US2451044A

US2451044A - Communication system employing pulse code modulation

Info

Publication number: US2451044A
Application number: US603934A
Authority: US
Inventors: John R Pierce
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1945-07-09
Filing date: 1945-07-09
Publication date: 1948-10-12
Anticipated expiration: 1965-10-12
Also published as: GB630098A; FR947270A

Description

J. R. PIERCE Oct. 12, 1948.

COMMUN I CATI ON SYS TEM EMPLOY ING PULS E CODE MODULAT I ON 4 Sheets-Sheet 2 Filed July 9, 1945 lNVENTOR By J. R PIERCE ATTORNEY Oct. 12, 1948 J. R. PIERCE 2,451,044

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Filed July 9, 1945 4 Sheets-Sheet 3 l 4W lll'r 5 WM ML:

5, F & P

I I I I Q o Z' F'I "a INVENTOR J R. PIERCE A TTORNE V J. R. PIERCE 2,451,044

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION 4 Sheets-Sheet 4 Oct. 12, 1948.

Filed July 9, 1945 lNl ENTOR By J R P/ERCE aw. J/L' LW.

ATTORNEY Patented Oct. 12, 1948 COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION John R. Pierce, Millburn, N. .L, assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 9, 1945, Serial No. 603,934

14 Claims. 1

This invention relates to a communication systern for the transmission of complex wave forms of the type encountered in speech, music, sound, mechanical vibrations and picture transmission by means of code groups of a uniform number of signal pulses of a plurality of different types or signaling conditions transmitted at high speed.

The object of the present invention is to provide a communication system capable of transmitting and reproducing a complex wave form over an electrical transmission path in such a manner that the signal-to-noise ratio of the received signal is materially improved.

Another object of this invention is to provide improved and simplified apparatus capable of transmitting and receiving signal pulses over a channel having a low signal-to-noise ratio and deriving therefrom signals having a high signal-- to-noise ratio.

More specifically, it is the object of the present invention to provide circuits and apparatus for transmitting in succession a group of pulses in sequence over a given channel representative of the amplitude of a complex wave at successive instants of time, all as part of a system, sometimes identified as PCM or pulse code modulation.

Still another object of the present invention is to transform a series of pulses representing the amplitude of a complex Wave at a given instant of time into a single pulse having an amplitude which is a function of the amplitude of the original complex wave at the given instant.

Another object of this invention is to recombine a succession of such single pulses of varying amplitude in a manner to reconstruct a wave form of substantially the same shape as the wave form to be transmitted.

A feature of the invention is that of accomplishing the above objects with a small number of pulses requiring a minimum of apparatus and equipment, taking advantage of the considerable distortion permissible in speech without loss of intelligibility. The invention is similar in some respects to my copending applications, Case 28 and Case 29, Serial No. 592,961, filed May 10, 1945 and Serial No. 603,989, filed July 9, 1945, respectively.

Other features of the invention relate to synchronizing and coordinating the various circuits and equipment at the transmitting terminal with each other and with the circuits and equipment of the receiving end so as to secure proper operation of the entire system.

Briefly, in accordance with the present invention, equipment is provided for generating a control pulse or a group of pulses of predetermined time relation one with another. These control pulses are employed to control a code element timing circuit, which circuit in turn generates a series or cycle of very short pulses some of which are positive and some negative and some a combination of the two.

Apparatus is also provided for rapidly sampling the Wave function and deriving an electrical quantity which is a function'of the amplitude of the complex wave to be transmitted, this sampling means being under control of the control pulse generator. code element timing circuit generates the series or cycle of code element timing pulses referred to above and these in combination with the sample amplitude derive the electrical quantity of a magnitude related to the magnitude of the complex wave at the time of the control pulse.

The electrical quantity takes on the character of a voltage drop over an impedance which drop is increased step by step and is tested after each addition to see if it is below or in excess of the sample amplitude voltage. If below, then the last addition is allowed to remain and if in excess, the last addition is removed. This form of comparing the sample voltage with the drop over the impedance is then carried on step by 7 step in smaller and smaller steps to as far a point as may be desired and in any case to an extent so t-hat the granularity of the signal finally produced at the receiving point will be Within the ated apparatus are provided whereby the received pulses are employed to produce a. pulse having a magnitude proportional to the magnitude of the complex wave sample at the transmission end of the system and thus the complex wave is reconstructed from a succession of such reproduced wave samples.

