DECEMBER 1938 - World Radio History

ELECTRONICSDECEMBERKEITH HENNEYEditor1938Crosstalk.TELEVISIONParamount, buying into DuMont, states that it willpipe programs (presumably by distributing film) from Hollywood into American homes by Christmas, or some suchdate. As in many things, "seeing is believing" and until at least the first ofthe year, our fingers are crossed.Misguided dealers and manufacturersare offering radio receivers on the basisthat they will receive, directly, thesound associated with television signalswhen they are available. These receivers must tune to frequencies at leastas high as 50 megacycles; there arefew, if any, receivers sold today whichtune to any such frequency.Therefore, if anyone wants to receivethe sound channel of television only,it will be necessary to purchase a converter which will change the 50 -odd Mcto the frequency accepted by the existing sound receiver. Any "vision only"receiver sold, however, will have a converter as an integral part. This converter, will not only deliver the picturei.f. but will (or should) deliver thesound i.f. (8 Mc) which can be utilizedby any existing sound receiver whichwill tune to 8 Mc.There is always talk about televisionreceivers becoming obsolete by the development of some-as yet unseenradically new method of transmissionand reception. All present thinking isto make the present standards do; andnot to anticipate violent changes inbasic principle. Within the structureof the present standards there is muchopportunity for improved service whichmay tend to make the early receivernot so good as later models-but thisis no different from the present soundsystem. Sets of 5 years ago still "play"but they don't have push buttons, shortwave, high selectivity or any of theother recent important improvements.International Television Radio Corp.,»(a Delaware Corporation) offers onemillion shares of stock at 1.60 making3,479,990 share outstanding. This company plans to build television centersconsisting of a transmitter and sets fordealers; to develop infra-red fog piercing navigation equipment; and aircraftdevelopment. The Priess scanning mirror, vibrating in two planes, is theheart of the television system.WELL MET . . . By one of thosehappy circumstances which occasionallytake place, Ken Andrews and F. J.Perillo, both interested in electronics,met each other in a restaurant-gettingtogether because one was reading acopy of Electronics. The firm of Andrews and Perillo resulted. Animatingdisplays for large industrial companiesfor such affairs as world's fairs is theirforte; but they do a lot of electronicwork too. Now John F. Dreyer, radioengineer with Atwater Kent and RCARadiotron background joins the association-and his forte is electronics. Thus,three good fellows with good technicalequipment join forces.CONTRAST. FCC spends a barrel of money investigating monopolyin communications; produces materialwhich will be forgotten in a year or so;raises big rumpus, gets in the papers,etc., etc. The engineering work of theCommission, however, is largely buried,unappreciated, disregarded by politicians. Who knows of the long seriousstudy of receiving conditions on shipboard where radio is of vital importance; of the investigation of tropicalnoise, static, engine room disturbances,all endeavoring for the first time tomake a real approach to the problem ofhow much power a ship transmitterought to have and of what signalstrength it ought to receive? Thiswork is of lasting benefit, and its effectwww.americanradiohistory.comwill be felt long after the fuss and furyof the non -technical investigations havebeen forgotten.RESEARCH . . . In five parts ofthe country, Electronics has researchmen talking to subscribers, finding outtheir reading habits, what they like,what they read, what they don't likein fact trying to discover at first handthe answer to all the questions aneditor has about his readers. Theseeditorial research men have nothing tosell, no axe to grind. They want facts;and it is to the advantage of the readernot to throw these fellows out whenthey appear. A successful technicalpaper must be a cooperative effort between publishers, editors and readers.SORRY . . . In the November issue, page 14, the statement was madethat "the ratings of many transmittingtubes have been pushed to the limit,and beyond, under the false standardof 'more watts per dollar' whereasmore watthours per dollar would indicate the true economics of the situation.We take this opportunity to state thatit was not our intention to impugn themotives of anyone using the aboveslogan or to imply that the use of theslogan indicates any unfair, dishonestor improper trade practices. Rather,it was the purpose of our statement topoint out that in our opinion the lifeof a tube is more important than theamount of power that the tube couldhandle at any particular time.REFUGEES . . . America gainsthrough the constant inpouring ofhighly trained men from Germany andAustria. Several have been in theeditorial office recently, hunting jobs;their names and qualifications are onfile. Physicists, mathematicians, chemists, engineers.

