US2185693A

US2185693A - Multiplex signaling system

Info

Publication number: US2185693A
Application number: US192471A
Authority: US
Inventors: Mertz Pierre
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1938-02-25
Filing date: 1938-02-25
Publication date: 1940-01-02
Anticipated expiration: 1957-01-02

Description

Jan. 2, 1940.

P. MERTZ MULTIPLEX SIGNALING SYSTEM Filed Feb. 25, 1938 /NVENTQR P. MERTZ' 3-Shee1;s-Sheet 1 FIG 2 YATTORNEY Jan. 2, 1940. P, MERTZ MULTIPLEX SIGNALING SYSTEM Filed Feb. 25,- 1938 v3 Sheets-Sheet 2 4a x9 1 l 48 f 2:2 7

' nfl i .39d J osc "5 /NVENTOR By P. MERTZ AT TORNEV Jan. 2, 1940.

P. MER-rz 2,185,693

MULTIPLEX SIGNALING SYSTEM Filed Feb. 25, 1938 3 Sheets-Sheet 5 FIGS AMP

P. MERTZ A T TORNE Y Patented Jan. 2, 1940 UNITED STATES PATENT GFFHCE MULTIPLEX SIGNALING SYSTEM Application February 25, 1938. Serial No. 192,471

5 Claims.

This invention relates to multiplex transmission of messages and particularly of message',

waves comprising bands of frequencies, for example speech or music waves, television or simi- 5 lar Waves, the multiplexing being on a time division basis.

An object of the invention is tosecure improved multiplex transmission of the type indicated, by means of simplified and improved apparatus.

In the transmission of speech and music or other complex waves it is necessary to reproduce at a receiving point a Wave which is changing from instant to instant in frequency, amplitude and phase in the same manner as the wave at the transmitting point. Moreover, it is necessary to avoid as far as possible the production of Waves other than the speech or music waves, such as all forms of noise and distortion, including cross-talk. In a time division multiplex system the speed at which the sampling of the different message Waves is to occur has to Abe high in comparison with the highest essential message Wave frequency, which in the case of speech or music, for example, is several thousand cycles per second. The sampling at the transmitter and the separation and reconstruction at the receiver must be carried out in close synchronism and especially Without an appreciable cumulative time difference in order to avoid cross-talk or the actual crossing of channels. These requirements impose severe restrictions upon the apparatus to be used in a time division multiplex system.

En accordance with this invention distributors of the electronic type and of special construction are provided at the sender and receiver, which cooperate tc effect the analysis and reconstruction or' the Wave bands constituting the messages. These distributors incorporate suitable multiple terminals for the incoming and outgoing circuits and control means for varying the electron stream in accordance with message waves in such a way as to sample the respective message n ri Waves at the transmitter and separately recon- The various features and objects of the invention will appear`more fully in the following detailed description of an illustrative embodiment of the invention as shown in the accompanying drawings.

In the drawings, Fig 1. is a simplified schematic circuit sketch of. a two-way terminal station for a system according to the invention;

Fig. 2 is a schematic circuit diagram of the transmitter; l

Fig. 3 is a' view of. the electrode structure looking toward the right in Fig. 2 from the plane 3 8;

Fig. 4 is a schematic circuit diagram of the receiver;

Fig. 5 is a schematic circuit diagram of the electrode structure of the receiver and of the synchronizing channel receiver; and

Fig. 6 is a partial view of an alternative type of receiver tube structure.

Referring to Fig. 1, a plurality of telephone lines. are indicated at It, I9 and 20 as coming into the station shown in this figure, these lines being indicated by single line circuits for simplicity. These lines are associated with transmitting electron distributor l0 and receiving elec- `tron distributor 50 which are respectively associated with transmitting radio terminal 2t and receiving radio terminal 58. While radio transmission is indicated in this figure, it will be obvious that the transmission could take place equally Well over line conductors of any suitable type.

The incoming telephone lines are associated with the transmitting tube i@ by circuits i3, M and i5 connected to the respective telephone lines by means such as the usual hybrid coil and line balancing network shown at il, il. Low-pass filters, such as it, are included in these connections to suppress currents of frequencies higher vthan the highest speech frequency which is to.

be transmitted. The individual lines i3, id and i5 lead to individual grids l, 8 and ii in the distributor tube i0. (The structure of distributor tubes it and 50 will be more fully disclosed in connection with the subsequent figures.) A common anode I2 cooperates with the individual grids. `A suitable rotating field is produced under control of oscillator 5 in a manner to be described for rotating the cathode beam indicated at II over the grids in succession such that the output current from the distributor tube l0 is composed of small fragments taken successively from the speech currents present in lines I3, I4 and I5. The scanning frequency is high compared to the highest speech frequency to be 55 transmitted so that the output current; is seen to comprise high frequencies. These output currents are transmitted through any suitable connecting circuits, such as a coupling circuit 22 and amplifier 23, to the radio transmitter 24.

