US2516752A - Coding tube for pulse code modulation signals - Google Patents

Description

2 Sheets-Sheet 1 ATTORNEY July 25, 1950' R. L. CARBREY CODING TUBE FOR PULSE CODE MODULATION SIGNALS Filed-Sept. 30, 1948 July 25, 1950 R. L. CARBREY CODING TUBE FOR PULSE CODE MODULATION SIGNALS Filed Sept. 30, 1948 2 Sheets-Sheet 2 Patented July 25, 1956 UNITED STATES orncs CODING TUBE FOR PULSE CODE MODULATION SIGNALS Application September 30, 1948, Serial No. 52,088

9 Claims.

The present invention relates to the automatic generation of pulses in code combinations indicative of instantaneous signal amplitude, under the direct control of signals to be coded. More specifically, the invention relates to automatic generation of code pulses by an electron beam coding tube in which the beam when deflected by different amounts by difierent signal amplitudes is caused to impinge upon certain ones of a set of electrodes in difierent combinations corresponding to the difierent digits of the code and to set up pulses on these electrodes.

Objects of the invention are, increased speed of coding, simplification of apparatus requirements, and improved operation.

Electron beam coding tubes of the kind to which the invention is applicable have made use of a quantizing grid and a feedback control from this grid to the beam deflecting means, for limiting the beam position to the grid aperture to which the beam was moved by the signal and for preventin the beam from assuming a position overlapping two grid apertures or only partially illuminating the intended grid aperture. Each grid aperture is lined up with a horizontal row of apertures in a code plate. Thus, registry of the beam with an aperture in the quantizing grid also causes proper registry of the beam with a given row of code apertures. The digit electrodes are located back of the code plate so that i the beam in passing through apertures in the plate strikes the corresponding digit electrodes.

In the use of such tubes a time considerably longer than the duration of a code pulse is allowed for the stabilizing of the beam position,

under control of the quantizin grid and its feedback action, in proper registry with a given row of code apertures. During this stabilizing period the beam is illuminating the digit electrodes and so producing pulses on them. In order to prevent these pulses from reaching the output circuit until the beam is stabilized and the pulses are wanted, gating amplifiers individual to the digit electrodes have been used and these ampliflers have been biased beyond cut-off until such time as the pulses were to be sent.

In accordance with this invention, the gating function is performed within the tube by blanking the electron beam used for the codin until such time as the pulses are to be impressed on the output circuit of the coding tube. In order to make use of the feedback principle to bring and hold the beam in proper registry with a given row of code apertures, an auxiliary, or pilot beam is provided together with a relatively narrow quantizing grid arranged in the path of the pilot beam but out of the path of the main or coding beam. The same beam deflecting means serves in common for both beams. This allows the feedback control, through the agency of the pilot beam, to establish the necessary conditions on the deflecting means such as will cause proper registry of the main beam with a given row of apertures when the main beam is unblanked. The main beam may be turned on at the instant when the code pulses are to be generated and not until then. It is left on for the duration of a code pulse and is then turned off until the next code combination is to be generated.

' Thus, the main beam is used both to generate the pulses in proper code combinations and to time the occurrence and duration of the pulses. One advantage following from this action is that the gating amplifiers heretofore used may be dispensed with and if amplification as such as needed it can be supplied in a common amplifler, for example in the output of the distributor as will appear from the fuller description to follow.

The nature and objects of the invention will appear more clearly from the following detailed description of certain illustrative embodiments of the invention as shown in the accompanying drawings.

In the drawings, Fig. .l is a perspective view of the electrode structure of an electron beam coding tube of one form according to the invention;

Fig. 1A shows a detail of the electron gun used in the construction shown in Fig. 1;

Fig. 2 is a schematic plan view of the electrode structure of a modified electron beam tube according to the invention;

Fig. 3 is a schematic view, in plan, of a tube of the Fig. 1 type showing the manner of connecting the tube into a pulse code transmitting system; and

Fig. 4 shows a timing diagram to be referred to in the description of Fig. 3.

Fig. 1 shows the electrode structure of a cathode beam tube, looking away from the small end of the tube at which the beam-forming means is provided toward the large end of the tube where the target electrodes are located. Provision is made for producing a pilot beam 30 of relatively narrow ribbon form and a main or coding beam 3| also of ribbon form and much wider than beam 30. These two beams impinge upon the code plate 36 and illuminate a horizontal line across the code plate, as shown.

