US2292816A - Picture transmitting apparatus - Google Patents

Description

mammina vPICTURE TRANSMITTLN. APrARATus Fiied zApril 2o, 193s P/C 7'U HE t P/c/fuP "7 c Df C' 1 7A/ HMP. HMH. I .ry/vc. GEN.

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Hld VB? dfordY attorney* Auk 11 l942- Y A. v. BEDFORD 2,292,816

PICTURE TRANSMITTING APPARATUS v Filed April 20., 1938 4 Sheets-Sheet 2 OSC/1212070 HG, 4l.` l y /1 /4 Y f6 l /37 Plc TURB Plc/f" UP lq. C. DIC. Y' Monuzuvron y "'9 mvp HMP. HMP. Los f .SY/vc. GJ-:I/v.y L f/omz. l .sy/vc. v A 41) Y l 19 I l l c, l #Serif/Ems',

, Y HMP. F'ZHS mmc/r LEVEL v '3a Y 1 Y Y osc/LL/vro f E', l A F1' G.

Gttorneg Aug. l1, 1942. Y BEDFORD. 2,292,81'6?,-i

PICTURE TRANSMIT'TING VAxf'lmsfus iledAprn'ao. Y1958, 4 sheets-shuts f I `rrcrmn:rnNsMrrTniGAPrARATs *Alda v. Bedford, Collingswood, N.'J., assigner to" l Radio Corporation' of of 'Delaware applicati@ Alim zo. 193s, seriaiN, 3,0152 j l, j ,1

- voisins. (ci. 1ra-*1.1)v4

lviyfinvention relates'toV the ltransmission of electricallsignalssuch as picture signalsland par.`

ticularly to a picture transmitterdesignedto transmit the direct current component of Dicture signals.

It has been found advantageous inl-.television systems to `transmit all frequency'components of the picture signals down toand including zero frequencythatis, direct current. .The principal advantagey of such transmissionis that the large vpowertubes of thetransmitter are used more eiiiciently for reasons which need not be discussedhere..V d

Transmission of the direct current. and-very low frequency components of signals cannot` be eifectedconveniently by employing.. direct current ampliiiers in -the entire amplifierf channel from: the inputto the output of, the transmitter. As..

is well known, itis much easier `to designsatisfactory A., C. ampliflersthan to design satisfactory D. Ci ampliers,especia11y if the amplifier' must give uniformv gain over .axwide` frequency band of the character :required in television' transmission."

It is,-,accordingly, anobject offmy invention to provide an improved method -of and meansfor transmittingthe direct current and low frequency components of picture signals.

A further yobject of my inventionlis to provide an'improved'method of andmeansfor reducing l" the effects oflow frequency disturbances in a* picture transmitter. 1..

lost component-:sis accomplished ,by supplying-vthe,Y`

` carrier wave, afterv it has been modulated by the ioy picture signals and synchronizing impulses, to a biasedrectifier circuit. vThis circuit is scadju'sted thatl when the modulatedqcarrier wave yexceeds 'a predetermined amplitude it is rectiiied 'and'fd back to a D.C. portion ofthe transmitter amplifier channel to reduce 'the 'amplitude of the carrier wave. Thecarrier wavefpeaks which are. rectified are the peaks representing n synchronizing impulses. In this way thefsyn-j chronizing impulses iarerestoredto their: original amplitude whereby the lost D.C. and low fre-` quency components are restored.

