US2445775A

US2445775A - Pulse time modulation multiplex receiver

Info

Publication number: US2445775A
Application number: US581037A
Authority: US
Inventors: Donald D Grieg
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC; Federal Telephone and Radio Corp
Priority date: 1945-03-05
Filing date: 1945-03-05
Publication date: 1948-07-27
Anticipated expiration: 1965-07-27
Also published as: ES182267A1; GB638371A; FR951261A; BE473659A

Description

July 27, 1948. D. D. GRIEG PULSE TIME MoDULATIoN MULTIPLEX RECEIVER Filed March 5, 1945 2 Sheets-Sheet 1 R R @7; LW. Lm 7 a M I 6 VMM WW@ ivm@ ww IMMW m E r 6% cZ/PPEA /8 v AMP /F/ff? /Ca SHAPE/P w W. f R HM CET a .www www u fr ou 1.111. WM Tami i. l. n 4

Mimi l l 1 l l l l f I N VEN TOR. 00A/HL D 0. GIP/EG ATTDRNFY Patented July 27, 1948 PULSE TIME MODULATION MULTIPLEX RECEIVER,

Donald D. Grieg, Forest Hills, N. Y., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application March 5,' 1945, Serial No. 581,037

6 Claims. l

This invention relates to muti-channel communicating systems and more particularly to a system for selectively receiving and demodulating the channel pulses of a train of multi-*channel pulses.

In the copending application of E. Labin-D. D, Grieg-G. J. Lehmann for Multi-channel communicating system, Serial No. 548,368, :tiled August 7, 1944, a multi-channel communicating system is disclosed having transmitting means at one terminal for transmitting a train of channel pulses wherein the pulses of one channel are provided With an identifying characteristic distinct from the pulses of other channels for use as synchronizing pulses. The signal pulses of the other channels are time modulated with respect to the pulses of the synchronizing channel according to signal intelligence of the respective channels. A receiving system is provided at a second terminal for receiving the train of channel pulses. The receiving system includes means for obtaining energy of the synchronizing pulses for use in selective segregati-on of the pulses of the different channels, and for applying the selected signal pulses to a demodulator for translation of the time modulation thereof into amplitude modulated energy.

It is an object of this invention to provide 4a receiver to utilize the synchronizing pulse energy of such a multi-channel train of pulses for both the selecting and demodulation of the time modulated signal pulses of the train.

Another object of the invention is to provide means for separating synchronizing or ben-chmark pulse energy from a train of interleaved synchronizing pulses and signal pulses and to translate the synchronizing pulse energy into a suitable signal pulse selecting and demodulating energy.

According to one feature of the invention, the synchronizing pulses, after separation from the received train of pulses are retarded in time to coincide roughly with the time modulated signal pulses of a given channel. The retarded synchronizing pulses are shaped to include a voltage variation characteristic such that when these reshaped or demodulator .pulses are combined with the received train of pulses in an electric valve such as an amplier or other suitable mixing circuit, the desired signal pulses combine with the demodulator pulses to produce composite pulses, the peaks of which represent the signal component of the signal pulses. The voltage variation of the demodulator pulses is preferably linear so that when the .signal pulses 'are superimposed thereon, they are elevated in amplitude according to the time modulation thereof.

The signal component may be obtained from the composite pulses thus formed by either a threshold clipping operation determined by the biasing of the demodulator amplier or the signal component may be obtained by a peak riding clipper connected to the output of the demodulator or combining circuit. In conjunction with the clipping operation, a low pass lter capable of rejecting the pulse and composite pulse components is used.

Other features of the invention include various ways of effecting reshaping of the synchronizing pulse energy. A simple transformer or capacitance-resistance network may be provided to differentiate the synchronizing pulse energy to produce a pulse shape having the desired voltage variation characteristic. A multiple-vibrator circuit may also be used for this purpose, the time constants being so selected as to provide the desired pulse shapes. Still another form may comprise an harmonic selector to pro-duce an harmonic wave of the desired frequency which, when combined With the synchronizing pulse energy, before or after retardation, will provide a composite demodulator pulse having the desired voltage variation characteristic.

