US2227066A

US2227066A - Television and like systems

Info

Publication number: US2227066A
Application number: US59022A
Authority: US
Inventors: Cork Edward Cecil; Bowman-Manifold Michael; Faudell Charles Leslie
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1935-01-15
Filing date: 1936-01-14
Publication date: 1940-12-31
Anticipated expiration: 1957-12-31
Also published as: DE893806C; US2210523A; GB455375A; DE754557C; FR800602A; FR49228E; GB489231A; US2212933A; FR832232A; GB490150A; BE413315A

Description

Dec. 31, 1940. E. Q CORK ET AL 2,227,066

TELEVISION AND LIKE SYSTEMS Filed Jan. 14, 1936 zfsneets-sheet 1 fm l 4 RAD/o Recs/vela 7a. arc/Lavez HOME? Dec. 31,1940. E. c. 'CORK Erm. Y 2,227,066

TELEVISION AND LIKE SYSTEMS Filed Jan. 14, 1936 2 Sheets-Sheet 2 (7a 076/1491, /Mvofr l Patented Dec. 31, 1940 2,227

UNITED STATES PATENT OFFICE TELEVISIGN AND LIKE SYSTEMS Edward Cecil Cork, Ealing, London, Michael Bowman-Manifold,` Worplesdon Station, and Charles Leslie Faudell, Acton, London, England, assignors to Electric & Musical Industries Limited, Hayes, England, a company of England Application January 14, 1936, Serial No. 59,022 In Great Britain January 15, 1935 15 Claims. (Cl. 178-75) The present invention relates to television and like systems, and is particularly but not exclusively concerned with the problem of synchronising a scanning operation at a receiver of a television system with theA scanning operation vtaking place at the transmitter.

It is usual to control the scanning operation at the receiver by means of impulses generated atV the transmitter; in mostsystems in which, for example, a cathode ray tube is employed to reconstitute the transmitted picture, these impulses are caused to `control the generation of saw-tooth scanning oscillations.

rFwo sets of synchronising impulses areusually transmitted; an impulse, known generally as a strip impulse, is generated'immediately before or after the scanning of each strip of the picture to be transmitted, and a frame impulse is generated between complete scans of the picture or, in the case in which the object `to be transmitted is a cinematograph film, between the scanning of successive frames. The strip impulses are generally arranged to be of substantially the same amplitude as the frame impulses. "In order that impulses of one set may be separated at the receiver fromv those of the other, it is usual to arrange that the frame impulses are oi longer duration than the'kstrip impulses; since then the frame impulses contain component frequencies lower than any inthe strip impulses, the strip impulses may be substantially completely excluded from the frame s'ynchronising apparatus at the receiver by means of a low-pass filter. `The effect of this filter is to attenuate the strip impulses relatively to the frame impulses, with the result that the impulsesare made to differ considerably in amplitude; they may then be separated from one another by an amplitude separation, or fed to a saw-tooth oscillation generator such for example as a blocking oscillator which requires an impulse of greater than a critical amplitude to trigger itv off. i

If it is necessary 'to exclude the frame frequency impulses from the strip synchronising apparatus at the receiver, vthis may be `achieved by means of a high-pass lter.

In systems employing a method such as that outlined above,for separating one set of synchronising impulses from another, it has been that the operation of frame synchronising has tended to be `upset by one or more elements of the low pass filter holding a residualcharge after the strip synchronising impulse last received has ceased. The reason for this is that between pulses. 'I'he residual charge adds residual charge held by the filter results blocking oscillator being triggered-ofi than it would otherwise be.

particularly in cases in which interlaced sc a different set of strips of the object to be successive scans of the object, the strips of complete scan being separated by blank To make this point more clear, it will differ by one half of a strip scanning will be clear that the residual charge due preceding strip impulse which is added added to the alternate framing impuls framing impulses thus trigger the blocki picture occur in pairs.

It may `also be foundv that interfering operation of synchronising. A pulse due cause that frame' impulse to take effect frame impulse may take effect at the ning is employed or not.