The code plate 36 is divided functionally into two distinct parts. The first part comprises the quantizing grid 31 which is in the path of the pilot beam 30 but is to the left and out of the path of the main beam 3|. The other portion comprises the coding section of the plate 36, this portion being in the path of the main beam 3| but out of the path of beam 30. It will be understood that the two beams 30 and 3| are maintained spaced apart from each other and quite distinct and separate in their effect. Positively biased concentrating electrodes l, 2 and- 3 are provided to shape the electron beams and to assist in maintaining the spacing. thereof.

All of the apertures in the code plate 316 are arranged in horizontal rows. In a seven-digit system, assumed in this figure, there will be- 128 horizontal rows. The quantizing grid 31 has 128 apertures arranged in a vertical column.

Just back of this column of apertures is the r the plate 3t differ in number in eachhorizontal row'from none to a maximum of seven in accordance with a binary system to form a permutation code and are positioned in seven vertical columns. A few of these apertures are shown at 38. The digit electrodes 4| to 41', inclusive, are located just back of the cod'epl'ate 38- in position to be struck by portions of the beam 31- which pass through aperturesin the correspond-- ing column.

The beams 36 and 3! are shown in the figure in their middle position corresponding to zero applied signal They can be deflected upward or downward to any chosen horizontal row of apertures by the application ofproper voltage to the deflecting plates 32", 34. These voltages may be applied over the leads 49 and 5! respectively: 7

Any suitable type of electron gunstructure may be provided for producing the beams 30 and 31 in ribbon form. The structure shown in Fig. 1 is a modification of that shown in United States patent to A. M. Skellett 2,293,567 granted August I8, 1942. The structurein Fig. I differs from that in the Skellett patent in that electrostatic focusing is used in Fig. 1 in place of magnetic focusing shown in the patent. The

electron gun comprises an elongated linear cathode 22, which may be of the indirectly heated equipotential type as shown or may be of other forms, for example, filamentary, a cy- 4 duce desired focusing. Portions of the shield are broken away in Fig. 1 to show the two grids 23 and 33 as distinct and separated from each other. If desired, suitable shield means may be included in the space between the two grids within the shield member 24.

In the operation of the electrode. structure of Fig; l a signal sample is applied across the leads it, 5! for the deflector plates 32, 34. (The circuit arrangements for carrying out the functions in this brief description will be given in Fig. 3.) During the time the signal sample is applied to the deflector plates themain beam 3! is blanked by the application of a proper voltage to the grid 23', but the pilot beam 38 is not blanked at this time. The pilot beam is therefore deflected. under control of the signal voltage to the vicinity of a particular aperture in the quantizing grid 3'! depending upon the amplitude of the signal. The beam may fall into exact registry with one of these apertures or may be only partially: in register with an aperture and part-way between two apertures. The portion of the beam that passes through an aperture strikes the target electrode Ml from. which a voltage is fed back through an amplifier to the deflector plates 32, 3 such as to exert feedback control on the: pilot beam- 30 This feedback control is such as to shift the beam 33 slightly when necessary tochange from the condition of partial registry of the beam with a given apertureinto full registry of the beam with that aperture. (The manner in which the feedback control effects this result is disclosed more fully in an article by R. W. Sears entitled Electron Beam Deflection 'I-u-be for a' Pulse Code Modulation published in the Bell System Technical Journal for January 1948, pages 44 to 57'.) If the beam is partly cut off on its upper side so that only, say; its lower half is transmitted, then ifthe feedback acts so that stronger current to electrode ill urges the beam downward (and weaker current urges it upward) an unstable condition exists and the beam will either move downward until further movement would begin to cut off' its lower edge or it will: move upward i'indrical grid 23 for the main beam and a similar 7 grid 33 for the pilot beam, each coaxial with the cathode 22, and a shield electrode including a U-shaped portion; 24' coaxial with the cathode 2-2; This shield portion has attached to it a pair of coplanar plates 25' extending normal to the longitudinal axis of the tube or of the beam and spaced from each other to define an elongated linear aperture 0r slit 26, parallel to and to a stable position in the next higher aperture.

The positioning of the pilot beam in the man.- ner described so as to be in full regiflry with a particular aperture in the quantijzing grid 3'! results in the establishment of such voltages on the deflector plates 34 that when the main beam is turned on it will immediately strike the corresponding horizontal row of holes in code plate 36 in full registry therewith. It thus becomes practicable to turn on the main beam for just the short interval corresponding to the duration of a code pulse and then turn the beam off again, since-the q'uantizing gridand feedback action through the agency of the pilot beam has prepared in advance the proper voltage condition on deflector plates 32', 34 for causing correct registry of the main beamwith the corresponding row of code determining apertures. Portions of the main beam pass through these apertures and strike the corresponding digit electrodes 41- to 4'? producing voltages on those particular; and only those, digit electrodes.

