US2465380A

US2465380A - Cathode-ray tube pulse separation and demodulation system

Info

Publication number: US2465380A
Application number: US565152A
Authority: US
Inventors: Labin Emile; Donald D Grieg
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC; Federal Telephone and Radio Corp
Priority date: 1944-11-25
Filing date: 1944-11-25
Publication date: 1949-03-29
Anticipated expiration: 1966-03-29
Also published as: FR939204A; CH261784A; BE469039A; BE474803A; GB603614A; GB624514A; ES172379A1

Description

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March 29, 1949. I E. LABIN ETAL ,2,465,380

CATHODE-RAY TUBE PULSE SEPARATION AND DEMODULATION SYSTEM v Filed NOV. 25, 1944 3 Sheets-Sheet 2 Arrow/EY Patented Mar. 29, 1949 UNITED STATES PATENT OFFICE CATHODE-RAY TUBE PULSE SEPARATION AND DEMODULATION SYSTEM lapplication November 25, 1944, Serial No. 565,152

8 Claims.

This invention relates to multi-channel receiver systems and more particularly tc means for separating and demodulating the pulses of the different channels.

One of the objects of the invention is to provide a method and means for separating the channel pulses of a multi-channel train of signal modulated pulses, and demodulating the signal modulation of each pulse during the operation separating it from the pulses of other channels.

Another object of the invention is to provide means employing a multi-channel separator of the cathode ray beam type for translation of the time modulation of channel pulses into amplitude modulated energy during the channel separating operation thereof.

It has been proposed heretofore to employ an electron ray tube for separating the pulses of diierent channels of a multi-channel train of amplitude modulated pulses. Such employment, however, merely separates the pulses according to channel with the View to preserving the amp-litude modulation of the pulses for the receiving channels. According to a feature of the present invention, a cathode ray tube or other electron beam producing means is provided together with means for controlling the beam and/ or the sensitivity of the beam responsive devices thereof, which are associated with corresponding receiving channels, for translation of the time modulations of the channel pulses into amplitude modulated energy. The electron beam is caused to sweep through a normal cycle in synchronism with the multi-channel period of the train of time modulated pulses. This synchronism may be controlled in any one of a number of ways. For example, the average timing of the pulses, wherea large number of channels are employed, may be used for the generation of a base wave to control the beam sweep. Preferably, however, the beam sweep is synchronized by pulses of a synchronizing channel which marks the periods of the multi-channel train of pulses. These synchronizing pulses may be distinguished from the other pulses of other channels by a shape characteristic such as width, build-up or decay time of the leading and trailing edges; or the synchronizing pulses may be of the same shape as the other pulses but modulated by a special signal which, when received in the synchronizing channel, locks the sweep cycle of the beam into synchronism with the multi-channel periods.

The beam, in its movement, is caused to either traverse the beam sensitive devices or to move along a path adjacent the devices. The time modulated pulse energy may be used to control coaction between the beam and each responsive device in any one of several ways. This coaction between the beam and the responsive devices causes a flow of energy in the channel receiving circuit associated with each device, the amplitude of such energy being proportional to the time modulation of the pulses of the corresponding channel. According to one feature of the invention, the coaction between the beam and the responsive devices is effected by keying the beam on and off according to the leading and trailing edges of the pulses. Where this type of control is employed, the beam sweep is caused to traverse successively the same responsive devices during the same corresponding parts of its cyclic movement, whereby the keying on and olf of the beam in relation to such parts controls the coaction referred to.

According to another feature of the invention, the control of the coaction between the beam and the sensitive devices is effected by deflecting the beam according -to the pulse energy. For example, the beam is caused to normally sweep, in the absence of signal modulated pulses, and at a given intensity, along a path adjacent the responsive devices. When the beam is deiiected by a channel pulse, it is caused to coincide with one of the responsive devices for a time interval proportionally to the degree of the time modulation of the delecting pulse energy, thereby producing a corresponding ow of energy in the receiving channel of the beam intercepting device.

This coaction, according to still another feature of the invention, may be effected by applying the pulse energy to a given element of each of the responsive devices whereby the devices are made responsive to the beam only when pulse energy is applied to such elements. The beam in this case is maintained at a constant intensity and its path of movement is caused to coincide l successively with the responsive devices.

