and the absence of a pulse in the same interval indicates thebinary value 0, the position of the interval within the beat period being indicative of the denominational or power value of the binary digit itself.

Such machines are desirably provided with a large storage capacity for recording data items and in practice cathode ray tube type stores become unnecessarily bulky and complex if used for retaining all of the required data items required in a long programme. Such machines are therefore frequently provided with a subsidiary store in the form of a magnetic drum MD having a magnetic recording layer around its circumferential surface to provide a plurality of endless data storage tracks DT in each of which a plurality of magnetic storage patterns each representative of one number or instruction word may be recorded. The total number of separate word 10- cations in any one track is made equal to an integral number, usually two, complete fillings for a cathode ray storage tube. Each track DT is provided with its own read/write head DH by which input electric signals can be caused to make the required magnetic recording and from which output signals representative of the previously recorded signals, can later be derived. Arrangements 4 are provided for selecting any required read/write head and hence any one track for use and then transferring signals between such track and the main data store MS whereby the contents of a cathode ray storage tube can be transferred en bloc to a magnetic storage track or vice versa. The selection of the required read/write head and its associated data track DT is effected by means of a track selecting relay tree circuit RT which is controlled in its operation by electric signals derived from the control unit CL of the computing machine in response to the obeying of a particular instruction in the machine. The signals transferred between the magnetic drum MD and the main data store MS pass through read and write circuits WRC whose purpose is to convert the normal dynamic signal pulse trains circulating within the machine to a form suitable for energising the heads DH to effect magnetic recording and, conversely, for converting the pick-up output signals derived from such heads back into pulse train signals of the type circulating within the machine. As such apparatus forms no part of the present invention and has already been described in the quoted literature it will not be further referred to.

For satisfactory operation during transfer of signals in either direction between the stores it will be apparent that both the speed of rotation of the drum MD and its angular position at any instant must bear a predetermined relationship to the beat period of the computing machine if the signals recorded in each of the word storage locations of a track around the magnetic drum by signal pulse trains delivered from the store MS are subsequently to be reproduced and converted back into signal pulse train form with a timing, relative to the machine beat period, which renders them suitable for direct use within the machine.

This required synchronism is obtained byarranging for the magnetic drum to be driven continuously by an electric motor DM, conveniently a 3-phase A. C. motor, energised over leads from a suitable supply source and with such motor arranged so that it inherently tends to drive the drum at a rather faster speed than that which iscorrect to maintain synchronism with the associated computing machine. Directly coupled to the drum MD is an electromagnetic braking means BM, conveniently in the form of a single-phase A. C. induction motor, which is supplied with a direct current of variable value whereby it operates as an eddy-current brake to maintain,by appropriate adjustment of the applied braking current, theresultant rotation of the drum at both the correct angular speed and with the correct angular relationship of any given point on the drum to the operating rhythm of the computing machine.

The drum MD isprovided with a further, slave pulse, recording track ST in which are initially recorded a series of short pulse signals, one foreach of the word storage locations in the other date storage tracks DT. Each slave pulse therefore corresponds to one line scanning motion of the main cathode ray tube data store MS. Thus, if each cathode ray tube of the main data store MS records 64 20-digit number or instruction words and if each magnetic recording track has a capacity of storing two complete fillings for a cathode ray tube store then there will be 128 separate word storage locations in each data storage track'DT and 128 individual slave pulse signals recorded at equally spaced intervals around the slave pulse track ST. This slave pulse track 'ST has an associated read or pick-up head SRH by which the series ofslave pulse signals may be continuously derived and made available on lead 101. These signals, after suitable amplification in amplifier SPA, are applied by way of lead 102to-one input terminal 103 of a phase discriminator PDC the other-input terminal 104 of which is supplied witha master-square'wavefor'm by way of lead from a master square Wave generator MSG. This master square wave generator MSG 'is controlled, in a manner described indetail later by certain timing waveforms of the computing machine which are made available over leads 106, 107 and 108 from the waveform generator unit WGU and also by output voltages supplied by way of leads 109 and 110 from a coarse servo control valve circuit CSC and a switch valve circuit SVC respectively.

The output from the discriminator over lead 111 is zero provided the slave pulse signals obtained from the slave pulse track ST are maintained in correct alignment with the so-called reference edge of the master square-wave delivered from the master square-wave generator MSG. Such reference edge is the point of abrupt change from negative to positive value of the master square-wave which occurs in synchronism with each nth or control pulse. If the slave pulses occur in advance of such reference edge the phase discriminator output will be of negative polarity whereas if such slave pulses are lagging on such reference edge the phase discriminator output will be of positive polarity. The output from the phase discriminator PDC is applied over lead 111 to a shaping valve SV whose output is fed by way of lead 112 to a power amplifier PA which in turn supplies the current for energising the electromagnetic braking means BM by way of leads 113. The output from the phase discriminator PDC is also fed by means of lead 114 to control the switch valve circuit SVC while the latter operates a visual Fast/Slow indicator IND over lead 115. A further output from the phase discriminator PDC controls the coarse servo valve circuit CSC over lead 116.

Referring now to Fig. 2 the Waveform generating arrangements of unit WGU of the computing machine comprise a stable-frequency master or clock oscillator CPG operating at 100 kc./s. and providing a square pulse or Clock waveform of microseconds periodicity as shown in Fig. 3a. This oscillator serves to control the basic operating rhythm of the machine. The output Waveform from this clock oscillator CPG is applied to a first pulse dividing circuit DV1 which counts down by a factor of 4 to provide an output waveform DV1 as shown in Fig. 3b consisting of a square pulse in synchronism with every fourth Clock pulse. The output from this divider circuit DV1 is, in turn, applied to a second divider circuit DV2 which counts down by a factor of 6 to provide an output waveform DVZ as shown in Fig. 3c, consisting of a square pulse in synchronism with every 24th Clock pulse. This output from divider circuit DV2 defines the beat period of the machine rhythm as comprising a total of twenty-four Clock intervals. Twenty of these clock intervals are used for signalling the respective binary digits of a -digit binary number and the remaining four clock intervals are needed for accommodating the fly-back motion of the scanning beam in the cathode ray tube store. This fly-back period is defined by the negative-going pulse period of the blackout or B0 waveform, Fig. 3a. This waveform is generated in a trigger circuit BOPG which is set into one state by each output pulse from the divider circuit DV2 and is reset back into its original state by the next following output pulse from the divider circuit DV1 whereby the blackout pulse period embraces the first four clock intervals of each beat and the remaining twenty Clock intervals are available for digit signalling purposes.