Similar problems arise in systems othe N,ttt

the time constant of the separating meansv is at least of the same order as the shortest interval to .the

charge due to the next succeeding impulse and increases its amplitude. In a case in which the impulses are applied to a blocking oscillator, the

in the earlier The effect referred to manifests itself more anning is employed, and it tends to produce distortion ofv the reconstituted picture. In interlaced scanning,

transmitted is scanned in each oi a small plurality of one of the sets being intercalated between the strips of another set. In such cases, the distortion above mentioned may take the following form: the strips in the reproduced picture may not be evenly interlaced, but may overlap one another, successive groups each comprising a number of strips equal to the number of part-scans making up a spaces.

be assumed that the object tov be transmitted is scanned completely in two complete scans; the phases of successive framing impulses must then period with respect to the strip impulses, and it is, convenient to arrange that one framing impulse occurs near to the endV of a strip, while the next occurs near to the middleof a strip period. It

to the to the frame .impulses occurring near to theend of a strip is less than the residual charge which is curring near to the middle of a strip. The latter cillator too soon, and the strips in the reproduced pulses,

such as those due to atmospherics, may upset the tov an atmospheric occurring just before a'irame impulse may produce a residual charge suiiicient to earlier correct time; distortion of the reproduced picture may arise from this cause whether interlaced scanr than The present invention accordingly providesa method of separating impulses of relatively long duration from impulses of a relatively short Vduration, which method comprises the step of converting said impulses into derived pulses, the amplitude of each of which increases during a period of time substantially equal to the duration of the corresponding impulse to a maximum value which is dependent on said duration, andthen falls to a minimum value at a rate which is greater than that of the increase and such that said minimum value is substantially fixed. In order'that one impulse shall not affect that which follows it, yit may be necessary to make the rate at which the amplitude of the derived pulses falls more rapid when the impulses succeed one another closely than when they are more widely spaced. i

f The invention further provides, for use in television systems, a method of separating frame impulses of relatively long luration -from strip impulses of `relatively short duration, which method comprises the step of converting said impulses into derived pulses, the amplitude of each of which increases, duringa period of time substantially equal to the `duration of the corresponding impulse, to a. value which is dependent on said duration.- and then falls, ata rate which is greater than that of the increase, and is such that, at least in the case of a pulse derived from a line impulse, a substantially fixed value is reached beforethe arrival of the next impulse.

According t0 a further feature of the invention, apparatus for separating impulses of relatively long duration from impulses of relatively short duration comprises an impedance element, and controlling means adapted to be fed with both long and short impulses and to cause the chargeA held by said impedance element to be changed in one sense during the impulses and in the opposite sense during the intervals between impulses, .characterised in that the time constant ofthe circuit determining the rate of change during impulses is much longer than the time constant 'of the circuit determining the rate of change duringintervals between impulses. The time constant of the first-named circuit may be veormore times that of the second-named circuit.

The invention also provides a method of operating apparatus according to the preceding paragraph, according to which the time constant of the second-named circuit is made much shorter than the interval between any short impulse and a longer impulse following it. According to another method of operating apparatus according 4to the preceding paragraph, the time constant ofthe rst-named circuit is made longer than the duration'of a long impulse.

In apparatus according to the invention, the controlling means may be arranged to prevent appreciable charging current flowing in said impedance element except when either a long or a short impulse is operative upon said controlling means. In these circumstances, since the impedance element only takes appreciable charge during the applied impulses, it will be clear that the Vlong impulses give rise to a greater maximum `charge than the short impulses. The impedance element may be a condenser, in which case two sets of substantially triangular voltage "pulses of diiierent amplitude are set up across the` condenser or it may be an inductance coil. In the latter case, the charging current is conveniently caused to flow in a resistance arranged in'series with the coil; triangular voltage pulses are then set up across the resistance. The triangular pulses oflarger amplitude may then be separated from those of smaller amplitude by an amplitude selection for example. Alternatively, the triangular pulses may be fed directly to a utilising device which in the case of a television system .for example, may comprise a saw tooth oscillation generator, and which is arranged to be responsive only ,toV the pulses of larger amplitude.