Fig. 4 is a fragmentary sectional View of the screen portion of the electron tube employed in either of the systems of Figs. l and 3 showing a variation of the beam responsive devices;

Fig. 5 is a side view of an electron tube, with parts broken away, together with certain of the associated channel receiving circuits of still another variation of the invention; and

Fig. 6 is a schematic illustration of the beam responsive devices and associated channel receiving circuits of the tube shown in 5.

Referring to Fig. 1, we sho-w one form of multichannel receiver for receiving a multi-channel train of pulses such as shown, for example, in graph a of Fig. 2. The train comprises interleaved pulses of seven signal channels, I, 2, 3, Il, 5, 6 and 1, each period thereof being defined by the pulses of 4a synchronizing channel 8. This train of channel pulses may be transmitted from a transmitter station by line communication, or

; by radiation on a given carrier frequency in `which case it is received on antenna 9 of a receiver unit I6. The receiver unit IIB includes the usual radio frequency ampliiier and detector means whereby the carrier is removed leaving the envelope of the pulses substantially as indicated in graph a of Fig. 2.

The pulse energy of the multi-channel train may be modulated according to any one of several principles of time modulation. While the pulses of different channels may be modulated according to different principles, for practical purposes one principle oi modulation is preferably employed for all channels. According to one principle of modulation, the successive pulses of a given channel may be time displaced toward and away from each other according to the push-pull time modulation or according to another principle, or the pulses may be modulated in width, either symmetrically or by displacement only of one edge thereof. According to still other .principles of modulation, the successive pulses may be time displaced relative to their unmodulated time positions or certain pulses of a series may be displaced in time relative to other pulses of the series. It will beclear as the descriptive proceeds that practically any type of time modulation of pulse energy, including the above examples and others not mentioned, may be separated according to channels and demodulated into amplitude modulated energy by means of a single electron tube according to the principles of our invention.

For synchronizing purposes, the synchronizing pulses 8 are shown to be distinguished from the other channel pulses according to width. By applying the train of pulses to a pulse width discriminator Il energy only of the synchronizing pulses 8 will be obtained which, in turn, may be applied to a base wave producer I2 for the generation of wave energy for controlling the sweep movement of the beam of an electron tube I3. For a circular sweep movement, the wave is applied to a phase splitter I4 whereby two sine waves of energy, separated in phase by 90, is produced for output circuits I5 and I6. These two waves of energy are applied to beam defiecting elements I1, I8 and I9, 20 whereby a circular sweep movement of the beam is produced for each period of the multi-channel train.

It will be understood, of course, that the circular sweep movement is illustrated herein for purposes 1 of illustration only, and that any other type of cyclic sweep movement may be substituted by merely providing the proper sweep. potentials for 4 the deflecting elements I1, I8 and I9, 2l). For example, the sweep pattern may follow the zigzag tracing commonly used in television.

The electron tube I3 is provided with a pluraiity of beam responsive devices or collectors 2i, 22;.23, 24,25, 26, 21..-and 28 Which correspond in number, in .spacing` and geometric conguration relative the beam movement to the com- :municating channels of the, train of received pulses. Seven of these channels, 2l to 21, are

each. connected to a receiving channel circuit which may include a low pass filter such as indicated'at-'Zl for channel I, the output of which is applied toiany desirable utilization circuit such as phones 3o. "The collector 28 corresponds in position to the synchronizing pulses 8 in the timing of the channel pulses and may be used for providing synchronizing pulse energy for any synchronizing purpose that may be required, for example, it may be used for synchronizing an associated multi-channel transmitter circuit.

The electron tube I3, of course, includes the usual electron gun equipment 3|, for producing a beam o electrons which is controlled by grid 3?: and suitably vshaped by the usual focusing means 33. The grid 32 is provided with a source of biasing potential at 34 which according to the principle of operation of Fig. l comprises a out off biasing value. The multi-channel pulses detected at receiver unit IE) are applied to control grid 32 to key the beam on and oil according to the leading and trailing edges of the pulses. The pulse output of receiver unit I0 may rst be shaped by pulse Shaper 35 to insurev a given amplitude and duration for the pulses prior to application to grid 32. The Shaper 35 is particularly useful in improving the signal-to-noise ratio of the transmission since the re-shaping of the pulses not only clip off any superposed interference, but by giving the pulses a given duration measured, for example, from the leading edges thereof, also removes any variations in duration of the pulses caused by interference occurring at the trailing edges of the pulses.