The dynamic form of the number and instruction signals used within the machine take the form shown in Fig. 3e and comprise a train of pulses, one in each of those of the 20 clock intervals assigned to digit signalling where it is required to represent the binary digit 1. The absence of a pulse in any digit interval indicates binary value 0. Each 1 representing or Dash pulse is negative-going and persists for the first 6-microseconds of the IO-microseconds clock or digit interval. These 1 representing pulses are derived, by suitable selection, from a continuous train of Dash pulses, shown in Fig. 3 and generated in a square pulse generator circuit DSPG which is repetivitely triggered by the Clock waveform. The position of a pulse in the group of 20 successive digit 6 intervals of each beat is indicative of the binary power value or denomination which it represents. Thus the first digit interval (the 5th clock pulse) in each beat may be indicative of the binary value 2 and is known as the p0 interval, the second digit interval then representing binary value 2 and being known as the p1 interval and For the purpose of selectively examining any one of the 24 Clock or digit intervals of each beat period there is provided a series of repetitive pulsed waveforms each on separate leads and each comprising one pulse in each beat period coincident respectively with each of the different digit or clock intervals. These pulses, known as p-Pulses, are generated in a unit PPG which comprises a series of 24 combined trigger-gate circuits P0, P1, P2 P23. Each trigger circuit controls its associated gate to open the latter when the trigger circuit is in one (triggered) state and to close such gate when the trigger circuit is in the opposite (reset) state. All the gates are supplied in parallel with the continuous train of Dash pulses, Fig. 3 f, and the trigger circuits are interconnected as a counterchain whereby each trigger circuit is triggered by the passage of a Dash pulse through the gate of the previous trigger circuit of the chain and whereby such trigger circuit is, in turn, reset to its normal or gateclosed condition upon triggering of the next following trigger circuit of the chain. The first trigger circuit P20 of the group is triggered by the output from the divider circuit DV2 whereby it allows the dash pulse of the next clock interval, p20, to pass therethrough whereupon that trigger circuit is reset and the next circuit P21 is triggered to allow the next following Dash pulse in clock interval 221 to pass therethrough and so on, thus to provide the series of so-called p-Pulse waveforms of which those coincident with the digit intervals p23, p0, p7 and p15 are illustrated in Figs. 3g, 3h, 3i and 3 respectively.

The slave pulse signals recorded in the track ST of the magnetic drum MD comprise a series of pulses coincident in timing with those instants when the storage positions in the other data tracks DT corresponding to the p23 Clock interval are passing the read/write heads DH for such other tracks whereby a transient output signal output as shown in Fig. 3l is obtained from the read head SRH once immediately before the corresponding twenty digit recording positions for a data word signal are scanned by the read/Write heads DH of the storage tracks DT. It will be observed that the positive transient pulse has its peak coincident in timing with the leading edge of the p23 Pulse when exact synchronism occurs. The master square-wave generator MSG is triggered by such leading edge of each p23 Pulse of the p-Pulse waveforms whereby it reverses its state to provide the reference edge at that instant of the machine operating rhythm as indicated at x in Fig. 3m and the synchronisation problem is therefore to maintain the slave pulse signals re ceived from the track ST of the magnetic drum in exact register with the abrupt change or reference edge of the master square-waveform. This is effected as will be explained in detail later, by causing the aforesaid reference edge of the master square-wave effectively to divide the positive transient portion of the slave pulse into two parts and then to compare them for equality whereby no re sultant output signal is obtained when the parts are equal. Whenever a slave pulse is out of exact register with the reference edge of the master square-Wave it will be predominantly in phase with either the negative-going portion of the master square-wave which precedes the reference edge or alternatively with the positive-going portion which succeeds such reference edge and the output signal will accordingly be of negative or positive value, the amplitude of signal varying according to the degree of divergence with exact synchronism until the degree of slip is such that the whole of the slave pulse occurs during the positivcor negative-going part of the master square-wave.

If the slip is large, thatis to say, the drum speed is widely in error then the slave pulses will rapidly passthrough the positive and negative portionsofthe master square-wave in succession and, over a few cyclesof operation, the mean output from thephasediscriminator will again be zero if, as hitherto, the master square-wave is made symmetrical about its axis with the positive-going portions equal in length to the negative-going portions. When this occurs all control willfaila In the present invention the output from the discriminator is arranged tocontrol-the-formof the master square wave so that it is asymmetrical, that is to say either its negativeor its positive-going portion'is of greater duration than the opposite negativeor-positive-going portion as the case may be-whereby the mean output from the phase discriminator will not be zero when the slip is large but will be of. a polarity such that the tendency is tocorrect the error. Thus, if the slave pulses occur at a lower frequency than the reference edges due to the drum speed being too low, the positive portion of the master square-wave following the reference edge region willbe extended so that, in the. time interval before the slave pulses pass through the next following reference edge, the number of instances when the slave pulse occurs during the positive portion of the master square-wave will outnumber the instances when the slave pulses occur during the negative-portion of the same master squarewave cycle thereby giving a mean output from the discriminator which is positive and which reduces the'applied braking effects. Conversely, if the slave pulses occur at a greater frequency than the reference edges of the master square-wave, due to the drum speed being too high, the preceding negative portion of the master square-wave will be extended togive, in similar manner, a mean output from the discriminator which is negative and which causes anincrease of the braking force. The aforesaid slave pulse dividing action will finally take charge when the speed and angular-position of the drum is nearly correct toobtain and maintain accurate synchronism. These arrangements are effective between about A correct speed and the exactly correct speed of operation of the drum relative to the machine rhythm. For ensuring satisfactory operation in the still lower speed range when the drum speed is building up from rest a coarse servo-control is provided which temporarily overrides the above described arrangements and causes locking of the master square-wave generator in the condition where it provides a positive-going portion subsequent to each reference edge which is longer thanthe related negative going por- The servo-control arrangement will now be described in detail with reference to Figs. 5 and 6. Referring to Fig. 5, the master square-wave generator MSC comprises the valves V1, V2 (conveniently a double triode valve) and diodes D1, D2 and D3, while the phase discriminator PDC comprises the valves V4, V5 (also a double triode valve) and valve V6 withdiodes D6, D7, D8 and D9. Valves V7 and V8 constitute the switch valve circuit SVC and valve V3 with diodes D 1, D5 constitutes the coarse servo valve circuit CSC.