The controlling means is preferably a thermionic valve whichV is arranged to allow the impedance element tocharge up during the time for `which kthe impulses last. The mixed impulses are conveniently fed to the control grid circuit of the controlling valve. When the impedance element isa condenser, the valve is arranged to be conductive, and to prevent the condenser from charging up, during the intervals between impulses, and -it is arranged to be insulating during `the impulses. When an inductance coil is employed, however, the valve is arranged to be insulating atall `times except when the impulses are operative.

Other yfeaturesrof` the invention will appear hereinafter.

Certain embodimentsof the present invention as applied to television systems will now be described by 4way of example withreference to the accompanyinggdrawings:` it -will be assumed throughout this description that it is desired to separate framingimpulses from strip impulses, the latter having 'the shorter duration; it will alsobe `assumed that separation is effected at the-receiver, `and :that the received signal, in which the twosets of impulses appear together, is of the form shown in Fig. l.

In the drawings, Fig. 2 shows schematically, by way of examplefa'television receiver, employing aicathode ray tube as a picture-reconstituting device, to which the present invention is applicable, and Figs. 3 to '7 show various circuit arrangements suitable for use in the vreceiver of Fig` 2.

Referring to Fig. 1, the line l represents a datum line corresponding to black in the picture signals 2. The strip impulses 3 and the framing'impulses; 'of which only two, numbered 4' and 5 are shown, are Iall ofY substantially the same amplitude and are transmitted in the blackerthan-black sense. The framing impulses are of longer duration than the strip impulses, and are adapted to give interlaced scanning in two traversals of the object to be transmitted. The framing impulse 4 commences at the end of a strip period, whilst the impulsev 5 commences at the middle of a strip. The signal of Fig. l is given for purposesof explanation only, and the invention is kof course not restricted to this particular form of signal. For example, a plurality of impulses may be transmitted at the end of lr2,227,066 each frame, and the synchronising impulses may be in the same sense as the picture signals, but of greater amplitude.` Furthermore, as has been stated, the invention is not limited to interlaced scanning; where, however, interlaced scanning the cathode-ray picture-reconstituting device d.

'I'he signal of Fig. 1 is also applied to a selector e which serves to free the synchronising signals (3, 4 and 5) from picture signals (2), and to feed the synchronising signals to a line-frequency scanning oscillation generator f and to a separator g, the latter serving to separate frame scanning impulses from line scanning impulses and to feed the frame impulses to a frame-frequency scanning oscillation generator k. The generator f feeds saw-tooth line-scanning o-scillations to scanning coils h and generator 7c feeds similar oscillations at frame frequencyto coils v.

Referring to Fig. 3, which shows one` form which the apparatus e and g may take, a screened grid valve having twoanodes 6 and 1, has its anode 1 connected through a load resistance 8 to a point in a suitable `anode current source, which is not shown for the sake of simplicity. All points in the gure bearing a positive sign are connected to this source, which has its negative terminal connected to the cathode of valve 5. The anode 6 of the valve 5 also has a load resistance, not shown, and is arranged tor feed the received synchronising impulses to the stripfrequency scanning oscillation generator f, Fig. 2 which, since the received picture is to be reconstituted by `means of a cathode ray tube, may conveniently comprise a saw-tooth oscillation generator such as a blocking oscillator.

It will be noted` that all of the synchronising impulses have an initial steep transient portion. With the exception of that of impulse 5, and every alternate framing impulse thereafter 'and therebefore, these transient portions recur at the strip scanning frequency, and all ofthe synchronising impulses, including the framing impulses, are fed to the strip synchronising apparatus. The framing impulses, with the exception of those occurringduring the strip periods, thus assist in maintaining the strip Ascanning operation in synchronism.