' Thefact that the two electron beams 30 and 3| are of widely different dimensions does not prevent the type of control just described from taking place. The important thing is to have the accelerating potentials for the two beams maintained equal and this result is achieved by generating the beams from a common cathode the digit electrodes 4| to 41 may be made use of in any desired manner. For example, they may be simultaneously transmitted over individual transmission channels or they may be applied to a distributor operating to arrange them in sequential order suitable for transmission over a single channel.

Referring to Fig. 3, the signal which may be speech or other suitable signal is shown as comingin on lead 93 to the signal sampling circuit 92a} The latter may be an electronic switch of the type which permits conduction in either direction when closed. Such switches are sometimes referred to as two-way clamps. The switch would be biased open to prevent conduction except for the time during which a signal sample is to be taken and applied to the storage condenser 95. The timing control for the sampling circuit 92 and for other functions to be carried out would in practice be accomplished by means of electronic timing circuits under the prime control of a crystal oscillator of highly constant frequency, with the various timing periods determined by capacitance-resistance timing circuits operating in conjunction with vacuum tubes in a manner generally well known in the art. For simplicity of illustration in Fig. 3 the timing control is shown as obtained from rotating brushes IOI, I02, I03 cooperating with segmented rings. A negative voltage from battery I05 is shown as applied to the common shaft I04 for the three brushes. The sampling circuit 92 is biasedopen during the portion of the cycle in which the brushis in contact with the conducting portion 98 of the segmented ring but this bias is removed during the time the brush IN is in contact with the insulating segment I06. It is during this latter interval that the sampling circuit 92 is allowed to take a sample of the impressed signal in lead 93 and store it upon the condenser 95.

During all or the greater part of this sampling period both the main beam 3| and the pilot beam 30 are suppressed by the application of high negative voltage to the grids 23 and 33 by brushes I03 and I02 in contact with respective conducting segments I00 and I01. The pilot beam is shut off during this period to allow the voltage conditions on storage condenser 95 to become stabilized. The high negative bias is removed from grid 33 near the end of the sampling period, allowing the.

pilot beam 30 to become projected.

The signal voltage stored on the condenser 95 isamplified in the push-pull amplifier 9| and is applied over the leads 49, 5| to the upper and lower deflecting plates 32, 34. The pilot beam 30 is thus deflected to an extent determined by the signal amplitude. The quantizing grid 31, electrode 40, feedback connection II to the input of amplifier 9|, amplifier 9| and deflector plates 32, 34 cooperate in the manner above described to position the pilot beam 30 in full registry with the aperture corresponding to the nearest quantized value of the signal. A considerable time is allowed during which this feedback quantizing operation is being carried out in order to permit the pilot beam to become fully stabilized in its finalposition. During this period the brush I03 is traversing the segment I00 and maintaining high negative bias on grid 23 thus suppressing the main beam. After a time suficient for stabilizedconditions to be reached, brush I03 arrives as quite short." After the projection of the main beam both beams are again suppressed and switch IOI permits sampling circuit 92 to take a new sample of the applied signal.

These timing relations are shown for further illustration in Fig. 4. It is assumed for illustration inFig. 4 that-the coder is being used in a 96-channel telephone system with speech samples taken at the'rate of 8,000 per second. The coding period for any one speech sample is therefore 1.3 microseconds. Beginning at time 0 in Fig. 4

the sampling periodis shown as lasting for about half a microsecond. The pilot beam is shown as coming on near the end of the sampling period and lasting throughout the remainder of the first 1.3-microsecond interval. Experience shows that it requires somewhat less than 0.5 microsecond for the action of the quantizing grid and its feedback control to become fully stabilized. The main beam is represented as not being turned on therefore until near the end of the 1.3-microsecond interval. The coder therefore produces a main beam ulse near the end of every coding period and pulses of the duration of the main beam pulse, shown as .186 microsecond, will appear simultaneously on those of the digit electrodes 4| through 41 for which the beam was in registry with apertures in the coding plate 36. Typical code groups of pulses obtained by distribution in time of the simultaneously appearing pulses are shown in Fig. 4 for channels 96 and I.

Fig. 3 shows a type of distributor suitable for use in distributing the code pulses sequentially in time so as to make them suitable for transmission over a single channel. This is done by the use of delay circuits 88, 89, etc. individual to the seven-digit targets 4| to 41. These delay lines are associated with the respective digit targets through isolating resistors 8| as shown. The first delay line D1 shown at 88 is represented in dotted outline indicating that this line may have zero delay and therefore that the delay line may be omitted, if desired. The amount of delay increases in succeeding delay lines D2, D3, etc. by .186 microsecond, the time interval between the beginning of one pulse and the beginning of the next pulse. Thus the code pulse generated on digit electrode 4! is delayed six full pulse periods longer than the one generated on digit electrode 4|, that on digit electrode 46 is delayed five pulse periods longer, etc. A common amplifier 90 is shown as serving for the transmission of the various permutation code pulses. This may also inelude shaping circuits to standardize the code pulse amplitudes if desired.

