US2426205A - Pulse selecting circuit for multiplex systems - Google Patents

Description

Aug. 2e, v1947. un. GRIEG ETAL 2,426,205"

PULSE SELECTING CIRCUIT FOR MUL'I-IPLEX SYSTEMS Filed March 2, 1946 2 sheets-sheet 1 (il P900/*Pur zal/WEG, cwi:

INVENToRS 2o/mm a., GR/EG HFA/OLD M. fV//VE ATTORNEY Patented Aug. 26, 1947 PULSE SELECTING CIRCUIT FOR MULTIPLEX SYSTEMS Donald D. Grieg and Arnold M. Levine, Forest Hills, N. Y., assignors to Federal Telecommunication Laboratories, Inc., corporation of Delaware New York, N. Y., a

Application March 2, 1946, Serial No. 651,652 6 Claims.k (Cl. 179-15) This invention relates to a circuit for selecting channels in a multi-channel pulse communication system in which the pulses are time modulated, and for demodulating the pulses of the selected channel.

In communication systems of the type to which this application has reference, the pulses forming the separate channels are interleaved and have dilferent time displacements with respect to regularly repeated marker or synchronizing pulses. The marker pulses may be single pulses having a distinct characteristic, such as a unique width distinguishing them from the signal pulses, or the marker pulses may consist of pairs of closely spaced pulses having a different time displacement from each other than have the signal pulses. The signal pulses are time modulated but the eX-V tent of modulation is so related to the spacing of the pulses as to prevent a pulse of one channel from moving into a time position received for a pulse of another channel.

An object of the present invention is the provision of an improved circuit for selecting channels, and for demodulating the time modulated pulses in the selected channels of a multi-channel pulse communication system of the type hereinabove described.

Another object is the provision of a circuit, of the type hereinabove described, which is characterized by its simplicity.

In accordance with a feature of the present invention, the circuit hereinabove described acts as a demodulator by producing rectangular waveforms of variable width (duration) corresponding to the time elapsing between the marker pulses and the associated signal pulses of the channel to be selected. Each of these rectangular waveforms is initiated terminated by the associated signal pulse of the selected channel. Consequently the width of each of these waveforms is a function of the time modulation of the pulse terminating them. These variable width rectangular waveforms are applied to a suitable utilization device. 'Ihe circuit also acts as a channel selector in that each of the rectangular waveforms can only be terminated by a signal pulse occurring during a given interval beginning at a selected time after the marker pulse, the duration of said interval being sulcient to include the extremes of time displacement of the pulse of the selected channel due to the time modulation of said pulse.

Other and further objects of the present invention will become apparent and the invention will be best understood from the following deby a marker pulse and scription of an embodiment thereof, reference being had to the drawings, in which:

Fig. 1 is a block diagram of a multi-channel communication receiver employing selector circuits in accordance with the present invention;

Fig. 2 is a schematic diagram of one of the selector circuits, shown in block diagram in Fig. I; and

Fig. 3 is a set of curves used in describing the operation of the selector circuit embodying our invention, these curves having no exact quantitative significance, but being introduced merely to enable easier understanding of the present invention.

In multi-channel communication systems of the type to which this application has reference, a plurality of signal pulses, as for example, signal pulses I--8, each forming part of a separate channel, are interleaved in sequence and progressively displaced in time with respect to an associated marker pulse 9. The marker pulse 9 is distinguished from the signal pulses for eXample by having a different width (as illustrated in curve A, Fig, 3) or by some other distinguishing means. The marker or synchronizing pulse S may consist of two pulses having a different spacing therebetween than the spacing between the signal pulses. The marker pulses 9 are regularly repeated and so are the signal pulses I--8 except for the variation due to the time modulation of said pulses within relatively narrow limits a and b. These limits a and b are so chosen that no pulse will overlap into a time Zone in which another pulse moves when it is modulated. Pulses l-9 may be generated, and the signal pulses I- may be time modulated, in a manner known in the art. They may be then used to modulate a carrier and radiated, or carried over a wire, or the pulses I-S may be transmitted to a receiver by any suitable transmission medium. Pulses I-B are preferably all of the same amplitudc.

Referring now to Fig. l, the incoming pulses may go through a receiver unit I 0 which may be used to assure that all the output pulses thereof are of equal amplitude. These equal amplitude pulses, similar to those illustrated in curve A (Fig. 3) are then fed to a plurality of channels Ii, I2, I3, etc., over lines I4, I5, I6, etc. The marker or synchronizing pulses 9 are separated by a suitable marker separator circuit I 'I and fed over lines I8, I9 and 2l] into various channels II, I2, I3 respectively. The marker separator circuit II may be any suitable known device Which utilizes the distinct characteristics of the marker pulses 3 to separate them from the signal pulses. For example, where, as illustrated, the marker pulses are distinguished in width from the signal pulses, the separator I1 may be any known form of pulse Width selector.