The received signal (Fig. 1) is fed to the control grid circuit of valve 5 in sucha` sense that the synchronising impulses drive the control grid potential in the positive direction. The control grid is biasedto a point just/beyond anode current cut-off, and thus of the received signal, only the synchronising impulses cause anode current to flow, the picture signals being ineffective.

'Ihe synchronising impulses, free from picture signals, `are se't up across resistance 8V and are fed through coupling condenser 9 to the control grid circuit of a screened grid valve I0. The control grid of this valve is biased to a small positive potential. The amplitude of the Synchronising impulses at the control grid of the valve I is made such that each impulse swings the potential of that grid to a value more negative than that corresponding to anode current cut-off, so

vthat the valve passes no anode current during :the synchronising impulses.

'Ihe anode circuit of valve I9 comprises a high resistance II, and two condensers I2 and I3 are connected in series between the anode and the cathode of this valve.' It will be clear that during the intervals between synchronising impulses, that is, when valve Iii is conducting, most of the current in resistance II flows through valve I0; little charge flows to condensers I2 and I3, and a certain small datum potential difference (which may be arranged to be approximately zero) is set up across condenser I3. However, during an impulse, when valve Ill is insulating, the condensers I2 and I3 chargeup, condenser I3 reaching to a potential which exceeds the datum potential, and is dependent, inter alia, upon the duration of the impulse. Furthermore, when the impulse ceases, the potential diierence across condenser I3 is relatively rapidly reduced to the datum value, and no residual charge due to the impulse remains to add to the next succeeding impulse.

In one arrangement, employing a received signal of the form shown in Fig. 1, the duration of each of the line impulses is 10 microseconds `(ms), the frameimpulses each have a duration of 4'0 ms., and the minimum interval between successive line and frame impulses is 40 ms.; `in this arrangement, the time constant vgoverning the charging of condensers I2 and I3 is approximately 200 ms., while the eiective time constant of discharge is approximately 5 ms.,

A series of triangular voltage pulses, the amplitude of each of which rises during the impulse period to a maximum 'value dependent on the duration of the impulse and then falls at a relatively rapid rate toa substantially xed minimum value, is thus set up across condenser I 3. The

triangularpulses due to the framing impulses are of greater amplitude than those due tothe strip impulses, and the `whole series of pulses is fed to a valve I4, which'is arranged to effect an amplitude selection.` t l 'I'he control grid of Valve I4 is biased relatively to its cathode bymeans of biasing resistance I5 and its associated shunt condenser to a point beyond anode current cut-off, and it is arranged that only the triangularpulses of greater amplitude are able to causean'de `current to ow. There is thus set up in the'output circuit of valve I4 a series of pulses each of which hasl the same phase as the framing impulsewhich` initiated it. The output circuit of valve I4`is coupledl to a blocking oscillator for example, which constitutes the frame-frequency generator 7c, and each pulse in the output circuit of valve I4 is arranged toy initiate a. return stroke of the saw-toothframe `scanning oscillation.

In the arrangement lshown in Fig; 4, the valve I0', which corresponds-to valve I0 in Fig. 3,\has a resistance I6f in its grid circuit, anda load resistance II of highvalue initsvanode circuit. The valve l'may be shunted by a similar screened grid valve (not shown) to feed the line synchronising apparatus, or a double-anode valve may be employed in place of valve Ill. The received signal is applied to input terminals I8 in such a sense that the picture `signals tend to make the control grid more positive; the control grid is biased so that picture black makes the grid slightly more positive with respect to the cathode than )the potential at which grid current commences to ow, and` the picture signals accordingly lie within the region of grid current, but owing to the voltage drop across resistance i6,

the actual grid potential remains substantially constant and unaffected by the ow of grid current. The synchronising impulses are arranged to reduce the grid potential to a point more negative 'than anode current cut-off.

' Connected between the anode and cathode of valve HJ are two condensers I2 and I3 arranged in series. Asin the arrangement of Fig. 2, a series of triangular pulses is set up across condenser I3, the pulses corresponding to the framing impulses being of greater amplitude than those corresponding to the strip impulses.