A special feature of the transmitter comprises a means of arriving at .a binary representation of amplitude digit by digit, the largest digit being For each of the control pulses the 3 obtained and transmitted before the smaller digits are known. To achieve this, double triodes with large cathode feedback resistors are used in switching on, and perhaps off, successively several constant reference currents in such a manner that the magnitudes of the currents are little effected by the switchingoperation. Theprocess of switchingthe curreritofi automatically transmits the proper digit for each positional notation place. A resetting pulse may be used also for transmitter blanking and sampling. The-voltage difierence between the sample and a voltage drop caused by the double triodes acts through a cathode follower tube to provide a bias:resulting in the selection of proper digits. turning one double triode off and the next on succeed one another and are of ipposite polarity so they may be obtained by producing two pulses of opposite sign by means of a network which produces a wave whose form is that of a derivative :of the wave .formof the single pulse from the :pulser.

.-A' feature=of the receiver is theme of a double triodewith a large cathode feedback resistor in combination with thedischarge of a condenser through a .resistor to change successive equal pulses, equally spaced, into .pulsesea'ch having one half the amplitude of the precedin :pulse. Another .:feature ;is the use of .asingle pulse to (1-) "recharge to a constant initialvoltage acondenser which then gives 1 an exponentially decayi g voltage to a double itriode, (2) to discharge the accumulated pulses .from .the double triode through an output filter.

A -feature of this system as a wholeqis that the measurement of the sample voltage is on a linear basis, 1. e.'the minimum variationin the sampled wave necessary to produce a significant variationin .the code re resenting the sample is the samemagnitucle forrlargeas forsrnall amplitudes of thesampled wave intead of being the same percentage of variationas in my application Serial No. 6 37,386, filed December 27,1945, now :Patent ;No. 2,437,707, granted March 16, 195:8.

flhczinvention, both as to its organization .and method of operation together with other objects and features thereof, will be -'better understood from .the following description taken with the accompanying drawings, in which:

Elias. 1.,and2 showin functional block form the various element and the lmanner in which they cooperate toform an exemplary communication system embodyin the present invention;

Fig. 3 illustrates the timing and nature of the pulses characteristic ofthe system at-the transmitting end;

.Figs. .4 to .6 give .in detail .the various circuits and operations of an exemplary system embodying thepresentinvention, and

,Fig. 7 .is.a modification .of aportion of Fig. 4.

Referring more specifically to Fig. 1, let M be asignal function representative of any complex wave suchasa speech wave,'.a.small portion of which is indicated .by curve 300 o Fig. 3. A pulse generator .Gi sets up 'a plurality of .cycles of pulses, the cycles coming in rapid succession, perhaps about 8,000 per second. The first pulse of a cycle activates .an electronic switch whereby a storage condenser .is charged to *a potential equal to the amplitude of the wave function at the instant of 'thepulsc. The remaining pulses in a cycle operate in coordination with the potential on'the storage icondenser *upona coder ,as a result of which there The pulses :ior

,pf these. however, is dependent on a pulse generatorshown in Fig. 5 and since the pulser op- .erationis independent of the circuit of 4 it willbe advantageous to describe it first.

Q'I'he controllingelement in thi portion Of the .systemis arelaxation oscillator comprising a gas tube 5E0. "-This relaxation oscillator is of a form vwelhknown in the art and includes a resistance 5 for charging a condenser 5|2. Assuming that :the-condenser EU is discharged then on closure of the circuit it-is charged at a rate determined .in part by the resistance 5| I. When the pote ntialiof the condenser and the plate of tube .5l0 ,risesto .a firing .value the condenser suddenlydischarges through .the tube and resistonSlA. The-duration ofthe discharge is. short and-gives rise to a sharppositive pulse acrossresistor 151 4. {I he duration-of this pulse .and the rate at which it is ,followed by identical pulses can be completely controlled by the parameters of the circuit; in particular, by the values of the elements 5H, .5l2 and .5l4 taken with the po tentialfof the gridof .thetube .5l0astdetermined by the potentiometer v 5| 5. While several forms of relaxation oscillator may be used-at this point the one shown is .simple and satisfactory.

' ThQpositit-e pulse' from 5M -is now used .to cpntrol'the emission of pulses tovarious parts of the circuit. This. is.,accomplished by connecting across the element till a delay circuitils made up of a transmission line, .an artificial transmission line .oranetwork o sections of inductance and capacitance .in. seriatim. .Ihepositive .pulse travels down this line or network, the ,time of arrival at each section .being spaced ,in accordance with the parameters of the various sections and givi ngfrise to corresponding .pulses going out overchan nels' to 4 for purposes to be described. While it is not .essentialthat each of themulsesbe'uniformly spaced intime from the preceding and the succeeding i ulses, thecircuits and apparatus of .theexmplary embodimentdescribed in .detail herein have .been arranged .to transmit and receive uniformly spaced pulses. Theldelay network is terminated by a load 540 ofmrfoper value to suppress any reflected wave.

