US2453454A - Coder for code modulation transmission - Google Patents

Description

NOV. 9, 1948. c, NORWINE 2,453,454

CODER FOR CODE MODULATION TRANSMISSION Filed Aug. 51, 1946 INVENTOR A. C. NORW/NE A T TORNEL Patented Nov. 9, 1948 CODER FOR CODE MODULATION TRANSMISSION Andrew C. Norwine, Short Hills. N. J., assignor to Bell Telephone Laboratories. Incorporated, New York, N. Y., a corporation of New York Application August 31, 1946, Serial No. 694,256

7 Claims.

This invention relates to coders for use in code modulation transmission systems of the type disclosed in a oopencling application for a United States patent, Serial No. 677.667 of J. C. Schelling filed June 19, 1946, and more particularly to improved apparatus for expressing the instantaneous amplitude of a complex wave to be transmitted as a binary code group of voltages suitable for transmission to a remote point.

In code modulation transmission systems the instantaneous amplitude of the complex wave to be transmitted is converted into a code group of signaling; voltages representative of that amplitude. In such systems the code may be based upon the binary counting system in accordance with which each of the plurality of signaling voltages may represent one of the denominational orders of the binary number and thus represent a fixed portion of the total possible amplitude of the complex wave expressed as a binary numbe Each of the signaling voltages has two values and in one value represents the presence of fixed portion. of the total possible amplitude while in the other value it represents the absence of that portion. In general. the fixed portions of the total are related in amplitude as powers of no. Since each of the signaling voltages may have either of two values, the total number of permutations which can be obtained between 11 signaling voltages is 2 Thus a three element code permits unique expression of 2 or eight discrete amplitudes and similarly a five element code permits expression of 2 or thirty-two different amplitudes.

One type of apparatus for generating such binary code groups of signaling voltages involves a coder for providing separate signaling voltages individual. to each of the denominational orders of the binary number code and suitable for transmission cver separate channels to a remote station. Since such coders must provide separate signaling voltages, each of which is referred to round potential to facilitate individual transmission. in separate channels they have necessarily been relatively complex employing numerous vacuum tubes and associated circuits. Such complex increase the space, power and maintenance requirements of the equipment and limit the usefulness of this type of equipment in many applications.

It is an object of the invention to provide means for continuously expressing the instantaneous 0.rnplitude of a complex wave in terms of a plurality of bi-valued signals each representing a 2 fixed portion of the total possible amplitude of the complex wave.

It is a further object of the present invention to provide a coder of the type referred to above which will be small, light in weight, have low power requirements and avoid the relatively complex circuit arrangements common to previous coders of this general type.

The invention relates in one aspect to a coder comprising a plurality of relays or vacuum tubes each controlling an output signal representative of a difierent fixed portion of the total possible amplitude of the complex wave to be transmitted,

means for applying the complex wave to all of the relays in parallel and means for applying individually to each of the relays separate voltages derived from all of the relays which control signals representing larger portions of the total possible amplitude, such voltages being respectively proportional to the output signals of the last-mentioned relays.

- The coder of the invention employs one relay or vacuum tube for each denominational order of the binary code. Thus, if an n--element code is" employed, the coder of the present invention comprises n vacuum tubes or relays, one for each element of the code, plus a single input tube common to all of the coding elements. of illustration a coder embodying the present invention is arranged to express the instantaneous amplitude of the complex wave by a three-ele-' or positive or negative signals approximately four units in amplitude.

The above and other features of the invention will be described in the following detailed specification and pointed out in the accompanying claims.

In the drawing, the single figure is a schematic diagram of a coding circuit in accordance with the invention.

' As shown in the drawing, the coder forexpress ing the instantaneous amplitude of a complex wave in three-element binary code comprises se-" lector tubes or relays l0, l2 and I4 controlling separate output signals which appear at terminals lii; l6 and 20-, respectively. Inaccordance with For purposes m i" the binary coding system considered above, each of these selectors controls a voltage represents tive of a fixedportion of the total possible amplitude of a complex wave to be transmitted, these portions being related in amplitude as powers of two. Thus selector ill controls an output representing one unit of amplitude .of the complex wave while selectors l2 and IQ control outputs representing respectively two and four units. By suitable control means, the operation of these selectors may be switched so that the combination of their output signals forms any one o'f eig'ht different permutations. Conveniently, the uriit of measurement employed may be one volt and in this case eight different amplitudes from O to? volts inclusive may be expressed uniquely.