While the beam responsive devices 2| through 28 may comprise any one of several different arrangements, they may be referred to for the present as electron collectors adapted to collect the electrons of the beam as the beam traverses the area thereof. Other forms of collectors or electron responsive devices will be described hereinafter.

In the operation of the receiver system of Fig. 1, let it be assumed that a train of multi-channel pulses according to those of graph a, Fig. 2, are detected by the receiver unit IU. The pulses of the synchronizing channel 8 being of a width greater than the pulses of the other .channels are separated therefrom by the pulse width discriminator II. The discriminator II may comprise any known circuit capable of distinguishing between pulses of diierent widths. The output of the discrimnator Il as represented by synchronizing pulses 8a of graph b are shown to correspond to the leading edge of the pulses 8. This relative timing of the pulses 8a with respect to the leading edge of pulses 8 depends on the character of circuit employed and is not important insofar as the operation of our systemis' concerned since the phase of the base wave produced at I2 in response to pulses 8a may be controlled as desired. The base wave output of producer I2 is split and shifted in phase as indicated at 36 and 31 by phase splitter I4. The phase diierence` of the two waves36'and 31 is chosen as 90 so as to provide by means of the deeeting elements I1, I8

and I 9, 20 a circular. sweep movement of the beam of the electron tube I3. The sweep movement of the beam may thus be controlled to traverse the collectors 2I through 28 as indicated at 38. However, since the beam is normally biased to cut-olf, there will be no interception of electrons by the collectors 2| to 28 in the absence of pulses from receiver unit I0.

Certain of the channel pulses l to 1 of graph a, Fig. 2, are shown to have various time displacements to illustrate the demodulation of the time modulation of the pulses during the separating operation performed by the tube I3. Pulse I is shown to be centrally positioned with respect to

limits

39, 40 and therefore may be regarded as representing a position of zero time modulation.

Pulses

2 and 3 are shown to be displaced different amounts to left representing negative displacement, the latter being displaced the maximum amount. Pulses 4 `and A5 representv different positive displacements to the right, pulse 5 being displaced to the limit. Pulses 6 and 'I are centrally positioned and therefore represent zero modulation.

The beam which may be of any suitable crosssectional configuration, such as circular, is turned on by the leading edge of each pulse and turned off at the trailing edge thereof. The beam while turned on traces paths indicated at All- 45.

The 50% overlap condition of the keyed on beam indicated at 4i relative to collector 2I cor-.- responds to the zero modulation position of pulse I of graph a. This amount of coincidence re,- sults in a mean amplitude ow of energy in the receiving channel associated with collector '2I.

substantially as indicated at Ia of graph d. The key-on beam portion 52 has a smaller degree of coincidence with collector 22, in accordance with the time position of pulse 2, thereby producing an output 2a, graph e, of an amplitude less than pulse Ia. The pulse 3 being in a full negative displacement position fails to coincide with collector 23 as indicated by beam portion 43, results in zero current flow as indicated by graph f. Pulse 4 being displaced to the right a certain positive displacement relative to the zero modulation position of pulse I, causes the keyed on beam portion 44 to coincide with collector 2li by a corresponding greater amount, thereby resulting in the greater an output pulse 4a, graph y, of amplitude greater than pulse la. y Since pulse 5 has a full positive displacement, the keyed on beam portion 45 is completely intercepted by collector 25 thereby producing an output pulse 5a, graph h, of maximum amplitude. The pulses 6 and 'l being centered the same as pulse I will produce output pulses of corresponding amplitude.

From the foregoing description it will be clear lthat while the pulses of channels I to l' are separated one from the other, the time modulation of the pulses of each channel is translated into amplitude modulated energy during the separating operation thereof. For a single channel the translation of the time modulation thereof may be represented by the series of amplitude modulated pulses 46 of graph i, the modulating signal wave being indicated at lll. It can be seen that in order to obtain undistorted audio output the geometric conguration of the collector plates, as well as the electron beam dimensions, must relate in the proper manner to the characteristics of the time modulated signal and displacement.