Qtherobjects, lfeatures and advantages oimy a invention will appear from they following descrip@ tion vtaken in connection with the accompanying drawings-inlwhich n i i l Figure` 1 is a block diagram of one embodiment or my invention, '1 f y i Figure 2 is a circuit diagram of the embodi' vment illustrated in Fig. 1,

` Figures 3a tov 3d,- inclusive, lare curves ywhich arel referred to in explaining the operation of the circuit showninFigs. 1 and 2, f

lFigured vis a blocktdiagram of the preferred embodiment of'myinvention, 'f 1 Figure 5fis a circuit diagram of the embodi- "ment showninFig.-4, f

Figure 6is a'curve which is referred to' in explaining the invention,

In the preferred.embodimentnofmy invention, l as applied to aitelevision transmitter; horizontal synchronizing impulseslare-produced at the end of e'ach .scanning line' andvertical synchronizing or framing impulses` aref-produced iat theV endv of each picture frame; The synchronizing impulses are-mixed: with the picturel signals 'for transmis; siontherewitm- 1 i' Figures'la, 7b and.7c arecurves` which are re-lferredztoV in explaining one. wayin which the apparatus shown yin Fig. 5 maybe operated; and 1 Figures 8 to 9c aregother curves which are rel ierredr to in explainingthe invention.`

Referring toy Fig. 1 one' embodimentfof my invention is shown applied toia television trans- A mitter which comprises suitable means represent- Suitable means is lemploye'dtomake 'the 'syn-1 chronizing impulses lhave fa .predetermined Aamplitude withA respect to black in the picture sig-f nal before theyiand the picturesignals are passed through 'the main A.C. amplifier of thetransf" mitter. l Y, After the signals have :passed through 'the main A. C; amplifier, the synchronizing impulses would no longer have theirV original `'amplitude (because ofthelo'ss of the DEC.,.and"---lovvfre-` quency components)unless a? special circuit were employed for lrestoring'` their original amplitude and thus restoring the lostoomponents. l

In accordance with one of the 'preferred eln-A bodiments 'of myV invention this 4restoration of 55 ed bythe-block I for generating picture .signals and synchronizing impulses. AThe apparatus indicatedsby4 referencenumeral IV may be designed accordance with'vtheteachin'gs of my Patent 2,192,121, issued Feb. 27, 1940,` entitled Televisio'nf systems and the method' of voperation thereof 'v and assigned to the Radio Corporation ofAnierl-A ica. The important `feature ofits-design is' that the .synchronizing signals have a 1:xedete1minedl yamplitude''with respect to =b1ack in the-pic-Q,

ture.

curve 2.

. picture vsignals `2a, and-synchronizing impulses 2bwhich, `in the illustration, areL the horizontal synchronizing impulses. The output .of block I,

The output of block is represented by the j It is a composite signal comprising in the example being described, is the same my above-mentioned patent. 'Ihe apparatus in. the block I may be identical with the apparatus shown in the saidv copending application which supplies a. composite signal to the said radio transmitter |58.. w

It win be lnoted'that curve 2f aan synohro-:'

nizing impulsev consists of a pedestal which always comes to the black picture level and a Seien. the free# super-sync impulse which is edge thereof.

It should be understood that this specifie-kind` of synchronizing signal isnot-necessaryQin-practicing my invention, that is', the pedestal may be omitted. For example, British Patent 406,672, l filed August 31, 1932, shows howto produce back' ground "controP impulses which alwaysreach va .MY synchronizing impulses may-be 'generated in this level representing black in the picture.

same 'waygifdesiredt n r, the` composite Esignal' may? be generated as taught bylFrenchfPatent 7383175,y having thefd livr date of Ociiobel' 17,4932-, Where the peak fv The V signal supplied' by-lblock I lis impressed@ upon an alternating 'current-amplifier 4 which greatly ampliiles .the composite signal. Amplifier' 4 is a high gain, wide band-pass, amplifier-which could not easily be designed? to pass' direct cura rent.

scribed in the abovelmenti'oned French patent) no longer reach black as other levels as'in Fig.` Bal 'I'he lost components are restoredf as follows:

The outputof amplier'4'is-supplied to al directv current amplifier 6 which is .provided Witlifang ative feedback circuit-including. a' D. C. 'ampliiierf` 'I -The feedback circuit'isfso `designed that'a' asA the output supplied to the transmitter` IS'II of The amplified signal appearingy in 'the' output-I of amplifier 4 'no longer contains thev direct' cur- Y l rentv and low frequencycomponentsf This melnsj referring to Figs. :8 and 8a,-=that' the syrichr'oni'z;V ing signals 2b (Shown Without pdes'talfs'as' de`` means ofa fixed biasing source I8 and by means of a. biasing potentialsupplied from D`. C'. ampliiler l, this last biasing potential being determined, aswill bel explained hereinafter, by the synchronizing impulses.