Conveniently, each of selectors it, 12 and it may comprise a triode type vacuum tube although other types of vacuum tube circuits having two different operating conditions or conditions of stability may be employed with equally advantageous results. The plates of selector tubes ill, l zand id are connected through load resistors 22, 24cm 26, 'rfiSDGCtiVely, to the positive terminal of battery 28, the negative terminal of which is connected .to ground. In addition the plates of these mo or-sa e respectively connected to out: put terminals l6, l8 and 2t]. 'lhecathodes of the selector tubes are connected directly to ground while their 'grids are connected respectively throughrcsistors' .30, 32 and 34- to .the negative terminal of bias battery fili'the positive, terminal of which is connected to ground In addition to the above connections the plate of .selectortube i2 is connected through resistor :15

3d tol'the' grid of selector tube it) while the plate of selector. tube M is connected throughresistor 41) to .thegrid of selector tube 12 and through rese ter; tothe grid of selector tube ill.

The complex wave to be represented in binary code form 'for transmission is applied atter ninal L4 and thence throu h a blocking capacitor it to id'of alcathodefollower .thegrid circuit of w 11 includes resistor 46 in series with bi'asbat: tel- 1; 'llhis cathode follower may conveniently cd nprise a 'triode' type Vacuum tube theplate of w h is connected to the positive terminal or a ry' .50 the negative terminal of which is grounded, the cathode of thejtube being. connected through load resistor52 to ground. ihe voltage appearingacross load resistor 52 is an, accurate'r'e; pro notion of the voltageapplied at terminal ill and is applied through resistors 54!. .555 and 58; respectively in parallel to the control grids of selectors i0, i2 and L4,

Inrlmnsidering'the operation of the coder of the invention, let it ,besassumed that the biasvoltage from battery M is positive and 'substantiallyequal tonne-half the largest possible peak to pealr amplitude of thecomplex wave tobe transmitted. Insurer words, the bias voltage is such'lthat' in the absence of applied complex wavedor when the complex .Wave is of zero amplitude) thcvoltage at the output'of cathod'e'follovver d8 will fall at the approximate center of the operating range of the coder. Then all of the voltages appearing across cathode resistor 52 (and applied through series resistors h 6 and-583cc the selectors .will be of positive polarity and will tend to oppose the negatiye-biasapplied. to these selectors from battery 36.

Inbrder to simplify the following description of operational the coderIit is convenientto measure the amplitudes of the voltages appearing across cathoderesistor 52 of cathode rouo erts inrespect tog round. It will .bere'rnembered "that the temples wave has zero amplitude, and

ill

. plitude of between three and f reaches itsmaximum possible posit). lngt'he-iollowing description all input vo t units) when amplitude of seven units when the complex wave amplitude. ges to exemplary coders total range oi oi the coder will be those appearing acro. fiflmeasured in respect to ground cor pm the summation of the complex wave a'op at terminal y! with the positive bias contri on from battery 41.

The negative bias from battery ef fective in the absence or voltages across cathode resistor 52hr cathode follower E8 to out off the flow of current in the selector tubes. Accordingly, the plate voltages of these tubes rise and relatively high 'voltages app'ear at output terminals i6, i8 and 25!.

Because of the interconnections between selectors I2 andifl, between selectors M and i2 between selectors M and l El, positive voltages are applied to the grid of selector ill whenever selectors {2 or M are cut off and to the grid of r ctor whenever selector l l is out on. F1 7 more, whenever selectors I? or M become satur ted due to the application of a sufficiently high positive ge totheirgrids, the positive voltage trib tions therefrom to the grids of other select'o'rs are removed.

