klESEARCH . DEl? A:t1'TMJl.1ifT

A COW~ARISONOF THREE RECORDING CHARP.CTERISTI CS

Report No • C.054/2 Serial No. 1946/2

Researoh Department ,REPORT NO~ G. 054/2

?erial N6.l946!2

'., , Maroh 1946

Wo~k ,oarried out by: Figs. Nos. C.054/2.1 . to C.654/2.7 D.G. Beadle

P. R.Terry

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. SUMMA.RY , '

ACOlVlPAIUSON OFT HREE' RECORD,ING CHARACTERISTICS ~ .. :;.' '~. ',-

This:Report follows, on from Report No. Lo023, 'whioh shouldb6 re-numbered C. 054' ----... '

, An, i,n~stigati9n of the relative ,meritr.· otthree rt3C6rdi~g,c'ha.racteristio.s ~Hs;;s been ,ctwried p~t'a.~a result of a req1~estbjr ~4e 'C6mll1?~ve?1~h. ~oni'erenoe •

. The thr.ee oharao.terist~(,s tested are the BBC' standard oharaoter~st~o, the N.iI:~B.' oh.a.racterist:i,o used in America for 33J/3 ,r.p.m., reoording,and'an imermediate 6~ract'eriE;tio. Gl-eat diffioul'9Y v.-as experi(3,nOadin nnking quantitative me~13uloements, arrl results shOWing the variability to be expected are given. As .r.~gards signal/noise 'ratio,.the N, .. A.B,. opare.oteristio~ bepause 'qf ~l1e h~gherreMrded levels' in the' upperf:r,equenoies" is, best, and on good 'equaPIIl.en:!:;gi,ves!94oellerit results, but 'on poor equipment whiohcannot.reoord

, the' hi~h'lev(3J:~ :liE:l~~'ssa'ry 'without dist6rtic:n it i'13 ini'er::Lorto tne,.,BBC ch8.racter'istio ~ :' .' ,

, INTRODUCTION

At ,the, Commonwealth' Co:rirer~noe th~ interohangeability of "d laos be-t;ween iinrious'br-oado@.sting 'organisations Was disoussed, and it 'mS agreed that standardisation of reoording oha;-aoterist ios-was . desirable. It: was . , impossible to arrive at a deoision'beoause oflaokof irifo:rrna.tion,'and the BBCms r~quested tooarry out dn investigat1.on'to asr:.ess the rela:t;ive mElrits of var iou's reoording omraoterist i?"s. ." It ~s generallYf:l.greed,thatthe . N.A.B. oha;raoteristio'should be adopted' if possible'. The t~sta'had tq cbe oarried oub at 33 1/3 r.p~:n1:. and 78 r.p.~, 'but as ;rar as inte I':na:bi'ona 1 . agreement .~s,6onoerned the ,m:Ol?t impori:;antspeGa:,is:331/3 r.p.m,.;.,.at which the Amerioa;ns use theN~A,B~ oharacteristio.'·· It s,hould ,be stated that the Amerioan33,l/3 r.p.m. ciharaoteristio;kno'wn as N~A.B~' (Nationald\s$ooia'bion of Broadoasters) 'has. been previously designated A.S.L. i]:J:the, BElC, .. but it is

,not'g:en,erally known outslde the, BBC, 'u.'1')dar this title. . "

Proposed methods of makingth~ test,s ~n l';ecofdi:hg characteristics have already been.set out in Researoh ReportNo.C, ).5~,and as the results were to be obtained in the. least possible t.ime it~sth61ight tlla:tthe "quioJr:est' way 'would:r>e to, measure distortion by the 'inte~cx1u:U:rl:;ionl\lethod desoribed on., Page 5 or this report .:. ',;: .. , ',' , , " , ,,'

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INTERMODUWIO~ METHOD

" Measnremenbs of distortion prod uaed very inoonsistent results ,foUnd to be due to a number of variables whiohare diffioult to oontrol, and it beoame oloo.rthat the investigation "WOuld ta,kBoonsid,erable time. Even if reliable results oouldha. vs' been, obtained, the' reoording characteristios derived from them 'WOuld have to be thoroughly tested under pro'gramme conditions" and it was deoided in the oiroumstanoes to conoentrate on t~, direot test ,method.

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THE.,VARIABLES OF DISC RECORDING -------~"--------------~~,

'In view or the, ,dif:f'iou~ties experienoed 'With the in:termodulationmethod, some time was spent in making observations on the varie,bles of diso'reoording, and' the'prinoipal oauses of variation 'Were fOull<:l to be as follows: ," ~"

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(1)' The variability of outter heads, in :response to diso loading, and in some oases.from day ~o day, oausing ol:J.ange in reo.orded, leve:I,s.