For demodulator equipment for a single channel reference may be had to our related copending application entitled, Demodulator systems,

ing elements

53, 55.

6 Serial No. 563,152, filed November 13, 1944, issued U. S. Patent No. 2,438,928, April 6, 1948.

In Fig. 3, we disclose another embodiment of the invention employing a synchronizing circuit 128, capable of selecting the synchronizing pulses Where the synchronizing pulses are of the same pulse shape characteristics as the pulses of other channels, the synchronizing pulses in this instance being modulated with a distinctive signal such as a steady frequency signal as distinguished from the varying frequencies of the usual sound signals. The circuit 48 includes a normally blocked amplifier 49 to which the detected multi-channel pulses are applied. A multivibrator 58 is adjusted for normal operation at a frequency slightly less than the repetition rate of the synchronizing pulses. The output of the multi-vibrator circuit is applied to the amplifier i9 to unblo-ck it for passage of a channel pulse for each operating period of the multi-vibrator. Since the multi-vibrator rate of operation is slightly lower than the synchronizing pulse repetitcn rate or the multi-channel period, the mixer will test the channels in succession passing the energy of each channel for a given time interval and then skip to the next channel, etc. The pulse energy passed by amplifier 49 is applied to a time modulation demodulator 5I capable of translating the time displacements of the pulses into amplitude modulated energy. Any time modulation detector may be used for this purpose, a suitable circuit for this purpose being disclosed in the copending application of D. D. Grieg, Serial No. 459,959, filed September 28, 1942, issued U. S. Patent No. 2,416,306, February 25, 1947.

The amplifier modulated energy output of demodulator 5I is applied to a lter 52 arranged to pass only signal energy corresponding to the synchronizing signal modulation. When the synchronizing signal energy is received by the circuit !8 it is arranged to tie in the multi-vibrator 56 for operation at the cadence rate of the synchronizing signals. Thus, the circuit 48 will thereafter provide a synchronizing pulse energy for application to base Wave producer I2.

For a further understanding of circuit 48 and its use for synchronizing purposes, reference may be had to the copending application of D. D. Grieg, Serial No. 514,998, filed December 20, 1943, issued U. S. Patent No. 2,418,116, April 1, 1947.

The wave produced at i2 is split and shifted in phase at i4 similarly as hereinbefore described in connection with Fig. 1 for application to deiiecting elements il, I8 and I9, 26 of electron tube 18a to produce a circular sweep movement of the electron beam thereof. The detected pulses at the output of receiver unit I6, Fig. 3, are applied through a pulse shaper 35 to deflect- The entire train of pulses is applied to the deflecting element 53 whereby a difference in potential is established between the element 53 and the grounded element 54 in accordance with the pulse energy. The pulse energy thus operates to deflect the beam during the movement thereof substantially as indicated by the trace line 55. In the absence of pulses, the beam follows .a circular path as indicated at 56 adjacent the beam responsive devices 2i through 28. Where an applied pulse is centrally located between limits of modulation as indicated for pulse l in graph a, Fig. 2, the deflection of the beam is such as to cause the beam to traverse the device 2| for a given mean time interval as indicated at ib, in Fig. 3. The deflection 2b produced by pulse 2 relative to device 22, reduces theamount of coincidence l: tween the beam` and the device ZE according to the degree oi negative displacement o the pulse. Fo the extreme negative displacement of the pulses such as indicated by 'pulse 3, the deflection 3b just misses the device 23 thereby resulting in zero energy iiow in the corresponding channel receiving circuit. Deflecton of pulses'll and 5 in the opposite or positive direction, however, produce a proportional increase in the coincidence between the reflected portions 4b and 5h and the devices 2P, and 2% acoording to the degree o modulation or the pulses. Itwillthereicre be clear that the deiiection control of the beam its sweep movement relative .to the beam responsive devices 2i to 28 not only. separates the pulses of the dilerent channels but also dernodulates the time modulation thereof into amplitude modulated energy.

