US2485821A - Translation of duration modulated code pulses into equal length code pulses - Google Patents

Translation of duration modulated code pulses into equal length code pulses Download PDF

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Publication number
US2485821A
US2485821A US90252A US9025249A US2485821A US 2485821 A US2485821 A US 2485821A US 90252 A US90252 A US 90252A US 9025249 A US9025249 A US 9025249A US 2485821 A US2485821 A US 2485821A
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pulse
duration
signal
pulses
stage
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Gloess Paul Francois Marie
Libois Louis Joseph
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/12—Analogue/digital converters
    • H03M1/14—Conversion in steps with each step involving the same or a different conversion means and delivering more than one bit

Definitions

  • the present invention relates to improvements in electrical codificationsystemsaand more particularly in the translation of duration modulated pulsesintolcode pulses.
  • the low frequency signal to .be transmitted modulates, in alknownmanner; pulses repeated at a .suitab1e.rhythm or repetition frequency, modifying, according tothe amplitude voi the signal, one of the characteristic parameters of the pulses used, this parameter being generally -.the amplitude, the .duration or theposition'in time.
  • the pulses thus modulated, from a number of communication channels are .thengroupedas is well known, inthe multiplex pulse transmission systems.
  • modulated pulses are transmitted to a coding device which converts each one .of them into a train of n codepulses, :each of which can assume each .of two valuesaccording .to the importance of themodulation of the.-,pulse of the corresponding channel, -insuch .a manner as to define .2 different levels .for the modulation parameter of the channel ,pulses, .It..islknown that, practically, one of the twoamplitude values of the codepulses is zero,.i..enan -absenceloflthe corresponding pulsathe other value having. generallya predetermined constant level.
  • the present invention -.has, as an object, a coding device enabling the conversion, into .code pulses, of pulses modulated in .duration Joya dis placement of their trailingedges.
  • the coding device successively compares duration modulated pulses with .,predetermined intervals, each one of which is half the. previous one, said time intervals offering a a simple relationship- [with the maximum duration of the initial pulses.
  • the coding device comprises .a number of successive stages equal to the number ofeelements of the codeconsidered, each stage beingconnected to'the next one by two: difierentcircuits, and, in eachvstagc, means, controlled v by the 'signal :applied ".to said stage, to direct the signal throughaone tor the other f said circuits, according to its-duration referred to a time interval characteristic o'i the stage 1 considered and, simultaneously, to render inoperative the circuit not utilized.
  • one-'o'f the circuits of each stage passes only thesignals having a duration longer than the-reference time interval of the stageconsidere'd andproduces, un-
  • the other circuit receives the signals which have a duration smallerthan the reference time interval of the stage considered, delays these signals by this time interval and'transmits them to the next stage without decreasing their duration.
  • the last stage of the coding device comprises only onecircuit and means are provided in this circuitto pass only signals having a duration greater thanithe shortest reference interval and toilpro'duce then .a code pulse.
  • the firstc'of'the circuits of each stage preceding the last one comprises an electronic device with two stability positions, controlled by a signal derived from the signal applied to the stage when said applied signal has a duration greater than the reference interval of the stage, said electronic "device controlling, when operated, the emission :of a code pulse and further creating a blocking signal'for the second circuit, this blocking signal having a duration substantially double that o'f'the reference interval of the stage considered.
  • Figure 1 shows, schematically, a coder in accordance with the invention
  • Figure 2 shows in greater detail, the electrical circuits of the two first stages of a coder in .accordance with the invention
  • Figures '3 and 4 are diagrams of pulses occurring at various points of the coding chain.
  • Figurefi shows an example of embodiment of the circuitsof the last stage of a coder in accordance with the invention.
  • Figure 1 shows, schematicallyga coder giving three codepulse positions and-giving, consequently, a definition corresponding to 2 different levels, but it will be obvious, upon reading the description, 'thatthe codwmightcomprise any larger number of elements and that, in such a case, a number of stages would be utilized equal to the number of these elements, each stage being similar to those whose functions will now be defined in connection with Figure 1.