(2)' Variations in,outter tip" radius" bevel size and bevel angle, whioh prodtioeohanges iJ::l outter head loading, ~eoorded levels and in groove' , .,. shape, and result "in var.iable reproduoed levels. 'The variability of outter tip radius isoonsiderable, for it is known to vary from 2 to 4 mils even in the produotof one manuf~cturer. Bevel ,size"

"oan be anything from 0.4 to 1 mil at the tip, and the angle of the bevel varies frqm 200' to 250 'bo the diso surfa,oe, and different orgamsattons u,se different faoe angles. '

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(3) " Diso materials are also extremely variable,' and depend very muohon the manufaoturer., The material affeots the outter head loading, and, also, the response of the piok-up which reproduoes" the diso ..

(4) The" s~ze of the replay stylus also varies a.t l~ast f':rom 2 to 4, mils , in radius throughout the various organisations, and as ,this deter- ,

\, '. 'mines:the level whiohis repl'oduoed from the Cj.isos, partioularly at ,the hi.gher fr~quenoi'el;l, cons iderable variation, oe.n be expeoted.

(5) " The number of. types of piok-up in use is not known, but is oonsid­erable. ThE;ly rang'e f':rom heavy piok-upswith 'armature reso;nanoes .in, the region of 5000 0/13 to UghtWeight piok .. ups with resonanoes as, high as 15,000 o/s, and; as their meohanical impedanoes differ,

" they :react diff~ently todisoS" of different hardness, to grooves of different shape, and to, needl~ points of various l3~zes.

All theeffeot's'whioh, ru3.ve been'specified are moreseribus at low groove speeds» and, ther eforeoaus~more trouble at 33 1/3 r.p.m. than ~ey' do !lot, 78 r.p.m. It is, unfortunate that it should be at 33 1/3 np.m. tha'bmi:J()l""' national agree:nent is most desirable, for it is impossible to eliminate the

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'variations ~oe:rrt: by the 'adoption of a. m1lohmO're i-igid ~tanaardi$ation ot ~ters,. disos" pic~~upi" and reoording .am r;eprOduoing' ·:ityli." An attempt hall been made to asseSs the· range of variation,;likely'to bee' niet under present ·oonditions at ~3 1/3 x~p.m. 'Theextent of thevari};i.tio;ts isshbvm in Fig.I, 'Whioh ,indioate:Jthe>~esult s o'&tained ·by using one outter head; with' eight

I, ' ,',"', ' ,',',' outter's of the same type and 'inanui'aotur,e,- and one make of (liso..The disos , werereplayed:byfou;t' differentp,iok-lipSt ;'tJle TeleNnken, R"C~A. ~ Voigt~ and E.M.I~, alld.in the ca,se 'of",theE.M.h piok-up', needle' point radii Of '2, 3 and 4 mils were used,The range ,of' variation is 'sh6wn to:: be':oonsiderable;at . 3000' o/s, 2 dp1inoreasing. to ,p db' at ,8000 o/s'and 12 ab at lP~OOO o/'{J, ,In practice 1Ia.riatl,olls ,due to c14tter heads; different types of cuttel?stdid d fsos must ~ added, ~

Fig,2 shows the' tesults~sing one outte~ hea.dand'on~ .. cutterandfbur types ,?f <iisQ~' the. replay being done by three piok-ups:;' TelefUnken:; R.C,A., and E.M"I.,',: a'ndagain the E.M8I. was used "nth the sa.'llethre~ needlepoint:; sizes.; The -yar:lationsup,:;f;o6006,o/s are not s,omarked, ,butab9ve'this.", f:requenoy'e~ 'v€lr~<wide range, is obtained, being 20 db '8:t 10,0.00 r-/s., Tll.ese

.:results;wB,re taken a.t 33 1/3 r.p.'rn.., but on the outside of a,'i6" diso,where the groovE), &~pe'ed i(3 reasonably. high. ,. At the, inside of the disc, 'at a r~dius of 4" ,the • range oi'varia:hilityhaj increased enormously,; a' typioal set of , '. ~\ _ ", '\' , _.r resul t3 being sho:w,ninFig.3, taken under the same ~ondit.ions ,as.Fig,~~ The

" :rangeat6000 0/8: has,'nowiooreased to.'16: fIbi : ,ana 111easuremenus ''Vkre not , possible, abo,ve thjgfreq ue~y~ ,,' 'Furtherm:o:~e~·the los,s'$.tf.r~quenc les above 2,000C/8 is 'large, ',and 'sd varIabl,e tha,t prOper'radius- compensati'onoannot,be'

, maintllined. 'rheseresultsimioate that a measure of standard:i bt ion may b?" ~,chieved, . Up, to, :f,',:r,. equenoi., es of 5,000 or :'600? ,o/a at ,thE> O,utsid,e:.of ~, 16 11

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dJ.soat33 ~/3' r.p.m .• ,.jJUt. ~hat standardJ. satJ.(;m.oV'€l!' the 'WholedJ.SO must' bi3, poorun,less' theI-ti ,is ,rigid .st~~ardiSl3.:tion:o.f cutter~,disos, and 'piok-ups.