The receiving distributor 5|) differs mainly from the transmitting distributor I0 in having a single l grid 55 and

individual anodes

52, 53 and 54. The received waves from the distant multiplex transmitter are amplified at 51 and impressed through any suitable coupling circuit 56 across the grid and cathode 55 and 59 of distributor tube 50. 'I'he oscillator 5' causes the beam shown at 5| to rotate and intercept the

plates

52, 53 and 54 in synchronism with the scanning motion of the beam at the distant transmitter. The grid 55 meanwhile varies the intensity of the beam 5| in accordance with the instantaneous amplitude of the received waves. There is thus produced in the output current, from each individual anode such as 52, a current impulse corresponding in all respects to the fragment of the speech eX- isting atthe same instant in the corresponding transmitting line at the distant station. These reproduced fragments of speech are transmitted through low-pass filters,A such as 60', and ampliers such as 60', into the respective lines 6|, 62 and 63, leading to the hybrid coils in the respective telephone lines |8, |9 and 20. Where the phase shift between terminals is sufficiently small, or can be suitably compensated a single oscillator may serve in place of the two shown at 5 and 5.

While only three telephone lines are indicated in Fig. 1 associated with the transmitting and receiving distributors, it is contemplated that a much larger number of lines might in practice be' arranged for multiplex transmission in the same manner, the actual number depending upon the design limitations of the tubes or other requirements.

Referring now to Fig. 2, the transmitting distributor tube I0 is shown in somewhat greater detail. The tube is shown provided with a' suitable cathode 36 in the form of a filament heated from battery 42. In axial alignment with the filament is the accelerator tube 31 positively biased from battery 43. Any other suitable form of electron gun may be used including so-called' electron lenses of known type. 'I'he beam emerging from the accelerator tube 31 or other gun structure passes between pairs of plates 38 'and 39 situated at right angles to each other and thence passes down the tube to the opposite end where the various grids and the anode are located. One pair of deector plates 38 is connected across resistance 4D while the opposite pair 39 is connected across condenser 4| in ac- .cordance with the usual practice, the resistance and capacity being bridged across the terminals of the oscillator 5. A rotating iield is thus produced. 'I'he individual grids 1, 8, and 9 are connected by

leads

28, 29 and 30, through individual secondary windings of repeating coils, one of which is shown at 26 to a common, grid 35 extending all the way across the tube, this latter grid being preferably grounded. Grid 35 is a retarding grid and is preceded by an accelerating grid 34 having a positive potential applied from battery 44. I'he retarding grid 35 produces a region of relatively low velocity in which the control grids are placed so that they are enabled to exert a more complete control over the anode current. Anode I2 is shown provided with suitable anode battery 45 which is connected in series with coupling impedance 45. Outgoing amplifier 23 is connected across this coupling impedance. 'I'he f control grids are each preferably shielded by being completely enclosed in individual screens 3|, 32, 33, which are preferably at ground potential. These screens provide electrical isolation of the individual control grids to guard against crosstalk eiects that might otherwise resultfrom the close positioning of the control grids. Tube I0 in this figure is indicated@diagrammatically as embodying more than three control grids. While elements 3|, 32, 33 and 35 are shown as at ground potential, it is within the invention to bias any of these elements as required. P

A typical arrangement for the control grids of the tube ||l is shown in Fig. 3. The grids are arranged in a circle with a suitable space between each two successive grids. Common anode I2 is shown back of the grids. Two

special grids

48 and 49 are shown in this figure connected to poles of respectively opposite polarity of batteries 4B' and 49'. These two grids serve for synchronizing purposes in a manner presently to be described. The orbit of the electron beam past the grids is indicated by a broken line.

Referring to Fig. 4, the receiving electron distributor 50 is shown in detail. This tube comprises a suitable electron source in the form of filament 66 heated from battery 68. Accelerator tube 61 is given a positive potential from battery 69. As in the case of tube I0 any other suitable electron gun structure may be used. Oscillator 5' is connected across resistance 10 and capacity 1| and the pairs of

plates

12 and 13 produce a rotating eld for the beam 5|. Located between the cathode 66 and accelerator tube 61 is control grid 55 shownv connected to the 'output of receiving amplier 51 following equalizer 15. The

individual anodes

52, 53, etc. are connected to one terminal each of the respective receiving circuits, the opposite terminals of which are connected in common to a guard'plate 14 and to the positive pole of anode battery 69. Guard plate 14 prevents'electrons not caught by

anodes

52, 53, etc. from accumulating into a space charge.