{The parameters of .the relaxation oscillator may be adjusted vso that pulses are delivered across resistor 514 at any frequency desired. For the purpose of my invention itis preferred tolhave .a sampling frequency higher than that of the highest .frequencycomponent in the complex wave .toibe transmitted. If, for example, this ,wavezis to .beaspeech wave and it is desired to transmit all components .up to 3,000 cycles then it is desirable that therelshould be at least twosamples per .cycl ,for this highest frequency component. .Asuitable value, therefore, for the relaxation oscillator frequency would be 6,000 cycles although a higher value .or a lower value maylbeusedif desired.

By means of the delay circuit or timestick 5I6"on e"has .availabl'esat the ends of the respective sections positive .pulses similar to that initiated in 5| '4 and spacedintime oneafter another a i e jmii e -ihe' ham n m th section of the timestick. These pulses will control the operation of three units, A, B, C, in a manner now to be described.

At time im when the pulse'is formed at 5M it is transmitted immediately to

tubes

520, 530 and 540 in the three units. The plate circuit of 520 in unit A includes the cathode resistor 522 and anode resistor 5'23 supplied from battery 524. Consequently at the time tm there will be available a positive pulse across resistor 522 and a negative pulse across 523, to be used in the circuit of Fig. 4 as described hereinafter. A similar circuit arrangement is provided in unit B and unit with the exception that the cathode resistor corresponding to 522 is omitted. Consequently, the negative pulse across 533 and 543 may be larger than that across 523 in a manner to be desired as pointed out later.

In the unit A there is also shown a triode 52l the output circuit of which includes resistor 525 as well as the resistor 523 with battery 524 in common with the plate circuit of tube 520. All of the tubes 52|, 53|, 54|, as well as

tubes

520, 530, and 540 are biased to cut offor so that only a small or substantially no current flows in their output circuits unless a positive pulse or potential is applied to their control grids. On the arrival of the pulse from l4 at the point 2 on the timestick this tube 52! is activated, yielding another negative pulse across 523 and a positive pulse across 525. A similar circuit comprising tube 531 in unit B is activated when the pulse from 5| 4 reaches the point 3 on the timestick and there is then available a negative pulse across 533 and a positive pulse across 535. Similarly when the pulse over 5|4 reaches the point 4 it activates the circuit of tube 54! to yield a negative pulse across 543 and a positive pulse across 545. These pulses due to the activation of tubes 52l, 53! and SM are of substantially the same magnitude, and in general are less than the initial pulses due to activation of tubes 520 530 and r 540. The character of these pulses and their timing is shown in Fig. 3.

Referring now to Fig. 4, there is shown a source of a complex wave to be transmitted, such as a speech wave. This source may comprise a microphone 4H] and suitable terminal equipment 4! l and transmission line to supply the signal function M to the transformer 4I3. Associated with the secondary of the transformer is a storage condenser 5 which tends to take on a potential difference equal to that developed in the transformer. However, since it is desired to sample the wave for brief intervals of time only, the circuit is provided with two

diodes

416 and 4". A biasing battery 4|9 normally prevents flow of current through diode 4!! so that the condenser 4" does not charge. A positive pulse going directly from the pulse generator resistor 5|4 activates for a short time the tube 420 converting this into a positive-negative pulse in the secondary of transformer 422. The transformer is so poled that the positive pulse will render diode M5 momentarily conducting in spite of biasing battery M8 and will therefore discharge the condenser 4l5. Immediately thereafter the negative pulse will render diode 4 l1 conducting whereupon the condenser 4l5 will be Charged to a potential depending on the instantaneous amplitude of the complex wave and yielding a sample to be used as hereinafter described. On completion of the pulse the condenser 5 will hold the charge till the arrival of the next pulse on tube 320. In parallel with the transformer 422 is the transformer 424, the purpose of which will be pointed out later.

It is now the function in this system to send out a code group of a limited number of pulses characterizing the voltage across condenser 5, that is the amplitude of the complex wave at the sampling moment. The invention will be described in terms of three pulses per group. To this end the coder previously indicated in Fig. l is used and the process consists, in part, of c0mparing the voltage over condenser 4l5 with that across a resistor R0, the current therein being adjusted step-by-step until the drop over it is equal to the voltage over M5 or as near thereto as the steps defined permit.

A feature of this invention is the use of three pairs of triodes V1, V2, V3, each having a cathode resistor R1, R2, R3 respectively, common to its two tubes. The grid of the right-hand tube of each pair is connected to an intermediate grounded point of the B battery 45!]. Normally, no current is flowing through the left-hand tube of each pair. However, making the control grid of any one of these left-hand tubes positive with respect to ground will give rise to a constant current through R0 while making the same grid negative will turn the current off. The characteristic of a pair of triodes connected as shown is that the current flowing through its common cathode resistor tends to remain substantially constant. If a positive potential of sufiicient magnitude is applied to the grid of the left-hand tube, the right-hand tube will be turned off, all the current then flowing through R0. The cathode resistors R1, R2 and R3 may be of magnitudes related to each other in any desired manner but are preferably so arranged that the current through Re which V2 controls is twice that which V3 controls, while the current which V1 controls is twice that which V2 controls. This is accomplished by suitable adjustments of the cathode resistors and the approximate adjustment is that in which R2 is twice that of R1 and R3 is twice that of R2.