  • T will designate the maximum duration selected for the modulation of the incoming pulses and t the duration of any pulse.
  • the pulses I modulated in duration and which are negative, for instance, are applied to a delay device 2, delaying said pulses by a time T/2, and delivering to a comparator circuit 3 a signal 4 whose shape will be defined in connection with Figures 2 and 3.
  • the signal 4 will be positive, for instance.
  • cuit 5 receives, on one hand, the initial pulse I and on the other hand the signal 4.
  • the circuit 3 delivers to a second delay circuit 8 an impulse l delayed by T/2 and having a duration (t-T/Z) further, it sends out a code pulse 8 which is transmitted to a mixing circuit 9 and, finally, it applies to device 5 a blocking signal l whose duration will be substantially equal to T so as to prevent this device from transmitting any signal to the circuit 6.
  • the device 3 does not operate and consequently does not supply any of the signals I, 8 and I 0.
  • the device 5 therefore is not blocked and it transmits to the delaying circuit 6 a pulse ll corresponding to pulse 1, i. e. whose duration is 75, but it delays this pulse H by T/2 with respect to pulse I.
  • the device 6 subjects the signals it receives to a delay T/4 and, as previously, the signal 12 produced is applied simultaneously to a comparator device l3 and to a device [4 which also receives the pulse 1 or H.
  • the devices l3 and 14 act, respectively, in a manner similar to that of the devices 3 and 5 of the first stage: if the pulse 1 or H entering the second stage of the coder has a duration greater than T/4, the device [3 projects a code pulse towards the mixer 9 and it also supplies a pulse l5 whose duration is decreased by T/4 with respect to that of signal 1 or II and which is delayed by T/4 with respect to said signal, and it sends to circuit I4 a blocking signal [6 whose duration will be about T/Z, this duration being sufficient to prevent the circuit I4 from operating while the pulse 1 or II is applied to it, since these signals 1 or II can only have a duration equal to or smaller than T/2 by virtue of the adjustment of the first stage of the coder.
  • the comparator circuit 13 cannot operate and the device l4 delivers a pulse I! which is identical but delayed by T/4.
  • pulses l5 or I! reaches the third stage of the coder which is the last Further, a cirstage in the example considered. It is applied to a delay circuit l8 causing a delay T/B (T/2 in the case of a coder with 12 pulse positions) and delivering a signal l9 applied to a last comparator circuit 28 which delivers a code pulse 2! if the signal I5 or I! has a duration greater than T/8 but delivers no signal in the opposite case.
  • the code pulses eventually delivered by the circuits 3, l3 and 28 are suitably distributed by the mixer 9 which may also, if desired, give them the characteristics suitable for transmission, and they are received at 22.
  • Figure 2 shows an example of embodiment of a coder according to the invention. For greater simplicity, only two stages of this coder have been shown, since all the stages are similar. However, an example of embodiment of the last stage is shown in Figure 5.
  • the incoming pulses 23 are modulated in duration by displacement of their trailing edges and that they are of a negative polarity, as shown.
  • Figure 3 shows schematically the shapes of the signals produced by the circuits corresponding respectively to the assemblies 2, 3, 8 and 13 in Figure 1.
  • Figure 4 shows the shapes of the signals procluced by the circuits corresponding to the devices 5 and M of said Figure 1.
  • the incoming pulse 23 is applied to the control electrode of an electron tube 24 whose anode circuit comprises particularly a delay line 25, open at its end 26 and closed at its input 21 on its characteristic impedance so as to reflect the signals applied to it.
  • the back and forth transmission time on line 25 has been selected equal to T/2.
  • the pulse 28, appearing on the anode of the tube 24 under the action of the pulse 23 is shown on line a of Figure 3.
  • This pulse 28 is transmitted simultaneously to the input 21 of the delay line 25 and over connections 29 and 30. These connections also transmit the pulse 3
  • produces the signal 32 shown at c, Figure 3; this signal offers a crest 33 of duration (tT/2) caused by the partial superposition of the pulses 28 and 3
  • the signal 32 is applied to the control electrode of a second electron tube 34, normally biassed beyond cut-off voltage by means, for instance, of a voltage divider 3536 designed in such a manner that the cut-off voltage, shown at 31 on line c of Figure 3 allows the tube to pass only signals having an amplitude higher than that of the pulse 28.
