Report No. T.01S fJer'lal .. No. 1911-8/11

Research Departmunt

Report written by: A.B. Howe

Investigation by: R.F. Vigurs

I&.JPORT NO '. T .015 ~-----

Drawing Nos. T.015.1-2

llTITESTIGATION l1""l"TO TBE POSSIBILITY OF COMj\,WN WAVE OPERATION ---'~~~~~~~~==~~~~~~~~~~~~~

OJP TEI,EVISION TRAN'SMITTERS

SUtviMARY

This report describes the results of an investigation whioh was carried out for the purpose of determining the degr:ee of interference which can be tolerated, in television recepticn, from a distant vision transmitter operating with the sarae nomihalcarrier frequency as the local transmitter. It, was assumed that the smnG programme Inaterial would be radiated from both transmitters, but the result showed that at least 40 db and possibly 50 db ratio of wanted to unwanted fiold would be necessary to give a ~ervice substantial~ froe from interference. At this interference:evel it would be immaterial '\I'J'hether the two tronsmi 'Hers were to radiate the same picture or not. It is concluded that common w~ve operation of tele­Vision transmitters working in the 40 to 70 l{c/s b8Jld 'within the confines of the British Isles is hardly practicilble, unless two new transmitters at a sufficient distance wore to commence service simultaneously.

'1. lNTRODUCTION

The types of interference to be expec ted 'when two or more vision trans-, mitters operate with small differences between their carrier frequencies are as follows:-

1. For beat freqncnciefc}' below the f'r&"D.e frequenoy of 50 cif? the inter­ference taken the form of' a more or less rapid flickering effect, depending on the vQlue of the boat frequency, similar to that which is cften procluced owing to reflections from aircraft.

2. At higher bent frequencies, up to about 2 Mc/s, the interference tnko8 the form of a patter.'), of' parallel stripes or bars superimposed on the picture, the spacing Md incli.nation of' the ,bars and the sta'bili ty of the pattern dopending on the value of the bpat frequency and upon its constanoy.

J. For low ratios of wnnted to unvv[mted sig::1,al EL ghost image will in general be seen, in addition to the effects described above, dis-, placed frorl the d0sired image by 8Jl 'amount proportional. to the total oath difference between the routes from camera to receiver--'viathe two transmitters !J.llbwance being made for the fact that the ' . . , w:llocity of propagation alongo:n:y cable route will be less than that

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associated with radio transmission. Af, ,the result ';pill show, 'however, the visibility of' ghost irm.lges is less than that of the interferenco caused by the beating of the carriel' frequencies.

Further information regarding the pattern formed by beat frequency interference is contained in Research Department Report Ne.' T .Ol~', recently issued..

In order to carry out measurements and make 8ubj(lcti"vc tests, an cxpcr:ullont Was arranged, to cimulatc the cond:i:tionn that would al"if:lC in reception from a pair of trannn"i ttors op0rating ,vi. th the BSmc. nomin:;1.l carrier frequonoy.

2. !§SC:amIOH OF .APPARA~

Tho equiprncnt oonsis ted of two indcpcnd.ont radio froquonc;r channels driven from a common o~cilla tor, ea.oh capablo of' being modtll3.ted with a. video sig..'YJ.al, vri th its output controlled by a calibratod attenuatoI'. The .::trrangomcnt, in 'blo(;L: schematic fonn, is Shl.)Vltl in Fig. 1. III

order tru,l,t ono channel, Clo:::;i:?,i1D.tod the 'tUlVf[ll1ted ch.:''l.nnol, Bhould haye n. frequency diff'crence of a oomparatively fm;! cyclec .l?or 8CCO~1tl rolr.'.tivo to tho vrantcd chEmnel, D, continuously ,advancin[~ phto'.se-silii't;j,;.:,g not-vvoJ.·k wns inserted. Tho rl?t;.J of phr.'f:lC cb.cJlge, ;tnd hOHoe the f:r'cqucncy differonce, "/(:\8 detm;''ld.ncd by -cho spool of a minintu:ix:' G1e(;h~:l.c motor o:C V'11!'iablo spood U;30(l, 'GO c1ri vo a var:L.'l.ble cond.enser. l .. f';n'klu(:m(w dif'fercnoe botwccn the J;;'IT{O ci·l.2J.lncls of from ono or ty.,ro cyoles por sooond up to about 100 c/G WtJ.S ;~hus c:.vailf'.blo at will.

