01938Reprinted 1990.

A Primerof

TELEVISIONFOR THE STUDENT

ANDLAYMAN

1ELEVISION, like our present-daymotion picture, is dependent upon

" the little known fact that the hu-man eye has a certain lag, and persistsin retaining for the fraction of a secondany image flashed upon the retina. Thusscientists found many years ago that, ifa successive series of pictures of a manrunning, each picture showing a slightadvance in the action, was flashed be-fore the eyes, the illusion of motionwas complete. In other words, by flash-ing say 16 progressive images on the eye,each a little more advanced, the viewergot the effect of a person running, eachpicture being stopped for the fractionof a second.

In television we have a similar sit-uation - we build up a picture in frontof the viewer, say 16 times or moreper second and, if we do this with

SIGHT CENTER OF BRAIN. IMAGESEEN RIGHT SIDE UP

00tICCT ISREVERSED

0/1 RETINAOf EYE

(OBJECT OPTIC NERVE. ADOUT50A000 FIBERS

FIG .1

The optical system connecting the eye withthe brain is actually a television process

2 Television

L/CHT-SENSITIVEPOTS IN RETINAOF EVE CS)

NOTE REVERSAL OFIMAGE IN DRAIN

OPTIC NERVE

SIMPLIFIEDROD & CONE

IN EYE

R

FIG 2Even though the object as seen on the ret-ina of the eye is upside-down, the brain

sees it right side up

enough lines to give good detail, weprovide as does the movie, a satisfac-tory representation of a person or ob-ject in motion.

The human eye is a very good ex-ample of television, so let us see hownature proceeds to transmit an imageflashed on the retina at the rear of theeye up to the brain. The illustration(Fig. 1) shows how the optic nerveconnects the eye to the sight -center inthe brain; also note that, while the ob-ject is seen upside down on the retina,it is observed right side up in the brain.The light-sensitive layer at the rear ofthe eyeball is divided into thousands

of tiny segments, and it is believedthat each of these segments transmitsan electric (nerve) impulse to the sight -center in the brain, over 500,000 oddfibers making up the optic nerve. Theoptical system of the eye, in otherwords, acts like a modern telephone ex-change; each wave or pulse represent-ing a light or dark spot on the imageis transmitted to the brain and here itis translated into the proper mediumto give us the sensation of sight.

Easy, you may say - all you haveto do is to duplicate that system andyou will get television or the trans-mission of a moving image to a dis-tance! Right you are, but the televisionexperts thought of that too - and infact the early television images weretransmitted in just this fashion. A dozenlight-sensitive cells (selenium - whichwas so slow in its action that it de-layed television many years) were con-nected with wires to an equal num-ber of light -reproducing devices, lampsfor example. If the light falling on oneof the selenium cells was weak, thenthe current passed over the circuit tothat particular lamp was weak too, andthe viewer at the receiving end saw

(OBJECT

LENS)

---"" LIGHT

id, -area0::00'00wa.:0.1190.00si TIVOOPVAW:OVOIT110*;;Isl131k11.!

DARtt AREA 142:LIGHT-SEN3ITiVE CELLS

TRANSMITTER

1 FOR EACHWIRES

CELL. PLUSCOMMONRETURN

FIG 3

4L3N LAMPS

RECEIVER

Simplest television system - a number of light-sensitive cells connected with anumber of neon lamps

Television

a weak light at that particular spot onthe picture. If the next cell received astrong light, then a strong current wassent over that circuit to the correspon-ding lamp at the receiver, and a brightspot was seen in the reproduced image;all similar to the action occurring inthe eye. But remember that the moderntelevision image is made up of over200,000 picture elements, and we wouldneed that many cells at the transmitter,plus two hundred thousand wires, con-nected to that number of light -repro-ducing devices,

Fig. 2 shows how the light-sensitivespots in the eye are joined to the sight -center in the brain by the numerousfibers (corresponding to the wires in atelephone cable) of the optic nerve.Fig. 3 shows how the early televisionexperimenters produced a crude imageat the receiving apparatus, simply byconnecting a selenium cell with a lampor other device (a relay for example)as previously explained.

Nipkow, over forty years ago, de-vised the scanning disc, a circular pieceof metal pierced with a spiral of holes.From a study of the construction of apicture, as exemplified by the modernhalftone - with its dots of many sizes(3-a) representing light, dark and in-termediate (half) tones, it becomes dearthat, if we break down the image orpicture into small elements, and trans-mit the light values of these elementsprogressively, at high speed, over anelectric or radio circuit, we should beable to recnstruct the image at the re-ceiving station,

This is what the scanning disc does- see Fig. 5. As No. 1 hole passes be-fore the light-sensitive cell, it scanspath No. 1 on the face for instance.

HALF -TONE IMAGE IsMADE UP OF LARGE8 SMALL DOTS

FIG. 3-aHalf -tone images are made op of

a series of dots

Hole No. 2 scans path 2 across theface, path 3 is scanned by bole No. 3,etc. When the last hole on the innerend of the spiral has scanned acrossthe image, the whole picture has beenscanned once: now revolve the disc fast

to scan the image 15 times or

SCANNING ,PATHS 3

4

7

9s

i4OW CURRENT VARJESIN PHOTO -CELL ASSCANNING SEAMSCANS OVER PATH -9

4T0-CELL FIG. 4 .

How image is scanned and how lightvaries along each scanning path

4 Television

PHOTO ELECTRIC CELL

15

FACE BEING SCANNEDHOLE 1 IN DISC SCANSPATH I. HOLE"2 SCANSPATH 2 ETC

SUBJECT

REFLECTED \\LIGHT RAYSFALL ON PHOTOELECTRIC CELLBEHIND DISC

0

a

04

Os

e`o.

LENS

AMPLIFIER

IMAGE FRAME

NEON TUBE BEHIND/ DISC

-, F. ,PHOTO CELLP elk

MOTOR

SCANNINGDISC

TRANSMITTER RECEIVER

7IMAGE HERE

FIG 5

Scanning discs at both transmitter and receiver - the fluctuating neon tube lightreproduces the lights and shadows of the image

more per second, in order to remove theeffect of flicker, and you have televisionas we knew it a decade ago. But theholes did not pass enough light, thedisc and its motor were crude mechan-ical devices after all, and it was notpossible to obtain much better defini-tion than SO lines per image. Thinkingin terms of 300 to 400 line definition,engineers bethought themselves ofBraun's cathode ray oscillograph -here was a device that used a pencilof light having practically no inertia.So today we have the electronic scan-ning in a cathode ray tube, as perfectedby Zworykin, Farnsworth and other sci-entists. But the principle of scanning theimage line -by-line is still the "back-bone" of modern television, so let usstudy this scanning action a little closer.

Fig. 4 shows how the illuminated ob-ject reflects varying strengths of lightand shadow onto the scanning disc anda photo -electric (light-sensitive) cell;the variations in the photo -electric cur-rents produced by the cell are amplifiedby several stages of resistance -coupledamplifiers, and if fed to a neon tubeplaced behind a second scanning discat the receiver, one perceives the recon-structed image of the subject's face atthe transmitter. Of course the two scan-ning discs must be driven at exactly thesame speed and kept in step with eachother at all times by the use of syn-chronous motors. Neon tubes or lampsproved useful for the earlier television;as they changed their brilliance veryrapidly and could therefore follow therapidly fluctuating pulsations of currentin the circuit coming from the photo-

Television

cells. The neon tubes used in the oldermechanical scanning systems were oftenmade with a large flat plate, 2 by 2inches or larger, so as to cover thewhole frame opening.