'I'he appearance of the anode structure looking to the right of the line 5-5 of Fig. 4 is shown in Fig. 5. It is seen that the individual anodes V52, 53, etc. are each narrower than the corresponding grids of the transmitting tube and are spaced apart so that the anguular distance between their centers is the same as that between the' grids of the transmitting tube. Slight deviations in phase between the transmitter and receiver are thus allowed for. shown at and the position of the beam is indicated at the instant corresponding to exact synchronism with the transmitting tube of Fig. 3.

The circuit of the oscillator 5 is shown in detail in Fig. 5 as comprising two triodes 8| and 82 having their plates connected through respective resistances 86 and 81 across the terminals of tuning condenser 95. The tuning inductance is shown as comprising the two halves 90 andv 9| mounted on magnetic core 93. The cathodes are connected together and to the heating battery 92 in series With the

resistances

88 and 89. The plates are connected to plate battery through primaries of output transformer 83, the secondary of which leads through a narrow bandpass lter 84 'to the phase Splitting circuit 10, 1|. Filter 84 suppresses harmonics so that a single frequency wave is supplied to the field plates. Phase splitting circuit 10, 1| acts similarly to the elements 40, 4| of previous gures but is arranged A special synchronizing anode is' to pass direct current so as to enable the normal potential of the deflection plates to be fixed. as noted below.

For synchronizing purposes the oscillator 5' is provided with a winding 94 on the magnetic core 93 of the tuning inductance to permit small changes in the frequency of the generated oscillations under control of current in the winding 94 which changes thedegree of saturation of core 93 and, therefore, the value of the inductance 9D, 9|. f

For this purpose a circuit is connected across special synchronizing segment 88 and anode battery 69 comprising conductors 99 and |80 .leading to direct current amplifier 91 `(if used) and low-pass lter 96 and thence to the winding 98.

The operation of the synchronizingy channel will be apparent from considering Figs. 3 and 5 together, Fig. 3 being assumed for this purpose to be the transmitting distributor at a distant station with which the distributor of Fig. 5 at the home station is to be kept in synchronism. The oscillators 5 and 5' are assumed to generate waves oi nominally the same frequency and the beams and 5| are assumed to be at the' instant under consideration in exact phase with each other as shown in the drawings. beam li of the transmitting distributor passes clockwise across the grids 58 and i9 a sudden reversal of line current takes place since grid 88 has a high negative potential and grid 89 a high positive potential. This sudden reversal of line current corresponds to the instant when the beam passes off from grid 48 onto grid 19.`

It may be of interest to note that this reversal is sent without the necessity of transmitting a direct current component. If the beam 5l of the receiving distributor reaches the middle of segment 8U at the instant when the reversal of line current is received, it is obvious that the beam has traversed the first half of the segment 88 when its controlling grid 55 has a high negative value and will traverse the second half of segment 38 when control grid 55 has a high positive value. Under these conditions a dennite average value of direct current is supplied to the winding 98 from battery 69', as determined by the synchronizing pulse.

Ii, however, the receiving beam 5| is slow the current in circuit 99, |88 is greater than average since beam 5| traverses the greater portion of segment 89 during the time when grid 55 is highly positive under the influence of grid #i9 at the distant transmitter. Conversely, if the beam 5i is fast the current transmitted into circuit 99, lill! will be lower than average. These values of current are properly related to the variations in the tuning inductance of the oscillator 5 to cause compensating variations in its frequency such as to advance the beam 5| or to retard it by the proper amount to restore exact synchronism. The advancing or retarding action may take place over a number of distributor cycles.

I have shown in Fig. 6 another type of ampliner for the synchronizing impulses. In this ngure a portion of a tube is shown which except for its anode end structure may be entirely similar to the tube 50 of Fig. 4. The tube 58', however, incorporates back of the guard plate 14 an electron multiplier type of amplifier comprising secondary emitter plates |80 to |83 and anode |84. The guard plate 'Hi is shown provided With an aperture at the proper point for permitting the beam 5| to strike the rst emit- AS theter platev |80 which corresponds in angular position to the anode 8|) of Fig. 5. By means of the successively higher'potentials` applied to the successive plates |80 to |83 from battery |88 over conductors |85 to |88, respectively, second-" the normal potential of the deflector plates would ordinarily be used in the circuits shown in each of Figs. 1, 2, 3, and 4 in place of the ungrounded connections that have been shown for simplicity. Such connections may follow `those shown in more detail in Fig. 5 where the grounded biasV battery lill is connected to the mid-point. of the secondary winding of the oscillator output coil, and the phase splitting circuit is arranged in bridge configuration to provide direct current paths to ground from all deflector plates.