In the master square-wave generator MSG, valves V1 and V2 are arranged as a two-stable-state trigger circuit by conventional cross-connection of their anodes and control grids by way of resistors R1, R2 with shunting condensers C1, C2 the anode load of valve V1 being constituted by resistor R3 and that of valve V2 by resistor R4, both connected to source of positive potential +300 v. Resistors R7, R8 constitute grid leak resistances for valves V1 and V2 respectively to a source of negative potential l50 v. The cathode of each valve is earthed. The, control grid of valve V1 is. supplied by way of input lead 106 and diode D1 with the pZS-Pulsewaveform,

Fig. 3g and Fig. 4a, whereby the triggercircuit is trig gered by the leading edge of such p23-pulse to-the condi-- tion -wherevalve-Vl is cut-offandvalve V2 is conductivewith its grid at or about earth potential. This provides the reference edge x, Fig. 3m and'Figs. 4d and 4e.

The grid ofvalve V2" is similarly-supplied by way of lead 107 and diode D2 with. the plfi lulsewaveform, Fig. 3i and Fig; 46 whereby, unless other conditions, to be described later, arise, the trigger circuit is reset by the negative-ed'geof the-p15 Pulse=waveform whereby valve V2 is cut-offend valveVL is rendered conductive. The output from the trigger circuit under these conditions isindicated inFig. 3m (full and chain-dot lines) and in Fig.- 4d, in which the return or intermediate edge y of the master square-wave occurs. at the instant of the p15 pulse in each beat.

the p7-Pulse waveform, Fig. 3i and Fig. 4c. D3 is controlled by the switch valve circuit SVC' and if rendered conductive thereby, the p7-Pulse waveform is then operative to reset the trigger circuit of valves V1 and V2 earlier ineach cycle of the master squarewaveto produce an output waveform as shown in Fig. 3m (full and dot'lines) and'in'Fig. 4e where the intermediate edge y occurs at the instant of the )7 pulse in each beat.

In the phase'discriminator PDC valves V4 and V5 have their respective anodes connected to opposite end terminals of the'primary winding of a centre-tapped transformer T which centre tap is connected to a source of positive .potential- +300-v. The control grid of valve V5 is taken directly to a source of positive potential v. whereas the control grid' of valve V4 is connected by way of a resistor R11 to the junction point of apotentiometer network of resistors F9 and R10. The opposite end of resistor R9 is connected by way of the input terminal 104- and' lead 105 to the anode to a source of negative potential 150 v.

up the operation of the circuit.- The junction point between resistors R9, R10 leading to the control grid of valve V4, is also connected to the anode of a diode D6 whose cathode is taken to a source of positive potential v. and to the cathode of a diode D7 whose anode is taken-to a source of positive potential +85 v. These two diodes serve to limit the voltage swing at the grid of valve V4 within the range of +115 v. to +85 v. When the trigger circuit of valves V1 and V2 is in its triggered condition, i; c. with valve V1 cut olf and valve V2 conductive togive the positivegoing portion of the ma er square-wave, the anode ofvalve V1 is higher than +115-v. whereby the control grid of valve V4-is driven to +115 v. When however the trigger circuit of valves V1 and V2 is reset andvalve V1 is conductive then its anode is lower than +85 v. and the control gridof valve V4 is driven to The two cathodes of valves V4, VSare. interconnected and joined'by way of a load resistor R12 to the anode of a pentode valve Vfi-whose screen grid is supplied with its operating potential byway of resistor R15 con-- head SRH-whichis associated with the-slave pulse track The control gridof' valve V2 is also connected fed byway of diode D2- and condenser C3 to lead-l08 which is suppliedwith The diode.

R16 and condenser C is joined by way of leak resistance R17 to a source of negative potential 12 v. whereby valve V6 is normally cut-off and is rendered conductive only during the positive transient peaks of the output waveform from the read head SRH which is shown in Fig. 31.

When valve V6 is cut-01f, valves V4 and V5 are likewise inoperative as there is no available supply of space current thereto. When valve V6 is turned on by the positive transient of the Slave pulse signal, anode current will flow in Whichever one of the valves V4 or V5 is at that time rendered conductive. If the trigger circuit of valves V1, V2 is in its triggered condition with the anode of valve V1 high in potential, then valve V4 alone of the pair V4, V5 will conduct and current will flow through one half of the primary winding of transformer T. If, however, the trigger circuit V1, V2 is reversed or in its reset state then the control grid of valve V5 will be more positive than the control grid of valve V4 and current will flow only through valve V5 and the opposite half of the primary winding of transformer T.