The tubes are turned on and off by pulses applied through the pairs of diodes V4, V5 and V6. The diodes of a pair are oppositely connected so that the one will pass a positive pulse and the other a negative pulse to the grid of the corresponding left-hand tubes of the pairs V1, V2 or V3. Positive pulses applied through the transformers P1, P2 and P3 will always turn on the respective left-hand triodes giving rise to a corresponding voltage drop across resistor R0. These positive pulses will also charge the

respective condensers

433, 443, 453 which will re-- tain the charges till a negative pulse arrives. Small negative pulses applied through transformers D1, D2 and D3 will turn the respective triodes oif, but, as will be explained later, only if the'drop across R0 exceeds the sample potential difference on the storage condenser 415. When a triode is turned oil? the current flowing through the control diode from the corresponding

condenser

433, 443 or 453 acts through transformers N1, N2 or Na to send a pulse to the transmitter 483 and this results in the transmission of a signal code pulse.

The operation of the transmitter end of the system may be understood now by combining the actions of the pulse generator with the coder. The sequence of operations is as follows: At the beginning of the cycle, determined by the gen eration of the positive pulse at 554, a negative pulse is developed across

resistors

523, 533, 543

asa; result of the activation of tubes 520, 530- and 540. These negative pulses act throughthe transformers D1, D2 and D3 and the corresponding diodes to turn off any of the lef thand tubes of the pairs V1, V2 and V3 that may have remained conducting during the previous coding cycle. At the same time a positive. pulse is developed over resistor 522 and arrives at transformer Pi, being delayed slightly by the delay network: $1. This pulse then turns on the tube 432.

At the sametime also, the positive pulse from 5M acting on the grid of triode. 420 gives a positive pulse over cathode resistor-Ml which in the secondary of the transformer 322 is converted into a positive-negative pulse. The positive pulse. acting through diode 4l6 discharges con denser 425 as already described and the negative pulse thereafter, operating on diode 4H, permits the condenser M5 to be charged to a potential,

corresponding to the amplitude of the complex wave at that instant. The charge on the condenser 455 then remains substantially constant until the pulse of the next coding cycle arrives.

The negative pulses over

resistors

533 and 543,.

operating through transformers D2 and D3, will be relatively large because of the absence of cathode resistors for tubes sac and 545 and they will be of sufiicient magnitude to overcome the biasing action of a triode V1 and so turn off tubes 4 32 and 452. The function of V7 will be described below. As stated, the positive pulse from resistor 522, operating through P turns on tube 432 but, because of delay Si, not until the initial negative pulses are completed. 1"he initial pulse may also be used, if desired, to blank the transmitter through transformer 424 so that the strong negative pulses in D2 and D3 at this time do not result in the transmission of a. signal.

The character and the timing of the pulses during one cycle are shown in Fig. 3. In this figure 305 represents a small portion of the complex wave and the dots indicate the successive sampling times. Line M is an expansion on, a time basis of one cycle interval an to tm+i, the first negative-positive pulse corresponding to the sampling at far and the second corresponding to th sampling at tm+1. The remaining lines show the timing of pulses within a cycle in the various circuits as will appear below.

With the tube 432 turned on, tubes 442 and 45?. turned off and a sample voltage stored in condenser 455, it is desired to compare this lat- ,er voltage with the drop across resistor Re. To this end a triode V7 is shown with its grid connected to the one terminal of condenser 415. With no current in R0 and no charge on M5, the grid of V1 is biased back by means of battery B2 to approximately ground potential in which case current of a definite value flows through the plate circuit of V7. The positive potential of the upper end of cathode resistor 41a is suitably adjusted and applied through resistors 4'", 412 and 4 33 to the cathode of the

respective diodes

435, 445 and 455. This provides the necessary positive bias on these cathodes so that the charge on the

respective condensers

433, 443, e53, does not normally leak off. Blocking

condensers

414, 415 and 416 prevent the shorting of a portion of the battery B3 in the plate circuit of V1 through resistance 418.