ROCHE S T1 9 3 8TELEVISION advancement wasthe main item of interest at theIRE-RMA Rochester Fall Meeting,where several hundred engineers andabout two dozen exhibitors congregated on November 14th to 16th foran informal discussion of engineeringproblems, to display new equipment,and to hold technical committee meetings. Papers were fewer in numberthan in past years, and a longer timewas voted to the presentation of eachpaper. As usual, all papers were ofhigh caliber. Never-E Rany radio services already assigned.The extensive use of the band between 450 and 470 kc made it advisable to select a frequency in thisrange if possible. By consideringthe frequency assignments in thisband, together with the three requirements given above, it becameevident that 455 kc was the bestpossible frequency for use for superheteredyne purposes. The RMA re -the same basic principle, were shown.All the tubes had the following incommon: First a steady source offree electrons from a plane cathodesurface or its equivalent. Second aplane grid structure, mounted parallel to and just in front of the cathode.This grid structure is made of a flatmetal plate, perforated with fineholes to the extent of about 160,000holes per square inch. The front surface of this plate istheless, specialTransparent anodeFour stage annularPhoto -islandcollectorinsulated with barelectronmultiplier--grid meshmention must beium fluoride andmade of papers precoatedwith a sputsented by Philo T.tered silver -cesium Farnsworth whooxide layer onPhotoelectric/described a newiemision'which the opticalimage tube of imimage is focussed.Scanninggunproved sensitivitymagnetic focus andcThis grid, called bydeflection)and making use ofMr. Farnsworth aMultiplierelectronic princiLu'Anode wall coaling-terminals"photo-island" gridples which, up tois roughly analonow, had not beenThe Farnsworth storage-type image-dissector tube, which displays nogous to the mosaicshading signal and has ten times the sensitivity of the ordinaryapplied to this typein the iconoscope.iconoscopeof tube; by HaroldWhenthe opticalWheeler of Hazeltine for his contri- quested that the channel between 450 image isfocussed upon it, it assumesbution to electric circuit theory in and 460 kc be assigned exclusively a distributionof potential which hashis discussion of "Interpretation of for intermediate frequency use.the same form as the distribution ofAmplitude and Phase Distortion inlight in the image, and the amplitudeTerms of Paired Echoes"; and by See- Farnsworth's New Tubeof the potential increases the longerley arid Foster for their demonstra"Image Amplifier Pick-up Tubes" the light is allowed to fall on it. Thetion of "Principles and Methods inwere revealed for the first time by distribution of potential on this gridTelevision Laboratory Practice".P. T. Farnsworth in a paper of that structure acts as the control potentialname written by Mr. Farnsworth and over the electrons flowing from theStandardized Intermediatecathode behind it. Consequently theseB. C. Gardner. The new tubes comFrequencybine several important features of electrons form an electron imageJ. E. Brown of the Zenith Radio the iconoscope and image -dissector, which passes through the meshes ofCorporation opened the technical ses- and in addition utilize the principlesions by delivering a paper on "Stand- of a control grid, for amplifying theardized Intermediate Frequency" picture signal before the image iswhich reviewed the past practice of scanned, rather than after it imanufacturers with respect to the se- scanned as in all prior pick-up delection of the difference beat or inter- vices. The resulting tubes are freemediate frequency for use in super- from the spurious "shading" signal,heterodyne receivers. Requirements and one form has a sensitivity aboutfor suitable intermediate frequency 10 times as great as the conventionalare that: (1) the i -f pick-up should iconoscope.be a minimum, (2) interference fromThe presentation of the paper folharmonics should be a minimum, and lowed the evolution of the new tube(3) the i -f selected should be such as over a period of 12 years and manyThe "photo -island" structure used inwill not result in interference from different forms of tube, embodyingthe Farnsworth tube8December 1938 -www.americanradiohistory.com-ELECTRONICS