While a preferred embodiment of the invention has been illustrated and described, it is to be understood that the invention is not limited to the specific details but that the'invention includes modications that may be made within the scope of the appended claims.

What is claimed is: l

1. In multiplex telephony, in combination, a cathode ray tube having means to produce a cathode beam, a plate against which said beam is directed, and a plurality of grids lying in a. plane normal to the axis of the tube and positioned to be scanned by the beam in its movements, a plurality of screens individually enclosing said grids and electrically connected together within said tube and to ground, means to move the beam to scan said grids in cyclic order at a cyclic frequency high in comparison with the highest essential speech frequency, a plurality of telephone transmitting circuits ccupled individually to the grids for placing speech potentials upon the respective grids, and an outgoing circuit from said plate.

2. In multiplex signaling, in combination, two cathode beam tubes, having beam deflecting field members, at respective stations, oscillators at said stations for producing rotary fields for said beam deiiecting members, a pair of grids in the tube at the transmitting station in a plane normal to the tube axis, in position /to be scanned by the beam, means placing a difference of potential between said grids, an anode common to said grids and an output circuit connected to said anode whereby the beam in scanning said grids produces an abrupt change of output current in passing from one grid to the next, means to transmit energy resulting from said abrupt change of output current to the receiving station, a grid in the tube at the receiving station, means controlling its potential by the received energy, an anode of small angular width in said tube at the receiving station, and means controlled by the current to said anode for producing small corrective changes in the frequency of the oscillator at the receiving station.

3. A multiplex system having transmitting and receiving stations with provision for the'transmission of energy between them, comprising at the transmitting station a yplurality of lines, an evacuated tube jhaving an anode and a plurality of individual grids or control members, said lines terminating individually on said respective grids, said tube having means for producing a cathode beam movable to scan said grids, and, at said receiving station, comprising a cathode beam tube including a plurality of anodes to which receiving lines are connected similar in number to the transmitting lines, means to control the beam intensity in accordance with energy received from the transmitting station, means, including an oscillator at said receiving station, for synchronously moving said beams over their respective grids and anodes and further comprising a pair of special grids side by side in' the tube at said transmitting station with means to impress a steady diiierence of potential between the grids of said pair, a special anode in said tube at the receiving station corresponding in angular position to said special grids, whereby the average current to said special anode is a, function of the departure from phase coincidence of the said beams and a circuit for utilizing variations in the current to said special anode to control the period of said oscillator to counteract departures from phase coincidence between said beams.

4. In multiplex signaling, in combination, two cathode beam tubes at respective transmitting and receiving stations, means at said stations for producing similar movement of the cathode beams in the respective tubes including at the receiving station an oscillator, a pair of grids in the tube at the transmitting station positioned to be scanned in close succession by the beam in the tube at that station, means placing a difteraisance ence of potential between said grids, an anode and anode circuit common tosaid grids whereby a change in anode current is produced by the scanning of said grids, means to -transmit energy corresponding to saidchange in anode current to the receiving station, a grid in the tube at the receiving station and a circuit for controlling its potential in accordance with the energy so transmitted, a segmental anode in the tube at the receiving station, said last-mentioned grid controlling the current to said anode, and circuit means operatively relating said anode and said oscillator for controlling the period of said oscillator in' direction and magnitude to maintain said beams in synchronism and phase with each other.

5. In the transmission of intelligence employing beam tubes as commutators, the method of keeping the beam in a receiving' tube in exact step with the beam in ai transmitting tube comprising locally controlling the movement of the beam in the receiving tube, causing the transmitting tube to produce an abrupt change in its output energy at a specified point in the movement ofits beam, transmitting energy in accordance with said abrupt change to the receiving tube, controlling the beam intensity of the receiving tube to produce normal, greater than normal or less than normal output current depending upon existence of exact agreement in movement of the two beams, departure from exact agreement in one direction or departure from exact agreement in the other direction,

as the case may be, and varying the rate of movement of the beam in the receiving tube in accordance with departures from the normal strength of said output current, in such. direction as to bring said beams into step.

PIERRE MER'IZ.