When condenser 4l5 has a sample charge and a current flows through R0, the potential of the grid of V1 will be raised if the drop over R0 is less than the voltage over M5. Consequently, the.

enlarged positive potential fromresistor 4'lo w ill prevent negative pulses coming over transformers D1, D2 and D3 from turning offthe corresponding

triodes

432, 442 and 452. If, however, the drop over Rois greater than thevoltage over M5, the potential of the grid of V7 will be lowered and the positive potential from resistor 410 will be decreased. A negative pulse will then turn off the last activated double triode and, by the discharge of the corresponding condenser through a transformer N, a. code signal pulse will, be transmitted.

As pointed out above, after the initiation of the first pulse in the cycle, triode 432 will be turned on. Shortly thereafter, a negative pulse is appliedito D1 corresponding to the time of arrival of the pulse in, the timestick 5H5v at the point 2, thispulse operating through triode 52l to give a negative pulse across resistor 523; If the drop across R0 is greater than the sample, then triode 432 is turned off and a pulse is transmitted through transformer N1 indicating a. zero in the highest order binary position. This pulse is indicated in line D1 of Fig. 3.

When the negative pulse over 523 is set up, a positive pulse is also developed across cathode resistor 525. This operates through a delay network- S2 to transformer P2 which, operating through its diode, turns on triode 422. Somewhat later tube 53! will be activated, giving rise to a negative pulse over resistor 533 which operates through transformer B2 on tube 422. If the drop across R0 exceeds the sample on the condenser M5, tube 422 is turned 01f and a pulse ,5 is transmitted by way of transformer N2, shown the second pulse in line D2 of Fig. 3. If not, V2 remains on and no code pulse is transmitted.

With the activation of tube 53! a positive pulse is also developed across oath-ode resistor 535' and this operating through delay network S3 turns on triode 452 very shortly after the testing, of. the eifect ofVz. Still later the activation of tube 55! develops a negative pulse over resistor 543 which operates through transformer D3 to test whether or not 452 should be turned off.

From the above description, it is seen that a combination or code of three off or on pulses will have been sent to the transmitter 480, the code group being representative of or characterizing the amplitude of the sample as given by the potential charge of condenser 4|5. This group of pulses, all rendered of the same amplitude by clipping if necessary, is accordingly trans mitted toa remote receiving station.

In the exemplary system shown in the drawing the pulses are transmitted from the transmitting station to the receiving station over a radio channel; This channel, may operate in the short wave, ultra short Wave or micro-wave regions where the Waves have quasi-optical properties.

The signals may be equally well transmitted overopen wire lines, cable conductors, a coaxial cable, wave guide, etc. or any combination of such path including a radio path. The transmission path may also include suitable terminal equipment amplifiers, gain and phase control One of these is theceives a pulse at the beginning of each cycle. This transformer is so poled as to block the transmitter 480 during the presence of the large negative pulses which turn off

tubes

442 and 452 on the initial pulse of each cycle.

The allowable range of operation of the circuit lies between a voltage on condenser 4l-5 which, when added algebraically to the voltage of B2 will brin the grid of V7 and hence the drop across 410 to certain specified values described above, and a voltage on condenser M5 which, when added algebraically to the voltage of B2 and the drop across R when the grids of

tubes

432, 442 and 452 are positive, brings the grid of V7 and the drop across 410 to the aforementioned values. It will be apparent that by a simple phase reversal in transformer 413 either of these voltages across M referred to above can represent the minimum of the AC signal. Furthermore, leav ing the circuit comprising 4"], 4| l, M3, M5, M6,

4H,- MS and 422 as they are, one may interchange the connections of the terminals of condenser 415 to B2 and the grid of V7. In this case the voltage across the condenser and that across R0 are in the same direction and the drop across R0 must be so adjusted by tubes V1, V2 and V3 that the sum of these two voltages will be constant within the operating limits. This sum combined with a reduced B2 voltage will then be the required voltage of the grid of V7. The signal then transmitted represents the magnitude of the drop over R0 as before but is now the complement of the voltage across the condenser, i. e., it is equal to a constant minus the condenser voltage. This complementary signal will carry the same information but with a 180-degree phase reversal.

Receiver A satisfactory circuit for receiving and decoding the groups of pulses from the transmitter station is shown in Fig. 6. In this figure the message indicated as arriving on a radio carrier is detected down to pulse frequency in any suitable terminal equipment BID and is then shown as being amplified by tube BIZ. The receiver circuit itself consists essentially of a decoding portion and a pulse generating portion, the latter of which will be described first. A positive pulse arriving at the input of N2 is inverted and is transferred as a negative pulse to the grid of a tube 62l which then inverts and amplifies it, if necessary, for application on the grid of a relaxation oscillator comprising the gas tube 623. This relaxation oscillator may be-of the same general form shown in Fig. 5 and by means of the resistor 524 and condenser 625 is adjusted to substantially the same frequency as the oscillator of Fig. 5; that is, to give one oscillation for each cycle or group of pulses characterizing a sample. Furthermore its adjustment is such that it is trigered off bya positive pulse of sufiicient magnitude on the grid of 623. The positive pulse generated over cathode resistor 62'! is transferred through condensers B3! and 632 to the decoder portion of the receiver circuit.