Fig. 6 illustrates a system used someyears ago to obtain a larger image -here the object was lighted by a flyingspot of light passed through a scan-ning disc from an arc light, and thereflected light rays fell on a series oflarge photo cells. The fluctuating cur-rent from the cells, corresponding tothe lights and shadows of the subject,was fed into a neon crater tube, whichgave a concentrated spot of rapidly va-rying light. This spot of light or craterwas scanned by a lens disc rotated infront of it at exactly the same speed asthe transmitting disc, and a brightimage was projected by the whirlingspiral of lenses on to a ground glassscreen. Surprising as it may seem, an8 by 10 inch image was obtained withfair brilliancy. Of course the detail waspoor; for think what would happen to

5

the image of a face 1 inch in height,scanned by only half a dozen lines.Such is the way of invention and prog-ress!

The diagrams show wire circuits con-necting the transmitters and receivers,for the sake of clearness; but it is un-

HORIZONTAL TUNING FORK( HIGH SPEED)

LAMP/

LIGHTBEAM

C.w

COUNTER(WEIGHT

(c-)

--- MIRRORS)

LENSSCREEN

VERTICALTUNING FORKC LOW SPEED

FIG. 7

A mechanical scanning system has beenadvocated by many inventors

REFLECTED RAYSFALL ON PHOTO-ELECTRIC CELLS -P -

SCANNINGDISC

NEON GROUNDCRATER TUBE

ARCLIGHT

AMPLIFIER

40b/ER SUPPLY

GLASS

LENSES INSCANNING

DISC

TO WIRECIRCUIT OR

s*-- RADIOTRANSMITTER

FIG. G

1

Here a small pencil of light scans the object - the reflections arc picked up byphoto cells and transmitted to the receiver

6 Television

PLIGHT

MIRROR SCANNING OI.SC. EACHMIRROR PLACED AT A SLIGHTLYDIFFERENT ANGLE

Cr,

LIGHT

HORIZONTAL MIRROR DRUM FIG.8ALSO USED

Scanning has been done with flat discs andcircular drums fitted with mirrors

derstood that the television transmitterwas often connected to a radio sendingapparatus, and that the image signalswere picked up on a suitable radio re-ceiver, the wave -length used in thosedays being in the neighborhood of 80to 180 meters. Today we are usingwaves only 6 to 7 meters long, becausethey furnish a much wider frequencyspectrum.

William H. Priess and other inven-tors have frequently advocated vibra-ting mirrors to do the scanning - seeFig. 7. One scheme shown utilizes tun-ing forks for vibrating the mirrors, onein a vertical direction and the other inthe horizontal. Priess has invented aclever device for vibrating a single mir-

ror in two directions simultaneously -whether it can be used to scan a 441(or higher) line image remains to beseen.

Mirror Scanning

Mirrors have been used widely forscanning in television. Fig. 8 shows ascanning disc made with a series of mir-rors arranged around the edge of thedisc, each mirror being tilted a frac-tion of a degree more than itsneighbor, so that in one revolution ofthe disc the reflected rays from the mir-rors will have completely scanned theimage. In some cases the mirror drumdesign was employed, as below. Themirror screw (see Fig. 9) was devisedin Europe, and the image was observedby looking directly at the rapidly re-volving screw. Each mirror was set ata slightly different angle, so that in one

Y,

IMAGE SEENIN SPINNINGMIRROR SCREW

SPIRAL Of GLASSPLATES EDGES AREMIRRORS

EACH \\NMIRROR AT \DIFFERENTANGLE

FIG 9

The mirror screw, used by some televisioninvestigators

Television 7

LOW -SPEED SCANNER THIS PART OF PATH LIGHTED --- 0CWHILE SPOT -C- SCANS A , SCANNINGACROSS PATH

SPOT\\

SCREEN

ILmAIARGO; 114 ZIIISCANNERSPEEDHIGH

CRYSTAL ,k,.

RTZ Ij,.-°

TO TELE,/ RECEIVER

.---} RECEIVER

FIG 10

C:. --'-- INI'

WERSONIC CELL __-(STORES LIGHT )

-..*--... -.

SUPERSONICLIGHTCONTROL 5COPHONY 51'57E11

"2In the Scophony television receiver, a light -storage cell causes the scanning spot to

persist for quite a period of time, giving a much brighter image

rotation of the screw, every path acrossthe image had been scanned. If therewere 60 mirrors then each mirror wasset or ground to cover 1/60th of thevertical angle of the image. A specialneon lamp with a long plate or heaterwas used to provide a narrow targetof light equal to the height of thescrew.

Scophony SystemThe Scophony optical scanning sys-

tem represents one of the most noveland radically different television systemsthus far devised. In practically all otherscanning systems, the only light ob-tained, at any given instant on thescreen, is that yielded by the focussedspot of the cathode beam for exa.nple- a tiny spot less than one fiftieth of

an inch in diameter (on an image 8inches high). Now suppose a plan wasdevised whereby over 200 times this a-mount of light was delivered at a giveninstant! This is what the Scophony ideainvolves - and more than that, thisEnglish invention provides a picture 20by 24 inches on a home type machine.

The two new features of the Scoph-ony system are the split focus and thesupersonic light -valve control. The dia-gram (Fig. 10) shows also the mirrordrum used as part of the scanning sys-tem. The split focus is an optical line-up of cylindrical lenses with their axescrossed, so that a beam of light is fo-cussed in two separate planes. One ad-vantage for optical scanners is that theycan be of much smaller size or, withthe same size of scanner, a far greater

8 Television

PHOTO rCELL\ A

-/rn

le

fel

A

MASK

0 LAMP

DISC MOLE *SCANS ACROSS FILM

FILM MOVES DOWN. GIVINGCOMPLETE SCAN IN I NEV. Of DISC

FIG. 11

In scanning motion picture film, the arrangement shown above has frequently

been used

aperture of the optical system can beusefully employed.

With this improved scanning systemany source of powerful light can bemodulated by the supersonic light con-trol, which consists of a container filledwith a liquid, at one end of which isplaced a quartz crystal. When thequartz crystal is actuated by a televi-sion signal, the mean carrier frequencyof which is the same as that of thequartz, supersonic (super -audible) wavesare set up at a speed corresponding tothe velocity of the sound waves in thatparticular liquid. The lens system per-mits passing light through the containerin a direction transverse to that of thesupersonic waves; by means of scannersand suitable lenses an image, of theilluminated light -control itself, can beformed on the screen, the width beingof one line of the picture, and thelength determined by the length of the

light -control liquid column. A remark-able feature of the supersonic cell isthe fact that light is stored up in it,and light from a powerful source isthus caused to illuminate the pictureelement being scanned at a given mo-ment, while in addition many other ele-ments in the train along a given scan-ning path will be illuminated too, pos-sibly half of the scanning path, thusincreasing the total controlled light ofthe image many times (200 times ormore).

Other advantages of the Scophonyapparatus are the small amount of radiofrequency (television) signal required tomodulate the quartz crystal on the lightvalve, and again - the simple type ofreceiver used - an 8 -tube T.R.F. setfor the image and a 6 -tube set forsound.

Movies for Television

Fig. 11 shows how a motion picturefilm may be moved by the scanningdisc (if a disc is used) in a continu-ous. manner. It will be seen that therevolving disc provides the horizontalscan and the moving film the verticalscan. With the modern cathode-rayelectronic scanning, the motion pictureis flashed on to the photo -electric platewithin the iconoscope; the tube thentranslates the variations in the light anddark parts of the picture into correspon-dingly varying photo -electric currents.