Since all the channels are similar, only channel II will be described. In channel II, there is provided a selector circuit 2| on which are impressed the separated marker pulses over line I8, as well as all the pulses over line I4. The selector circuit produces a rectangular waveform in response to pulses of the selected channel, the width of said waveform varying in accordance with the time modulation of the pulse of the selected channel. These waveforms are then fed to asuitable integrating circuit 22 which may be for example, an audio lter and are thereafter fed to asuitable utilization device 23, which may consist for example, of audio amplifiers and a sound reproducer.

Referring now to Fig. 2, the selector circuit 2| is a multivibrator which is adapted to be tripped by a marker pulse and to be returned by the following signal pulse of the selected channel. In the absence of a signal pulse in the selected channel (which in the systems to which this application has particular reference only occurs inadvertently), the multivibrator will return to its ,original state or output voltage level after a time has elapsed from the initiation of its oscillation equal to the maximum time between any synchronizing pulse and the most extremely displaced pulse of the selected channel. The constants of the circuit are moreover so chosen that the multivibrator cannot be returned by any pulse occurring before the minimum time elapsing between any synchronizing pulse and any associated one of the pulses of the selected channel. In accordance with a further feature of the invention, the synchronizing or marker pulses 9 each generate a blocking voltage waveform which has a duration substantially equal to said minimum time, and which blocking voltage preventsI return of the multivibrator to its original level.

For the purposes of operation mentioned in the foregoing paragraph, the multivibrator 2| may include twoelectron tubes 24 and 25 each of which may be a triode and may be contained within a single envelope. Tube 24 is normally blocked to cut-off as for example by means of a potential derived from the positive side of a source 2S which is applied through a suitable resistor 2'! to the cathode oftube 24 and which may further include acathode resistor 28 shunted by the usual by-pass condenser 29. The marker pulses 9 are fed to the grid oftube 24 over line I8 and asuitable grid resistor 30 is provided between line I8 and ground. The marker pulses 9 applied to the grid oftube 24 are sufcient in amplitude to cause said tube to conduct. Since these marker pulses are positive, the anode oftube 24 becomes negative. The anode oftube 24 is coupled over a variable condenser 3| to the grid oftube 25. A negative voltage is thereby impressed upon the grid oftube 25 which causes the anode of said tube to become positive. The anode oftube 25 is connected over acoupling condenser 32 to the output. This the potential in the output of multivibrator circuit 2| rises steeply in a positive direction as illustrated at time T1 curve E in Fig. 3, time T1 corresponding to the time when the marker pulse is impressed over line I8 upon the grid oftube 24. The anode oftube 25 is coupled by means of the1usual condenser 33 to the grid oftube 24. In accordance the known operation of multivibrators of the type hereindescribed, whentube 24 conducts, condenser 3I is charged negatively, preventingtube 25 from conducting. After a selected time has elapsed, depending upon the adjustment of condenser 3I, and its R.-C. discharge circuit which includes avariable resistor 34 andresistor 35 coupled toground 36 in series with each other, the charge leaks off the grid oftube 25, andtube 25 again conducts whereupon the anode oftube 25 becomes negative and the output voltage drops from the level it reached at time T1 back to its first or original level.

In accordance with the present invention the time for a complete oscillation of multivibrator 2| is selected so that in the absence of any signal pulse, its duration is equal to the maximum time displacement between a signal pulse of the selected channel and the marker pulse with which it is associated. For example, referring to curve A (Fig. 3), assuming that pulse 3 is the pulse to be selected, and that pulse 3 has a maximum time displacement due to modulation between the limits 3a and 3b, the oscillation of multivibrator 2| will terminate substantially at a time T4, corresponding to the maximum displacement 3b of pulse 3 from marker pulse 9. Thus pulses occurring after time T4, will have no effect upon the rectangular wave form produced by the multivibrator since their application at point I4 to the grid oftube 25, will have negligible effect especially in view of the fact that the voltage swing of the multivibrator may be, for example, of the order of volts, whereas the amplitude of the pulses may be for example, l0 or 20 volts. Moreover since the pulses applied over line I4 cannot trigger the multivibrator, it will not be tripped again until the next marker pulse 9 arrives.