The phase-shifting C,.I'XOMgCMO;1:l; UJ ~:'\hmvn in Fig. 2. It consists oss(mtially of 0., cont:iJ.1'lGu::.;ly r'Ot[d;ing I).ir conclonncr with f()ur -sope.r-n,to sb,tors fed :i.n qU~1.d:c,r~t1).rc fr.\)m the Gircuit 01, HI, C2, H20 nw rotor consists of et oircular plato mourd;crl eccent:d.oaUy 0,11(1 providos th:J output c0rmection. 'rhis Drr~'1:gtm)cnt, whi::h was c1ovclepodin th8 Bell TelephCl'!"J 1a;;;()rn.tor:Los, e;iy,s::l E~ pk1SG shift .. ",hid', is et li:l1,oG.!' fV"'1ction of ·t.ho ,i:n,':;13 of:' r?:ta·~io:..-:). :1l1<1 DTl ou1:;put yoltc:,g8 ;:"lu'b::d':'l,ntL1.11y in.c1opcYldent of p;:)(:~sc. In the npF,;t:cO,'t;Us dese~::l.1)ed7 a 1~ .. :r;11 tor 7ms u,,:o<1 following the rlw:.Y:l sbii'''~ cil'c'",l.it, ns frec;;o.om i':!:\);n amplitude moc1ul:;.tioll 'Was of some importanoe:. .

Separo.te vidoo f.1.J:!pliflers Vf81'O u;3cd to mocluJ_ate the two chc,nY'.ol~), which were lined up so <1.:3 to gi7c identical on:tputs. ID, ·th:) Un\l,ClnGed ch, ... ~'YJ.llel, how6vqr, the yj.fleo s:i.b.i1D.l '!V~H:j clGlc,yc(l by et? pr,)ximo.tely .5 11,. [:h)C.,

equivalent to abolit one mile of tl'[mS,ni:3;3i:m path by racl.io, tu ropx'u;,::ont t'ho path difference; bo.1l.{"\yoon the two receivod si[;Dn.ls. D.C. m.oduJ.:\t:loll vn~s used .:>11 tho suppressor grids of the output stD.[1;CS of th<J two cha.""l.l10ls, the val Yes used hrwing n. short SU'pp:rOI:;::lOl~ :}dl1 baso nnd bo:~ng biassed t\) cut-off in the f:'.i;sonoc of t:\. vi(1oo signal.

.Arrangements were :.::.110;0 mf1.(k t'J dri ~:O the secona. cho.nncl from on cntiroJ.y separate oscil18.t0r~ so thn,t the effoct of c[\:rricr fregw:.m.cy differoncos up to sever;"l mG gacyclos per seoond oculd be ObHeryed.

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All the circuits were screened so as to minimise cross-talk effects between the.two channels er s~~rious radiation into the circuits of the receivers and such effeots were found to be inappreciable. . ,

The attenuated outputs of the two ohannels were passed to a mix­ing oircuit and thence, via trap valves, to the input Circuits of four typical commercial television reoeivers in order that oomparative tests could be made. In order that unmodified reoeivers might be used, the carrier frequency employed for the tests was approximately 45 MC/s.

3.. OBSERVATIONS AND RESULT§.

i .. In the first series of tests, the system' was modulated with too output. of the laboratory vision ohannel, us,ing the same standards as Alexandra Palaoe· and a statio pioture derived from a nlOnoSOOpe. The result showed, in brief', that an interfering signal at least 40 db below the wanted signal was necessary in ordcr that its effeot should be negligible. This would be an exoeedingly difficult oondition to realise in practice, particularly at the outer limits of the service area o·r a local, transmitter, and it was thought possible thB.t the effeot of other fotms of interference, evon general r~ise, such as is experienced in,such areas, might be suoh as to relax the abo,~ require­ments somewhat ..

Accordi..ngly El. S600i.1.d. sc:des of observations Wc.~,S mnde at BagJ..ey Croft, in 'Which the system WI1.S modula·ted v.lith the ou-cpu:c of a eomm-. e1"eiaJ. receive:::' uset3. to recei vca picture from Alexnndt'c. Pf~.l.?06. The reoeiving aerial used. Wf'l.3 El. s:Lilple d.ipole erected at such a height that the measured ambient fielo. strength v.ras of the o::-der ef 100 /-tV/m for peak white. This was considered to represent the outer limit of the service area of a local station and the signal was accompapiedby noise and. intertercnoG appropria. te to avcrage reasonably quiet con-d.i tions, as the experiment was done during the evening tronsmissicn when sources of interferenoe originating within the building were absent.

AlthoUgh the expeoted degree of noise and interference was observed, the,results obtained did not differ in anr w~ from those recorded under purely, laboratory cond.it~ons, with no interference whatever. It is evid.e~t that the eye can deteot one source of inter­ference in the presence ~~ .another, provided that the latter :l.s r..ot'· severe. The subjootive d.isturbing effect otone particular souroe 1.8' moreover \.'IDCMngoc'l, particularly when tIlt;; ~.a.di tion.'ll in tcrference-C0l.1-'· sis~s of general noise.

A r..:umber of independent observe:::-s took part in tr.,e tests end their opinions 'I!fe;r:-e pra.ct:toally ur.animous. The impressions were graded \U1.der the same headings as have been used in assessing intel"­ferenoe in conneotion with frequenoy-modulation sound broadcasting, and the levels of interference at the input to the receiver corresponding with the various grades are tabulated below.