One of the schemes employed by sev-eral well-known television engineers todemonstrate large image television in-volves the use of the famous Kerr cell,which acts as a light valve when mod-ulated by a television signal (See Fig.12). Here we may again use anystrong source of light and pass itthrough a diaphragm and a pair of Ni-

Television 9

col polarizing prisms on to a whirlingscanning disc. This disc is similar tothose already discussed and is sometimesfitted with a spiral of lenses.

The Kerr cell is a small glass jarcontaining a grid made of brass or othermetal and immersed in a nitrobenzolsolution. (This is highly inflammableand should he carefully watched: also,if discolored, it should he redistilled orelse replaced with fresh solution.) Aprojecting lens is used to throw the pic-ture onto the screen. In setting up theapparatus one of the Nicol prisms isturned until no light passes through theoptical train: then, when a televisionsignal modulates the Kerr cell, a changein the polarizing effect takes place,which causes the light beam to betwisted so that light once more passesthrough on to the screen. The amountof light is in proportion to the strengthof the modulating signal (this in turnis in proportion to the degree of lightor shade on the particular spot beingscanned at the moment).

The efficiency of this system is quitelow, and the scanning disc makes itunsuitable for modern 441 -line defini-tion (in previous demonstrations where

C,®0 ORO 0!) (04fP) to0 ®000000000000 \

flON )LAMPS

BRUSH MANESCONTACT WITHONE PIN AFTERANOTHER

FIG 13

TELE*.RECEIVER

111' "II!

\.COMMONWIRE

MODULATiONCURRENT

SYNCHR ("NousMOTOR

TINFOI

I.01E.!D(NEON TUDE

4 -

TINFOIL SEGMENTSALONG COMMON NEONTUBE MAY BE UJEO

In this "big image" television reproducer,sections of a neon tube are progressively

illuminated

this system was employed the scanningwas done with 60 lines or thereabouts).

A plan for producing large televisionimages - and actually demonstrated bythe Bell Laboratories a decade ago -is that shown in Fig. 13. Here the scan-ning was accomplished in a very in -

STRONG KERR CELL

1.1

A

( DIAPHRAGM

SEAM OF LIGHT PASSESTHROUGH THE PLATESCARRYING TELEVISIONSIGNAL CURRENT

TO TELEV. RECEIVER

NITRO BENZOLSOLUTION

`LENSES

WHIRLINGSCANNING DISC

N NICOL PRISMSSCREEN B

FIG. 12

A light valve using a chemical solution is shown above, in connection with a"large image" television apparatus

10 Television

ONE MODERN WAY TO GETLARGE IMAGE HIGHVOLTAGE -EXTRA BRIGHTIMAGE' CATHODERAY TUBE

Air. MAGNIFYINGLEN5E3

FIG. 13 a

One of the more recent ideas for largetelevision images employs magnifying lenses

in front of a cathode ray tube

genious manner. Instead of using ascanning disc, a rapidly revolving com-mutator switch was employed at thereceiver. As the switch blade made cir-cuits with the successive contacts onthe switch, successive tinfoil segmentsspaced along a neon tube were progres-sively excited. This had the same effect

as if light from the holes in a spiralin a scanning disc were being thrownonto a screen. Baird (in England) andothers used a similar idea, but used abank of small lamps which were pro-gressively lit up as the scanning pro-gressed. A scanning disc of the orthodoxpattern was actually employed at thetransmitter, and needless to say, theswitch controlling the lamp bank wasdriven by a synchronous motor; so thateach lamp (or section of the neon tube)was switched into action just as eachrespective hole of the disc at the trans-mitter was passing before the scanninglens aperture.

The high -power (20,000 to 30,000 -volt) cathode ray tube seems to be thepresent hope of many television inven-tors for big -image presentation (Fig.13-A), but the apparatus is costly andsome scheme like Scophony's seems tohold great promise for large theater -screen television.

(7:1

Television 11

THE KINESCOPE OR CATHODERAY TUBE FOR RECEIVING

THE IMAGEScience often works out a problem

for which there seems to be no imme-diate application - such was the casewith many of the vacuum tube experi-ments carried out late in the last cen-tury, by such men as Zender, Crookes,and others. The marvellous cathode-ray(C -R) tube, which is reproducing suchfine television images today, is basedon a discovery made years ago that,when a cathode ray was generated orprojected in an evacuated tube, thisray could be deflected by a magnet heldnear the tube (See Fig. 14). It was also

The action of the cathode ray tube dependsupon the fact that the electron beam pro-duced inside the evacuated tube, may bedeflected in any direction by a magnetic

or an electrostatic field

TO TELEV.RECEIVER/

-BIAS/

HEATEDCATHODEEMITSELECTRONS

FOCUSSINGCYLINDER

FLuORE5CENT SCREEN

CATHODERAY VERTICAL

DEFLECTINGPLATES

HORIZONTALDEFLECTING

PLATES

HIGH VOLTAGE 4 -ANODE )

FIG 14 a

ELECTRONEMITTINGCATHODE

SASE

HEATERFILAMENT

INTENSITYCONTROL

GRiO

FOCUSSINGANODE FLUORESCENT

SCREEN")HIGHVOLTAGEANODE

VERTICAL &HORIZONTALDEFLECTING

PLATES

SPOTOF

LIGHT

`GLASS

/METAL /LATES

SOURCE OFCATHODE

RAYS

MAGNET

DEFLECTEDCATHODE RAY

BEAM

FIG. 14

SPOT OF 0LIGHT

RETURN,- 1TO

START 7

7.7

INTERLACEDSCANNING

45

8

2 2 Q

LEACH -

12 Television

3LOW00WWNOTION

RAPID"HORIZONTAC

MOTION

(*ON

TELEV.SIGNALT. a.

A

SWEEPPLATES

A

1,1

HORIZONTALSWEEP

B RETURNor SWEEP

CATHODE RAYRETURNING

T. S.

VERTICALSWEEP

LAST (4415T .)LINE

AA

11,111I1'1111111 A1

-TIME -

LINE Nat

55 RETURNOF SWEEP

orAov, %oo AA FIG. 14 b

The action taking place during the scanning of an image on a cathode ray tubeis shown in the above diagram

The electrical control impulses deflected in the beam are timed to give quick returnof the beam after each scanning sweep

discovered that, if an electric chargewas put on a metal plate arranged inthe tube near the path of the cathoderay, it likewise would deflect theray one way or the other, dependingupon the polarity of the charge. (Thesame holds true for the magnets - anorth pole having the opposite effect toa south pole on the ray.) It at oncebecame apparent that, if we apply analternating current to a pair of deflec-tion plates within the C -R tube, weshall cause the cathode ray to oscillateback and forth in exact step with thealternations of the current. (See Fig.14-A) This property you may have no-ticed if you have ever observed theC -R oscillograph now used by radioservice men to align modern receivers.

Not only does the cathode ray haveno inertia(practically none), enabling itto respond to almost infinite frequen-cies, but scientists have found that, ifa metal sleeve of the right shape andcarrying the correct electric charge, is

interposed in the path of the cathoderay, the electrons composing the raymay be focussed similar to light rays.Fig. 15 shows this focussing effect andalso an early experimental C -R tube,in which the cathode ray was causedto project an image of a metal crosson the end of the tube. In the moderncathode ray tube as used for televisionreception, the end of the tube has acoat of certain chemicals depositedover its inner surface, so that wher-ever the cathode ray impinges on thiscoating, it causes a fluorescent spot toappear. The higher the voltage appliedto the anode in the tube, the brighterthe spot on the screen. In some roses,a series of anodes are placed progres-sively along the tube and by connect-ing these with higher and higher vol-tages (D.C.) the velocity of the elec-trons is tremendously increased.