For a better understanding of the objects and features of this invention, reference may be had to the following detailed description t-o be considered in connection with the accompanying drawings in which:

Fig. 1 is a schematic block diagram of a multichannel receiving system according to this invention;

Figs. 2 and 3 are graphical illustrations useful in explaining the operation of the receiving system;

Figs. 4, 5 and 6 illustrate diierent pulse shapers that maybe employed in the system of Fig. 1; and

Fig. 'l is a graphical illustration useful in explaining the operation of the system illustrated in Fig. 6.

Referring to Figs. 1 and 2 of the drawings, the receiving system shown in Fig. 1 includes a plurality of receiving channels for selective reception of the channels of a train of multi-channel pulses such as illustrated in graph A of Fig. 2. While the train of pulses is shown to include

signal channels

1, 2 and 3 besides a synchronizing channel S, it Will be understood that many additional channels may be provided depending upon the length of the train cyclelthe width of 3 the channel pulses, the maximum time modulation of the pulses and the width of guard interval required Ibetween pulses of adjacent channels.

The receiving system of Fig. 1 is shown with two receiving channels, the provision for additional channels being indicated by broken connections. The receiving system is shown, for purposes of illustration, as capable of receiving over either a direct line connection 4 through impedance matcher 5 or over a radio link by shifting switch 6 to the output connection of radio receiver l. The received train of channel pulses may have the appearance shown in graph A of Fig. 2 or according to graph B. Graph A shows the train of pulses with interference fluctuations varying the amplitudes of the pulses. Where the system is subject to such interference, as in the case of a radio link, either the radio receiver 1 may be provided with clipping means or a separate clipper amplier may be provided as indicated at 8 for clipping the pulsetrain bey tween levels 9 and I0, thereby eliminating substantially all of the interference occurring between pulses and during pulses.

After clipping, the pulse train is applied to a synchronizing pulse selector I I which may be of any known character capable of discriminating between pulses of two different widths. Since the synchronizing pulses are provided with a pulse width greater than the pulse width of the other channel pulses they are readily segregatable and the output, depending on the circuit or pulse shaper means that may be included in such selector circuit, may comprise wider width pulses S1, graph C. The synchronizing pulses S1 thus separated from the other channel pulses are applied over connection I2 to a delay network I3. The network I3 is provided with output connections I4, I5, etc., to provide different amounts of retardation from the synchronizing pulse energy in accordance with the time spacing between the pulses of the successive channels. The energy of output connection I4, which is retarded an amount t1 for demodulation of channel pulses I, is applied to a shaper I6 whereby the delayed pulse energy is shaped similarly as illustrated at S2, graph D. The shaper I6 comprises a transformer of a known characteristic capable of differentiating the synchronizing pulses S1 into the shape represented at S2. The leading edge of each pulse S1 produces a sharper increase in voltage and the duration of the pulse Vin conjunction with the time constants of the transformer produce a Voltage variation decay such as indicated at Il. It is this voltage variation of the pulse Sz that is desirable for translation of the time modulation of signal pulses. A

The pulses S2 are applied to a selecting demodulator I8 which may comprise any one of several known circuits. One common circuit that may be employed is a threshold clipping amplifier biased to cut oi at a clipping level such as indicated at I9'in graph E. The signal pulse output of clipper 8 is applied over connection lia to the demodulator I8 where the pulses thereof combine with the demodulator pulses S2 in accordance with their timing relationship. The duration of the linear portion I1 is selected preferably to overlap the interval between the limits of modulation of the signal pulses desired. This is illustrated by the relationship shown in graph E. The signal pulse is superimposed, in effect, upon the inclined portion of the ,demodulator pulse thereby producing a peak corresponding to the signal component of the time modulation of the signal pulse. These peak portions exceed the clipping level I9 and a corresponding amplified current flow is produced in the output of the demodulator I8 as indicated by pulses la. By applying the pulses Ia to a low pass filter 20, the pulse component is removed and the resulting audio signal is applied to phones 2| or other utilization apparatus.