Some of the distributed code pulses are represented in the timing diagram of Fig. 4. At the end of thecoding cycle immediately prior to time 0 the main beam pulse is assumed to have set up a coding pulse on each of digit electrodes 4|. 4'3, 44, and 47. Corresponding pulses I, 2, 4, 5 and l are therefore represented in the timing diagram as they would be sequentially arranged in time by the distributo delay lines. Another sequence generated. near the 'end of the first l 3-microsec and interval is shown by the pulses i, 2, 3'. andfi.

- Fig. 2 shows a modified type of electrode construction in which eight individual electron beams are used, one of these, 64 in the figure, being the pilot beam and. the other seven being beam; deflecting; platea of; which-theaupuer; Qne-.is.

shown. at' 32 f is; .usedas the: previous flatness The-manner of. connecting; into, the. circuimwould.

be thersame as-. described'imconnectionrwith. Rig. 3; ,the w'arious: coding.- beam controlling-. grids-obes in paralleled; I

What is claimeddsz. v

1.. 'ilhe method. of. generating. pulses in-v code. combinations. by; electronic. action. comprising; pigoducing. two :parallelelectron; beams subjecttd lilr-edeflection under,- signal control, initially, ap plying thesignal.controhtoonly the first. of said, beams. to. deflect. thesame. to. a. desiredv position dependent. unon, the, signal, amplitude. while.- blanking. thesecond; beam, establishing; by feed-- bacleaction, efiectiueupon thafirstbeam, ,VOItage. conditions determinative. of the deflection. oi, the second. beamwhen: exis-tent,, such. as to." causethe. second. beam to ,assuma-av position.limited. to. electrode. areas. determinative: of. one. particular code group.and.theneafter. and. while :said condi tions. are. stillmaintainedturning on said secondi beamifor a time corresponding to the le-ngth'ofcodev pulse to. be. generated.

2. In; an electrombeam. pulse code, producer, meanato,1r0duc.e.;t\vo electron beams parallel to each other, common, ,defiectimg; means, ion both. beams. aquantizin grid. losatedr in,-.the pathof. the-first .orpilotbeam but out, oi the pathr-ofi the.

second. or. main: beam, code. generating; target.

areasin positionizo.beengagedby. said main beam. when deflected by differenuamounts-,, afeedback; control. from. said: quantizin g,. g-rid to J said. deflect. ing, means. for. establishing; and maintaining; proper, registry oil said, mainabeam with. thez particular. target. areas-producing. the. desired: code,- and means-tosuppresssaidmai' ,beanudurine; the time such pnopecregistry is.being, established:by;- said fecdbackhcontml beam during, the. timessuchspropert registrypis .b.e.- ing..maintained=.,

3. In, an. electronlbeam; coding: tuba, a main: beam, a. pilot-b.eam,-,.common-.deflecting means; for saidv beams,. a. quantizine. grid in; thevpath saidz pilot. beambutoutiet .the:-p.ath-.ofi said'main, beam; a feedback.beamacentering:control from-said: grid-. to:saiddefiectingmeans;coderdetermining;target. areas positioned to be engaged-mysaidmainrbeam; when deflected by successiveramounts; said areas coordinated spatially with the grid apertures. suchthat said control centers,the-mainbeamonrespective code-determining areas, and means: to suppress said main beamzindependently of'said pilot beam.

4.. In: an electron beam codingtube, means: to-produce amain beam, a plurality of. targetex-- posureareas arranged in patterns according to-a" code, means to defiectthe main beam under con-- trol of a signalvoltage'to-becoded, so as toibe intercepted by certain of said exposure areas. depending upon the signaii amplitude, meanszto suppress said beamexcept for times duringwhich; codes are to be generated; means-"to produce: a: pilotbeam, special target exposure: areasar but toz turn. onsaid main.

rangedlopposite said'plurality of exposure: areas,

means:including, said. deflecting means, for moV-e ing said pilot beams-to, one; of saidspecial areas underi-contr l =;ot,a-.signal toibe coded, while said means. also including, a. feedback control from.