The whole series of pulses set up across condenser I3 is applied between the control grid and cathode of a grid-,controlled gas discharge `tube I9, the grid of which is so highly negatively biased by means of biasing resistance ZIJ and its associated shunt condenser that only the pulses of greater amplitude set up across condenser I3' are able to initiate lthe iow of anode current in the tube I9.

The anode circuit of the tube I3 comprises coil 2|, resistance 22 and condenser 23 arranged as shown; the circuit I9, 2I, 22 and 23 is selfoscillatory, and operates to set up a potential difference of saw-tooth wave form across condenser 23, and it is arranged that the frequency of this saw-tooth oscillation is equal to the frequency of the voltage pulses of greater amplitude, and hence to the framing frequency. The saw-tooth oscillation set up across condenser 23 is employed for scanning purposes. The valve I9 and its associated circuit thus constitutes the frame-oscillation generator k of Fig. 1.

In a modification of the arrangements of Figs. 3 and 4, the condensers I2 and I3, or I2 and I3 respectively, are connected in series between the anode of controlling valve I0 or I0 and the positive terminal of the anode current source. Now in the arrangements of Figs. 3 and 4, when an impulse arrives, the potential at the junction point of the two condensers becomesmore posi'.- tive, and the potential difference across condensers I3 and I3 increases, In the modified arrangements, although the potential at the junction point becomes more positive when an impulse arrives, the potential dierence across condensers I3 and I3decreases. The modified arrangements are fundamentally equivalent to those shown in Figs. 3 and 4, and in both cases the change in the charge on condensers I3 and I3 maybe regarded as due to the ow of charging current tending to make the junction point of condensers I2 and I3, or I2' and I3 more positive.

Fig. 5 shows an arrangement in which the synchronising impulses are converted into triangular voltage pulses of different amplitudes by causing them to initiate the flow of vcharging current in an inductance.v

Referring to Fig. 5, a screened grid valve 24 has in its anode circuit an inductance coil 25 in series with a resistance 26. Means (not shown) are provided whereby the control grid of valve 24 is so biased, relatively to the cathode thereof, that no anode current flows in the absence of applied signals, and av received signal (such as that of Fig. 1) is applied to input terminals 2'I in such a sense that only the synchronising impulses cause anode current to flow, the picture signals having no effect. The time constant of the coil 25 and resistance 26 is made longer than the duration of a framing impulse, and during each synchronising impulse, the valve 24 conducts and a current gradually builds up in coil 25. At

the end of the pulse, the; valve insulates and the charging current is shut off; triangularvoltage pulses are thus set up across resistance 23.

Means (not shown) are preferably provided,

in the arrangement of Fig. 5, for damping the back electromotive force set up across the coil 25.

A backed-off diode valve may be employed, if desired, to separate the triangular voltage pulses of greater amplitude from those of smaller amplitude. An arrangement employing a diode valve f is shown in Fig. 6. 1

Referring to Fig. 6, a screened grid valve `2B has an anode resistance 29, and a condenser 33 connected between its anode and cathode.

Means (not shown)` are provided whereby thel is applied to the input terminals 3| in such a sense that the picture signals lie within the region of grid current, the synchronising impulses reducing the anode current in the valve to zero.

As in the arrangement of Fig. 3, triangular volt- I age pulses are set up across condenser 33.

These voltage pulses are established across a circuit comprising a winding 32 of a three coil transformer, a diode valve 33, and a source of rent commences to ow, and the received signal Z0 potential difference-33 which is arranged with 30 its positive polev connected to the cathode of diode 33. vThe potential of the source 35 is greater than the potential 4set up at the anode of the diode by the voltage pulses of smaller amplitude set up across condenser 33, but is less f than the potential due to the pulses of greater amplitude. Each ofy these latter pulses thus causes current to flow in the diode 33 and winding 32; in a modified arrangement, which is sometimes found preferable, the positions of g diode'33 and coil 22 are reversed, the` anode of the diode being connected to the anode of valve 28.