The plate I9 of the tube I1 is suppliedr with a directv current'operating potential from a suitable source such ias 2.3250 volcbattery 2|. In the specie example-illustrated, the screen grid of the tube II is also suppliedv with operating potential from the battery 2I while the. suppressor grid is 'vc'onnecteddirectly' to the plate I9. It will be un- .derstood Athat.-the tubej Il may be athree-element -1 tube," if? desired;

Thedire'ctcurrent amplifier I includes ampliiler tubes 22land23,' 'the output circuit of tube 22 bias is applied Yto the `input circuit ofl amplifier 6f' I which has a value-depending `upon .the amplitude of the synchronizing impulseswhichappear in the' output circuit of Aamplifier 6. Thus, as will be more fully explained in the' following paragraphs, the synchronizing impulses-are again brought upto the black level," whereby?-` there is suppliedtoa modulator .8 ya signa-l of the same character as that shownfby .the curve' 2.

'I'he carrier .wave whiohismodulatedin modu-f lator 8 by-thecomposite video signal issupplied from a suitable oscillator The operation of.I the. direct. current :and =lowz frequency reinserting circuit will be better underL-.l stood by referring to .the circuit. diagram in Fig.'2.

Referring; to` Fig. 2 the-last .amplifierv tube. of. the A. C. amplifier 4 isgindicated at .I I; -.I'he."" number ofampliflerstagesjin the amplifier. '4. is such that the synchronizing impulses whichlarev impressed through the coupling condenser z I2'- upon the grid of tube II are4 of `.positive.Dolarity'.

. nals with thai). C.`componentremoved as they appear.k at the gridofftube ,I,I.Y' Considering the J being coupledtdthe input circuit of tube 23 by l. means of a', suitable direct current connection.

'I'he input circuit of tube 22 includes an adjustableilxed lbiasl which may consist of a-battery 24v whiehis shunted by a .potentiometer 26. This input-circuit also-includes in series with the adjustable fix'edbias 'a resistor 21' which is shunted by a condenser 28 whereby a control voltage vmay be` built upacrossthe resistor 21. The relative impedance values of'resistor 21 and condenser 28. are such thatimpulses supplied to this resistorcondenser circuit `at the frequency at which synchronizingfimpulses. occur are integrated vor filtered to iproduce substantially l'a steady direct v A control voltage is caused to appear' across the resistor 2l by the utilizationof afrectiiler whichl has its platel connected tothe plater I9` of tube II'andliwhich has its cathodeV connected to the high-potential 'endof thevfresistort 21. As will beexplained morezin: detailclater, thesynchroniz'- ingimpulses,;when1l .theyj exceed a' certainy ampli-- tude",.-cause` a flow of currentitlirough-the rectifier 29i-whereby the;A condenser.` Zlreceives-y a charge ands. whereby;.thef .-.direc' t` current output of the DtC. amplifier lisoh'axige'd.'A This direct current;

output is applied. tothe. control grid I4 of the I: tube II invnegativefeedbaclerelation whereby'the tendency .for currentfilow ythroughrrectiner 29l is Preferablylthe'output circuitlof the D. C. am, pliiler I includes-' iilterfconsisting ofy a resistorl 3 I and a condenser.`32gforfurthet smoothing out` the current impulsessupplied'by the recti'iier29.

The time constant. of the ill'ter' 3.I'.-f,32 may be the same' as=fthat ofthe filter 217-2817 the time constant beingI suchfthat4 therefisjonly al small dis- A.charge of` the' condensenx32 through the resistor 55' 3 lj between'successivesynchronizingimpulses.