Fig. 14 shows the principal elementsof the cathode ray tube. The heatedcathode generates the cathode ray: the

Television 13

focussing cylinder does what its nameimplies, the amount of focussing beingdependent upon the degree of negativebias applied to the cylinder. Note thatthe varying modulation current fromthe television receiver is applied to thisfocussing cylinder also (or else to agrid member), for the purpose ofchanging the strength of the ray inaccordance with the light and dark por-tions of the image being scanned.

For television purposes a sawtooth(named from the shape of the wave)form of scanning wave or current isapplied to the respective pairs of verti-cal and horizontal plates for this pur-pose. The purpose of the sawtoothwave for scanning is to give a slowprogressive sweep of the cathode rayacross the screen and a quick returnof the ray to its starting point. For in-stance, the ray moves across the screenon scanning path No. 1 (Fig. 14-B)as it reaches the end of this line thecondenser in the neon tube circuit dis-charges, and the ray suddenly flies back,to begin the scan along path (line) No.2. But in the meantime a secondforce has acted on the ray - that ofthe vertical scanning oscillator and thesecond scan begins, a fraction of aninch lower down on the screen. Event-ually the whole screen is scanned in aprogressive line -by-line manner andthis action - when repeated 16 to 24times per second - results in a movingimage being flashed on the fluorescentscreen at the big end of the tube. Thecolor of the image can be changed byusing different chemicals; the presenttubes favor a bluish -black, but manyC -R tubes show a greenish image. Theblack -and -white image seems to be themost popular now.

,CONDENSER

NEONTUBE

JAW TOOTHOUTPUT

RESISTANCE

5AW TOOTH -WAVE SHAPE OF CURRENTDESIRED TO GIVE RAPID "FLYBACK"OF SCANNING BEAN IN CATHODERAY TUBE

FIG. 16

Above - The wave form of the currenproduced by a neon tube oscillator ishere shown

Below - The principle of the electronlens for electron focussing in a cathoderay tube is shown

\GRIDFIRST ANODE

tcrOCVSStNG ELECTRODE

CATHODE

(LENS

,s

2ND ANODE

LENS

EQUIVALENT OPTICAL SYSTEM

EARLYCATHODE RAY

TUBE FIG 15

I4

SPOTOF

LIGHT

TRACEOP THE

MOVINGSPOT

ELECTRON SEAM

SPOTOP

L111147

TRACEOP THE

MOVINGSPOT

ELECTRON SEAM

FIG. 17

Further explanation of the movements ofthe cathode ray beam in making the hori-

zontal and vertical movements acrossthe end of the tube

The neon tube sweep oscillator, Fig.16, was one of the first, but a moreconstant type of oscillator is now fa-vored, using new type radio tubes ofthe gaseous type. Fig. 17 shows therespective scanning action of the ver-tical and horizontal traces or sweeps.Fig. 18 shows the action of the mod-em thyratron tube oscillator at A,and how the condenser action repeated-ly breaks down the gas in the tubeand then starts the cycle all over againat C. The saw- tooth form of the waveor current is seen at B.

For the larger cathode-ray tubes(above 5 inch diameter) the magneticscanning method is preferred, and thearrangement of the vertical and hori-zontal magnetic sweeps is shown inFig. 19 ; also an end -view of the mag-nets. These are sometimes made witha split iron yoke, so that they can beopened for the insertion of the neckof the glass C -R tube.

Television

It is interesting to note that the sizeof the spot on the target or screen ofthe tube remains constant in size, butvaries in brilliance as it is modulatedby the television signal. Another pointoverlooked by many students is thatthe cathode ray itself does not passbeyond the glass tube, or at least theeffective part we are concerned with ;and it reacts on the chemical screendeposited on the inner surface of thetube. If the image is projected beyondthe end of the tube, we really see ashadow picture projected on to thescreen, and not a cathode ray picture.

SynchronizingA synchronizing signal is transmitted

after scanning each line, as Fig. 20NO CURRENT

-ar Off LEGTINGTO

PLATES

-A-

TNYRAT RON

TODEFLECTING

PLATES

-C-G/1.5 BREMEN DOWN

POTENTIAL OF POINT P RISES DURINGTHIS INTERVAL

-5-3

POTENTIAL or POINT -10- DROPSNIZTANTANEOUSLY HERE

The wave form of the current and also thecircuit connections for modern type

sweep oscillators

Television

shows, and in this way the scannerat the receiver is kept constantly instep with the scanner at the trans-mitter. Note how the current fluctuatesup and down as the scanning beammoves across the object being viewed,in accordance with the lights andshadows on the image. At the start ofeach line, a stronger impulse comesthrough from the transmitter, and bymeans of a synchronizing filtering cir-cuit this signal is impressed on thetwo sweep oscillators and holds themin perfect step. Fig. 20 shows the fre-quency relation between the imagecarrier frequency and the soundcarrier.

Right - The arrangement of sweep controlor deflecting magnet, placed about theneck of the the cathode ray tube

Below - We see the relation between theimage and sound carrier frequenciesused in modem television

.15

IRONCORE

MAGNETICCONTROL FORCATHODE RAY

TUBE

NECK OF- C R TUBECATHODE RAY

VERTICAL

...i_HoRizONTAL FIG 19

SYNCH. SYNCH.IMPULSE

(IMPULSEPICTURE BLACKERSIGNALS THAN

BLACK'

FBLACK

.J GRAYSO coo "" ETC.

O

Ei WHITES

SCAM-'

en ey

SCAN

RE TURN

1/60SEC

67;-0

(RAMO)sEc RETURN /

D C7)ONE LINE ---"1 BACKGROUND

LOWER1E.-TIME . LEVEL

pti I

SIDE BAND SOUND

PARTIALLYSLIP PRESSf -

(CARRIER F REG%0 RETURN

73.-`.5 TIME (RAPID)Tlic7T`i ,/132,000 SEC

1.,j,(61

I3, 200SEC.MC. FOR

PICTUREh4

-4 I. KC SAW TOOTH WAVE FORMSAND RETURN

(I-4 PK 6-4;.25EC .49.26/1C, 49.75ME50 MC) FIG 20

TUNER

MT. DET.

16Television

VOICE L

RECEIVED ONAMP. Del (AMP. .11(1:IMAGE I.F. NO. AF.

FREGUENCE5 MIRRORDIFFERENT

{Mr I .11 1011

Mgt C. R. TUBE

MAGNETCOILS

OilVIDEO

"COMMON "OSCILLATOR I,,F

SYNCHRONIZING LINK

SOUND

OSCILLATORS TOCAUSE BEAM

TO SCAN

rTCRUSE. MIAROR

MAIMTUNING

AUX.CONTROLKNOBS

SPEAKERV.T. POWER SUPPLY ETC. FIG 21

The general arrangement of the sound and image receiver components are here illustrated

A typical television receiver circuit standards, it is possible to use a com-

b shown in Fig. 21. As the spacing mon single oscillator for both the image

between the picture and the sound and the sound receivers, as the diagram

carriers is set definitely by the R.MA. indicates.

Television

TELEVISIONreceiver Antennas

17

TWISTEDPAIR

cAeLr

TELEV.REC-

.3" SPACE

5 ARMS

INNERWIRE

ARMS OF DOUBLETEACH SFT LONG(QUARTER WAVE)Va To 416 BRASS

TELEV. RECEIVING AERIALS

)OUTERTUBE

INSULATORS

COAXIALCABLE

TRANS. -.

REFLECTOR ROO 10.1.0N9(NO CONNECTION WITWO'l

INSULATORS

WOOER STANDCAN SE ROTATEDFOR BESTRECEPTION FIG.22

The picture shows how the television receiving doublet is arranged "broadside" to thetelevision transmitting station. How the signal can be intensified by the use of areflector rod is indicated at the right of the picture.