The secondary winding of transformer T has one end connected to earth and its opposite end connected in parallel, by way of condensers C8 and C9 to diodes D8, D9, the latter being reversed in their polarity whereby the cathode of diode D8 is connected to the condenser C8 and the anode of diode D9 is connected to the condenser C9. The cathode of diode D8 is connected by way of leak resistor R18 to a source of positive potential +12 v. while the anode of diode D9 is connected by a similar leak resistor R19 to a source of negative potential l2 v. The anode of diode D8 and the cathode of diode D9 are interconnected and joined to one terminal of a condenser C whose opposite terminal is earthed. The interconnected anode of diode D8, cathode of diode D9 and condenser C10 are also joined to the phase discriminator terminal 124 In the operation of this phase discriminator circuit as so far described and assuming that the valve V6 is rendered conductive for a period which is accurately bisected by the reference edge produced in the master square-wave signal upon the changeover of the trigger circuit of valves V1 and V 2 from its reset to its triggered condition, then current will flow for an equal period of time in each of the two halves of the primary winding of transformer T and equal and opposite charging currents will be supplied to the condenser C10, first in positive-going sense by way of diode D9 and secondly in negative-going sense by way of diode D9. The net charge acquired by the condenser C10 is therefore nil and no output voltage will be provided at the phase discriminator output terminal 120. If however the positive transient of the Slave pulse supplied to the control grid of valve V6 is late, relative to the changeover instant of valves V1, V2 and the reference edge of the master waveform, then current will flow through valve V4 for a longer period of time than it will flow through valve V5 with the result that the condenser C18 will receive more current through diode D9 than it receives through diode D8 and the upper terminal of condenser C10 will acquire a positive charge a negative output control voltage will be produced at terminal 120. Similarly if the positive transient of the Slave pulse on the grid of valve V6 is early relative to the reference edge of the master squarewave then more current will be supplied through the valve V5 than through the valve V4 with resultant increase of current through D8 compared with that through diode D9 whereby the condenser C10 will be charged positively to provide a positive output voltage at terminal 120. If the transient of the Slave pulse on the control grid of valve V6 occurs wholly during the time when the trigger circuit of valves V1 and V2 are triggered then all of the current will flow through valve V4 with resultant charging of the condenser C10 positively. Con versely if the said transient of the Slave pulse occurs during the period when the trigger circuit of valves V1 and V2 is in its reset state then all the current will flow through valve V5 with consequent charging of the con denser negatively.

In the switch valve circuit SVC the valves V7, V8 (conveniently a double-triode) have their cathodes interconnected and joined by way of a load resistance R22 to a source of negative potential 150 v. The anode of valve V8 is directly connected to a source of positive potential +200 v. while its control grid is connected by way of resistance R21 and lead 114 to the output terminal 120 of the phase discriminator PDC. The gridof valve V7 is connected directly to earth and by way of condenser C11 to the anode of the valve which is joined by way of load resistor R23 to a source of positive potential +300 v. and also to one end of a potentiometer network of resistors R24 and R25 the opposite end of which latter is connected to a source of negative potential l50 v. The junction between resistors R24, R25 is connected by way of lead to the cathode of the diodes D3 in the master square-wave generator circuit MSG.

In the operation of this switch valve circuit, as the control grid of valve V7 is held at earth potential, current will flow in valve V7 and not in valve V8 whenever a negative voltage is available at the output terminal 120 of the phase discriminator PDC and under these conditions the potential at the anode of valve V7 will be lowered with a consequent lowering of the voltage, relative to earth, of the junction point between resistors R24 and R25. Under these conditions the potential on the cathode of the diode D3 will be lowered sufiiciently to render it conductive to the p7-Pulse waveform. Such p7-Pulses then operate as a resetting medium for the trigger circuit of valves V1, V2 whereby the master square- Waveform is reversed back from its positive-going to its negative-going state at the time of pulse p7 instead of at the time of pulse p15 as shown in Fig. 4e. When, however, the output potential on terminal 120 rises above earth, the control grid of valve V8 is likewise raised whereby this valve becomes conductive and V7 is rendered non-conductive. When this occurs the anode potential of valve V7 moves positively and accordingly raises the potential on lead 110 to bias the cathode of diode D3 positively to an extent whereby it blocks the transmission of any p7-Pulse therethr-ough. The resetting trigger circuit of valves V1, V2 of the master square-Wave generator MSG therefore has to await the arrival of the subsequent p15 Pulse as already described.

Connected to the anode of valve V7 by way of lead 115 and resistor R26 are two neon indicator lamps N1 and N2. The opposite electrode of lamp N1 is connected by way of feed resistor R27 to a source of positive potential +200 v. while the other electrode of lamp N2 is connected by way of resistor R28 to a source of positive potential +85 v. Condenser C12 connected between the junction of resistor R26 and the two neon lamps and earth serves as a reservoir condenser whereby the neon lamp N1 has sufficient potential impressed thereacross to cause its striking when the anode voltage of valve V7 is lowered i. e. when that valve is conductive, to indicate a Fast running condition and the opposite neon lamp N2 has sufiicient potential impressed thereacross to cause its illumination when the anode voltage of valve V7 is raised due to the valve being cut-off thereby indicating a Slow condition of running of the mag.- netic drum.

The coarse servo valve circuit CSC comprises valve V3 having its anode directly connected by way of lead 109 to the anode of valve V2 of the master square-wave generator MSG. The cathode of the valve is connected directly to earth while its control grid is joined by way of resistor R30, diode D4, condenser C15 and lead 116 to the anode of valve V6 of the phase discriminator 11 PDC. The junction of resistor R30 and diode 'D4' (anode) is also connected by way of resistor R311to-a source of positive potential+300 'v. and to one terminal of a condenser C14- whose other terminal is earthed. Diode D5 is connected by its anode to the junction between the cathode of diode D4 and condenser'ClS and has its cathode earthed.

In the operation of this coarse servo valve circuit, the valve V3 is normally conducting heavily due to the positive bias supplied to its control grid by way of resistor R31. Its anode current is drawn through the anode load resistor R4 of the valve V2 whereby the potential on the interconnected control grid of valve V1 is held down and-the trigger circuit of valves V1, V2 is locked over in the condition Where the output on lead 105 to the phase discriminator PDC iscontinuously at its high or positive value and indicative of slow running condition of the drum MD. Each positive transient of the slave pulses applied to the control grid of valve V6 causes a corresponding negative-going pulse on lead 116 and this is effective via diode D4 to neutralise, in part, the positive charge on the grid-connected terminal of condenser C14. Only when the frequency of the negative-going output pulses at the anode of va1veV6 reaches a value sufficiently fully to neutralise the positive bias voltage on the grid of valve V3, is that valve moved towards anode current cut-01f and the trigger circuit of valves V1, V2 freed to operate as previously described. This is arranged to occur at about normal running speed of the drum.