In accordance with a feature of the present invention, use is made of one of the known characteristics of multivibrators of this type which enables a pulse applied on line I4 to cause said multivibrator to return to its original level before time T4. This characteristic is as follows: When a positive voltage is applied to the grid oftube 24, the anode thereof becomes negative and remains negative from time T1 when the marker pulse 9 is applied to the grid, until time T4 which is the selected period of vibration of the multivibrator as determined by the R.-C. constants of the circuit. Thus a voltage having a rectangular waveform 3'! appears on the anode of tube 24 (see curve B, Fig. 3). The grid oftube 25 is coupled to the anode through condenser 3 I, but due, however, to the constants of the condenser 3| and its associated resistors, a voltage having awaveform 38 appears at the grid oftube 25, as indicated in curve B in dotted lines, said waveform having aflat portion 39 which starts to curve positively a short time before time T2 to form acurve 40. The shape of the curve 4I) which is made as linear as possible, and the time it begins may be adjusted by adjusting the value of condenser Ei and its associated resistors. Pulses I and 2 occurring before time T2 are insufficient to causetube 25 to conduct and return the multivibrator to its original state. However, any time after time T2, the pulse 3, no matter what its time displacement between modulation limits 3a and 3b, =Will be suflicient to causetube 25 to conduct since pulse 3 will coincide with the risingcurve 40 ofwaveform 38 and therefore is sufficient to raise the potential of the grid oftube 25 above the lcritical level 4| at whichtube 25 begins to conduct.

From the foregoing it will be seen that only the marker pulses 9 and signal pulses 3 will affect the multivibrator 2 I. While pulses 4-8 occurring after the multivibrator has returned to its origina1 level of stability, are not likely to produce any eiect in the output, to make doubly certain thattube 25 does not act as an amplier and feed pulses 4 8 through to the output, the grid oitube 25 is made so positive by applying energy from the positive side of a source oi potential 42 thereto, that normally when multivibrator is not vibratingtube 25 is saturated. Thewaveform 38 of curve B has been illustrated as having a higher potential than the rectangular waveform 31 (curve B, Fig. 3) to indicate to the potential fromsource 42 applied to the grid oitube 25.Tube 25 being saturated the pulses 4 8 applied over line I4 to the grid oftube 25 produce no effect in the output thereof.

In accordance with a further feature of the present invention, to guard against any of the pulses prior to the selected pulse returning the multivibrator to its original state after it has been tripped by the associated marker pulse 9, the following additional means are provided. In the plate circuit oftube 24, there is provided an adjustable tunedtank circuit 43 which may be in the form of a permeability-tunednductance coil 44, (which for example may be tuned by moving slugs in and out of said coil to vary its permeability) said coil having aninherent capacity 45 making it a resonant or tuned circuit.Coil 44 is shunted by a rectifier 4S. When the positive marker pulses are applied to the grid oftube 24, thev anode goes negative, thereby shock-exciting tank circuit 43 and producing a negative undulation 41 (curve C, Fig. 3) Thenegative undulation 41 has a duration slightly less than the minimum interval between the selected pulse and the marker pulse and slightly more than the maximum interval between the pulse preceding the selected pulse and the marker pulse. For example, assuming pulse 3 is the pulse to be selected, thenegative undulation 41 will have a duration from time T1 to slightly less than time T2 but slightly more than the widest separation of any pulse 2 from its associated marker pulse 9. The positive undulation which would normally follow thenegative undulation 41 is damped out byrectier tube 45, which preferably has as low an internal impedance as possible to assure rapid and complete critical damping of the positive undulation.

Thenegative undulation 41 is combined withwaveform 31 to produce a voltage at the anode oftube 24 as indicated in thesolid line 48 in curve D, Fig. 3. At the grid oftube 25, the combined voltages produced consist of a combination ofwaveform 38 and a waveform corresponding tonegative undulation 41, the resulting waveform being designated by the numeral 49. From an examination ofwaveform 49, it will be seen that there is very Aslight possibility of any pulse occurring before time T2 lto have suilicient amplitude to causetube 25 to conduct and return the multivibrator to its original level after it has been tripped by marker pulse 9. Theundulation 41 thus serves as a blocking voltage to safeguard against any accidental returning of the multivibrator to its original level. Between times T2 and T4, however, pulses will be capable of causing`the multivibrator to return to its original position because these pulses will cause the level of the grid oftube 25 to reach the critical level 4| at which the tube conducts. Assuming that pulse 3 is in its middle or unmodulated position at "time T3, the multivibrator will be returned to its original level at time T3 and the resulting output voltage will have a duration equal to time T1 to T3 and having arectangular waveform 50. If pulse 3 is in position 3a at one extreme of modulation at which it is nearest to the marker pulse, the resulting output voltage Will have a rectangular waveform as shown in curve E at 5|. Finally, if pulse 3 is at its other extreme of time modulation, the rectangular waveform of the resulting voltage will be that designated bynumeral 52. Accordingly, voltages of rectangular waveforms of different duration, depending upon the time modulation of the signal pulses will be produced. These voltages will have diiferent energy content and any one of various suitable utilization means, which may include for example, an integratingcircuit 22, may be used to respond in accordance with the energy content of these rectangular waveforms.