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DIRECT TEST METHOD . , ,:"'

',1, , .' Havi~g '~bandon:ed meas';~nlenl:;s' by'~h:e intermod~:taoio~method,: it was

decided ,to find out by .dire'et 'Ustening'.tests whiohcha~cteri'stio "gave ·the : be strec ord ingof : pro gra.:mme.':This is ,the d ireot'te it meth<?<f ,:d:escr ih~dUnde:r , ,,$ootio'll '9 'of,R~port, C ~ 054. .'; . " " , . ,

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TheoharaoteristicswerE) ,set l;tp with as great ac(niraey as ,pos'sible on the various types of equipment to be te~ted, and a panel"cf 'obserV?rs listened' 't6'~eproduotion. during re09rding" o01?-pared it with ,the "incoming progra.mri:te, am adjusted therfic~rding level unt il>t here: "WaS neglig'ibledis;:;orlion.' 'The , me,thod i.s 'very, lia1:H?rious ,forthetestshaveto,be conduoted' 0l.1' alltY.l?es·of programme; but -it gives comprehensi vei'eS'ults' onastat;istioal basfs if'enough observationsare·inad'e. " ""

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. 'Thethree\ohl;ti:aote~:i.stios tested,'·shcvvn:inFig.4;' are N.A.B'.dra1imaS " ,p'ubli~hed,b.nd plo'i:;-t;od :,on ,bhe St~l.ho:,bCl;riis'~ th6' r.tn.licti',rd· DDG ~ . t.Jld n mOdified

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characteriStic oalled BBC/2.The testswerecarrled':,out with the BBC TyPe D reoorder, arrestor eoorder :Ci tted with a Type Boutter hea~ like 'that on the TypeD'reoorder, and ,a Presto reo6rderfitted with a Presto o utter' ,head. . '

, I Each 'E:quipment was' 13gb up to oue the three'oha:raQter:tstios. Tests were, carried out on mtlny types of programme by obserVers who did not know to 'Whigh,'

.charaoterist,icthey.were listening, am the relative levels a:rrivedat for. the . ;r.;tP6 D r.eoorderare'those shown in F1g.4. . It "is of interest ,to :note t~t the ,obs,,~v:ers:preferredthe standard BBCoharaoteristio reduoed2', db below the '

':1.,~1~~ffi;:;norma11! '~sed, ,and that thelewls .. reoorded in .t~~rf:~~r'o:no:e~ooc/s to 200~jo'%s·are. ~tn:J.lar for all oharaoteristJ.cs. ., "r!': .' .

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. T,he same prooedure 'was oarried out fer Presto equipment, axil' the resu1t~ / e with a Type B outter head are· given in Fig.5. ~he level of the N.A.B. oharacterist'io 'was reduced by 2 db compared with Fig.4, d.ue to di·stortion, but the. levels of BBC/2 and the BBC ol'..a.raoteristio d id rot, require modificatiol1.

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, The results for a Prest~--:'reoorder with a Presto Gutter head a'ppea.r in ·Fig.6,.'and in this case, 'oocause of, odn;:dderaqle distortion 'with the N.A.B. ~l1araoteri'st io , the level 1JlfaS reduoed by 6 db as oompared w.i th Fig.4.Slight distortion on BBC/2 am' BBC charaoteristics caused. reductions ave:ragil1g 'about 1 db as compared with the other tests.', ., " .

" . ,All, 'bhe tests on the adjustment of level Were oarried out without 1:iJniters

in' the, reoordil1g, system, beoaus'e·the control. of1evei had,to beplaoed ,entirely in the ,hands of the observers at :this . juncture. It ms discovered, however, that on oertaintypesof progremme whi'oh oomain oonsiderablelevels at high freque~oies,such as certain types of orohestras and sibilant speakers" . particularly women, distort:l.on occUl');-edwhich required reduotion in level of the order of 6 dbtd 10 db. . This', ocourred particularly on the. hesto equipment ,but subsequent experiments with limiters in oirouit indioated thab the Jimiter usually pre~nts serious di st ort ion. On the Type D 'record,er ··e. there appeared to be no diffioulty in using the N.A.B. oharaoteri~tic at·the 1eyals shown, but .1ess satisfactory equipment might give trouble, as mdioated

. by the results obtained with the Presto apparatus shown in Fig.S. .With the N.A.B. oharacteristic, the level had to be :reduo6d because of inoreased distortion at thehighi'req\~enoi~s; so that turntable' rUmble beoame mor~ audible. In this re'speot the BBC'charac.teristic was better than the oN.A.B. oharaoter-~stic beca.use the high fr'equency le,vels were less. .