Referring'more specil'eally'tol the-'operation of theabove `Y descrit'ied,` circuit, reference will be made tothe curvelsgshown'invFigsl3a to 3d. The

curvesshown in'Fi'gs. 3a to crepresent signals having a lightl .background and: having a dark background', respectively. Also,these vare the sigdark signal shown iuiFigl 3c`,.the circuit may be Tube H is suitamy biased by means ofy a battery.A I3 or the like to make it function as arr-,undis-f` torting amplifier or type A amplifier for video.;

signals. a i f n. The amplifier lI :ube II tis coupled byymeans of resistance capacitypoupling to the controlgrid I4 and cathode I6.of the'ampliertube 'Il-in the; direct current amplifier 6. z v l 'I'he control grid I4 is biased negatively by adjusted as follows: The bias supplied by the batter-y4 ltoth grid I4I is lso adjusted vlthat the positive peakxof. the dark. signal causes the platev IBfto go to a potential of 220.volts. Also, a V,

Lbias-potential of 220 volts has been applied from thegbatter'ys 2I`z through .theconductor 33 to the rectifler2il.'A .It=will be seen that thisisan ad justment for` the ,condition orne-current flow throughV rectifier 29; Due to the voltage drop in "the plate resistor 3410i' tube IT', the' plate of the rectifier 29 is at the same potential as the cath- .low frequency 'co condenser. io'r'v further filtering thefee1;lb'ack` control-voltage..-y The plate-of the tube :6251's connected through a conductor-Sato the control grid of amplifier tube 42 whereby A-thei direct current output of ampiierv4-I'biases-the'ftube 4Z and thus controls its direct current output! From .the forgoingidescription it-willbe-seen that, with the -switches L: infgthe fpositions' illustrated. in- Fig.- v5,--f1they television transmitter will" transmit the .direct cur-rent and low frequencycomponents-for the-.condition oi? negative ."morlu'-4` lation of' the carrier 'waveibythe-synchronizing impulses,-that 'is, Yfo'r' the-condition where 'the synchronizingimpulses :increase "the amplitude ofthecarrierwave;

In :thecircuit-fof 5Fig. .5.' Athe adjustment is' substantiallyqthe-same as that`ldescribed infcon-i nection f with Fig..12. .The circuiti-isv so 'adjusted by v'adjusting' a .bi'asing' battery" 1l in' the input circuit of the modula tor"and vv'by adjustingthe value 'of `biasing iba'ttery 49`=that` when the' carrier .wave vis being'lmodulated-by a 'dark signal-` such as represented "by Fig.4 3cthe'r'e is' little or no'fiow of current throughfthe 'rectifier-41.' The-v adjustments' are made such that' the synchronize' ing peakslcausefthe' inaxirnur'n carrier-which is desired for any picturecondition'. Noww'hen a light signal,v sucli`^``as -"represented by "Fig.' 3a; modulates' thecarrie'r`wave',` the peak amplitude of vv-tl'ie '-carrier representing the-"synchronizing curately restore lowfrequency response.- To operate-the system'o'f Fig'.'5 in this manner, switches SI-'and 64 are thrown y"to the 'dotted line A. C. positions." With the vswitch. 51| in the A. C. p0- sitloni thefbiasing batteryl 49 is removed from the circuiti'and'there is 'substituted therefor a lter comprisingjav 'resistor"12' 'and a condenser 13. In this case, thecondens'er-JS gradually charges up to any value required' to substantially stop the synchronizing peaks e from Y causingv current flow through rectifier-'41 -soithat the voltage acrossventio'n itmay-be desired to transmit a signal impulses' increases to-cause charging current to flow into 'condenser' 54 whereby a positive 4voltage is supplied over conductor-'59 to the vgrid of tubeil. This in turnmakes theitco'n'trol grid or tube 42 more positive" wnereby'the `contrer grids of tubes- 43' and'44` are made more negativeV anclfwhereby-A the peakvalue'of thecarri'n wave is held down to approx'ixnately-v the v'original -pre-set level.