In Fig. 3, a graph is shown illustrating the demodulation of the signal component of a particular channel such as channel 2 according to a demodulator circuit having a out oi I9 selected belo'w the channel pulse levels. The signal pulses of channel 2 are superimposed on the demodulator pulses S2 according to the time modulation of the signal pulses, thereby producing pulse peaks which define the signal component 22 thereof. The low pass filter in this case removes the pulse formations and passes the frequencies of the signal component represented by the peak portions of the pulses 2.

In Fig. 4 a shaper 23 is shown comprising a capacitance 24 and a resistor 25 connected as a differentiator circuit. By proper choice of the time constants of the

elements

24 and 25, the synchronizing pulses S1 may be transformed into a demodulator pulse S2 of the desired shape. The output of shaper 23 in this embodiment is shown applied to the selecting demodulator I8 which as shown in Fig. 4 is connected to a peak riding clipper 26. The peak riding clipper is i1- lustrated in block form since it is of known character. The peak riding clipper operates to produce a current output corresponding in amplitude to the peaks of the composite pulses such as illustrated in graph E of Fig. 2.

In Fig. 5, a shaper circuit 21 is indicated as of the multi-vibrator type. Since multi-vibrator circuits of thecharacter useful in this invention are well known, no circuit diagram is necessary. The point to be made is that by selecting proper time constants for the multi-vibrator circuit, the synchronizing pulse energy S1 may be translated into a wave form, indicated at 28, having discrete pulse portions with inclined tops 29 of substantially linear characteristic capable of translating time modulation of the signal pulses.

In Fig. 6, I have shown a further shaper arrangement for the multiplex receiver Yof Fig. 1. In graph G of Fig. 7, I illustrate the train of received pulses corresponding to the train shown in graph B of Fig. 2. Graph I-I represents the synchronizing pulses S1 after retardation in delay device I3 as it appears on connection I4, Fig. 6. Shaper 30 may comprise any known ampliiier whereby t'wo pulse potentials are combined for controlling the output thereof. Connection 3| of the delay device I3 is so selected as to provide a retardation of the synchronizing pulse energy substantially as indicated at S3. The pulse output S3 is applied to an harmonic selector 32 whereby an oscillatory or other suitable wave shape 33 is obtained, the frequency of which is an harmonic of the repetition frequencyl of the pulses Ss. The ascillatory wave 33 is applied to the shaper 30 to mix, or in other words, combine with the pulse energy S1 thereby producing eilective demodulator pulses S4. This pulse is the result of adding pulse S1 to wave 33 and removing by clipping all wave energy except that of the wanted pulse S4. It will be noted that the inclined Voltage variation 34 of the pulse S4 is opposite to the inclination of the pulses shown in Figs. 2, 4 and 5. It will be understood, of course, that the retardation of pulses S3 may be so selected as to phase the wave 33 for coincidence between the pulse S3 and the trailing edges of the undulations of Iwave 33 instead of the leading edges shown.

While I have shown and described particular forms and systems by which the principles of my invention may be practiced, it is recognized that many variations may be made without departing from the invention. For example, the receiver system may be made for one channel only, and also may be used for single channel transmission such as might include, besides the signal pulses bench mark pulses similar to the synchronizing pulses S. Also the delay device may comprise circuits other than the inductance capacitance delay network shown. It is to be understood, therefore, that the forms herein shown and described are to be regarded as illustrative of the invention only and not as restricting the scope thereof as set forth in the objects and the appended claims.