The accompanying sketches (Fig. 22)show several designs of television re-ceiving antennas - the simplest is thedoublet having two horizontal arms,each about 5 feet long. The doubletis placed broadside toward the trans-mitter, and should be moved about un-til the clearest and brightest image isobtained. In some locations, due to

reflection from other buildings, theremay be several waves received at agiven point (multiple path). Thus, ifa ghost (double) image is seen on thescreen, then the aerial should beshifted about until the ghost imagedisappears.

A twisted pair of rubber -coveredwires may be used to connect the

18 Television

4'ik '41/p:,\O

HOW DOUBLET GETSSIGNAL IN POCKET

SOME POSITIONS PMAY BE POOR.TRY OTHERLOCATIONS CGIDooD

REFLECTED 11G 23WAVES

The ultra short waves used in televisiontransmission are reflected between tall buil-dings as shown, and the best location for

the antenna is found by experiment

doublet to the receiver; but, for dis-tances of over 75 feet, it is preferableto use coaxial cable. (See Fig. 22-B.)The aerial may be placed at quite adistance if coaxial cable is employed.If interference from automobile igni-tion is experienced with the aerial atthe front of the house, put it at therear of the building. It is best to getthe aerial as high as possible to min-imize pickup of interference from man-made static, electric light wires, etc.,and also because of the line -of -sighttransmission properties of the ultra -short waves used for television. Wherethe signal is weak, it may be intensifiedby placing a reflector rod a quarterwavelength (about 5 feet) behind the aerialas the sketch shows. The metal rodsof copper, brass or aluminum can bemounted on porcelain or glass insula-tors and all connections should be sol-

dered. Don't forget to slowly rotatethe aerial, to ascertain the best posi-tion for strongest reception.

Fig. 23 shows how a reflected wavemay account for the reception of atelevision signal in a location hemmedin by high buildings. The multiple re-flections of these waves often producethe difference between failure and suc-cess. The strongest signal may not al-ways be the best signal: move the aerialaround - sometimes a few feet willmake all the difference imaginable, dueto multiple reflections. The aerial mayhave to be raised up a few feet toget the best signal. Also try puttingthe doublet on an angle, as shifts inthe polarization of the wave may de-mand this. If you live on a lower floorof a high building, try to get the aerialon the roof and run the lead-in downthe side of the building, as Fig. 24shows. Apartment houses will event-ually have a common doublet or othertype of television receiving aerialmounted on the roof, with coaxialcable feeding the signal down throughthe floors to the various apartments.Of course, you can try using the aerialdoublet inside the apartment -a num-ber of experimenters have bad sur-prising success with indoor antennas.

Many people have asked the ques-tion -"Can I obtain a converter topick up the television image signalon my present broadcast or all -wavereceiver?" The answer, in general, isemphatically NO! The reason is thatthe image receiver requires extremelybroad amplifier stages to pass the imagesignals, and this requires special inter-mediate frequency transformers, etc.,all of which cannot be done in a simpleconverter. However, the televisionsound channel can be picked up on a

Television

1-11111

DOUBLETAERIAL "

TWISTEDPAIR ORCOAXIAL

CABLE

AERIAL ONRION BUILOIN6

T. TELEV.. OUTLET

. edY "'MN'

Apt o

AERIAL FORAPT. HOUSE

FIG.24

For apartment houses the receiving an-tenna is best placed high above the roof

converter, and one may be built fromplans given in the television magazinesor else purchased on the market. (SeeFig. 25).

The television sound channel can alsobe picked up on a regular short-wavereceiver, capable of tuning to the properfrequency. Some of the modem all wavereceivers tune down low enough tointercept the television sound channel,and in this case the acquirement of atelevision image receiver is all that yourequire, in order to enjoy the completetelevision program. Many of the smallHam type receivers will tune in thesound channel too. However, the broadfrequency bands allotted to sound intelevision broadcasting, give excellentquality to the sound picked up onvision programs, and if you wish to

19

- REC. DOUBLET

A. & G. POSTS ONS.W. OR BROADCAST

RECEIVER

U.5 W CONVERTERfOR TELEV."SOUND'CNANNEL ONLY

coven/TAERIAL

110 V. Ae

SPEAKER

TELEV IMAGE RECEIVER)C.* IMAGE 'MOE

FIG. 25

An ultra short wave converter may ba usedfor the reception of television "sound'

fully enjoy this superior sound quality,it will pay you to, obtain the bestquality receiver possible. Otherwise thebeautiful high-fidelity feature of thetelevision sound channel will be lost,to a great extent, if poorly designedreceiving circuits and a low -qualityloud -speaker are employed.

One of the latest innovations in therealm of television receiving aerials, isa loop aerial. This is still in the researchstage but the experimenter will wantto try it out. The two halves of theloop are connected together through ahigh resistance of about 900 ohms. Byplacing the aerial in different positions,the best one for a given location isreadily determined. The dimensions ofthe loop vary somewhat for differentfrequencies.

20

HOW TELEVISIONPROGRAMS

ARE BROADCASTA few years ago, the subject to be

televised was "viewed" by a scanningdisc with a strong beam of light sweep-ing over his face: today the subjectmerely stands before the televisioncamera and he hears no sound of whirl-

iCONOSCoPE

PHOTO ELECTRIC MOSAIC PLATE

AMP1

TOTRANS-MITTER

DIODULATOR

IMAGE REVERSED

GLASS/ TUBE

OBJECT

LENS

Ills

CATHODERAY

MAGNETSTO SWEEPCATHODE RAYACROSS PLATE

FIG 26 so.

Pi

GUNHEATER

Simplified arrangement of the image pickup tube, used at the transmitting station

Television

ing discs or motors. Strong lights il-luminate the actors and the image ispicked up by a lens the same as in acamera; only in the Iconoscope (as thetelevision camera is called) the imagefalls on a special mosaic plate. Thismosaic plate is made up of thousandsof tiny photoelectric cells and, everytime a spot of light strikes a spot onthe mosaic plate in the evacuated tube,an electric current proportional instrength to the light is developed by theiconoscope tube. See diagram (Fig. 26and 26-A).

The image is thrown on the mosaicplate upside down, as in a camera;next, we find that the scene is scannedwith 441 lines by magnet coils placedabout the neck of the iconoscope tube.One set of coils causes the cathode rayinside the tube to sweep across thescreen horizontally, while the second setof coils acts on the ray to sweep itup and down vertically thirty times asecond. At the end of each horizontalline sweep, a synchronizing signal issent over the radio transmitter; andthe reception of this signal at the re-ceiver serves to hold the sweep oscil-lators in step or synchronism with thetransmitter.

Television

New light -weight television pick-upunits have been perfected by RCA andothers, so that all of the amplifiers,cameras. etc., can be packed into sev-eral bags or portable cases, and carriedabout by a small crew of men. In manycases the mobile television pickup unitis plugged into the nearest speciallycompensated telephone or other circuit.In one demonstration the televisionimage was sent over a telephone circuitabout one-half mile in length, the cir-cuit having been carefully balanced asto its capacity and inductance beforehand. There is a far greater loss ofsignal strength of course on such tele-phone circuit transmissions of televisionimages, compared to the special coaxialcable designed by the Bell TelephoneLaboratories. In the television trans-mission between New York and Phila-delphia the images are sent over thecoaxial cable installed between the twocities by the Bell Laboratories.

Television relay apparatus so com-pact and self-contained that it can bemounted on poles located 25 to 30miles apart, has been perfected by theRCA. This relay system will eventuallyuse frequency modulation (F -M) andin fact in some of the test demonstra-tions F -M was used. This will obviatethe effect of static and other undesir-able factors, which show up in the tele-vision image at the receiver in the formof white splotches or streaks.

