US2326584A - Multiplex telephony system - Google Patents

Description

Patented Aug. 10, 1943 UNITED STATES PATENT oFFica MULTIPLEX TELEPHONY SYSTEM I Johannes Jacobus Zaalberg van Zelst, Eindhoven,

Netherlands, assignor, by mesne assignments, to Hartford National Bank and Trust Company,

v Hartford, Conn, as trustee Application February 28, 1940, Serial No. 321,342

' In Germany February 20, 1939 3 Claims.

- lvlyinvention relates to multiplex telephony systems in which a plurality of telephonic conversations are simultaneously transmitted over a common transmission medium for instance a common transmission line.

In such systems mechanical or electrical-commutator means, for instance a cathode ray tube, is employed to successively and periodically connect each speech channel to the common transmission medium for a very short time period. Thus, the conversation of each speech channel is transmitted in theform of periodically-occurring short impulses whose amplitude is modulated in accordance with the conversation being transmitted. In this case the frequency of the impulses is the same for each speech channel but the impulses of each channel are shifted with respect to each other, v

I have found that in such systems difficulties are caused by the fact that the impulses are distorted due to the frequency-dependent attenuation of the transmission medium i. e. cable.

The object of my invention is to overcome the above diificulty and for this purpose I provide means to make the transmission medium or the speech channel impassable, at least for voltages below a definite threshold value, during the time periods between successive impulses.

I prefer to make the transmission medium impassable by using a suppressor to which is supplied a voltage whose frequency is determined by the successive impulses being transmitted and which periodically removes the blocking of the suppressor. According to another embodiment, I use a device whose threshold value is so selected that the disturbing voltagesoccurring during the time periods between successive impulses do not exceed this threshold value.

In order that the invention may be clearly un- 15 the preceding impulses I.

traversing the cable are different for the various frequencies contained in each impulse. Furthermore, as the cable has a frequency-dependent attenuation, the impulses are strongly distorted. 5 In Figure 2 the solid line indicates the distortion 5, indicated in dotted lines in this figure. Due

. to this, the magnitudes of these latter impulses are effected by the magnitude of impulse I so that each impulse 2, for instance, isenlarged to a degree which depends upon the magnitude of I This efiect causes cross-talk between the various channels.

When the transmission takes place through a relatively short cable the above described eflect is less, so that by a proper selection of the length of the cable, the impulses shown in Fig. 1 can be distorted to a degree corresponding to that shown in Fig. 4 in which each impulse is only slightly influenced by the preceding impulse. However, when the impulses are to be transmitted over a long distance as is usually the case in practice,

the distortion introduced in the first part of the cable must be removed ordecreased before they can be transmitted to the next part of the cable.

To overcome this difiiculty I make the transmission medium or the speech channel impassable, at least for voltages below a definite threehold value, during each time period between two successive impulses. This, will be explained in more detail below. I

As stated, Fig. 1 shows the impulses as they are emitted at the beginning ofthe cable and Figure 4 shows them as they reach the first intermediate station. The distortion has now proceeded to a much smaller extent than in Fig. 2 so that, for

derstood and readily carried into effect, I shall 40 instance, the tail 1 of impulse I has scarcely describe the same in connection with the prior art and with reference to the accompanying drawing in which,

Figures 1 to 9 are voltage-time graphs for use penetrated the impulse 2.

As will be described in more detail hereinafter, the first intermediate station may comprise a suppressor which is controlled by the voltage in describing the principle of the invention, and 5 illustrated in Fig. 3. At the moments at which Figs. 10 and 11 are schematic diagrams of portions of systems embodying the present invention. Fig. 1 shows a series of impulses I, 2, 3, 4 and I5 as they are transmitted and corresponding to the the impulses I, 2, 3, 4 and 5 shown in Fig. 4 occur the suppressor receives, for instance the impulses II, I2, I3, l4 and I5 of Fig. 3. At these moments the suppressor is passable, whereas it is imfirst, second, third, fourth and fifth conversation passable betweenthese impulses. Consequently,

respectively. In such a telephony system a very broad frequency band must be transmitted through the transmission medium. When making use of a transmission line or cable as a transduring the time elapslng between the impulses II, I2, I3, I4 and iii no voltage originating from the impulses I, 2, 3, 4 and 5 shown in Fig. 4 can be transmitted. As a result the impulses leavmission medium the transit times required for to ing the first intermediate station have the shape also subject to another type of distortion. I particularly, due to the presence of Pupin coils,

shown in Fig. 5 and the tail 1 of Fig. 4 has completely disappeared.

After the impulses have traversed the cable between the first and second intermediate station they reach the last-mentioned station in the form of the impulses shown in Fig. 6. There they I are again subjected, before being transmitted, to

the above described treatment until they finally reach the final station without any of the impulses being influenced appreciably by preceding impulses.