I claim:

1. In a receiving system for a train of time modulated signal pulses including bench mark pulses with signal pulses interleaved therewith, means for segregating the bench mark pulses from the signal pulses and give them a substantially rectangular shape, means to retard the rectangular pulses and produce a wave having an harmonic relationship with respect to the recurrence rate of said bench mark pulses, means to phase said wave to cause a substantially linear part thereof to coincide with each of said rectangular pulses, said rectangular pulses being combined with said `Wave to produce discrete demodulator pulses having substantially linearly inclined top portions.

2. In a receiving system for a train of time modulated signal pulses including bench mark pulses interleaved therewith, means for segregating the bench mark pulses from the signal pulses and give them a substantially rectangular shape, means to retard the rectangular pulses, means to produce a Wave having an harmonic relationship with respect to the recurrence rate of said bench mark pulses, means to phase said Wave to cause a substantially linear part thereof to coincide with each of said rectangular pulses and to combine said wave and said rectangular pulses to produce discrete demodulator pulses having substantially linearly inclined top portions, and means to combine said demodulator pulses with the signal pulses to translate the time modulation of the signal pulses into amplitude modulated energy.

3. In a receiving system for receiving a desired one of a plurality of channels of time modulated pulses interleaved in sequence in the form oi a single multi-channel 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; means to separate the synchronizing pulses from said single train, means to retard the pulse energy oi' the synchronizing pulses of a width as great as the time modulation displacement to produce demodulator pulses timed to overlap the signal pulses of the desired channel, means to give the demodulator pulses a voltage amplitude variation characteristic and means to mix said demodulator pulses with said train of pulses, the pulses of the desired channel combining with the demodulator pulses to produce composite pulses having peaks representing the 6 signal component of the time modulation of the signal pulses of said desired channel.

4. A receiving system according to claim 3 wherein said means for giving said synchronizing pulses a substantially linearly inclined portion includes a means to give said synchronizing pulses a substantially rectangular shape, and a differentiator means having time constant characteristies which respond to each of said substantially rectangular pulses to produce a sharp build up potential followed by a substantially linear decay portion.

5. In a receiving system for receiving a desired one of a plurality of channels of time modulated pulses interleaved in sequence in the form of a single multi-channel 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; means to separate the synchronizing pulses from the single train, means to retard the pulse energy of the synchronizing pulses, means to produce substantially rectangular pulses from the retarded energy, means to produce from the retarded energy a wave having an harmonic relationship with respect to the recurrence rate of said synchronizing pulses, means to combine the wave with said rectangular pulses to produce demodulator pulses having inclined tops, and means to mix said demodulator pulses with said train of pulses, the pulses of the desired channel combining with the demodulator pulses to produce composite pulses having peaks representing the signal component of the time modulation of the signal pulses of said desired channel.

6. In a multi-channel receiving system for selectively receiving a plurality of channels of time modulated pulses interleaved in sequence in the forni of a single multi-channel train, the pulses of one of the channels being provided with an identifying characteristic distinct from the pulsesI of the other channels for use as synchronizing pulses; means to separate the synchronizing pulses from said single train, retardation means to retard by different amounts the pulse energy of the synchronizing pulses to provide different trains of demodulator pulses of a Width as great as the time displacement of said modulated pulses each such train corresponding in time to one of the channels, a plurality of receiving channels each having mixer means to mix a train of demodulator pulses with said single train, and means including a Shaper circuit to apply said demodulator pulses to said mixer means, said shaper circuit being arranged to vary the voltage amplitude of said demodulator pulses in a manner to elevate the voltage oi the desired signal pulses according to their time modulation to produce peaks representing the signal component thereof.

DONALD D. GRIEG.

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

UNITED STATES PATENTS Number Name Date 2,172,354 Blumlein Sept. 12, 1939 2,199,634 Koch May 7, 1940 2,361,437 Trevor Oct. 31, 1944 2,391,776 Fredendall Dec. 25, 1945 2,413,023 Young Dec. 24, 1946