Referring now to Fig. 6 the output from the phase discriminator PDC on lead 111 is applied to the shaping valve V which is arranged in a feedback type circuit. The input to the control grid of valve V9 is by Way of resistors R33, R34 and R35 of which the resistor R34 is shunted by condenser C17. Condenser C18 is connected between the junction of resistors R34, R35 and earth and is shunted by a manual control switch S which, when closed, serves to disable the servo control system. The anode of the valve V9 is connected to a source of positive potential +300 v. by way of load resistor R36 and feedback to the control grid is by way of condenser C19. A further feedback for controlling the response of the circuit and to limit the degree of overshoot in the servo action is by way of a series network comprising resistor R37 and condenser C20 connected between the control grid of the valve and a tapping point on a potentiometer network of resistors R40, R41 and R42 which is connected between the anode of the valve and a source of negative potential +150 v. The tapping point of this feedback path between resistors R40 and R41 is also connected to the anode of a diode D10 whose cathode is connected to a source of positive potential +12 v. and to the cathode of a diode D11 whose anode is supplied from a source of negative potential +12 v. The cathode of valve V9 in addition to having a cathode load comprising variable resistor R39 shunted by fixed resistor R38'for varying the gridto-cathode bias potential and thereby the mean brake current when the system is running in synchronism, is also connected by way of a series network of resistors R43, R44, R45 and R46 to a source of positive potential +200 v. The tapping points on this resistance chain are each decoupled to earth by way of condensers C21, C22 and C23 and provide stabilised output points giving +ll v., +l00 v. and +85 v. for use in the other parts of the circuit as already indicated. The required biassing voltages of l2 v. and +12 v. already noted are provided from a separate potentiometer network comprising resistors R53, R54, R55 and R56 connected in series between a source of positive potential +200 v. and a source of negative potential 150 v. The tapping point between resistors R54 and R55 is connected directly to earth while the remaining tapping 12 points between resistors R53 and R54 and between resistors R55 and R56 are decoupled to earth by way of condensers. C24 and C25 respectively and provide respectively the supply potentials of +12 v. and -12 v.

The power amplifier circuit PA comprises a pair of parallel-connected valves V10 and V11. The input to the control grid of each valve is by way of resistor R47 from the tapping point between resistors R41 and R42 on the output network from the shaping valve V9 already described. The screen grids of valves V10 and V11 are independently supplied with their operating potentials by way of resistors R50 and R51 from the source of positive potential +300 v. while both valve anodes are connected together and joined to the source of positive potential +300 v. by way of resistor R52. The operating coils of the electromagnetic braking means BM are connected across this anode resistor R52 by way of leads 113.

The operation of the circuit arrangements described is as follows. Assuming first that the magnetic drum MD is at rest and that current is then supplied to the driving motor DM; Initially the frequency of the slave pulses derived from the recording track ST, will be low and the resultant negative charging current delivery to the condenser C14 of the coarse servo valve V3 will be insufficient to neutralise the positive bias-on the control grid of that valve, which therefore remains fully conducting continuously. This locks the trigger circuit of valves V1, V2 in the master square wave generator MSG, in the condition where valve V1 is continuously cut-off. In consequence a steady potential equal in value to that of the positive portions of the master square wave is applied over lead to the phase discriminator PDC and as a result a steady positive potential is available at the phase discriminator output terminal and by application to shaping valve V9 provides a steady negative potential at the control grids of the power amplifier valves V10 and V11 sufficient to reduce the braking current to zero or substantially so. The driving motor DM is thus unhindered in its acceleration.

These conditions prevail until the drum speed reaches about three quarters that of the proper operating speed whereupon the coarse servo valve V3 becomes cut off by the increased negative bias current delivered from the anode of valve V6 to the condenser C14 and thereafter the trigger circuit of valves V1, V2 becomes fully operative.

In each subsequent beat period of the machine rhythm the trigger circuit of valves V1, V2 will be put into its triggered condition by each 223 pulse delivered on lead 106 through diode D1 and as the discriminator output potential is still positive on lead 114, the valve V7 of the switch valve circuit SVC will still be cut off and a positive bias potential still maintained on the cathode of diode D3 whereby the next applied resetting pulse the p7 pulse on lead 108 is ineffective to reset the trigger circuit of valves V1 and V2; such resetting to generate the intermediate edge of each cycle of the master square waveform must therefore await the arrival of the :15 pulse on lead 107. In consequence the positive-going portion of the master square-wave immediately following each reference edge is materially longer than the negative-going portion which precedes the next subsequent reference edge, and, as indicated diagrammatically in Figs. 41 and 4g, there will be, over a plurality of cycles, more slave pulses during positive-going master square wave periods than there are during the negative-going master square wave periods, and as a result the smoothed potential at the output terminal 120 of the phase discriminator will still be positive, thereby maintaining the braking current applied to the braking means BM at a relatively low value.

Eventually. when the motor speed is nearly correct several successive slave pulses will lie in approximate register with therelated reference edges of the'square 13 waveform as shown in Figs. 4k and 4i whereupon the smoothed output voltage at the phase discriminator output terminal 120 will drop to zero and, after some degree of oscillation or hunting about the zero point, will settle at a value which adjusts the braking current delivery from the power amplifier valves V and V11 at the correct value to maintain synchronism.

Under these conditions any divergence of the slave pulses from the position in time where their positive transients are bisected exactly by each reference edge of the master square-wave, will produce a compensating change in the output potential at terminal 120 thereby to increase or decrease the braking current as may be required. Thus, if the slave pulses tend to creep forwardly, i. e. to be of slightly greater frequency than the master square-wave, then such pulses will become predominantly in phase with the negative portions of the master squarewave which precede each reference edge thereby providing an increased negative output on lead 111 which, acting through the shaping valve V9, will increase the braking current through valves V10 and V11. Conversely any creep or slip of the slave pulses backwardly, i. e. because the slave pulse frequency is slightly lower than that of the master square wave, will make such slave pulses predominantly in phase with the positive portions of the master square wave and thus provide a positive voltage on lead 111 which, through shaping valve V9, reduces the braking current through valves V10 and V11. It will be observed that during such near-synchronised conditions the position of the intermediate or resetting edge y of the master square wave-form is irrelevant.