The

windings

35 and 36 of the transformer form a part of the frame frequency blocking oscillator, which is of a known kind, and comprises a blocking oscillator valve 3l; for the sake of simplicity, it is not proposed to describe either the construction or the method of working of this blocking oscillator, since these details are not of primary importance from the point of view of the present invention. It will suffice tol say that the blocking-oscillator can be arranged to generate a saw-tooth oscillation of the same frequency as controlling impulses applied to the blocking oscillator valve 31; vwhen current flows in diode 33 and winding 32, due to a pulse of greater amplitude across condenser 33, the impulse which is accordingly set up across winding 3I`is fed to the grid circuit of blocking oscillator valve 31, and initiates a returnstroke of the sawtooth oscillation.

A dry contact rectifier may be employed in place of the valve 33 in the arrangement shown in Fig. 6; preferably the winding 32 of the threecoil transformer` is electrostatically screened from the remaining windings.

Referring now to Fig. '7, which yshows diagrammatically a preferred form which the apparatus e, gand lc of Fig. 2 may take, a screen grid valve il has its anode connected to the positive terminal of a source (not shown) of anode current, the negative terminal 0f which is earthed, through a resistance 42, and, has its control and screen grid circuits coupled together by means of an iron-cored transformer 43. 'The valve 4| is adapted to operate as a blocking oscillator, and to generate saw-tooth scanning'oscillations which are taken oi from'a suitable condenser (not shown) connected between terminals 44. A condenser 45 is connected in the controlgrid circuit of valve 4|, and the grid of this` valve is connected to its cathode `through a leak resistance 46.

Received television signals of a form such as that illustrated in Fig. 1 already referred to, are fed to input terminals 21 and, for example, by an arrangement similar to that disclosed by Fig. 5, are established between the control grid and cathode of a pentode valve 48 in such a sense that the picture signals cause the controlgrid potential to increase in the positive direction relative to the cathode potential. l

Now the D. C. component of the picture signals is arranged to be present in the received signals, and in the case of a transmission by modulated carrier, it is arranged that the output from the signal detector at the carrier amplitude corresponding to black in the picture is somewhat positive relative to earth; valve 48 is connected through a conducive coupling to the output electrode of the signal detector, and the control grid is accordingly biased negatively to a corresponding suitable extent relatively to its cathode by means of biasing resistance 49, which is in series with resistances 5l, 52 and 53 between the positive and negative terminals of the anode current source, and grid current thus flows while vpicture signals of any amplitude from black upwards are present on the control grid. A highresistance 50 is connected in series with the control grid-cathode path of valve 48, and the now of grid current thus produces substantially no change in the control grid potential of the valve. The picture signals accordingly produce substantiallyV no changes in anode current, and the synchronising impulses are thus effectively freed from them.

The anode of valve 48 is connected to the positive terminal of the anode current source through two

resistances

54 and 55 in series, and the junction point of these resistances is -connected through condenser 58 tothe inputcircuit of a strip frequency blocking oscillator (not shown) by means of lead 5l. If

Aresistances

54 and 55 are made sufficiently large,` then, with a suitable screen-grid potential, the synchronising signals are freed from picture` signals by anode current limitation, and the separation is then not dependent upon the flow of grid current. The resistance 50 is retained in this case, however, and serves to reduce the load on the source of signals due to the input capacity of valve 48.

Resistances

54 and 55 may have respectively the values 50,000 ohms and 200,000 ohms. y

Current is fed to the screen grid of valve 48 through resistance 58, and a condenser 59 is connected between the screen grid and earth. In operation, the synchronising impulses swing the control grid of valve 48 negatively to such an exshown) by the valve 48; the latter may arise, for.

example, should the shape of the synchronising signal be such that current commences to flow in valve 48 before the oscillationg'enerated by the blocking oscillator ceases. During each synchronising impulse appearing at the control grid of valve 48, condenserv 59 receives a charge. When lthe impulse ceases, the condenser 59 discharges through the screen grid-cathode path of Valve 48.