  • a voltage divider 3536 designed in such a manner that the cut-off voltage, shown at 31 on line c of Figure 3 allows the tube to pass only signals having an amplitude higher than that of the pulse 28.
  • the crest 33 of the signal 32 goes through the tube 34 which delivers on its anode a negative signal 38 shown at d on Figure 3 and whose duration is that of the crest 33, i. e.
  • the anode load of tube3 l1 consists, in. addition to resistor 43 at whose terminals appears the negative signal 38, of a transformer i l which re verses and differentiates thepulsetfl, producing a signal 45 as shown by line-e of Figure 3.
  • This signal 'comprisesa short positive pulse 4*,(30119- spondiugto-thefirst edge of the inverted signal from the pulse 38, anda short'ncgative pulse. il, caused by the second edge of this inverted signal.
  • the signal 45 energizes-an electronic device havingzone position of permanent equilibrium and capable of assuming, during. apredetermined time intervahanother position of equilibrium under the control of a suitable signal.
  • Such devices are well known and the one which is represented by way of example in Figure 2 consists of a double triodc tube 48, connected in a known manner.
  • the left hand element of this tube is normally non-conductingbut it is made conducting by the positive potential applied to its control electrode by the pulse 46.
  • the potential at the terminals of the load resistor 49 ofthis element thus dcreases suddenly when this pulse 46 appears, which gives a negative pulse 5.
  • the time constant of the circuit formed by the resistor 49 and by thevcoupling capacitor 5! is determined in such a manner that the tube 48 comes back to its position of permanent equilibrium by unblocking of its right-hand element, only after a timeinterval T, i. e. at a timei3T/2 afterthe beginning of the pulse 23;.
  • An examination of Figure 4 shows that this duration is suflicient to include the duration of the pulse 4! produced by the tubeEEl even if the initial pulse 23 had the maximum duration T.
  • the negativepulse is transmitted to the control electrode of the tube 42 which thus receives the signal shown on line 2 of Figure l.
  • the circuit elements are proportioned in such a manner that the portion 55 of signal 54 caused by the appearance of signal 4
  • the incoming pulse'having a duration greater than 172, the device 48 also controls the emission of a code pulse produced as follows:
  • the pulse 38 whose duration, (t-T/Z), by hypothesis, issmaller than TM, is applied to the control-electrode of an electron tube T0 is theinput tube to thesecond .coderstageand which corresponds in that stage to the tuber-34.
  • the tube 1!] vconverts this pulse into acpositive pulse 1 l, shownon line 1" of- Figure 3zand whichris appliedito thevinput of a-delay line l2 similar-to line 25.
  • the line-l2 comprises an open end l3 but is closed-at its input IA-.onits characteristic impedance so aslto reflect. at ,tone of its endsthe signals appl-iedt it.
  • This line 12 has abackand .iorthtransmission time equal :to T/4 or T/22.
  • the delay line of any stage p of the coder hasaiback and forth-transmission time equal to T/Z
  • the pulsell is further transmittedthrough'aa lead 15 vto the controlielectr-odeaof anxclectron tube 16 andthrough a lead "it is alsoapplied to the control electrode of another-electron tube 18 such as a pentode tube.
  • the delay line 12 causes the; appearance oina pulse 79 shown-on line lc of v Figure 3, identical with the! pulse H but having with respect toithat pulse ll a delay T/4. Since, by hypothesis, the duration of the pulse H is smaller than T/ -i, the total signal-180 transmitted over leads l5 and" consists of two identical successive pulses as shown on line. I of Figure3.
  • the tube 16 is .biassed beyond-cutsofi voltage, like the corresponding tube 34 of the firststage, by means, for instance, of a voltage :dividerdl H82, so as topass only-.signalshaving an :amplitude greater than that of the pulse 1 I. 'Thusthisatube does nottransmit the signaltfl, sothat no control signal is sent through the differentiating transformer 83 to the device fl lysimilar/to the device 48 of the first stage and, likeit, shown in the form .of a double triode tube.
  • This tube thus remains .at its permanent equilibrium position when the signal applied tothe-stage has'a 'duration smaller than the'back and forth propagation time of the delay line ofthi stage.
  • the signal is applied; on theother hand, through theilead TI to the control electrodev of thetube 18.
  • On thesuppressor grid of this tube is also applied the negative signal 38 which, having given rise to'the signal-H coincides in time with this signal.
  • the signal 38 is again shown'on'line h of Figure 4.