If, for any reason the speed of the drum increases appreciably above that required for correct synchronism whereby successive slave pulses slip continuously forward with respect to the reference edges of the master square wave, then over a plurality of cycles from the point where the increase of speed commenced, the slave pulses will be predominantly in phase with the negative-going portions of the master square-wave whereby the output potential at terminal 120 of the phase discriminator PDC becomes negative. This applies a bias to valve V8 which causes valve V7 to become conductive and thereby produces a lowering of the voltage at the anode of valve V7 which correspondingly lowers the potential at the cathode of the diode D3 to an extent whereby the p7 pulses arriving on lead 108 are rendered effective to reset the trigger circuit of valves V1 and V2 ahead, in time, of the previously used p pulses so that the master square Wave now has its negative-going portion of appreciably greater duration than its positive-going portion. Thereafter any further increase of the relative frequency of the slave pulses with reference to the reference edges of the master square wave will still provide a negative control voltage terminal 120 even although such slave pulses may slip past the master square wave at a rate which, with the prior art arrangements using a master square wave of symmetrical form, would result in complete failure of control.

Although the invention has been described particularly with reference to the synchronising of a magnetic drum store forming part of an electronic digital computing machine having a cathode ray tube type of main data store and wherein the slave pulse signals are synchronised with a selected one out of each of a plurality of cyclically repeated trains of pulses, it will be apparent that the invention is capable of much broader application and may be used to synchronise a series of slave pulses with a corresponding series of related single pulses. The use of other pulses such as (nm)th and (n+m)th pulses for resetting the master square wave generating means, while convenient in the present application where such pulses are available, can be avoided, for instance, by constructing the trigger circuit of valves V1 and V2 as a single stable-state trigger circuit having two alternative durations of its unstable state. One duration could arrange normally to be operative and the other duration brought into effect under the control of the switch valve SVC. Similarly, numerous departures may be made even with the particular arrangements described. For ex ample the portion of the master waveform which follows each reference edge may be negative-going rather than positive-going provided that appropriate modifications are made to the phase discriminator circuit or to the servo sstem, while similarly the servo output may be positive when the slave pulses are predominantly in phase with the negative half cycles of the master waveform, and vice versa.

The values of the circuit components shown in Figs. 5 and 6 are as follows:

R1 ohms 100,000 R2 do 390,000 R3 do 27,000 R4 do..- 27,000 R5 do 470 R6 do 470 R7 do 330,000 R8 do 390,000 R9 do 100,000 R10 do 390,000 R11 do 470 R12 do 100,000 R13 do 5,600 R14 do 1,000,000 R15 do 68,000 R16 do 100,000 R17 do 100,000 R18 do 120,000 R19 do 120,000 R20 do.. 3,300,000 R21 do 470 R22 do 13,000 R23 do 47,000 R24 do 330,000 R25 do 390,000 R26 d0 100,000 R27 do 220,000 R28 do 220,000 R30 do 470 R31 do 15,000,000 R33 do 470,000 R34 do 2,200,000 R35 do 470 R36 do 82,000 R37 do 2,200,000 R38 dn 3,300 R39 do 1,000 R40 do 470,000 R41 do 15,000 R42 do 470,000 R43 do 18,000 R44 do 3,300 R45 dn 3,300 R46 do 18,000 R47 do.. 100,000 R48 do 470 R49 do 470 R50 do 15,000 R51 do 15,000 R52 do 15,000 R53 do 39,000 R54 (in 2,200 R55 do 2,200 R56 do 27,000 C1 picofarads 47 C2 do 47 C3 do C4 d0 47 C5 do 100 C6 do 470 C7 (in 470 C8 microfarad 0.01 C9 do 0.01 C10 do 0.002 C11 picofarads 470 C12 microfarad 0.1 C14 do 0.1 C15 picofarads 68 C17 microfarads 0.05 C18 do 0.01 C19 do 0.001 C20 do 2.0 C21 do 0.1 C22 do 0.1 C23 do 0.1 C24 do 0.01 C25 d0 0.01

Valves- V1, V2 12AT7 V4, V5 12AT7 V7, V8 12AT7 V3, V9 12AT7 V6 EF91 V10, V11 807 orN78 D1 6D2 D4 6D2 D5 6D2 D6 CG4C D7 CG4C D8 6D2 D9 6332 D11 CG4C I claim:

said signal output a master signal of square waveform I having a period time equal to the time interval between successive controlling pulses and said generator also includingsignal-controlled means for rendering the respective time durations of the positive and negative portions of each square Waveform period asymmetrical, a phase I discriminator having a first signal input for connection to said source of slave pulses, a second signal inputconnected to said signal output of said generator anda sig: nal output, said phase discriminator operating to, compare the phases of said derived master square wave signal and said slave pulses and providing at its signal output an output voltage which is of one or other polarity according to whether said slave pulses are predominantly in phase with the positive or negative portions,.as the case may be, of said master signal, servo means having a control signal input supplied with the signal output of said phase discriminator and an output connected to operate said timing control means of said source of slave pulses, said output of said servo means operating to 3djust the frequency of said slave pulses in a sense dependent upon the polarity of said output voltage of said phase discriminator and frequency discriminating means having a signal input also supplied with the output from said phase discriminator and a control signal output connected to operate said wave-asymmetry control means of said generator when the relative frequencies of said master,

Cir

16 signal and of said slave pulses are unequal to render the time durations of the positiveand negative-going portions of the master square wave signal asymmetrical in such sense that the mean value of said phase discriminator output over a plurality of slip cycles is of the polarity appropriate to correct the frequency of said slave pulses.