It will be apparent that by adjusting various elements in multivibrator 2| such as for example, variable condenser 3|,resistor 34 andtank circuit 33, etc., the multivibrator 2| may be made to select any desired channel. To enable this to be accomplished readily, thevariable inductance 44, condenser 3| andresistor 34 may be ganged together to enable simultaneous tuning of these elemenits in one manual movement. Thus a single selector circuit may be employed to successively select different ones of a number of pulse signal channels. If, however, it is desired to simultaneously receive several channels, then an equal number of selector circuits, as illustrated in Fig. 1, may be employed, each of said selector circuits being tuned to a separate channel.

While we have described the selector circuit as being primarily Ia multivibrator, it will be apparent to those versed in the art that other types of circuits, such as various forms of flip-flop circuits, may be used in place thereof.

While We have described above the principles of our invention in connection with specic apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of our invention as set forth in the objects hereof.

We claim:

1. In a receiving system for receiving a desired one of a plurality of channels leaved in sequence in the form of a single multichannel pulse train, the pulses of one of the channels being provided with an identifying characteristic distinct from the pulses of the other channels 'for use as synchronizing pulses, and the pulses of at least the selected one of said channels being time modulated signal pulses; an adjustable circuit having an output variable between two levels and adapted, after being tripped from one of said levels to its other level, normally to return to said one level after an interval selected to be substantially equal to the maximum interval between a synchronizing pulse and an associated signal pulse ofthe selected channel, said circuit being capable of being returned sooner by a suitably applied pulse occurring after a predetermined minimum time has elapsed after said circuit has been tripped, said minimum time being selected to be substantially equal to the minimum interval between a synchronizing pulse and an` associated signal pulse of the selected of pulses interchannel, means 4fcr applying the synchronizing pulses to said circuit to trip it, means for applying the other pulses of said channels to said circuit so that each tends to return it, and utilization means responsive to the length o-f time said circuit remains at said other level.

2. A receiving system according to claim 1, wherein said circuit is a multivibrator.

3. A receiving system according to claim 1, iurther including adjustable means associated with said circuit and responsive to each of said synchrcnizing pulses to produce a blocking voltage having a duration substantially equal to the minimum interval between la synchronizing pulse and an associated signal pulse of the channel to be selected.

4. In a receiving system for receiving a desired one of a plurality of channels of pulses interleaved in sequence in the form of a single multichannel pulse train, the pulses of one of the channels being provided with an identifying characteristic distinct. from the pulses of the other channels `for use as synchronizing pulses, and the pulses of at least the selected one of said channels being time modulated signal pulses; a circuit having 'an output variable between two levels, means for applying the synchronizing pulses to said circuit to trip it from one of said levels to its other level, means for applying the other pulses of said channels to said circuit so that each tends to re-trip it to said one level, adjustable means controlled by each of said synchronizing pulses to produce a blocking voltage of selected duration for blocking said re-tripping of said circuit for a selected interval, and utilization means responsive to the length of time said circuit remains at said other level.

5. A receiving system according to claim wherein said circuit comprises a multivibrator.

6. In a receiving system for receiving a desired one of a, plurality of channels of pulses interleaved in sequence in the form of a single multichannel pulse train, the pulses of one of the channels being provided with an identifying characteristic distinct from the pulses of the other channels for use as synchronizing pulses, and the pulses of at least the selected one of said channels being time modulated signal pulses; a multivibrator adapted to have a normal period of oscillation substantially as great as the maximum interval between a synchronizing pulse and the associated signal pulse of the selected channel, means for applying the synchronizing pulses to said multivibrator to trip it, means for applying the signal pulses to said multivibrator tending to re-trip it, adjustable means associated with said multivibrator and responsive to each of said synchronizing pulses to produce a blocking voltage having a duration substantially equal to the minimum interval between said associated signal pulse of the channel to be selected and its synchronizing pulse, and utilization means responsive to the output of said multivibrator.

DONALD D. GRIEG. ARNOLD M. LEVINE.

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