A series `of substantially triangular voltage pulses is thus set up across condenser 59, the pulses each having an amplitude 'dependent on the duration of the synchronising impulse to which it is due, and on the magnitude of con-` denser 59. The frame impulses thus produce triangular pulses ofgreater amplitude than do the strip impulses.` r

The whole series of triangular pulses is established between the anode and cathode of a diode valve 50. The cathode of the diode 60 is connected to the junction point of resistance 52 and 53 and is thus biased positively relative toearth, the magnitude of this bias being made greater than the steady potential of the anode of diode 60, that` is, the potential of the screening grid of valve 48 in the absence of vsynchronising signals by an amount such that current owsin diode 89 only when a triangular pulse due toa frame impulse is applied to it. The diode is connected in series with one winding of transformerv 43, and when a frame impulse is received on the grid of valve 48, a pulse of current ows in the diode and in the transformer winding, and the blocking oscillator valve 4l is thus triggered off.

It will be observed that since the valve 48 is a pentode, disturbances from the strip-frequency blocking oscillator which may be fed back alongk lead 5l to the anode of valve llirare prevented by the screening action of the'suppressor grid, from affecting the frame frequency oscillator. Furthermore, disturbances from the frame-frequency oscillator `are to some extent prevented from` reaching the strip-frequency oscillator by the diode 60. However, in general, the scanning oscillation generated by the frame-frequency oscillator lasts for several line-intervals, and during a part of the cycle of this oscillation, the diode 60 may conduct, and this mayresult in the potential of the screen grid of valve 48 being reduced to such an extent that the anode current of this valve is cut oil. Should this happen, strip synchronising impulses are prevented from reaching the strip-frequency oscillator, whichA may accordingly fallout of synclfironism.` This effect may be avoided by giving the screening grid of the valve .48 a suitably chosen high positivepotential, or by connecting the cathode of diode (illl not to the lower end of the control grid `winding of transformer 43, but to a suitable tapping point therein, in which case resistance 52 may be short-A circuited.. l 4

The invention is, of course, not limited to the arrangements described, and many modifications and elaborations'of these arrangements, within the scope of the appendedclaims will be apparentV to those versed in the art.

We claim: 1

1.-,In a method of separating impulses of relatively long duration from impulses of a relatively short duration, the steps of converting the impulses into derived impulses, increasing `the amplitudeof each ofthe derived impulses in accordance Vwith the duration of the first two named impulses, and decreasing theamplitude'of:

relatively long duration from line impulses of relatively short duration, the steps` of converting the impulses into derived impulses, increasing the amplitude of each of the derived impulses in accordance with the duration of the rst two named impulses, decreasing the amplitude of the derived impulses at a rate greater than that of the rate of increase to a predetermined minimum value, and initiating successive derived impulses only after the cessation of the preceding derived impulse.

3. Apparatus for separating impulses of relatively long duration` from impulses of a relatively short duration, comprising an impedance element, means for storing energy in said impedance element, means for controlling the time periods during which the storing means is operative by received impulses, means for enabling the stored energy to change in one sense during the time said impulses are received, and an electrical network having a time constant much shorter than said last nam-ed means for enabling the stored energy to change in the opposite sense during intervals between received impulses.

4. Apparatus as claimed in claim 3 and wherein the time constant of the electrical network is made shorter than the interval between any short impulse and a subsequent long impulse.

5. Apparatus as claimed in claim 3 wherein the time constant of the last named means is made longer than the duration of a long impulse.