However, in addition to the distortion shown in Figs. 2 and 4, the impulses shown in Fig. 1 are More resonance phenomena may occur so that the impulses shown in Fig. '7 are distorted as shown in Fig. 8 when they reach the first intermediate station. A resonance phenomenon occurs at the beginning and at the end of each impulse, the resonance phenomena associated withimpulse I being denoted by reference numerals 8 and 8 in Fig. 8. The resonance phenomenon 8 may at the moat only slightly alter the average height feature of the invention is that the device provided for the above purpose has a threshold value of such a value that the disturbing voltages, which occur during the time elapsing between two impulses, do not exceed this threshold value. In Fig. 8 this threshold value is indicated by a dotted line It and only the voltages above this threshold voltage are transmitted to the second intermediate station.

A threshold voltage is used again at this station so that the disturbing voltages between the impulses are made inoperative. This threshold voltage is indicated by the dotted line I! in Fig. 9.

The last-mentioned method may also be used for eliminating the distortion illustrated in Fig. 4.

The two above-described methods will be more fully explained with reference to the circuit arrangement shown in Figs. 10 and 11 and available in an intermediate station.

The circuit arrangement illustrated in Fig. 10 comprises a discharge tube 20 having two control grids 2| and 22, an anode land a cathode II. The impulses are received over a transmission line 23 and supplied to the control grid 2| and for this purpose the conductors of the transmission line are connected to the primary winding of a transformer 24 whose secondary winding has one end connected to the control grid 2| and its other end connected through a grid bias battery 42 to the one terminal of cathode 4|.

When the control grid 22, for instance, is almost at cathode potential, an anode current modulated by the voltages set up by the control grid 2| will flow in tube 20, so that the amplified impulses are transmitted by a transformer 25 interposed in the anode circuit of tube 20 to a transmission line 26 leading to the next intermediate station. If, in contradistinction thereto, the control grid 22 is strongly negative with respect to the cathode, the anode current will be suppressed so that voltages originating from the transmission line 23 cannot be transmitted to the transmission line 26.

asses Periodical suppression of the electronic stream in-tube 20 may be eifected, for instance by supplying to the control grid 22 a pulse-shaped voltage as shown in Fig. 3. As an alternative, an alternating voltage may be supplied to the control grid 22 by a transformer 21, which has its secondary winding connected between cathode 4| and grid 22 with the interposition of a resistance .22. This voltage has a large amplitude and a frequency determined by the frequency of the succeeding impulses. When the control grid 22 is made negative relatively to the cathode by the alternating voltage, the anode current is suppressed. A grid current fiows inthe other half elapsing between two successive impulses so that the tails or the oscillation phenomena directly behind each impulse are not transmitted.

In Fig. 11 impulses received over a transmission line 23 are supplied to an amplifying tube 30 having a cathode 45, an anode 46 and a control grid 3|. For this purpose the transmission .line has one conductor connected through a grid bias battery 32 to grid 3| and its other conductor connected directly to cathode 45. After being amplified the impulses are transmitted through a transmission line 38. v The control grid 3| is negative biased by means of the battery 32, which bias has such a valuethat voltages below a predetermined value are not amplified and that the resonance phenomena, which occur at the end of each impulse, are not transmitted.

Although I have described my invention with reference to specific examples I do not desire to be limited thereto because obvious modifications will present themselves to one skilled in this art.

What I claim is:

1. In a multiplex telephony system, a common transmission line adapted to transmit a train of elementary pulses each representing the signal in one of a pulrality of channels, and suppressor means in said transmission line to block the transmission line, only during the periods between successive pulses, against the passage of disturbing voltages which lie below a predetermined threshold value, said means comprising a controlled discharge tube, and a control circuit for said tube including a source of voltage which varies in synchronism with the pulses to thereby make the tube operative in the rhythm of the pulses.

2. In a multiplex telephony system, a common. transmission line adapted to transmit a train of elementary pulses each representing the signal in one of a plurality of channels, and suppressor means in said transmission line to block the transmission line, only during the periods between successive pulses, against the passage of disturbing voltages which lie below a predetermined threshold value, said means comprising a controlled discharge tube having a plurality of control grids, a control circuit for one of said grids and connected to the transmission line, and a control circuit for another of said grids and including a source of voltage which varies in synchronism with the frequency of the pulses to thereby make the tube operative in the rhythm of the pulses.

3. In a multiplex telephony system, a common transmission line adapted to transmit a train of elementary pulses each representing the signal in one of a, plurality of channels, and suppressor means in said transmission line to block the tarnsmission line, only during periods between successive pulses, against the passage of disturband connected to the transmission line, and a 10 control circuit for another or said grids and including a source or voltage varying in synchronism with the frequency or the pulses to thereby make the tube operative in the-rhythm of the pulses and a resistance element interposed between the said latter grid and the said voltage source.

JOHANNES JACOIBUS ZAALBERG VAN ZELST.

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