Digest of technical papers delivered at the Rochester Fall Meeting, where an attendanceof 500 gave evidence of an improved business outlook, and which proved once again thatthe laboratories have not been idle, business or no businessthe grid, to be collected by an anodeand conducted to the external signalcircuit. In this case the scanning ofthe signal is obtained by moving theelectron image, after passage throughthe control grid, bodily past a smallaperture in the collecting anode,using magnetic deflection similar tothat employed in the conventionalimage -dissector.The most modern form of tube doesnot employ a plane cathode, but usesa beam of electrons from a gun, magnetically focussed and deflected. Theelectrons from this beam are directed,in the usual scanning motion, towardthe back surface of the "photo-island"grid, which emits secondary electrons under the influence of the beam.These secondary electrons are drawnthrough the interstices of the control grid in an amount dependingupon the potential existing at thepoint then scanned, the potential being. determined by the degree of illumination at that point. The secondaryelectrons, after passing through thegrid, are collected by an electronmultiplier structure, built in the formof an annular ring, located at thecircumference of the tube face andcontaining four stages. The outputof this amplifier is conducted to anexternal coupling resistor of about2000 ohmä,-and thence to the conventional videò head amplifier. A signalof at least 10-i volts is required toovercome the noise, in this arrangement. Actually under .optimum conditions, the signal output is approximately 0.01 volt.It was pointed out in the paper thatthe charge accumulation on the gridmay be allowed to continue for verylong periods, since the only paths fordischarge are leakage and grid current resulting from the collection ofprimary or secondary electrons bythe grid. Storage times of as longas 15 minutes have been observed.Obviously this is far too long astorage time if moving objects are toELECTRONICS-The ordinary storageinterval employed in the iconoscopeis the frame interval time of 1/30thof a second. Mr. Farnsworth statesthat storage times as long as 1/5thof a second might be tolerated, butthat in any event, additional sensitivity may be obtained by employedstorage at least as long as 1/15th ofbe televised.Substitution of a voltage regulatortube fo: a filter capacitor (Acheson).,. io4 .-----.'--. .,Entrancewindow11-stagemultiplierPhotosensitive Anode Scanningcoating aperturecathodeNon -storage image dissector usedfor television film pick-upa second. The difficulty at present inthe new tube is to limit the storagetime to this amount.Since secondary electrons are generated, it was to be expected thatthese electrons might be redistributedon the control grid in such a way thata spurious shading signal would begenerated, as in the case of theiconoscope. However, no such signalis observed, and the conclusion is thatthe secondary electrons are forced toproceed directly through the grid interstices by the high value of spacecharge existing at the back surface.After Detail, What?In the paper "Gamma and Rangein Television" I. G. Maloff of RCAVictor pointed out that up to theDecember 1938present the attention of televisionengineers has been directed almostexclusively to the problem of obtaining adequate detail in the reproducedimage. This problem has becomecorrespondingly well understood, andthe necessity of proper phase andfrequency response is appreciated.However in picture reproductionthree other factors are also of greatimportance: the average brightnessof the scene, the range of brightnessbetween the highlights and the shadows, and the linearity of reproduction between differences in brightness in the reproduced image. Thepurpose of Mr. Maloff's paper wasprimarily to draw attention to thelatter two factors, which can be described in the corresponding photographic terms of range and gamma.Rough electrical analogs of thesequantities are the usable gain andthe harmonic distortion of the amplifiers or other transducers in thesystem. Unlike sound transmissions,television pictures may be improvedby harmonic distortion.The gamma of the television system may be described as the exponent of the curve (assumed logarithmic in form) relating the lightlevel in the studio and; the corresponding light level in the reproduced image. If the relationshipbetween these quantities is linear,the exponent of the curve betweenthem is unity, that is, the gamma ofthe system is unity and distinctionsin brightness in the televised objectare reproduced accurately in the reproduced image. That an overallgamma of unity is desirable seemslikely, but it is true that the response curves of intermediate transducers may be far from linear, providing that there are compensatingdepartures from linearity, so thatthe overall effect is linear. It issuggested, for example, that the response of the transmitter have acompressive characteristic corre 9www.americanradiohistory.com