' In the grid circuit of selector l2, resistors 5B and 32ers so'proportioned that at the grid the positive voltage contribution from coder l l when out off bears the same relation to the portion of the total amplitude represented by coder H!- in this example, 4 units) as the input voltage contribution at the grid applied through resistor Eiil s to the amplitude of the complex applied at input terminal 44. In other words, the voltage contribution at the grid of selector'lii correspondto an input signal of amplitude A has an amplitude of a arbitrary units, then the positive contribution from selector i when cut oil has an amplitude of 4 in the same arbitrary ibution I. when out off is of amplitude 4 in the same arbitrary units and that from selector i2 is of amplitude 2 in the same arbitrary units. It should be noted that the arbitrary units employed measuring. the relative amplitudes of the voltages applied to the several selectors needs not be the same for each selector so long as the relationships defined .aboveare preserved in a consistent system of units at the input or" each selectcr.

It will be recalled that in the absence of other applied voltages, the negative bias from batry is effective tocut off each of the This bias voltage is also sufficient to r a item. lcctor. [2 cut oli despite the positive contribution from cut-off selector i l and to maintain selector Hi cut off "despite the positive contributions from cut-bit. selectors Hand M. Since two such posi tive contributions are applied to selector ID a lower input signal amplitude is required to saturate this selector than is required to saturate selector [2 in which the input signal is aided by only one positive contribution.

More specifically, in the exemplary system considered herein, the negative bias voltage and the proportions of the resistances in the grid networks of the selectors are so chosen that, when all selectors are cut off an input signal of at least one volt is required to saturate selector Ill. Similarly, when selector I4 is cut oil an input of 2 volts is required to saturate selector l2 and an input of 4 volts is required to saturate selector M. If the positive contributions from selector I4 to selector l2 and from selectors l2 and Hi to selector ID are removed, the input voltage must be increased correspondingly to cause saturation of the selector involved.

Turning now to the detailed operation of the circuit, let it first be assumed that the input signal across resistor 52 is zero. Then each of selectors I0, 12 and I4 is cut off and high voltages which may denote the zeros of a binary number, appear at output terminals I6, 18 and 20. The binary code group obtained by setting down the values (one or zero) of the outputs at these terminals is thus 000. Hereinafter such binary code groups will be written by listing the signals appearing at terminals 20, I8 and it in that order. This corresponds to listing the code element representing the highest denominational order of the binary number first reading from left to right in the usual manner.

If, for example, the instantaneous amplitude of the complex wave to be transmitted is such that one volt appears across resistor 52, one volt is applied to each of resistors 54, 56 and 58. As to selector ill, the positive voltage corresponding to the applied l-volt signal plus the positive contributions from cut-off selectors I2 and i4 is suffi cient to overcome the negative bias from battery 36 and to cause the flow of saturation current through selector ID. The voltage applied to selector l2 in response to the l-volt signal is in suflicient, even with the positive contribution from selector l4, to overcome the negative [bias and this selector remains cut off. Similarly, the voltage applied to selector I4' in response to a l-volt signal is not sufficient to overcome the negative bias on this selector. Accordingly, with the application of a signal of one volt, selectors i2 and 14 remain cut off and selector lo becomes saturated. The voltage at output terminal l6 drops to a relatively low value and the binary code group obtained by listing the signals on the output terminals becomes 001.

Let it now be assumed that the complex wave amplitude is such that an input signal of two volts appears across load resistor 52 of cathode follower 48. In the absence of other considera tions, this voltage would be sufficient to maintain selector ID at saturation. It would also cause the application of a positive voltage of sufiicient amplitude in addition to the positive contribution from selector II to cause the flow of saturation current in selector l2. Resistors 58 and 34 associated with selector [4 are so proportioned that an applied voltage of two volts is insufficient to overcome the negative bias applied to this selector. It will he noted, however, that when selector 12 becomes saturated due to the application of two volts, its plate potential is lowered and the positive potential applied to the control grid of selector I0 from this tube is reduced. Accordingly, in'the' absence of this positive contribution, which is proportional to two volts as applied to selector ID, the negative bias from battery 36 is again eiTective to out 01f selector it. Thus, for an input of two volts selectors it) and Id are out ofi while selector i2 is saturated so that the binary code group obtained at output terminals as, it and i6 is 010 corresponding to an amplitude of two.