2. Apparatus according to claim 1 which includes coarse servo control means having a signal input supplied with a signal derived from said slave pulses and having an output connected to said master square wave generator, said servo control means operating to override the operation of said synchronising means when the absolute frequency of said slave pulses falls below a predetermined value;

3. Apparatus according to claim 1 wherein said slave pulses are derived from the movement of a rotating body and wherein said servo-means operate to adjust the speed of rotation of said body.

4. Apparatus according to claim 3 in which said rotating body is driven by a motor inherently capable of rotating said body at a speed in excess of that required for synchronism of said slave pulses with said controlling pulses and in which said servo means operate to control a braking force applied to said body.

5. Apparatus according to claim 4 wherein said control pulses comprise a predetermined one or nth pulse in each train of a number of cyclically repeated trains of pulses.

6. Apparatus according to claim 5 wherein said square waveform generator is adapted to be triggered by each of said nth pulses and to be reset to its opposite condition by selected other pulses of said cyclically repeated trains of pulses, the selection of said resetting pulse being determined by the direction of slip.

7. Apparatus according to claim 6 wherein the required asymmetry of said master square waveform is obtained by using either the (nm)th pulse-or the (n-l-m)th pulse of said cyclically repeated trains of pulses for resetting said trigger circuit.

8. Apparatus as claimed in claim 7 in which said master square wave generator comprises an electronic trigger circuit having two-stable-states with two independent controlling inputs, one of said controlling inputs being continuously supplied with said nth pulse signals and the other controlling input being continuously supplied with the (nm)th pulses and further supplied by Way of a gate circuit with the (n-l-m)th pulses said gate circuit being controlled by the output derived from the phase discriminator.

9. Apparatus according to claim 8 wherein said coarse servo control means includes a thermionic valve having at least'a cathode, a control grid and an anode having its control grid connected to a capacitance which is continuously supplied with current of one polarity by way of a resistance and which is also supplied with current which is of the opposite polarity and is variable in accordanc with the frequency of said slave pulses, said thermionic valve having its anode connectedto the anode of that one ofthe trigger circuit valves which will ensure that the trigger circuit is maintained continuously in the condition which provides an output appropriate to increase the drumspeed until the frequency of the slave pulses exceeds the predetermined value.

10. Electric signal synchronising apparatus for synchronising each pulse of a continuous train of slave pulses derived from the movement of and dependent upon the speed of rotation of a'rotating body having means for adjusting its speed of rotation, with the related pulses of a continuous train of controlling pulses which comprises an electric square waveform generator triggered by each of said controlling pulses to generate a master signal of square waveform having a period time equal to the time interval between successive controlling pulses, a phase discriminator connected to compare the phases of said derived master square wave signal and said slave pulses 17 v and providing an output which is of one or other polarity according to whether said slave pulses are predominantly in phase with the positive or negative periods, as the case may be, of said master signal, servo means actuated by the output of said phase discriminator and operating said speed adjusting means of said rotating body to adjust the speed of rotation of said body in a sense dependent upon the polarity of said output of said phase discriminator and frequency-discriminating means actuated by said phase-discriminator output when the relative frequencies of said master square wave signal and of said slave pulses are unequal for rendering the time durations of the positive and negative-going portions of the master square wave signal asymmetrical whereby the mean value of said output over a plurality of slip cycles is of the polarity appropriate to correct the frequency of said slave pulses.

11. Apparatus according to claim in which said rotating body is driven by a motor inherently capable of rotating said body at a speed in excess of that required for synchronism of said slave pulses with said controlling pulses and in which said speed adjusting means comprise a braking device controlled by said servo means.

12. Apparatus for synchronising each pulse of a continuous train of slave pulses with the related pulses of a continuous train of control pulses, said slave pulses being derived from and dependent for their timing upon the movement of a rotating body which is driven by a motor inherently capable of rotating said body at a speed in excess of that required for synchronism of said slave pulses with said control pulses and which has variable braking means for altering its speed of rotation, which comprises an electric square waveform generator triggered by each of said control pulses to generate a master signal of square waveform having a period time equal to the time interval between successive control pulses, a phase discriminator connected to compare the phases of said derived master square wave, signal and said slave pulses and providing an output which is of one or other polarity according to whether said slave pulses are predominantly in phase with the positive or negative peirods, as the case may be, of said master signal, servo means actuated by the output of said phase discriminator and operating to control said variable braking means to adjust the speed of rotation of said rotating body in a sense dependent upon the polarity of said output of said phase discriminator and frequency-discriminating means actuated by said phase-discriminator output when the relative frequencies of said master square wave signal and of said slave pulses are unequal for rendering the time durations of the positive and negative-going portions of the master square wave signal asymmetrical whereby the mean value of said output over a plurality of slip cycles is of the polarity appropriate to correct the frequency of said slave pulses.

13. Electric signal synchronising apparatus for synchronising each pulse of a continuous train of slave pulses, whose timing is controllable, with the related pulses of a continuous train of control pulses which comprises an electric square waveform generator connected to be triggered by each of said control pulses to generate a master signal of square waveform having a period time equal to the time interval between successive controlling pulses, a phase discriminator for comparing the phases of said derived master square wave signal and said slave pulses and for deriving an output which is of one or other polarity according to whether said slave pulses are predominantly in phase with the positive or negative periods, as the case may be, of said master signal, servo means adapted to be actuated by the output of said discriminator and when so actuated operating to adjust the frequency of said slave pulses in a sense dependent upon the polarity of said output, a first source of resetting signals for said generator, said first resetting signals occurring at instants subsequent to said control pulses which are more than half said period time later, a second source of resetting signals for said generator, said second resetting signals occurring at instants subsequent to said control pulses which are less than half the period time later, and switching means actuated by said phase-discriminator output for controlling which of said first and second resetting signals are effective on said generator so that when the relative frequencies of said master square wave signal and said slave pulses are unequal the time durations of the positive and negative-going portions of the master square wave signal are rendered asymmetrical in such sense that the mean value of said output over a plurality of slip cycles is of the polarity appropriate to correct the frequency of said slave pulses.