Eventually, all of our large cities willbe linked by ultra short wave relaytransmitters. The relay station has areceiver tuned to pick up the image asbroadcast, and the amplified vision sig-nal is fed right back into a transmitterand sent on its way again, usually on adifferent frequency so as not to interfere

21

with the original transmission. Each timethe signal is retransmitted the frequencyis slightly changed, for the reason afore-mentioned. The relay apparatus, or atleast its antennas, are placed on steel orwood masts as high as possible, to givethe maximum range. The higher the an-tennas the greater the range. A recentdemonstration of television pickup byan airplane in Hight, showed what thefuture of television in warfare (as wellas peace) may be like. The airplanecarries a television camera and a smallcompact transmitter, with which itsends out the image signals to a specialpick-up station; this station relays theimage signals to the television broad-casting station, from which the imageand sound is telecast to the receiverslocated in your home and mine.

Where desired the plane can have avision receiver aboard and pick up theimage as broadcast from the televisionstation. Images of the enemy terrain,gun emplacements, etc., can be trans-mitted back instantly in war -time, andcan also be transmitted to other planesif necessary. In fact, by means of tele-vision, it becomes possible to send outrobot bombing planes (with no oneaboard) ; the directing officers behindthe lines send out radio impulses whichcause the plane to rise or fall as de-sired, and at the crucial moment releaseits load of bombs! The effect of thebombing can be seen at once - thanksto television.

The staging of television plays andthe handling of speakers, etc., is quitedifferent than the technique followedin regular stage and radio broadcasting.The television studio is as quiet as agraveyard when a play is ready to goon the air. The play director speaks his

22 Television

ICONO5COPE

LENS

BATTER Y

TO TRANSMITTER

LIGHT--..,... CONDENSER FORMED ON.......

SCREEN -- MOSAIC SCREENMICA

SCANNINGCOILS

CAESIUM

REPRESENTSEACH CELL OfTHE MOSAIC

METALPLATE

CATHODE RAYBEAM

C.R.. CATHODERAY

-

TELEVISIONPICK-UP CM1ERA

a+

FIG. 26 aAnother view of the television pickup tube and first amplifier stage

"cues" into a microphone in a closedroom provided with a glass window,through which he watches the actors.The stage manager down on the studiofloor hears his cues and) other correctionorders in a pair of phones worn on thebead. As the play progresses, we seethe camera men (or rather their assis-tants) pushing the rubber -tired cameradollies in and out, in order to catchclose-ups, long -shots, etc., as the di-rector orders. These actions are re-hearsed in the same way that stageplays are rehearsed. At present theactors are viewed in small groups, butas the optical range of the camera andinconoscopes improves, the groups ofactors viewed at one time will growlarger and larger.

In picking up the image of footballand baseball games, a great deal ofimprovement in technique is still to beworked out. One camera is not enoughand a second or third camera is re-quired, so that close-ups of the actionat certain spots can be focussed on and

transmitted to the waiting televisionaudience. If a baseball player slides toa base during an exciting play, the tele-vision camera should be there to catchit, and place it on your television re-ceiver screen ! The technique of tele-vising such sports is now being greatlyimproved by the engineers, and thesports telecasts are rapidly becomingmore popular with those owning tele-vision receivers.

In producing plays and special "newsevents" in the studio, some very inter-esting devices are resorted to in orderto give continuity to the action beingportrayed. For example, if a navalbattle is to be shown, it is impossibleto show the actual battle, so the tele-vision experts resorted to an old movietrick - they used miniature models ofwar ships. When skillfully handled, withsmoke pouring from their funnels andguns blazing, you - the televiewer, sit-ting before your receiver, can not tellbut what it is the real thing! The housesand other figures are cleverly built by

Television 23

MOSAIC PLATEOBJECT jag*

.11111100'"- AMPLIFIERLENS

<.,5wEEp.

CAMERA

\ SWEEP COILS

ToTRANS.

I *I

HORIZONTAL DEFLECTIONSYNCH GENERATOR

OSCILLATOR GPOWEP AMP.

MODULATOR

ICONOSCOPE

VERTICALDEFLECTION &SYNCH. GENERATOR

SOUND ANTENNAII

'SOUNDRA010

TRANSMITTER

,IRAN S.ANTENNA

_tI \

C

FIG. 27

At the transmitting station the image and sound pickup units arelined up in this manner

artists and painted so faithfully, thatwhen you see the whole ensemble -ships moving about, etc., you are com-pletely fooled - and, let it be said totheir credit - completely entertained.

Another trick of television broadcast-ing is the method of rapidly flashingfrom one scene to another. A closeup ofa box of matches mysteriously openingitself can be staged easily; also suchweird effects as candles that snuff outthemselves, glasses that empty them-selves, thanks to a glass tube fittedthrough the bottom of the glass, butwhich you can't see on the televisionscreen of your receiver, etc., etc.

One of the most ingenious machines

imaginable produces dissolving titles andall sorts of fancy designs to serve asentertainment between scenes.

One of the diagrams (Fig. 27) showsthe simplified line-up of the oscillators,the amplifier, modulator and finally,the transmitting narillator and poweramplifier.

The sound is picked up by a micro-phone, supported above the actors ona boom, and the sound currents areamplified in the usual manner; thenthey are passed into a modulator andradio transmitter, finally being led tothe aerial and broadcast on a slightlydifferent wavelength than the image. The

24

R. M. A. television standards committeehas fixed the ratio between the soundand image frequencies, so that onetuning control can be used at the re-ceiver to tune in both the image andthe sound. Furthermore, a single os-cillator can he used for both theimage and the sound sections of thereceiver.

A typical television broadcasting sta-tion set-up is portrayed in Fig. 28. Inthe studio the actors are brightly il-luminated : the control room is placedto one side and higher up than thestudio floor. The director usually sitsin the control room, and all ordersarc given to the stage manager andthe camera men on the studio floorby telephone lines connecting with

Television

headphones worn by these men. Severaltelevision monitors are arranged be-fore the sound -and -image control ex-perts in the control room and the di-rector can cooperate with these mento produce the best possible show.

"Spot news" may be picked up bya mobile camera unit as shown, andthe image as well as the sound sig-nals is picked up by a suitable receiverand then fed into the coaxial line lead-ing to the transmitter. The televisiontransmitter is placed as high as possible,in order to reduce losses and to getit as close as possible to the antenna.In the case of the N. B. C. installationthe antenna is placed on top of the1250 -foot Empire State building, andthe transmitter on the 85th floor of

SCENERYACTORS

LIGHTS

TWO ORMORECAMERAS

HOME TELEV--- RECEIVER

MIKE

STAGEDIRECTOR REL

9W/TCNBOARD

NCONTROLDESKS

DIRECTOR

P. HEADPHONE CONNECTION WITHDIRECTOR IN CONTROL ROOM

SOUND & IMAGE ANTENNASAS HIGH AS POSSIBLE

TELEVISION TRANS.NEAR TO ANTENNA

POWER ICAR

CABLE

,TELEV. TRUCKS

ICONOSCOPE

rr

MIKE -) LCAMERAtt,a-o

f

MOVIECAMERA

FIG 26"Spot news" is often picked up by a mobile television unit, connected to thetransmitting station by an ultra short wave "link"

Television 25

CONTINUOUSor LTFILM

ICONOSCOPE

MOBILE UNIT

COATS FILMWITH EMULSION

30 SECONDS

MOVIECAMERA

SCENE

SHORT WAVE LI NK TOPICK' UR STATION )

RECEIVER LAMPLIPIER

CLr HOMETELEV. SET

EV. TRANS.

rrtr

flG 25

In this "spot news" system images are photographed on a film, rapidly developedand dried, then transmitted via radio

the building. The studio is in theR. C. A. building over half a miledistant. The mobile truck can transmitthe spot news or other programs pickedup by its cameras for a distance of sev-eral miles if need be.