While. the beam responsive devices in Figs. l and 3 have been shown as a simple form of electron conductor it wi i be understood that such devices may be o various forms and operate on other principles. in ll, for example, a beam responsive device is shown associated with a fluorescent coated screen 5l which may form the large'end of the electron tubes i3 and ita. The screen 5lis coated with an opaque mask 56, provided with apertures 59 each or" an area corresponding. to the desired beam responsive area of a beam responsive device. The responsive device in this case is a light sensitive oell 553, for example, a selenium photo cell. The selenium cell Sli is responsive to the light produced by the beam on the fluorescent screen so that when the beam is keyed on or deflected for coincidence with the fluorescent screen of an aperture area, the cell associated therewith responds produce a iiow of current in output circuit 5l, the amplitude of which corresponds to the degree of coincidence or the beam with respect to the aperture area.

Besides employing selenium cells, other photoelectric cells may be employed such as the ordinary gas photocell. It is preferable, however, to

use a lens between each aperture of the member '58 and the screen 5l. In such case, the two parts 5l and 58 will be spaced apart to provide adequate room for their use.

Since the selenium or other photocell is disposed externally of the tube they may be adjusted without destroying the tube, which in such case may lalso be of a standard type. Danger of crosstalk. resulting from halation effects in the iuorescent screen and glass is largely minimized sincel the beam intensity is maintained substantially constant during its active condition due to the average constant energy characteristic of the time modulated signal. rThe use of a fluorescent screen allows its light retention characteristics to be obtained. By proper choice of screen, the energy output for each channel may be increased by making gradual decay of the trailing edges of the pulses substantially as indicated by broken lines Sla in graphs d and z', Fig. 2. The retention characteristic must be such as to reduce each` output to zero before the next pulse occurs for the same channel.

In Figs. 5 and 6 we show an electron tube 13b which may be used in either of the circuits shown in Figs. 1 and 3. The tube ith is provided at the larger end thereof with a plurality of beam sensitive devices of the secondary emission type. The beamnsensitive arrangement includes a barrier plate 52 provided with a series of apertures 63 otsuitable dimensionsrarranged according tothe sweep path ofithebeam. In-back of thesek aperturesarefa plurality of dynode elements 64 each ofrwhich is .provided with an output circuit B5. The barrier plate 62 is provided with a high positive bias-.HB'at G6 while each of the dynode 64 is provided with a suitable bias through resistor '58,. 6l depending upon the principle of operation desired. For example, the bias at El, whether the beam iskeyed on and oi or deiiected as hereinbefore' described, is selected of a lower value than the-potential on plate 62. This dilerence in potentialzbetween the barrier plate and the dynode causes a secondary emission from the dynode to the barrier plate each time the beam traverses thenaperture of the plate 62. That is to say, whenever the electrons of the beam implngeuponfthe dynode 54, an electron emission of 'corresponding vgreater value occurs, thereby causing :a flow of current in the output circuit thereof; This secondary emission feature of the tubein Figs; 5 and 6 has the advantage of greatly ampliiyingthe. signals as well as separating the pulses of diierent channels and demodulating the time modulation thereof. It will thus be clear that the tube: of Figs. 5 and 6 may be used in the circuits'. ofFlgs. l and 3, if desired.

The :output circuit of Figs. 5 and 6, however, are arranged for illustration of a further principle of control of the coaction between the electron beam and therbeam sensitive devices. For this purpose the dynodes are biased at 5l to cut 01T, that is,at a potential substantially the same as thatprovided atv Ell for the barrier plate 62. The pulse input, instead of being applied to the control grid or deiiecting elements, is applied to the barrier 4plate through circuit connection B9. In this embodiment the beam is maintained at a constant intensity while the input pulses control the sensitivity of the dynode-barrier plate system. That is to say, the pulses change the bias from out-off vto a given value depending on the amplitude of the input pulses whereby a secondary emission is produced corresponding to the actual degree. of coincidence between the beam and the pulse sensitized elements.

While we .have disclosed the principles of our inventionin connection with specio apparatus, we are aware that many changes and modifications thereof are possible without departing from theinvention, it will be clearly understood therefore that such apparatus is given by way of illustration only. and not in restriction of the scope of the invention asset forth in the objects and the appended claims.