1twiu be understoodf that the above 'deseribedf circuits will 'removelow frequency disturbances which are introduced ttt pontsinthe transmitter following the-equipment represented by` the block i. For exan'ipie,(ifv a lowl frequency `disturbance, such as a 60 cycleripple,l occurs in-the.

amplifier 4, or in thi'e"oscill atorl 38, it 'will cause the synchronizing impulses to followjthe form. of this sine wave'and the C reinserting' circuit which follows will remove the cycle .ripple by again levelling off the`'synchronizing impulses.

The-action of the4 circuits -in restoring low Y,

frequency components which have vbeenfilt'ered out by the 'alternating current'ampiifler 4 is' il`- represents a lowv frequency component the" picture signal, which-it is dsi'xiedft'o'transmit, the output of the block i may be represented by j the curve in Fig. 917;" Here'i'twillbefnted that the synchronizing impulses allcome .up' to a pre determined' level with respect to jvblaekf 1n the' pioture' signeL 'In effect.' the syn'ehronizing' imn pulses have beenniodulated bythe loivffrequen-,

cy component as is evident by referring to Fig.

9c where the lowv'frequency componentof' Fig.' 9a 4 has been filteredout' by the A. C. amplifier.' It will be apparent that, after the signal shown 'in Fig. 9c has been passed through the reinserting network and the peaks of the synchronizing irnl pulses brought beek; to 'their originel level, the;

been reinserted.'

It may be desirable to ytransmit picture vsignals without the direct :current component but still retain the .property of my improved system to buck out low frequency interference and acmp'onent ofjFig. 'Qafwill have' .by the rectifier 16.

carrier' representative of synchronizing impulses.'-

in which the -.synchronizing impulses are represented byv a decrease in the carrier wave amphtude; this being known as flpositive transmission" in R. A. standards-*In vthis case the syn.- chronizing impulses are-of positive polarity on the` control grid -oftube 42. -With this type of transmission the radio frequency peaks are not representative of synchronizing peaks' so that the` vradio' frequency' must be detected-before it is applied-to that rectifier which responds to the'heightbf the synchronizing peaks.

In'Fig. 5 *this detection is accomplished by means-'oi a rectifier 16. In this case the diode 16 is" supplied with energy from the tank circuit 46 by means of' a coil -11.coupled thereto and tuned tol the 'carrier frequency by means loi a .condenser-18. The rectified current is'fedinto' a load circuit comprising a filter resistor 19-and a filter condenser' 8|. This load circuit 19-8I has a very low'impedance compared` with the impedance of the circuit comprising rectifier 16 and coupling 'coil 11 in series whereby tube 16 functionsl as an ordinary detector.- Further filtering maybe provided by means of an inductance coil SZ'and-'a resistor 83. ing across yresistor 83 is now supplied toa rec- `tifier tube 84 which corresponds in function to the rectifier tube v41 for the case of negative modulation. f

For the'transmiss'ion `of the directcurrent com- I ponet a switch'-V 86 is on the D. C. position :whereby-a 'biasing-battery 81 connected inv series with'th'e resistor 8 3',y the rectifier 84 and a filter resistor- 88, the lterfresistor 88 being shunted by'a filter V` ':'o'n denser 89'. The parallel circuitcomprisingjresistor 38 and condenser 88 hasa'j'hi'gnimpedance compared 'with the'circuit consisting of' rectifier 84 and resistor 19 and 83 in series therewith, whereby tube. 84 functions as a special peaklvoltage responsive device.

The vbattery 81 is.so poled4 that it vtends to "causea fiow of currentthrough the rectifier 84.