  • the tube 78 is blocked pending the duration of the signal 38 and thus passes only the first pulse H of the signal 80. Only the pulse 19 goes through this tube and after inversion by the transformer 85 supplies a pulse-86 :delayed'by T/4 withrespect to the pulse H and which is appliedto the control electrode of an electron tube 81.
  • This pulse 89, delayed by T/4 with respect to the pulse 38 is also applied to the control electrode of the input tube 90 of the third stage, and also, eventually, to a pentode tube not shown, such as 39 or 18 of the third coder stage, through the lead 9
  • the load resistor 94 of the left hand element of the tube 84, and the capacitor 95 for connecting the control electrode of the right hand element of this tube must be chosen so as to offer a time constant equal substantially to T/2.
  • the tube 84 leaves its equilibrium position under the control of a pulse such as 38, whose duration is greater than T/ i, it thus delivers a negative blocking pulse 95, with a duration T/2 sufiicient for blocking the tube 8'1 until the end of the signal transmitted in this case by the tube I8 since, in any case, the signal from the second coder stage cannot have a duration greater than T/2.
  • the blocking pulse 96 must not have a duration substantially greater than T/2 in order not to hinder the operation of the coder when it receives the following pulse, which occurs substantially at a time T/2 after the appearance of the pulse 96.
  • the right hand element of the tube 84 delivers a positive pulse which is differentiated by the capacitor 9! and applied to the mixer device 64 as was seen during the description of the operation of the first stage.
  • the pulse 89 delivers, at the output of the tube 90 a positive pulse 98 represented on line is of Figure 4, a pulse which is transmitted to the delay line 92 and eventually through the lead 99 to the control electrode of the tube of the third stage, such as previous tube 39 or I8.
  • the nth stage it is quite obvious that it does not have to transmit signals whose duration is greater than T/Z Its circuits are therefore simpler than those of the previous stages, as shown by Figure 5.
  • the last but one coder stage delivers a signal I whose duration is greater than T/2
  • This signal is applied to the control electrode of an electron tube IN, and there is received at the terminals of the load resistor I02 of this tube a positive pulse I93, which is applied to the input of a delay line IB S.
  • this delay line is open at its end I95 and closed at its end I06 on its characteristic impedance and its back and forth propagation time is taken equal to T/Z.
  • the line I04 thus delivers a signal Ill! built up by the partial overlapping of the pulse I03 and of the delayed reilected pulse, delayed by 172.
  • the signal I01 is applied to the control electrode of an electron tube I08 biassed beyond cut-off voltage by means, for example, of a voltage divider I09I I 0 in such a manner as to pass only signals having an amplitude greater than that of the pulse I03.
  • a positive pulse III delivered by the peak of the signal IN.
  • This pulse III is differentiated by a capacitor I I2 and. thus produces a signal II3 applied to the mixer device 64, for instance, on its terminal II4. Owing to the detector 65, there is received on the terminal 63 a positive signal II5, transmitted to the shaping device 61 ( Figure 2) to produce a code pulse.
  • the system described in connection with Figures 2 to 5 delivers code pulses at fixed times with respect to the position in time of the leading edge of the initial pulses 23.
  • these pulses under the action of the delay lines of the successive stages, and due to the clipping caused by the comparator tubes, these pulses, when they do exist, appear with respect to this leading edge, with respective delays T/2, (T/2+T/4), (T2+T/4+T/8) It is known that it is advantageous in practice, to distribute regularly, in time, these coded pulses so as to facilitate transmission.
  • this result may be obtained very easily by means of a mixer circuit 64 consisting, as shown on Figures 2 and 5, of a delay line closed at its two ends on its characteristic impedance. If it is assumed, for instance, that the code comprises four elements, the corresponding pulses may be spaced by a regular time interval equal to T/4. As the two first code pulses occur with a shift of T/ i as mentioned, they will be received successively on the input terminal 63 of the mixer circuit.
  • the third pulse oiTers a delay T/8 with respect to the second one; it will be applied to a terminal I IS ( Figure 2) located at a distance from the terminal 63 corresponding to a propagation time T/B so as to have a total delay (T/8+T/8) with respect to the second pulse.
  • the fourth one will be received on a terminal I I? such that the propagation time from this terminal to terminal 63 is equal to 3T/IB, etc. the code signal being derived from the terminal 53 to be transmitted to the shaping device 81.