If a signal of three volts is now applied to the selectors the following action occurs. Because of the relative proportions of resistors 58 and 3t, selector l4 remains cut off, the applied voltage being insufficient to overcome the negative bias from battery 36. Selector 92 becomes saturated since an input signal of only two volts is required to saturate this selector so long as selector 14 remains cut oil and the positive contribution therefrom to selector i2 is maintained. On the other hand, the input of three volts produces a suffi-- ciently large positive voltage at the grid of selector 10 to overcome the negative bias from battery 36 despite the absence of the positive contribution (which is proportional to two volts) from selector i2. Accordingly, selectors iii and iii are saturated and the binary code group obtained on the output terminals is 011 corresponding to an amplitude of 3.

Further, if the input voltage across resistor 52 rises to four volts selector M is saturated. When this selector becomes saturated, however, its plate voltage drops and the positive contributions from this selector through resistor til to the grid of selector l2 and through resistor A 2 to the grid of selector It are removed. In the case of selector l2 the removal of the positive contribution from selector i is effective to balance the 4-volt input signal for this selector and to permit the negative bias from battery 38 to cut ofi the tube. When this occurs the plate voltage of selector l2 rises and a positive contribution is again applied from selector !2 through resistor 38 to the grid of selector Iii. It will be recalled, "however, that an input signal of one volt is required to saturate selector iii even when positive contributions from both selectors l2 and it (proportional to two plus four or six volts as applied to selector i 0) are applied to augment the signal.

It is thus apparent that the sum of the positive voltages applied to selector it must at least be proportional to seven volts to saturate this selector. Here the sum of the positive voltages is proportional to six volts, four from the input and two from selector i2, sot-hat selector is cut oil and the code group appearing at the output terminals becomes 100.

In a similar way the application of input signals greater than four volts causes appropriate changes in the conditions of selectors iii, i2 and M. For example, upon the application of five volts selector l4 remains saturated. This increased signal voltage, however, is suficient to make the sum of the positive voltages applied to selector l0 proportional to seven volts, five from the input and two from selector l2, and this tube is saturated. On the other hand, the increase of the applied voltage is not suflicient to saturate selector 12 which remains cut off and the output code group is 3.0. The operation of the circuit in response to inputs of six and seven volts may be analyzed in an entirely analogous manner.

It will be understood that a wider range of amplitudes may be expressed in binary code group form by increasing the number of selectors. In each case connections must be made between each of the added selectors and the grid'circuit's of all selectors representing smaller portions of the total possible amplitude of the complex wave to be transmitted. Such connections must be so made that the contribution from each selector to each selector controlling a signal representative of a smaller portion of the total possible amplitude bears the same relation (in the selector to which it is applied) to the portion of the total amplitude represented by the originating selector as the signal voltage applied to'the selector rethe contribution of does to the input signal.

The coding circuit of the invention may be used in the transmitters of various types of code modulation transmission systems. For example, the separate voltages at terminals 1 0, l2 and [4 of the illustrative embodiment may be individually and simultaneously transmitted or each output terminal may be sampled and the voltages thereon transmitted in sequence.

What is claimed is:

1. In a communication system, a coder for generating representations of the instantaneous amplitude of a Wave to be transmitted according to a code of a plurality of code elements each representative of a different fixed portion of the total possible amplitude of said Wave, said coder comprising a plurality of relays each controlling an output signal corresponding to one of said code elements, means for applying said Wave to all of said relays, means for deriving from the outputs of all of said relays except that representing the largest portion of the signal amplitude volta es proportional to the amplitude portions which the relays respectively represent, and means for applying the voltage derived from each relay to each other relay controlling a larger derived voltage.