However, the .battery voltage is opposed by the voltage drop,y across resistor 83, the direction ofcurrent fiow through resistor A83 being determined Thus, :when the modulated decreases 'in amplitude '(due to 4an increase in impulse amplitude at the grid of tube 42) thev flow of current through resistor BI decreases, and

' current iiows through rectifier 84 to charge con- The video signal appeardenser AI9, whereby the switch terminal marked L .rhusfwitntheswitchen@ the Posx becomes :more negative.

.tionforfpositivefrnodulationtransmissioru stantilally steady C.,.voltage Willappear condenser Y"89' whereby a negativ be ensued through tneffconduptqr, s; anatnrgh i the switchl. (Whchgis in thegD C; position) Vto* l tube? lz'more negative and making .the of y the ymod'ulatorfsmoray positive,;thereby :increasing I.

the amplitudelfof.-thecarrier wave .aItimeconstant such that-the condclel diS charges `only ai very small:A amount :between the.

currents of synchronizing impulses For ,positive ltransmission, asinthegcase of negative transmission, my control circuitgmay bey utilized lt0.'..reduce low` frelzluency ldisturblances when it is not desired toitransmitthe ,direct current component. In this case, the switch 60 being ony the 'positive transmission position, the

switches 86 and 64 are set on the A. C. positions.

Here, again, the biasing battery in series with v the rectifier is `replaced by a filter such as the modulating means whereby there is produced a resistor B2 and condenser 93, thislfilter corresponding to thev filter 'l2-13 in the. other Vembodiment. y

In accordance with still another embodiment of my invention, which is claimed in my divisional application SerialfNo. 426,811, filed January 15, 1942, the vswitch 52' may be thrown to the right hand position to -produce the feedback signal illustrated in Fig. 7c whereby the control voltage is derived from impulses l96 which are set on top of thepedestals. This type of control may be employed for either A. C. or D; C. Vtransmission.

The signal of Fig. '7c is produced Vby adding the signal of Fig. "la (representing one side or polarity vof a modulated carrier) which appears in the tank circuit I6 and the signal of Fig. 7b which appears across a resistor 91. The side of the carrler represented by Fig. 7c is rectified by the rectifier lt1, but because of the biasing battery 49 (or filter 'l2- 13) only the peaks of impulses 96 pass through the rectifier to charge condenser 54.

Theimpulses 96 may be derived from the horizontal synchronizing impulses supplied 4from unit l and passed through a phase shifter or delay network 98 and amplifier and/or modulator 99 to the resistor 91.

When the control is `derived from impulses 9S. the pedestals :are maintained at a. fixed level regardless of what happens to `the super-sync impulses. This would -be useful in case the"su per-sync impulses were subjected to saturation orvother alteration causing them to be unsuited for the purpose of reinserting D. C. and low frequency components.

It should be noted that the signal represented in Fig. 'la is the one which is radiated toa re-.

ceiver eventhough the signal of Fig. 7c is used' inthe feedback control.

It'may vbe noted'that additional -intelligence or TO nals which periodically reach a predetermined information rmay be transmitted by modulating the impulses 9B Abefore 'they are impressed across the resistor" 91. The effect of such modulation is to Aintroduce an additional `modulation on the transmitted composite signal comprising picture signal and synchronizing impulses. At the tele- "-visio'n receiver, this modulaton is removed before,

the signal isimpressed upon the cathode'ray tube byfmeans of a-v `leveling or `D.1C.'reirlserting'circuit .r Y, such :afsl that *disclosed in Pechapplication'vlon' -f 15 Television systems, 'SerialfNo.v7-18,192, filed March d 30;;19.34,*andjassigned tothe VRadio Corporationv "ofielca meunier'-`v could be transmitted in j this'way. It

wouldfbe taken from 'a Vportion yofV the -receiver circuit preceding ltheleveling or D; C.y Ir'einserting` *stage -1 I claim asrmyfinventionlz'4 Y:1 rn .apicture transmitter 'wherein picture sig- ;vasffaf'compositef signal over a common 'channel` `*witlr theA said' impulses of greater" lamplitude than `picture signals.' of'like polarity, r'nea'ns"forapro` ducing said fcoinposite'signal: witiifthe4 peaks of 20 saidimpulses always reachinga ycertain-"prede--Ll ,termned level with respect to picture signals representing black in thex transmitted 'picture'.an