  • the pulse from the last stage of the system may be collected on the output terminal H t of the circuit and the total propagation time will then be equal to (T/8+T/I6+T/32+. .+T/2
  • time interval between two consecutive code pulse positions may be different from the one just indicated by way of example, especially if the code comprises more than four pulse positions.
  • a simple calculation will allow the determination of the positions of the various terminals of the circuit 94 capable of giving the desired recurrence frequency.
  • Method for translating recurrent electric pulses modulated in duration by time displacement of their trailing edge into coded pulses according to a chosen code, whichcomprises comparing successively, onthe one hand, the duration of the initial pulses or the signals. successively derived as modifications thereof with, on the other hand, a series of reference time intervals based on' the maximum modulation duration and themselves serially inter-related by a com mon factor, and producing a code signal each time a comparison determines that the initial pulse or its signal modification exceeds in duration the corresponding reference interval.
  • Method according to claim 1 including staggering the produced code pulses in time in a predetermined order and with predetermined time intervals.
  • each comparing operation includes for the next comparison either diminishing the duration of the modulated pulse or of its signal modification proportionally with the change in the corresponding reference interval if said duration exceeds said interval, or retaining its duration in full if it does not exceed said interval.
  • a device for translating recurrent electric pulses, modulated in duration by time displacement of their trailing edges, into coded pulses comprising a cascade of coupled coding stages equal in number to the code elements, means in each stage whereby the stages compare successively the duration of said modulated pulses with predetermined reference time intervals one for each stage and successively related in value by a common multiplier, the first one of said time intervals having a predetermined value amounting to a substantial part of the maximum modulation duration, and each stage also having means to transmit a code pulse corresponding to that stage in response to excess value of the modulation duration of the incoming pulse over the reference interval of that stage.
  • a translating device wherein the coupling between stages comprises two signal transmission circuits, and the means to compare includes devices responsive to incoming signals to direct the signal selectively over one of said circuits in dependence on its duration respecting the reference interval of the directing stage, and to block the other circuit.
  • a translating device wherein the comparing means includes devices responsive to incoming signals to pass over one of said circuits only signals having a duration in excess of the reference interval of the same stage, to produce a code pulse responsive to said signals, and to supply the following stage with a signal having a duration decreased by the reference interval of the passing stage and delayed by this last mentioned duration.
  • the comparing means includes devices adapted to respond to incoming signals, to delay signals having a duration smaller than the reference interval of the instant comparing stage, and to transmit them over a selected one of said circuits in their full duration.
  • a first one of said circuits in each stage having two circuits comprises an electronic device having two positions of stability, means by which said device is actuated by a signal derived from the signal applied to that stage, when said applied signal has a duration greater than the reference interval of that stage, connections whereby the said electronic device, when actuated, controls the emission of a code pulse and otherwise produces a signalblocking the second circuit, said blocking signal having a duration substantially double that of the reference interval of said stage.
  • a translating device wherein the last stage of thecascade has a single signal transmitting circuit and the comparing means has devices for passing thereover only signals having a duration greater than the shortest reference interval, and for thereby producing a code pulse.
  • a translating device wherein, with T representing the maximum modulation duration, and n the number of code elements, the reference intervals are related successively as T/2, T/2 T/2 11.
  • delay circuit staggering said short duration pulses in time in a predetermined order and with predetermined time intervals, a circuit for shaping short-duration pulses received at output terminals of said mixer circuit into code pulses of predetermined wave shape and duration, and means for impressing said code pulses upon a transmission circuit,