2. In a communication system, a coder for generating representations of the instantaneous amplitude of a Wave to be transmitted according to a code of a plurality of code elements each representative of a different fixed portion of the total possible amplitude of said wave, said coder comprising a plurality of relays each continuously controlling an output signal corresponding to one of said code elements, means for continuously applying said Wave to all of said relays, means for deriving from the outputs of all of said relays except that rep-resenting the largest portion of the signal amplitude voltages proportional to the amplitude portions Which the relays respectively represent, and means for applying the voltage derived from each relay to each other relay controlling a larger derived voltage.

3. In a communication system, a coder for generating representations of the instantaneous amplitude of a Wave to be transmitted according to a code of a plurality of code elements each representative of a different fixed portion of the total possible amplitude of said Wave, said coder comprising a plurality of electron tubes each controlling an output signal corresponding to one of said code elements, means for applying said Wave to all of said electron tubes, means for deriving from the outputs of all of said electron tubes except that representing the largest portion of the signal amplitude voltages proportional to the amplitude portions which the electron tubes respectively represent and. means for applying the voltages derived from each electron tube to each other electron tube controlling a larger derived voltage.

4. In a communication system, a coder for generating representations of the instantaneous amplitude of a Wave to be transmitted according to a code of a plurality of code elements each representative of a different fixed portion of the total possible amplitude of said wave, said coder comprising a plurality of electron tubes each controlling an output signal corresponding to one of said code elements, means for applying said wave to all of saidv electron tubes in parallel, means for deriving from the outputs of all of said elec-- tron tubes except that representing the largest portion of the signal amplitude voltages proportional to the amplitude portions which the electron tubes respectively represent, and means for applying the voltage derived from each electron tube in parallel to each other electron tube controlling a larger derived voltage.

5. In a communication system for transmitting a message Wave, apparatus for generating code representations of the instantaneous amplitude of the message Wave to be transmitted comprising a plurality of electron tubes each controlling an output signal representative of a different fixed portion of the total possible amplitude of said message Wave, means for applying to each of said electron tubes voltages proportional to the instantaneous amplitude of the message Wave, and means for applying individually to each of said electron tubes separate voltages from all electron tubes controlling signals representing larger portions of said total amplitude, said voltages as applied being proportional to the parts of the total amplitude represented by the electron tubes from which they are respectively derived in the same units as those in which the voltage proportional to the complex Wave is measured.

6. In a communication system for transmitting a message wave, a coder for generating code representations of the instantaneous amplitude of the message wave, said coder comprising a plurality of selectors each controlling an output signal representative of a diiierent fixed portion of the total possible amplitude of said message wave, means for applying a fixed bias voltage to each of said selectors, means for applying a voltage proportional to the instantaneous amplitude of the message Wave in parallel to each of said selectors and in opposition to said bias voltages, and means for applying to each selector except that controlling the output signal representing the largest portion of the total possible amplitude additional control voltages, these voltages in each case also opposing said bias voltages and being respectively proportional to the amplitudes represented by the output signals of selectors controlling signals representative of larger portions of said total amplitude.

7. In a communication system for transmitting an intelligence Wave, a coder for generating code representations of the instantaneous amplitude of an intelligence wave to be transmitted, said coder comprising a plurality of electron tubes each controlling an output signal representative of a different fixed portion of the total possible amplitude of said intelligence wave, means for applying a fixed bias voltage to each of said electron tubes, means for applying to each of said electron tubes and in opposition to said bias voltages voltages proportional to the instantaneous amplitude of said intelligence wave, and means for applying individually to each of said electron tubes separate additional voltages from all electron tubes controlling signals representative of larger portions of said 9 10 total amplitude, these voltages also opposing said bias voltages and, as applied to each electron REFERENCES CITED t b being proportional t th parts of th t t l The following references are of record in the amplitude represented by the electron tubes file of this P t t! from which they are derived in the same units 5 UNITED STATES PATENTS as employed in measuring the instantaneous amplitude of the intelligence wave applied to the re- Numb Name Date spective tubes. 2,272,070 Reeves Feb. 3, 1942 2,282,046 Goldsmith May 5, 1942 ANDREW C. NORWINE. 9 Valensi Mar. 9, 1943

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