A. LC. amplifier connected "to, amplifyisaidV com- .pfoh'sites-ignalt after said 'impulses have been"y leveled 'V25 with respactto'black whereby theD.7C. andlow frequency components ofy said signal are lost, a source of carrier wave and modulating means for modulating said carrier wave, means for supplying the output of said A. C. amplifier to said carrier wave modulated by said composite signal, a negative feedback circuit comprising a D. vC. amplifier having an input circuit and an output determined -level and for converting said energy into substantially steady` direct current, means 40 for applying said steady direct current'to said D. C. amplifier input, and means for applying the output of said D. C. amplifier to the input circuit -of said modulating means and in negative feedback relation whereby any tendency .for that portion of the modulated carrier representing synchronizing impulses to go beyond said level is counteracted.

2. The invention according to claim 1 characterized in that .said means for deriving energy from said modulated carrier comprises'a rectifier,

a biasing voltage source, and a condenser, all connectedin series with each other, the series combination ybeing coupled to the output circuit of said modulating means, said condenser being connected in the input circuit of said negative feedback circuit whereby a charge on said condenser causes a change in the voltage feed back, a resistor across said condenser, the time constant of said condenser and said resistance in series being such that said condenser discharges only a slight amount between ythe occurrence of successive synchronizing impulses, and the value of said biasing voltage being such that current flows through said rectifier only in response to the carirer representative of said synchronizing impulses exceedinga predetermined amplitude.

3. In a device for inserting the D. C. component in` a signal, the combination of means for producing a carrier wave, means for producing sig- '5 ents, modulating means for modulating a carrier wave `bysaid signals in such sense that said 5 predetermined level is represented by maximum "FOX, example; 'acontrolf'tone for av volum -ex'y nais and synchronizing impulses `are transmitted.' Y

amplitudeofsaid carrier wave, said modulating g means having an input circuit and. an output circuit and being capable ofltransmitting lowy frequency components. a degenerative: feedback?.

circuit having an input circuit and' anloutput circuit and being capable of transmitting said low frequency components.. means for deriving from said modulated carrier wave a D. C. voltageV having avalue which is ameasure ofthe amount.`

said periodically recurring signal portions y.depart ,from said level, means for impressing said D. C. voltage upon the input circuit of ysaid feedback circuit, and means for coupling the output circuit of saidfeedbackcircuit to the input circuit 5. .The invention'according to claim 3 charll acterized in that said Lmeans forvderivlnguthe D. C, voltage comfprises-` a rectiiierfand'a con-` fecser in series, said. condenser being shunted -aaeasic by a resistor, and the' circuit comprising said condenser.v and `said resistor inwseries havingA a timejconstan't 'suchthat the condenser discharges.l only a small amount between successive periodi-I y calls' recurring signal portions.

6; The inventionV according to vclaim ,3.

acterized in that said means for deriving the D. C. voltage comprises means'for demodulating -said modulated carrierv wave, and means for im-` pressing vthea-resulting dernodulatedl signal upon ak circuitcmpr'isi'ng a rectier-fand'a source of.: biasing voltagein series1.:'= -1 v v'1. The invention accordingto claim 3-charac,4

V terized in that-said'means Vfor deriving the D; C.-

voltage comprises 'means 'for demodulating -said modulated `carrier wave,` and means for impressing the resulting demodula'tedsignalupon'f'la, circuit comprising av rectiiler and a condenser in series, said condenser being shunted by a resistor of sxmlcientlyv high resistance vto prevent said condenserfrom discharging a substantial amountbetween the vsuccessive periodically recurringsignal portions. l l ALDA V. BEDFORD.

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