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. . After agreement hadbeenreaohed' on the levelS whioh c.ould be reoord~d, observations on the signal/nOise ratio were made.. The N.A.B. oharaoteristio showed an advantage of aboue ,2 db over the BBC charaot.e:dstios with the 'TyPe D .'

. eg,uipment I but with Presto equipment the, reduction in level required to over.' ' come d iSt prt io:t;l. 0 ana e11e cl t1;ti s' advant; age. .

The se tests had been carried out wi ththe BBC standard 10uispeaker, a' B~T .H. RE: in, a box baffle. The design:ts nOl.V ob~olescent, 'and it "Wa~ .

thel"ef'ore oonsidered to be advisable to oh~ok the results using a more modern J type of loudspea.ker. The only one ~va::L1able 'Wl:l.S an Altec,,:,Iansing, which is

· an Amerioan wide-range loudspeaker using !:I. oone for the lower and a oellular horn for :che higher frequenoies. A series of tests showed that as rElgards

,the reoording levels this loudspeaker gave re(3ults similar t.o those obtained with the RK, but the 1I';ider fr€!quenoy range l$.de baokground noise more notiooo'ble. . The N ~.B. charaoteristio thw. had marked advantages oompared . with the BBC charaoteristios, the improvement' :'n Signal/nOise ratio varying" from 4 to 10 db" depending on\disos., ou-':;ters, eto.'With the Presto apparatus, reoorp.ing was at the reduoed level required' to avoid distortion, but there was still some advantage as regards no~se in favour of the N .A. B~ oharaoteristio. l.bst of the tests were ma.dewith reasonably good cutters and disos, and there­fore 10'N baokgrounJ noise, and it was found that when the baokground noise was serious the advantage of the N.A. B. chaJ:'1t.oteristie' was more,pronounoed.

This investigation ha~ been confined to a oomparison of three specifio recording oharaoteristios" but there is somo evidonoe to suggest' thatbE:ltter results might be obtained with a oharaoteristio resembling that used experimentally by the BBC belore the' ~"rar (Fig.7). ' In our opinion, hO'Vrever j it is unlik91ythatthe advantages :vvouldbe suffioient to seoure its adoption by organisations now oOllunitted to the H.A.B. oharaoteristio.

CONC'LUSIOHS

The' experiments which have been oarried out show that a reoording ·oharaoteristio oannotbeestablishOd vr.th any aocuraGY without oontrol of outter headS, outters, discs, and pick-ups. The purpose of standardisil~ reodrding charaoteristios is ~o enSL~e interohangeability of disos, and even if different organisations oould reoord the same frequep..cy characteristio 011 the . outside of a 33 1/3 r.p.m. o.iso there would st.i11 be "vide, variations in the r6sponse at the inside, unless acourate standardisation of styli, piok-ups and diso naterials is naintained. The:refore, the adoption at·,33 1/3 r.p.m. ofa standard reoording ohara.oterist ic a,+one is largely illusory, since it vdll not ensure interchan€',eability of pr'o~rariune discs. . ",', ' '

.f!. test JI oontrolled with as muoh aoouracy' as possi ble, indicates thm; the N.A.B. oharaoteristic gives a ,better s5.gnal/noise ratio than BBC when higl). grEj.de reoording equipment' is used. With low grade equipmEmt, much of Whioh is in use througholltthe broadoasting systems, this advantage, is . lost beoause reoording ~evels must be reduoed to avoid distortion, anj rumble maybeoame

, troublesome. There therefore appears to be no reason why theN.A.B. oharaot­eristio should not boadopted as standard at 33 1/3r.p.m. provided good ~quipment is available, but with poor equipment a oharacteristio of the BBC type is like],y to give more satisfaotory results. If standardisation of the N.A.B. oharacteristio -cook place, it 'would. be advantageous to adopt it i'or . 78 r.p.m. reoordlng within the BBC, except in the London rranscription Servioe,

'1Nhich has to oonform t9 'oommeroial standards for gramophone reoords ~ We have notoom:gnred the results obtained from pro,essing disos out with theN .A.B. and BBC oharaoteristios.

, RS (H. L. Kirke)

This drl!lwing is the properly (;)f the BRITI S.­BROADCASTING CORPORATION, alld ma,. not be reproduced or t!liscle<o ed to a third party in any form without Ihe writte" permission of the CorOf) ra ' i'

CUTTER HEA-D DISCS

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cu~v~ _SHOW THe: REC.ORDING CHARACTE.RL5T1CS AT THE: LEVELS bItTERMINE.b BY LISTENING TESTS WITH PRE5LO ~CORDa & T'1P..E 'B _.c.u:rIER .HEAD. __ _ ZERO LEVEL: VELOCITY OF e eMS/SEC AT IOOOCjs.

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