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  • Engineering & Computer Science (AREA)
  • Theoretical Computer Science (AREA)
  • Dc Digital Transmission (AREA)
  • Plasma Technology (AREA)
  • Compression, Expansion, Code Conversion, And Decoders (AREA)
  • Manipulation Of Pulses (AREA)
US90252A 1948-08-05 1949-04-28 Translation of duration modulated code pulses into equal length code pulses Expired - Lifetime US2485821A (en)

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US (1) US2485821A (sv)
BE (1) BE490359A (sv)
CH (1) CH278421A (sv)
DE (1) DE824067C (sv)
FR (1) FR970030A (sv)
GB (1) GB659020A (sv)
NL (1) NL82618C (sv)

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2560289A (en) * 1948-12-31 1951-07-10 Olive S Petty Pulsed distance measuring instrument
US2616965A (en) * 1950-03-22 1952-11-04 Raytheon Mfg Co Binary coding device
US2625600A (en) * 1951-05-03 1953-01-13 Bendix Aviat Corp Decoding information translator
US2643819A (en) * 1949-08-11 1953-06-30 Research Corp Apparatus for computing correlation functions
US2645712A (en) * 1949-12-01 1953-07-14 Rca Corp Reading circuit for storage tubes
US2646548A (en) * 1948-07-21 1953-07-21 Alsacienne Constr Meca Electron tube coder device
US2807783A (en) * 1953-04-28 1957-09-24 Radio Electr Soc Fr Pulse code modulation systems
DE969435C (de) * 1952-08-01 1958-06-04 Int Standard Electric Corp Anordnung zur Umwandlung phasenmodulierter Impulse in Codesignale
US2894254A (en) * 1953-12-10 1959-07-07 Raytheon Co Conversion of binary coded information to pulse pattern form
US3188624A (en) * 1959-11-17 1965-06-08 Radiation Inc A/d converter
US3247487A (en) * 1960-04-19 1966-04-19 Electronic Associates Analog storage device
US3500387A (en) * 1963-12-10 1970-03-10 Int Standard Electric Corp Ptm to pcm and pcm to ptm conversion circuitry
US4630031A (en) * 1983-12-27 1986-12-16 Motorola, Inc. Pulse-width discriminating A/D converter

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR1071074A (fr) * 1952-08-13 1954-08-24 Labo Cent Telecommunicat Circuit de codage d'information sous forme d'impulsions électriques
DE1225234B (de) * 1965-05-17 1966-09-22 Siemens Ag Verfahren und Einrichtung zur binaeren Codierung von dauermodulierten Nachrichtenimpulsen

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
None *

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2646548A (en) * 1948-07-21 1953-07-21 Alsacienne Constr Meca Electron tube coder device
US2560289A (en) * 1948-12-31 1951-07-10 Olive S Petty Pulsed distance measuring instrument
US2643819A (en) * 1949-08-11 1953-06-30 Research Corp Apparatus for computing correlation functions
US2645712A (en) * 1949-12-01 1953-07-14 Rca Corp Reading circuit for storage tubes
US2616965A (en) * 1950-03-22 1952-11-04 Raytheon Mfg Co Binary coding device
US2625600A (en) * 1951-05-03 1953-01-13 Bendix Aviat Corp Decoding information translator
DE969435C (de) * 1952-08-01 1958-06-04 Int Standard Electric Corp Anordnung zur Umwandlung phasenmodulierter Impulse in Codesignale
US2807783A (en) * 1953-04-28 1957-09-24 Radio Electr Soc Fr Pulse code modulation systems
US2894254A (en) * 1953-12-10 1959-07-07 Raytheon Co Conversion of binary coded information to pulse pattern form
US3188624A (en) * 1959-11-17 1965-06-08 Radiation Inc A/d converter
US3247487A (en) * 1960-04-19 1966-04-19 Electronic Associates Analog storage device
US3500387A (en) * 1963-12-10 1970-03-10 Int Standard Electric Corp Ptm to pcm and pcm to ptm conversion circuitry
US4630031A (en) * 1983-12-27 1986-12-16 Motorola, Inc. Pulse-width discriminating A/D converter

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CH278421A (fr) 1951-10-15
BE490359A (sv)
GB659020A (en) 1951-10-17
NL82618C (sv)
DE824067C (de) 1951-12-10
FR970030A (sv) 1950-12-28

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