Subjective Impression

Intolerable

Very disturbing

Slightly ,disturbing

Perceptible

Just porceptible

Imperceptible

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Level of Unwanted ReIL-I. ti vc to Wanted Signal

- 10 db

- 20 db

- 30 db

- 40 db

- 50 db

- 60 db

The effect of a ghost image becomes imperceptible at about - 20 db and therefore doos not in itself present a pr,oblem, as other aspects of the interference persist at levels as low as - 50 db. For high or moderate levels of interferencc some displacement of the image on tho screen in a horizontal direction, doubtless owing to compromises in the design of the sC~Dning circuits of the receivers, was observed. ' The most important aspect of the interference wE'.s, however, the patternirig of' the screen due to the beat frequency bctween the carriers, or in tho case of low frequency beats the flickering effeot referred to earlier in this report. '

iJ,'JlllO tests were undertaken to aiscover whether the effect of' the interference could be reduced by suitably choosing the order of the beat frequency between. the wo carriers. It was found, however, thllt whilst all observers wero agreed that at low levels of interference a slow beat or flutter was slightly less irritating than a pattern composed of parallel stripes, the visibility as a funotion of level was substantially unchanged. Horizontal bars produced by an audio frequency beat were equally disturbing as 'vertioal or diagonal bars due to beats of some ,hundreds of kilocycles per second,. .

The effect of complete' synchronisntion, i.e. no boat frequency, was also investigated. In this case if it vvere cnpable of being real­ised in practice, the limit of tolerable interference would be the appco.rancc of the Chost imnge at 2. level of about - 20 db. The result is however of academic interest only as 0ven if it were poss-" , i1le to hold the carriers Jf two distant transmitters in absolute synchronism with no relative phase chanses, propagation conditions would always impose irref:,ular and often rapid phase chc.'U1gos on the interfering signal if not on the 1000.1 si[)'l8.l. The result would be a fluttering effeot to be d:istinQlishcd only by its- irref:,ularity from that of a low frequency beat as describo(1 above.

CONCLUSIONS

The result of the tests is that at least 40 db difference in level between the local and the interfering sir~al must be aohieved in common wave opeI'['.tion of television stations over the whole servioe '

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area if tho picture is net tobo appreoiably impaired. Further, the general opinion waG that e..n interfering signal in excess of - 45 db was not entiroly tolerable. Accepting, howevor, the more optimistic figure of 40 db, the result quoted inc;1.icates that if the vmole of the servico area of a televil::lion. station is to be pro­tected, 8. i'iel(l strength from Et distont trnnsruitter must not exceed 1 !-tV/m.

strttistical oviclenco rJ.!?r.wd on prolonged obsprvations of the field I3trungth of' distrnt tr<'ll1.smitters opurat1ng in the television bend is necessary before a final decision can be made as to whether Common wave operation of television stations is entirely out of the question in -I;his country. Sueh data .:.;tro not nt prosent Llvf.';.ilable. A foy'! measurements 11'wC been made, however, which m,'1.y give some idOl3. of thE': possibili tics.

Measurements of the fhJlc1 strongth of Alexnndia Palace made in the Leoc1s district have shDwn th::'l.t the quasi-maximum field strength (the field exceedecl for 5~J of' the total time) Varies from about 10 p.V;)J\ in the SUlOoor to about 5 /lV/')1 in the winter. A station of' the same radio. tcd power o.s AlexanClrr. Palace. in the N ()r~h of Englnnd would tb.lJrefore reduce th0 sex'Vicc areo. of Alcxa.ndra Palo.cc to the limits of the 0.5 r;iV/:a contour in winter ffi1(1 the 1.0 mY/m cont,our in su.'Illner, vlhilst f'or a 35 Klf'l interfering station the respoctive values would be 0.7 mV/m and 1.". mV/m.

. MC2.surements made at Wcstorglon (luring the winter only havo indicated maximum field. strengths of the order of 2 /lV/m, s:> that a 35 kW t-rf'insmi tter ID SC'Jtlr"l1d 'would presumably spoil reception in the LoncL:m area outside the 250 /lV/m contour or thereabouts, with the likelihood of' 'Il/orse interference in the summer months, p~tic­ull:trly as experienoe in television reccption inc.1.icates that i'iold strengths at thc limits of the local service ['.rea nre appreciably lower in thc summer than in the winter.

T.) sum U1J, the possibility of common wave working in this country in the yvave-bflntl at present u~lcd fr)r television seems s:J!newhat remote. Thore iH, however, ·the possibility th..'1.t n. station in Scotland might be synchronisod with one in the South of' England, particularly onc, such ~s 1.1 transmitter in the Brisbl areE'. , whos() service area has nut D.l:c(.:;a<~y beC01:1e 88 tablishucl. Pri:>r t:> the establishment of' a synehr-::>niseclDorvioo, howevor, a considerable progrcunmc of f'iel(l strongth re(;Qrcling would be essential in order to obtain tho necessary '::ata.

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(H:L. Kirk:e)

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