6. A television synchronizing system including apparatus for separating impulses of relatively long duration from impulses of a relatively short duration, said apparatus comprising an impedance element, a circuit for .enabling a charge held by said impedance element to change in one sense during said impulses, a circuit having a time constant much shorter than that of said first-named circuit for enabling the charge held by said impedance element to change in the opposite sense during intervalsA between impulses, controlling means for preventing substantial charging current flowing in said impedance element except when one of said impulses is operative upon said controlling means, means for feeding both long and short impulses to said controlling means an oscillation generator, and means to initiate oscillations of` said generator by potentials arising from the charge in said impedance element. 7. Apparatus according to claim 6, for use in television systems, wherein said controlling means comprises a discharge device, means for feeding composite signals comprising picture signals and frame and line synchronising impulses to said controlling means'and means for operating said discharge device to control the charging of said impedance element and amplitude separation means to segregate said impulses from picture signals.

8. Apparatus for separating impulses of relatively long duration, from impulses of a relatively short duration, said apparatus comprising a condenser, a thermionic valve having the anode-- cathode path thereof arranged eiectively in shunt with said condenser, means for Vfeeding said long and short impulses to said valve, means for causing said path to be conducting in the absence oi an impulse of either duration and means to make said path non-conducting when an impulseis present, and a source of charging current for said condenser.

, 9. Apparatus according to claim 8, comprising a thermionic valve having a cathode,v an anode j and a control grid, means including the condenser for increasing the potential of the control grid in' a positive sense relative to said cathode, and

sense of the potential difference of said source being such that current flows in said device only when a derived pulse of greater amplitude exists across said condenser.

l1. A television synchronizing system including apparatus for separating impulses of relatively long duration from impulses of a relatively short duration, saidl apparatus comprising an inductance coil, a thermionic valve having the anode-cathode path thereof arranged eiectively in series with said coil, means for feeding said long and short impulses to said valve, means for rendering said path non-oonducting in the absence of an impulse of either duration and to become conducting when an impulse is present, a source of charging current for said coil, an oscillation generator, and means to initiate oscillations of said generator in accordance with a flow of current through said inductance coil. f

12. A television synchronizing system including apparatus for separating impulses of relatively long duration from impulses of a relatively short duration, said apparatus comprising an inductance coil, a thermionic valve having the anodecathode path thereof arranged effectively in series with said coil, means for feeding said long and short impulses to said valve, means for rendering said path non-conducting in the absence of an impulse of either duration and to become conducting when an impulse is present, a source of charging current for said coil, a resistive impedance adapted to be traversed by said charging current, an oscillation generator, and means to initiate oscillations oi said generator in accordance with the iiow of current through said inductance coil.

13. Apparatus according to claim 12,.compris` ing a thermionic' valve having a cathode, an anode and a control grid, means for causing derived voltagerpulses set up across said resistive impedance to cause the potential of said control grid to increase in the positive sense relative to said cathode, and means for biasing said control'grid relative to said cathode to anegative potential which is of such a magnitude that, in operation, only those voltage pulses which correspond to longer impulses cause anode curr-ent to flow.

14. Apparatus according to claim l2, comprising a unidirectionally-conducting device, a source of' potential difference in series with said device, and means for establishing derived voltage pulses set up across said resistive impedance across said device and said source in series, the magnitude and sense ofI ,the potential difference of said source being such that current flows in said device only'when a derived pulse of greater amplitude exists across said resistive impedance.

15. In a television system in which synchronizing impulses of two diierent time durations but with equal amplitudes are transmitted sequentially with picture signals, the method of synchronizing a picture reproducer at a. receiving point, which comprises the steps of receiving the picture signals and both the synchronizing impulses, separating the picture signals from the synchronizing impulses, converting the synchronizing impulses into impulses having diierent amplitudes determined by the time duration of the corresponding impulse, separating the lower amplitude converted synchronizing impulses from 10 the higher amplitude converted synchronizing impulses, initiating energy iiow of a. first source of wave energy by the separated lower amplitude impulses, initiating energy flow of a second source' of Wave energy by the separated higher ampltude impulses, and controlling the picture reproducer by both of the controlled sources of wave energy.

EDWARD CECIL CORK.

MICHAEL BOWMAN-MANIEOLD.

CHARLES LESLIE FAUDELL.