14. Electric signal synchronising apparatus for synchronising each pulse of a continuous train of pulses, whose timing is controllable, with the related pulses of a continuous train of control pulses, which comprises an electronic two-stable-state trigger circuit having a triggering input and a resetting input, a first source of resetting signals for said trigger circuit, said first resetting signals occurring at instants subsequent to said control pulses which are more than half the time interval between successive control pulses later, circuit means for applying said control pulses to said triggering input and said first resetting signals to said resetting input of said trigger circuit to generate a master signal of square waveform having a period time equal to the time interval between successive controlling pulses, a phase discriminator for comparing the phases of said derived master square wave signal and said slave pulses and for deriving an output which is of one or other polarity according to whether said slave pulses are predominantly in phase with the positive or negative periods, as the case may be, of said master signal, servo means actuated by the output of said discriminator and operating to adjust the frequency of said slave pulses in a sense dependent upon the polarity of said output, a second source of resetting signals for said trigger circuit, said second resetting signals occurring at instance subsequent to said control pulses which are less than half the time interval between successive control pulses later, circuit means including controlled switch means for applying said second resetting signals to said resetting input of said trigger circuit and frequency discriminating means actuated by said phase discriminator output and controlling said switch means whereby the time durations of the positve and negativegoing portions of the master square wave signal are rendered asymmetrical in such a sense that the mean value of the output from said phase discriminator over a plurality of slip cycles is of the polarity appropriate to correct the frequency of said slave pulses when the relative frequencies of said master signal and said slave pulses are unequal.

15. Electric signal synchronising apparatus according to claim 14 which includes coarse servo control means for over-riding the operation of said synchronising means when the absolute frequency of said slave pulses falls below a predetermined value.

16. Electric signal synchronising apparatus according to claim 15 which includes frequency sensitive means controlling said switch means to maintain said switch means open circuit whenever the slave pulse frequency is below said predetermined value.

17. Electric signal synchronising apparatus according to claim 16 wherein said coarse servo control means comprises a thermionic valve having at least a cathode, a control grid and an anode, a capacitance connected between said control grid and said cathode, a resistor connected between said control grid and a source of current of one polarity, a source of current of opposite polarity and variable in accordance with the frequency of said slave pulses connected to said control grid and a connection from the anode of said thermionic valve to a point on said two-stable-state trigger circuit which will operate to clamp said trigger circuit continuously in the condition which provides an output appropriate to increase the frequency of said slave pulses until said slave pulse frequency exceeds the predetermined value.

18. Apparatus for synchronising the operation of a magnetic drum type store in an electronic digital computing machine which comprises an electric motor for driving said magnetic drum store at a speed which is inherently faster than that required to effect synchronisation, electromagnetic braking means connected to said drum store, said braking means being variable to control the speed of said drum store, a recording of slave pulse signals at predetermined positions in a recording track around said drum store, reproducing means for reading said recorded slave pulse signals as electric slave pulse signals as said drum store rotates, a source of control pulses synchronised with the signalling rhythm of said computing machine, a master square-wave generator controlled by said control pulses to generate a master squarewave signal having a period time equal to the time interval between successive control pulses, said square-wave generator including signal-controlled means for rendering its square-wave output asymmetrical in either of two senses of which one has the half cycle following each control pulse longer than the subsequent half cycle and the other of which has the half cycle following each control pulse shorter than the subsequent half cycle, a phase discriminator supplied with the output from said slave pulse reproducing means and said master square-wave signal, said phase discriminator providing an output which is of one or other polarity according to whether said slave pulses are predominantly in phase with the positive or negative periods, as the case may be, of said master square-wave signal, braking current control means controlled by said output from said phase discriminator and supplying said electromagnetic braking means, and circuit means for applying the output from said phase discriminator to said signal controlled means of said master square-wave generator to control the sense of asymmetry of said generated master square-wave in accordance with the polarity of the output from said phase discriminator.

19. Apparatus for synchronising the operation of a magnetic drum type store in an electronic digital computing machine which comprises an electric motor for driving said magnetic drum store at a speed which is inherently faster than that required to effect synchronisation, electromagnetic braking means connected to said drum store, said braking means being variable to conpulse signals at predetermined positions in a recording track around saiddrum. store; reproducing means for reading said recorded slavepulsesignals as electric slave pulse signals as said drum store rotates, a source -ofron trol pulses synchronised with the signalling rhythm of said computing machine, a master square-wave generator controlled by said control pulses to generate a master square-wave signal having a period time equal to the time interval between successive control pulses, said squarewave generator including first signal controlled means for rendering its square-wave output asymmetrical in either of two senses of which one has the half cycle following each control pulse longer than the subsequent half cycle and the other of which has the half cycle following each control pulse shorter than the Subsequent half cycle, and second signal controlled means for suspending its operation so as to provide a continuous potential output, a phase discriminator supplied with the output from said slave pulse reproducing means and said master squarewave signal, said phase discriminator providing an output which is of one or other polarity according to whether said slave pulses are predominantly in phase with the positive or negative periods, as the case may be, of said master square-wave signal, braking current control means controlled by said output from said phase discriminator and supplying said electromagnetic braking means, circuit means for applying the output from said phase discriminator to said signal controlled means of said master square-wave generator to control the sense of asymmetry of said generated master square-wave in accordance with the polarity of the output from said phase discriminator, a frequency-sensitive circuit supplied with said slave pulse signals and providing an output signal of predetermined polarity whenever the frequency of said slave pulse signals is below a chosen absolute valueand circuit means for applying said output from said frequency-sensitive circuit to said second signal controlled means of said square-wave generator.

References Cited in the file of this patent UNITED STATES PATENTS 2,406,978 Wendt et al. Sept. 3, 1946 2,495,946 Schuler Jan. 31, 1950 2,558,358 Hales June 26, 1951 2,614,169 Cohen et al. Oct. 14, 1952 2,652,554 William et al Sept. 15, 1953

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