Signal grid side rodG304screen -----7 'i '\.signal grid-,sary.\ - -K,-Screeni\\-// 'b; ;\t,, ,i --r\\.--,,rocol,i//¡X" o/IAccordingly, an examinationwas made of the use to which thevarious base pins were put and itwas found that by not providing aseparate pin for the plate of theoscillator section, the tube could bemade a single ended device using astandard octal base. However, thischange required the development ofnew and unusual oscillator circuits.A large portion of the paper wasdevoted to the development of newoscillator circuits, for use with thisnew tube, and the operating characteristics of the tube with the recommended circuits were given.6 SA7ti,%\ Collecting--plates,'/-- xii-Anode--Shell'GridGIThe electrode structure of the new single -endedpentagrid converteremploys some of the principles of the beam powertube in obtainingSingle -Side Band Televisionbetter performance with one less terminalsponding to a gamma less than unity,and that the receiver have an expansion characteristic (gamma aboveunity), the product of the twogammas being equal to one. Suchcompression in the transmitter wouldallow more efficient use of the iconoscope, and could be used to emphasize the picture information relative to the noise.Feedback Applied toLoudspeakersHugh Knowles of the Jensen Company presented an analysis of theapplication of inverse feedback toloudspeakers. By including the electro -acoustical system in the feedbackpath, distortion generated in thissystem is reduced by the feedbackaction. A very simple and directmethod of accomplishing this resultis to set up a microphone before thespeaker and to feed the microphonevoltage to the input of one of theamplifier stages. The difficulty hereis that the phase velocity of the soundis so low, relative to the electricalpropogation of the signal, that thephase relations cannot be accuratelymaintained and so the system becomes unstable.A more practical solution consistsin mounting a voltage pick-up coilon the voice -coil of the speaker, withits own magnetic field, and feedingthe voltage generated in this coilback to the input of one of the amplifier stages. Any non -linearity in themechanical system, but not that inthe sound field itself, is thereby corrected to some degree.The impedance looking into thespeaker terminals changes with frequency and this effect must be takeninto account if instability is to beavoided at the frequencies where thephase shift is great. One method isto derive the feedback voltage froma lumped -impedance network, in series with the speaker terminals having approximately the same impedance variation as the speaker itself.In addition thetransfer characteristics insertedin the feedbackpath must beIt became apparent at the convention that single side-band transmission of television images is to bethe rule in this country, due toprogress made in transmitting methods, and that as a result a bandwidth of about 4 Mc (as againstabout 2 Mc in double-sideband) isavailable for picture signal. Thatthis doubling of the available picturefrequency band would result in acorresponding doubling of the hori-zontal picturedetail is certainly to be expected,bu t theoreticalconfirmation wasnot forthcominguntil the presentation by Dr.Stanford Goldman of G.E. insuch that theproper phase andamplitude relations, requiredfor stability, arepreservedOscillator circuit for use with the6SA7; no oscillator anode terminalis requir edBridgeport,throughout the frequency range. Under these conditions it is possible todeliver power to the speaker indirect proportion to the square of thevoltage input to the amplifier, whichimplies a linear system throughout.ofthe paper "Television Detail andSelective Sideband Transmission".Dr. Gol

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