This decoder comprises the double triode V9 wth the common cathode resistor 634. The plate circuit of the left-hand section of V9 includes the condenser 642. A resistor 64! may be included, but not necessarily, in parallel with condenser 642. The bias of that section is such that it is at or below cut-off. If, however, its grid receives a positive potential its current will rise at the expense of the current inthe right-hand section. If an on pulse arrives from the transmitting station, the relaxation oscillator is triggered, a positive pulse of short duration is impressed on the grid of tube and a charge is stored on condenser 652 which condenser then discharges exponentially through cathode resistor 653. The potential of the condenser 652 is impressed on the grids of both sections of V9 and it will be 2 observed that this is always a positive potential which, however, will decrease exponentially. The

circuit is so arranged that the potential across the condenser 652 decreases by half its value in the time interval between successive digit pulses. If at the beginning of this cycle there is received an on signal then a positive pulse appears through transformer 635 on the grid of the lefthand section of V9 and the potential of this grid s therefore equal to the potential of the pulse minus the drop through cathode resistor 634 plus the potential across condenser 652, plus any additional bias 636. A pulse of short duration and proportional to this grid voltage will flow through the left-hand section of tube V9 and charge the condenser 642 correspondingly. Another pulse of a code group arriving at a later time will give rise to an additional pulse through V9 and an addition to the charge on condenser 642. But, although the amplitude-of the arriving pulses will be the same, the potential of the condenser 652 will have decayed so that the charge received for a pulse corresponding to tube V2 at the transmitter will be less than that for one corresponding to tube V1 and for a pulse corresponding to tube V3 will be still less, since the potential on condenser 652 will have decreased to a still lower value. At the conclusion of the cycle, condenser 642 will have a charge determined by the particular code group which has arrived. Then with the beginning of the next cycle a pulse will be transmitted from tube 623 through condenser 632 and transformer 63! to the grid of tube 66], normally below cut-off. Arrival of this positive pulse, however, activates the tube and a pulse from condenser 642 will flow through the plate circuit of tube 66!, the quantity of the charge passin being equal to the charg on condenser 642 and therefore substantially proportional to the original sample.

The code groups will follow each other at a rate equal to the sampling rate at the transmitter and these may then be passed through the low-pass filter 610 into suitable terminal equipment 680 and appropriate receiver 68! (here shown as a head-set) in which there will thus be reproduced th original complex Wave function.

It will be evident that the tube 623 may be triggered off by any pulse in a code group inasmuch as they all arrive with the same amplitude, and the particular place in a code cycle where the tube will be triggered off is one of chance depending on when the receiver circuits are closed. .It is important, however, that its triggering shall be brought into coincidence with the first pulse in a cycle. To this end an additional feature is included with the relaxation oscillator. This may comprise a tube BSI, here shown as a pentode. Its output circuit is connected across the condenser 625. Its input circuit comprises a battery 692 and a key 693 in series with an in-. ductance 694, paralleled by a potentiometer resistor 695. There may also be included in series a current limiting resistor 696. On closure of the key 693, a. large voltage appears across the inductance 694 which voltage, however, rapidly falls as current through 694 becomes established. The connection is such that the positiveend of -storage condenser.

11 the inductance, operating through potentiometa- 695, impresses a positive voltage on the grid of pentode 69! for a moment only. During this interval charge will be withdrawn from 625 delayingthetime when thetube 623 is again ready tobe triggered oil. The magnitude of the pulse ap'plied'to the grid of SM is adjusted so that the delay in triggering time is approximately onethird of the cycle period, i, e. about one digital period. By at most two or three taps of the key 693 the triggering of tube 623 may be brought into coincidence with the arrival of the first pulse in a cycle. The opening of key 693 will give rise to a large reverse voltage over inductance 694 but this has no efiect inasmuch as it drives the grid of pentode 691 to a negative value.

Normally the relaxation oscillator at the receiver will be held in synchronism with the transmitter by the first digit pulse in each cycle. If the first digital signal of a cycle is an off-pulse, the relaxation oscillator will then trigger off itself and sufliciently close synchronism will be maintained, if there is not an excessive number in su'ccession of off-signals for the first digit.

The'arrangement of the diodes M6 and 4H as shown in Fig. 4 is such that the condenser 4| 5 will 'alwaysreceive a negative charge on the top plate,'so that the potential from this condenser plate to the grid of V7 is positive. This, however, is not necessary. By reversal of the direction of the diodes and their biasing batteries and a reversal'of transformer 422 the condenser will be charged in the reverse direction and appropriate behavior of the circuit is still obtained by-a com- 'an operative system requiring as few and simple parts as possible.