These mobile units are, in the caseof N. B. C., arranged in two sections -one is the power supply unit, and theother truck houses the radio trans-mitters for sight and sound. The truckscan be joined together by quick coup-ling cables and' they can hasten to thescene of a disaster or other event withfire truck speed. The crew erects thespecial antenna mast and gets out theparabolic mike reflectors and theiconoscope camera in a few minutes.

For studio movie pickups, the motionpicture is flashed onto the iconoscope

camera and the sound taken off sepa-rately and sent through its respectiveamplifier to the sound transmitter.

The Intermittent Film System hasbeen used in Europe for reporting "spotnews" and is quite interesting as thefilm can be developed, washed andfixed - all in 30 seconds or less. Fig.29 shows this system: in one methodthe film is a continuous link or loop,as fast as the picture is flashed on thetelevision pickup, it keeps movingalong until it reaches the emulsion re-mover; then it goes to a device whichputs another coat of emulsion on itand the film is ready to pass throughthe standard movie camera and registerthe next scene. In other systems, thefilm is rolled off a stock spool and isnot recoated in the mobile truck.

26 Television

COLOR TELEVISIONBY SCOPHONY USES

THREE SPECIAL TUBES LARGE screen projected televisionpictures have been made possibleby the Skiatron, a new develop-ment of the Scophony system, describedin Electronics and Television & Short -Wave World, London. The process isdifferent from the more usual methodin that the subtractive method of colorseparation is employed). The accompany-ing illustration shows the path of thelight beam from the source of illumina-tion to the screen. Three complementarycolor screens on each of which a mo-mentary color deposit is produced areutilized.

A complete description of the systemfollows as taken from the British pub-lication.

Three cathode-ray tubes 1, 2 and 3are provided, each comprising a trans-parent image screen indicated at 4, 5and 6, respectively. Each screen is sit-uated in an electric field provided bypairs of transparent electrodes 7, 8 and9, across which is maintained a poten-tial difference by means of a sourceof potential 10. The screens can alsobe maintained at a suitable temperatureby means of a thermostatic control ofsuitable form, which in the example

shown consists of a source of current11 for each tube which passes a currentthrough one of the electrodes 7, 8 and9 of each screen, the amount of thecurrent and hence the temperature foreach screen being controlled by athermo-couple 12 in each screen. Meansindicated at 13, 14, 15 are associatedwith the tubes for producing a scanningcathode-ray beam and the amplifyingarrangement 16 is fed by signals repre-sentative of each of the primary colorsof the object being transmitted.

The sense of the modulations appliedto the beam is such that the densityof the deposit produced is inverselyproportional to the intensity of the cor-responding primary color. The threebeams are caused to traverse the threescreens 4, 5 and 6 in synchronism bymeans of scanning coils 13a, 14a and15a.

The material and/or temperature ofthe screens are chosen so that the col-ored deposits produced therein havethe required complementary colors.White Light from a suitable source 17is projected successively through thescreens 4, 5 and 6 in such a way that

Television 27

I0

3

22

2,

8

21

13

20

717

12

19

"?a.), 11

18 a

Three color filters operate from incoming signals in new system

the screen images are superimposed inregister on a reproduction surface 18.This can be done by fully illuminatingthe first screen with a condenser system19 which also forms an image of thelight source 17 on a first projectionlens 20 situated between the first twoscreens 4 and 5. This projection lensforms an image of the first screen 4on the second screen 5 in register, andthe light is focused by means of a fieldlens 21 on to a second projection lens22 which forms an image of the secondscreen 5 on the third screen 6 in regis-ter. A field lens 23 focuses the light

passing through the screen 6 into a finalprojection lens 24 which forms an im-age of the third screen 6 on the repro-ductive surface 18, forming thereon thefinal color picture.

It will be noted that with the in-verting optical system shown, the imageon the screen 4 must be inverted, thaton screen 5 upright and that on screen6 inverted, so that the projection lens24 forms an upright image on the screen18. This can be arranged by applyingthe deflecting currents to the coils 13a,14a and 15a in a suitable sense or di-rection.

28 Television

network television NETWORK television, necessary togive vision to the radio sets ofAmerica, was successfully demon-strated at Schenectady, N. Y., in Jan.1940. The program, which originatedin NBC's New York studios, was pickedup in Schenectady homes, 142 airlinemiles away, thanks to the GeneralElectric's relay station, working in con-junction with its main transmitter atopthe Helderberg Mountains.

-

Although telecast programs had beenreceived at the Helderberg relay stationbefore in tests, it was the first time(Jan. 1940) that such programs wererebroadcast for the entertainment ofpersons in the area served by the localstation. Both image and voice were ex-cellent, in fact, equally as good as pro-grams originating in the Schenectadystudio, thus proving that networktelevision is practical. The 1940 New

ALBANY ' - .a

\\- ,;,01,:(fy

Artist's sketch, showing how programs from New York are relayed by General Electric'stelevision transmitter, as well as being fed to the transmitter from the

main studio in Schenectady. Left -1'2 miles to Schenectady.

29

grams is then relayed on a carrier waveof 156-162 megacycles from a smalltransmitting antenna to the main Hel-derberg station. This transmitter issimilar to the diamond -shaped one usedto pick up the programs from NewYork but is only 10 feet across ascompared with the 400 feet of the re-ceiving antenna.

At the main transmitter a dipole an-tenna picks up the picture part of therelayed program and feeds it to thetransmitter where the frequency is con-verted to the 66-72 megacycle level andamplified to 10 kilowatts.

The sound part of the program isrelayed from the receiving station tothe main transmitter by wire line. Thereit modulates a standard 10 -kilowattultra -high frequency transmitter, andthe programs are then broadcast fromtwo antennas above the transmitter tolisteners in the Capital (Albany) Dis-trict.

The relay station is located 129 airline miles from New York City andstands 1,700 feet above sea level, withthe rhombic antenna 128 feet above.The main transmitter is at an altitudeof 1,502 feet with 60 -feet antennasabove.

Television

York -Schenectady test and demonstra-tion marked the first time a televisionprogram was actually rebroadcast overany such distance and from a pointmore than a mile below the lineof sight.

By the use of the new relaying equip-ment, located 1.2 miles from the maintransmitter atop the Helderberg Moun-tains, 12 miles from Schenectady, tele-vision broadcasts from New York aremade available to Capital District resi-dents within the range of the com-pany's station W2XB.

The programs transmitted from NewYork City are received at the relay sta-tion on the 44-50 megacycle band bymeans of a rhombic antenna that resem-bles two diamonds placed end to end andsupported by four 128 -foot towers. Theprograms then pass through an ampli-fier, a part of the antenna structure,that increases the signal strength about20 times before entering a wire lineleading to the relay receiving stationlocated beneath the antenna. Here theradio signals from New York arechanged to sound and picture signals.