ing proper; negister: of said main; beam-with;.said. certain exposure areas. 1

5. In; an electronbeam; coding tube-, a coding element... having agpluralitm undefined areas! each representing.;a-,sig;nal amplitude, means, ionestabdefieetingsaid .bea-m: in accordance; with. the amplitude. of; a ignal to. becoded: tothe ,corresponding one ot= said; defined. areasmeans; for limiting thQzDOSSiblC: positions. ofisaid beam -to;.those. corre,

, spending; to.-said.;- defined areas saidmeanscom prisinameam; for generating, an auxiliary; electron, beam-within the-tube, a. quantiaingq electrode dis posed-beside .saidcodingelement. to interceptisaid auxiliary; beam;.. andhaving openings positioned. oppositesaid-.,d;efined-. aneaa, said auxiliary; beam being deflected. b the. signal, to. be. coded acting;

through .t'he' n eansefor deflecting. theifirstemen tioned; beam. and,.- meansi for feedin back from.

said. quantizing. electrode; a. controls voltage. for. application tozsaid deflecting-means;

6.. A-bcam, tubeefors producing permutation; code. pulses, comprising means to project. a-main beam and a pilot. beam side by, sidawithin and along said. tube, common deflecting means for bothbeams, aseparate and-independently controllable. beam. suppressing; meansforsaid main beam, said maintbeam. being many times. wider than its yer ticalor thicknessdimension, targetexposure areasarranged in horizon-tall rows one above. another,

conformingto. a, permutation-code, a vertical column of other targetexposureareas each horizonpliedwith deflection, controlling voltages to 1 move bothlof. saidLbfiamsio..be...interc.epted. by. any given.

one. of said horizontal...rowssof. aiieasand. the: aligned therewith respectively, and means: operating response, tointerception of. the pilot. beam said. latter; area, for. maintaining. said. beams. in..r,egi'ster,= with: the areasto. Whi'chthey. were;moyedrespeotively..,

'7. Iman. electronbeam coding, tube. means; to.

produce, a main beam, andl to project. the. beam.

toward a targetarea means. to) deflect the, beam.

to, difierentpositions. Qnsaidtarget. areadeuend ent, upom the... amplit d e of an. impressed; signal,. said target. area havinma di fierentexposure; pat.- t'erni'or each different a mp litude, of. signal to be; codc'-,d',, endl'imeans. tnrediicez ambig ut incod'i'ng. compnisi'nzgmeans. to producegandproj ectla. pilot. beam; saidldeflectingsmeans. for. th main; beam alsoideflectingthe, pilotlbeamhy thes ameamount speciaLtarget. exposure areas arranged, to. be? in, tercent'edlby, saidqpilotibeam.vandiihdiiridually cor.- responding.,with' the difierentexposure area pat}; ternsmeansoperatihg in responseto.interception; of... saidlpilotbeamby a. given. one .101 said special. exposure. areas tor feed-ins. backayoltage td saidl defiesting' means, such as-to. maintaimsaid. main,- beameisteswitn the. corresponding. exposure. area; nattern and an schemata, and. independently controllable; bean'risupnressing; means. fcn said.v maimbeama- 8:; A3. two-beam. electron: coding tube: including;

mainbeam-is: suppressed. to predetermine thepo- I sition to. be assumed. bag-said;main-beam said last lishing; am eleetronpbeam in; said, tube, means for.

the;v individual areas being arranged. in a ,pattern.

main target areas and pilot target areas aligned therewith, means for initially directively projecting a pilot beam, under control of a signal voltage to be coded, against one of said pilot target areas while maintaining the main beam suppressed, a feedback control from said pilot target for the pilot beam for properly centering the pilot beam on said one pilot target area, and means subsequently operative under control of said signal voltage and feedback centering control for projecting the main beam against the respectively aligned main target area.

9. A two-beam electron coding tube including code determining target areas and guide target areas aligned therewith, means to produce a pilot beam and a coding beam, beam deflecting means, means for initially suppressing said coding beam and for causing said deflecting means to direct said pilot beam, under control of a signal voltage to be coded, against one of said guide target areas, a feedback control from said guide target area to said deflecting means for establishing proper register of said pilot beam on said guide target area, and means including said deflecting means subsequentl operative under control of said signal voltage and feedback control for projecting said coding beam against the corresponding codedetermining target area.

ROBERT L. CARBREY.

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

UNITED STATES PATENTS Number Name Date 2,108,097 Bedford et a1 Feb. 15, 1938 2,179,205 Toulon Nov. 7, 1939 2,180,944 Linsell Nov. 21, 1939 2,289,205 Nagy et a1 July '7, 1942 2,449,339 Sziklai Sept. 14, 1948 2,451,484 Gould et a1 Oct. 19, 1948

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