Fig. shows an alternative input to that shown in-Fig. 4. In this alternative form a pentode Vs replaces V7 and the storage condenser is connected across the input circuit, which input circuit includes -the=cath'ode resistor 'liil. The plate circult ofthis pent'ode includes the resistor R and a fairly large resistor 103. The screen grid is connected to'positive'of battery; that is, to the positive end of Ru. Normally, current flows through'the pentode of su'fiicientmagnitude so "that considering the'drop through R0 and 103 the potential of the plate is slightly positive and is directly connected to-the resistors 4', "412 and- 41-3, with the elimination of V7 and its circuit.

With no signal on the storage condenser and with the double triodes turned ofi the potential of the cathodes of the left-hand sections of the diodes is-sufficiently positive so that the negative pulses coming up through the D transformers will not render the said diodes conducting. A signal potentialon the storage'condenser, lowering the potential of the control grid, will raise the potential of the 'plate and the corresponding diode cathodes.

What is claimed is:

1. In a communication system apparatus responsive to permutation code groups ofpulses of signaling conditions, a relaxation oscillator, apparatus responsive to a particular-pulse of each of said code groups of pulses for accurately controlling the frequency of said relaxation oscillator, and other apparatus for shifting the controlof said relaxation oscillator from one of said Pulses of each code group to another of the pulses of each group.

2. In a communication system apparatus responsive to permutation code groups of pulses of signaling conditions, a relaxation oscillator, apparatus responsive to a particular pulse of each or said code groups of pulses for accurately controlling the frequency of said relaxation oscillator, and other apparatus for shifting the conirol of said relaxation oscillator from one of said pulses of each code group to another of the pulses of each group, decoding apparatus for decoding said permutation code groups comprising apparatus for varying the efiectiveness of each of the pulses of a code group and other apparatus for adding said changed pulses of each code group together, and means controlled by said relaxation oscillator for generating a-single pulse having a magnitude proportional to the sum of said added pulses.

3. In a communication system in which the amplitude of a complex wave is sampled at recurrent intervals and the sample amplitude epresented by a permutation code group of pulses each of either of two difierent signaling conditions for transmission, means for storing a volt-age proportional to the sample amplitude of the wave, means for building up a reference current of components of successively smaller amplitudes, the number of components corresponding to the number of pulses of said group, means for comparing the stored voltage with the voltage produced by the flow of the reference current through an impedance upon the addition of each component current, and means responsive to said means for comparing for establishing the signaling condition of each of said pulses.

l. A combination according to claim 3 in which the means for comparing the stored voltage with I the retention of that component reference current-lastadded only if the sample voltage is in excess of said-voltage produced by the-flow of the referencecurrent through an-impedance.

5. In a communication-system in which the amplitude of the intelligence wave to be transmitted is sampled at recurrent intervals and the sample amplitude represented by a-series of permutation code pulses each of oneof a plurality of different signaling conditions, means for storing a voltage proportional tothesample amplitude of the intelligence wave,-a resistor, a; series of constant current devices -for controlling 'the flow ot-current through said-resistor, means-for actuating all of said devicesto a first-condition, means for subsequently operatin said i devices -in succession to a'secondcondition; means responsive to the relative values of the -stored-voltage and of the voltage drop across said'resistor for controlling the restoration of each of said devices-to said-first condition prior to'the actua- 13 tion of the next device of said series to said second condition, and means also responsive to the relative values of said voltages for controlling the signaling condition of the respective pulse of said group.

6. In a, communication system in which the amplitude of an intelligence wave is sampled at recurring intervals and each sample amplitude represented by a group of permutation code pulses each of one of a plurality of different signaling conditions, means for storing a voltage proportional to the sample amplitude of intelligence waves, a resistor, a series of constant current devices for controlling the fiow of current through said resistor, the number of said devices correponding to the number of pulses of said group, a flesistor, means for actuating all of said devices to a first condition, means for subsequently operating said devices in succession to a second condition, means responsive to the relative values of the stored voltage and of the voltage drop across said resistor for causing the restoration to said first condition of the last of said devices operated to said second condition when the voltage drop across said resistor exceeds the stored voltage and prior tothe actuation of the next device of said series to said second condition and for maintaining said last operated device in said second condition when said stored voltage exceeds said voltage drop across said resistor, and means also responsive to the relative values of said voltages for controlling the signaling condition of the respective pulse of said group 7. In a communication system in which the amplitude of an intelligence wave is sampled at recurring intervals and the sample amplitude represented by a series of pulses of one of two different signaling conditions, means for storing a voltage proportional to the instantaneous amplitude of the intelligence waves, a resistor, a series of constant current generators connected to supply current to said resistor when actuated, means for actuating said generators in turn to each produce a difierence voltage drop across said resistor, means for comparing the voltage drop across said resistor with the stored voltage for producing a control voltage when the stored voltage exceeds the drop across said resistor, means operating after the actuation of each generator and before the actuation of the next generator of the series for rendering inactive the generator last actuated except in the presence of said control voltage, and means responsive to said control voltage for transmitting a pulse of one of said signaling conditions.