By means of a low -power 10 -watttransmitter the picture part of the pro-

A THEORETICAL PATH OF SIGNALS

Pooros or 'roLE,oluon MO NT BETWEENNEW YORK CT, ND 0 E RECEIVER 1 M NELOONIENOS

Sketch, showing curvature of the earth between New York and Schenectady and howthe relay receiver, although mounted atop a mountain, 1800 feet above ,sea level, is still

more than a mile below the line of sight from New York and 129 miles distant

30 Television

EFFECT OF ANTENNA HEIGHTON TELEVISION DISTANCE

THE accompanying chart providesan approximate method of quickly de-termining what the height of the trans-mitting and receiving aerials should beto cover a given distance in miles. Ultrashort wave transmission, in general,obeys the laws of light and while freakdistances are covered from time to timeon the five and six meter bands, theaverage range of an ultra short wave

11". "2W0 24007,4 2020

1610 1110.111111111M11111111111E

1360a 1060

;1% 620 0111111110.1111111111MIN11110111011110.111MMEN

ti416267

k Mk MIL 1

1.01111011 50 11111.NOME0:c

10.101..NEM 40 WOO. N7

ArT ONO.so

110.111011101

V'n-.NI§EHEac8HEIGHT OF Pt ECOVING AERIAL

ABOVE SEA LEVEL

Chart giving "line -of -sight" transmissiondistances

transmitter is 40 to 60 miles, when theantenna is placed at fairly high eleva-tion.

Here is a chart that can be used incalculating distances in miles when theheight of both the transmitting and re-ceiving antennas are known. As an ex-ample: find the height of the transmit-ting antenna and your receiving an-tenna. Follow over from the left margin

(transmitter height) to the vertical linecorresponding to the height of the re-ceiving aerial. The nearest diagonal lineis the normal limiting distance for thatparticular station.

That the ultra short waves proceed ina fairly straight line and form a tangentwith the curve of the ea.rth's surface,was proven in one television test con-ducted above the city of Washington,D. C., by the RCA engineers. No tele-vision reception from the transmitterin New York City was possible atWashington, a distance of nearly 300miles, but when an airplane carryinga television receiver went aloft to anelevation of several thousand feet, theimages were picked up at once. By thesame token, if we place a televisionaerial high enough, all sorts of longdistace reception becomes possible. Dur-ing 1938 the RCA television receivingstation located on Long Island pickedup the image being broadcast by theBritish Broadcasting Station in London,England. This was a freak performance,made possible by certain atmosphericconditions and shifting of the reflectinglayers of the upper atmosphere.

The reception of the New Yorktransmitter's television image signals inChicago by RCA engineers, is anotherexample of freak transmission. The im-age in this case was only held for about20 minutes; the engineers concludedthat it was received because of a spora-dic ionized layer at a certain criticalheight in the upper atmosphere.

Television SI

TELEVISION TERMSAMPLIFIER - In radio parlance usu-

ally considered as comprising oneor more stages of vacuum tube am-plification, in which the voltage orcurrent (or both) is amplified.

ANODE - The positively charged elec-trode in the cathode ray tube, alsothe positive terminal of a battery.

ARC - The effect produced when twoelectrodes such as carbons, connectedto an electric supply circuit areseparated, producing an arc or flamedischarge between the separatedelectrodes.

AUDIO FREQUENCY -A frequen-cy range falling within the range ofaudibility of the human ear andusually considered to cover 25 to16,000 cycles per second. Somepeople can hear sounds of muchhigher frequency.

BEAM ANTENNA - An antennawhich can be swung around so asto pick up the maximum amount ofenergy from the advancing wave.

BRAUN TUBE - The first cathoderay tubes were demonstrated at theturn of the century by Dr. FerdinandBraun, famous German scientist. Itwas the first device capable of show-ing graphically on a screen at theend of the tube just what was goingon in high frequency circuits.

CARRIER - The oscillatory wavewhich carries the television or soundsignal and which is modulated bythe signal.

CATHODE RAY - In the cathoderay tube a heated cathode producesa stream of electrons, known as thecathode ray, and its great value totelevision lies particularly in the factthat this ray can be moved back and

forth in any direction very rapidlywithout any appreciable lag. The raymay be acted upon by an electro-magnetic field or an electrostaticone.

COAXIAL CABLE - This cable wasdeveloped by engineers of the BellTelephone Labs. for carrying severalhundred telephone conversations sim-ultaneously, or for carrying televi-sion image signals. The cable com-prises a hollow tube, usually of cop-per, with a lead sheath and a smallwire is placed at the center of thetube, supported in that position bya series of small circular insulatorsplaced every few inches.

ELECTROMAGNETIC FOCUSING- The method of focusing a streamof electrons, such as the cathode ray,by causing the ray to pass throughthe field of a powerful magnet.

ELECTROSTATIC FOCUSING -A great many cathode ray tubesnow used for television image repro-duction employ electrostatic focus-ing -i.e., the cathode ray stream isconcentrated or otherwise changed bya high voltage electric charge placedon an electrode within the tube.

KINEOSCOPE - The name given tothe cathode ray tube when used forreproducing television images at thereceiver.

ELECTROSTATIC DEFLECTION- Many cathode ray tubes used fortelevision reception use electrostaticdeflection in contradistinction tothose using the electromagnetic re-flection. A series of four plates, ar-ranged in two pairs - placed at rightangles to each other, are fitted withinthe neck of the tube and by causingalternating charges to be applied to

32

these two pairs of plates, the cathoderay is rapidly deflected back andforth across the screen and thus"paints" the image on the screen atthe end of the tube.

FLUORESCENT SCREEN - T h echemically prepared coating on theinside of the large end of a cathoderay tube. When the cathode ray im-pinges on this chemical screen, itcauses a fluorescent spot to appear,the color of which depends upon thechemicals used. Green is a commoncolor seen on such tubes, while someof the later tubes produce a blackand white image.

ICONOSCOPE -The special vacuumtube somewhat resembling a cathoderay tube, and which is used in thetelevision camera to pick up the im-age at the transmitter. It was in-vented by Dr. V. K. Zworykln.

INTERLACED SCANNING - Amethod of scanning which helps toeliminate flicker and in which thepicture is scanned not in progressivepaths, such as 1, 2, 3, 4, but 1, 3, 5,etc., on the first scan, and 2, 4, 6,etc., on the second scan.

KERR CELL - A light modulationvalve using a special solution, suchas nitro-benzol, and having two ormore thin metal plates separated aslight distance and between whichthe beam of light to be modulatedis passed. The incoming televisionsignal is connected to the metal platesin the cell and thus modulates thebeam of light passing through thecell by electrostatic reaction on thelight beam.

MAGNETIC DEFLECTION - SeeElectrostatic deflection.

MICRO WAVES - Very short radiowaves, usually considered as those

Television

having a wavelength shorter thanone meter.

MIRROR DRUM OR DISC - Pre-viously used by television experi-menters for the purpose of scanning.

OPTICAL LINE-OF-SIGHT-Refersto the fact that a person on a highbuilding can only see to the horizon,not beyond; the ultra short waves,in general, follow this law of opticsand thus the range of a televisiontransmitter is limited by this phe-nomenon.

PHOTOELECTRIC CELL- Usual-ly an evacuated glass tube containingtwo or more electrodes, one of whichis coated with a chemical sensitiveto changes in light. Fluctuating lightimpressed upon such a cell causescorresponding fluctuations in theelectric current, controlled or gen-erated by the cell.

REFLECTING LAYERS - Refers tothe various ionized layers in the up-per atmosphere which reflect radiowaves, the angle and degree of de-flection depending upon the frequen-cy of the wave.

SCANNING - The process of breakingup the image into paths or stripswhereby the cathode ray, for ex-ample, scans the image - path bypath - in a progressive manner, un-til the whole image has been com-pletely scanned. The whole processis repeated many times per second,similar to a motion picture, at aspeed which obviates any flicker.

SYNCHRONIZING - The methodwhereby the scanning of the imageat the receiver is kept in perfect stepwith the scanning of the object atthe transmitter, usually effected bytransmission of regular synchronizingimpulses.