We claim:

1. A method of separating and demodulating pulsesof a multi-channel train of time modulated pulses comprising producing a beam of energy, causing said-beamto have a movement in synchronism with Vthe average pulse period per channel so that the occurrence of the pulses of the dierent channels always correspond, in abscence of modulation, to corresponding parts of vthe beam movement, the time interval of coincidence between the pulses of each channel and the corresponding part of said beam movement being varied in proportion to the amount of time modulationof 'the pulses of such channel, and causing-a ow of energy for each channel proportional to the -time interval of coincidence of the pulses of such channel and the corresponding part of said beam movement.

2. Amethod according to claim 1, wherein the operationcfcausing said ow of energy includes the step of keying the beam on and off according to the leading and trailing edges of each oi said pulses.

3. A method according t-o cla-im lJ wherein the operation of causing said flow of energy includes the step of deecting the beam with respect to the parts of beam movements corresponding to the diierent channels by energy of the channel pulses.

4. A method according to claim 1, wherein the operation of causing the flow of energy includes the steps of causing said beam :to traverse a different one of a plurality of beam sensitive devices for each of the channel corresponding parts of the movement, and rendering said devices sensitive only for the duration of each of said pulses.

5. A method of separating and demodulating pulses oit' a multi-channel train of time modun lated pulses comprising, producing a beam of energy controlled -by said modulated pulses for impingement on a plurality of targets, one target for each channel of said train, sweeping said beam past said targets in synchronism with the average .pulse period .per channel so that the occur- 1,

rence of the pulses of the different channels always correspond -to said targets, and adjusting the time of duration of said pulses to produce linear paths across said targets, which paths correspond in length -on the targets to the time modulation of said pulses.

6. In a receiver system for receiving a multim channel train of time modulated pulses, means for producing a beam of energy, means for causing said beam to have a movement in synchrog nism with the average pulse period per channel so that the occurrence of the pulses of said channels always correspond, in the absence of modulation, to respective parts of the beam movement, a plurality of receiving channels, separate devices for each receiving channel arranged in a given relationship yto said parts of the beam movement, means for controlling coactive coincidence Ibetween said beam and said devices according to the time modulation of said channel pulses to cause a flow of energy in the associated receiving channel in accordance with the degree of such coincidence, said devices including a barrier disposed in the -path of said beam, said barrier having an aperture therein, and -a dynode disposed in alignment with said aperture for interception of said beam when said beam coincides with said aperture.

7. In a receiver system for receiving a multichannel train of time modulated pulses wherein said train includes synchronizing pulses distinguished in some characteristic from the channel pulses; means to produce an electron beam, means to produce a sweep potential in synchromsm with said synchronizing pulses, means reing relation according to the time modulation of said pulses, said beam responsive device including a barrier member having a plurality of apertures therein corresponding to said plurality of receiving channels for passage of said beam during parts of the sweep movement thereof, a dynode element disposed in alignment with each of said apertures for interception of the beam passing therethrough, and means for biasing differently said barrier plate and the dynode elef ments whereby secondary emission therebetween is produced upon interception of the electrons of said beam by said elements.

8. In a receiver system for receiving multichannel pulses wherein the pulses of signal channels and the pulses of a synchronizing channel are interleaved together in a single train, a plurality of receiving channels, means for producing a beam of energy, a plurality of beam responsive devices, one for each of said receiving channels, means responsive to said synchronizing pulses for producing a sweep voltage to cause said beam to sweep in a given relationship with respect to said devices, means for effecting a degree of coaction between said beam and each of said devices in accordance with the ltime modulation of the pulses of the corresponding channels, means for effecting coactive relation between the beam and said devices including means for controlling the sensitivity of said device to said beam in accordance with the energy of said pulses.

EMILE LABIN. DONALD D. GRIEG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,757,345 Strobel May 6, 1930 2,057,773 Finch Oct. 20, 1936 2,144,337 Koch Jan. 17, 1939 2,189,898 Hartley Feb. 13, 1940 2,250,528 Gray July 29, 1941 2,256,336 Beatty Sept. 16, 1941 2,262,838 Deloraine et al. Nov. 18, 1941 2,265,216 Wolf Dec. 9, 1941 2,308,639 Beatty et a1. Jan. 19, 1943 2,405,252 Goldsmith Aug. 6, 1946