8. In a pulse code modulation system, means for reourrently sampling the Waves to be transmitted to produce a voltage proportional to the amplitude thereof at each sampling instant, a resistor, a series of sources of constant current connected to supply current to said resistor when operative, the current supplied by said sources to said resistor being so proportioned that the voltage drop across said resistor increases as the sum of the ascending powers of two as successive sources of said series are rendered operative, and timing means for: first, rendering all of said sources inoperative to supply current to said resistor; second, rendering a first source of said series operative; third, rendering said first source inoperative except when the voltage proportional to the sample is greater than the voltage drop across the resistor and transmitting a pulse of one characteristic upon the rendering of the source inoperative; and repeating said second and third steps for each source of said series.

9. In a communication system in which the amplitude of the intelligence wave is sampled at recurring intervals and the sample amplitude represented by a permutation code group of pulses of either of two different signaling conditions, means for storing a voltage proportional to the sample amplitude of the intelligence wave, a resistor, a plurality of electronic devices each connected to supply a respective constant current through said resistor when operative, means for normally maintaining each of said devices inoperative, a control capacitor associated with each of said devices and adapted when charged to maintain the corresponding device operative, a discharge circuit for each of said control capacitors, means for supplying to said discharge circuits a biasing voltage for preventing the discharge of said control capacitors except when the voltage drop across said resistor exceeds said stored voltage and a control pulse is applied thereto, and timing means for charging said control capacitors in succession.

10. In a communication system in which the amplitude of the intelligence wave is sampled at recurring intervals and the sample represented by a permutation code group of pulses of each of either of two signaling conditions, means for storing a voltage proportional to the sample amplitude, a resistor, a series of constant current devices connected to supply to said resistor component currents producing across said resistor valuation voltages that decrease in proportion to the descending powers of two as said devices are individually switched from a normal to an operated condition in succession in said series, means for switching the devices of said series from a normal to an operated condition in succession, and means operated after the switching of each device of said series from a normal to an operated condition and prior to the switching of the next device and responsive to the relative value of said sample voltage and said valuation voltage for restoring the device last switched to the normal condition or for maintaining it in the operative condition dependent on the relative value of said voltages.

11, A combination according to claim 10 in which the last-mentioned means operate to maintain the device in the operative condition when said sample voltage exceeds the valuation voltage.

12. In a communication system in which the amplitude of the intelligence wave is sampled at recurring intervals and the sample amplitude represented by a code group of pulses each of either of two different signaling conditions, means for storing a voltage proportional to said sample amplitude, a resistor, a series of constant current devices normally non-conductive and each connected to supply a component current to said resistor when conductive, a capacitor for each of said devices adapted when charged to maintain the respective device conductive, a circuit for charging each of said capacitors in response to a control pulse, a circuit for discharging each of said capacitors in response to another control pulse, means responsive to the relative voltages on said means for storing and across said resistor for rendering said circuit for discharging nonresponsive to said other control pulses when the voltage on said means for storing exceeds the voltage drop across said resistor, means responsive to the operation of said circuit for discharging said capacitors for producing a signal pulse of '15 one condition, and timing means for producing a series of pulses comprising: first, a clearing pulse tooperate all of said circuits'for discharging irrespective of the relative voltages of said means for storing and across said resistor; second, a control pulse for charging the capacitor of the first constant current device of said series to render said device conductive; third, a control pulsefor discharging said capacitors; and subsequently corresponding control pulses for each of the other 10 constant current devices of said series.

13. A combination according to clair .12 in which the component currents supplied by said series of constant current devices decrease in proportion to the descending powers of two.

'14. A combination according to claim :12 in which each of said constant currentdevices comprises a pair of electron tubes eachhavinga cathode, an-anode and a control electrode, :a

resistor common to the cathode circuits of:.sa-id tubes, a control electrode circuit for one:of.said

tubes'for maintaining that tube normally conducting, connections from the anode of the'other of said tubes for completing the circuit to said resistor in the so-called conductive condition of 16 the *device, and connections vfrom said capacitor forxeac'h .of said devices-to supply a voltage tto thegrid of said other device to render it conducting.

JOHN R. PIERCE.

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

UNITED STATES :PATENTS OTHER REFERENCES Review of scientific Instruments, vol. 9, :March 1938, pages 83-89.