, Research Department

Work carried out' by W. Harris' T. Somerville

SUMMARY

REPORr NO. C.050

Serial No. 1949/19

DIRECT RECORDING DISCS

14th June 1945

Figs. Nos. ,C.050.1 ,to C.050.2

Recent work has made it clear that the discs available in this country do not reach the standard required by the B.B.C. The various faults are discussed in this report, and the steps which are being taken to overcome the faults are described.

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It has always been the opinion of the B.B.C. that British discs are inferior in many respects to the discs obtainable in America. DuG to war­time conditions the quality of British discs has deteriorated still further, end in some cases a level has now been reached et which action must be taken to effect an improvement. Furthennore, the Corporation is actively engaged on the improvement of sound recording , I"1l'ticularly with the di sc system, and the standard of discs necesS2ry is therefore higher than before the war.

British discs suffer from a number of defects. The metal base is not as flat as is deSirable, .and in this respect much behind the flatness obtainable on American discs. To some extent this is attributable to war conditions, because it is extremely difficult to g~t the high grade rolling capacity necessary to give the requisite flatness, and in due course the situation may improve. ' As regards the lacquers used, there is also great cause for complaint. To be satisfactory, the lacquer should be very uni­form in thickness, and in the past, particularly with M.S.~. discs, this has not been so. E.M.1;. discs are better, but the standards which we accept leave much to be desired. '

As regards hardness and surface noise the E.M.I. lacquer approaches that produced in America 'by a number of good manufacturers, but the M.S.S. lacquers are much harder and more noisy. In fact, to overcome the surface noise of M.S.S. discs cOrlpletoly it would be necessary · either to record an extremely high level which results in distortion in the recording system, or else to use an extremely large bevel on ' the cutter, which results in lack of upper frequencies accompanied by amplitude distortion. Then appears to be some connection botween hardness and surface noi~e in that hard discs tend to produce more noise than soft ones. The wearing properties are also ~f importance, and hard discs as a rule wear better than soft discs, but the

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toughness of the lacquer also comes into the wearing properties, ' and this point requires investigation before a statement oan 'be made; it is known, howeve r, that ~erioan disos, which are softer than M.S.S. disos, give adequat§l wear even with heavy reproducing heads of the B.T.H. type. Similar :', remarks apply to the E.M.I. discs. ' ' '

An investigation of direct recording disos is a~ present in progress in conjunction with the manufacturer~. ' This has ' come about as a result of the research wo rk on cutters, for out ter research has reaohed a stage where it is possible to make rutters with various degrees and types of bevel. Aft(,';rthe, eqUipment for grinding and bevelling cutters had been manufect'tred, experi­ments , on , ~utte;I:I shapos and b~vel.ling were t:ommmced, and it was ' roon discov­ered that the. bevel whioh was requ~red to produce a quiet cut was nru.eh in :.:' excess . o;Lt·he value · which could ·be tolerated if upper frequency re.sponse was " not to be sacrificed. The preliminary work on outter ahapes had been oommenced on E.M.I. disos beoause they were known to be more uniform than M.S.S., but as the Corporation uses more M.S.S. · discs than E.M.I. it was soon found that a cutter shape satisfaotory for E.M.I. would often be quite unusable on an M.S.S. diso. ' " .. ,

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TESTS ,ON- M.B. S. mscs )

. ,Conside~able work was .then done us ing M.S.S. discs of various sizes, and it was ·found that ;1,n general terms the M.S.S. laoquer (No. 1'2) gives 8. very' patchy performanoe; ocoasional batches of discs would be quite good, ana .a nOise-free cut would then be possible with a reasonable bevel si ze, but usually the noise level obtained with a reasonable bevel was too great to be tolerated. One particular batch of discs obtained in October last year wes so bad that , it was generally rejected by the Operations Department. When outting di scs from this batch it was: discovered that a sticky material accumulated on the tip o,f the rutter and' produced a noisy groove, and this

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oondition would continue until the material was removed from the outter. e When this condition arose ,it was found that oleaning the ou tter, preferably, with .thinners, would restore oonditions ·to normal, but that quite often on the same disc the cutter would once again become contaminated and a noisy cut would ensue~ The matter Was taken up with the ~st Office .and M.S.S., who were rather puzzled by the phenomenon, and undertook to investigate it. They do not appear to have found the oause, elthough it w~s suspected that the effect might be due to unevaporated solvents .in t .he disos, and this is probably confirmed by the experienoe of R.M.U., that a batch of noisy discs if left for· several days unsealed or heated' on a radiator for an hour or so would often beoome normal.

During the oourse of the discussion it transpi:redthat· the Post Office was investigating laoquers for us.e .byhlJ.'.S.S. , whio:h were ' intended to give up to 100 playings with 'an Il.M.I • . Type .12 reproduoing :head', whioh of .oourse is an pbnonnally high number for use 'by the B.B.C.-: ::iIt was suggested,.;that a maximum of 25 playings would 'be ao:ceptable,. and :~i's ' the lacquer situation was

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still such ~bfJ.t ncisy discs were being prcduced, the M,S.S. Co. deGided~" t0/' , .. , -, ch~ge over Cluickly to a new lacquer (No.185A) capable of giving only 25 , plo.yin~s with ari E.M ... I. ,.'l'ype 12 reproducing head. This ;was '",intended , in ,tho first instance for l,'SO witl:!- P" discs,as ftis a:t' 33 -1/3rd.'1' .. :p .. mo ,: that the worst noise , conditions Ei,rise .. , "J'hls ,l accd.ue rhas proved to be" more , satis- ' " " I

factory lhar.. thoold M.SoS. le,~Qu.er , .. dnct: under the condition's just. specitiod it gives the requisite amount of playings. When first tried on 12" discs and w:l;,th B.T.H. reproducing heads- the wear with lacquer 185cA.was excessive, bu:t c'.1bsequent investigatiqn in c,c..njunp;t;ion vii th tl].e Post Office prov'o d that these :cl:i.scs are" bott91" ':'.t):i6"1f'E o M.I~ . as ' ~egards wear and noise after 25· play-ings. The. reastm for t~9 :poor .performanc:e of the 'first discs : is not known, but may bu due to uS,e '1?oo: 'sooli aftor manufacture, and this point will have to' be checkod ,J}JhB!l ,produ:c'tion di 'scs comeintosorvi ce . Operat 'iOIis DepartmEmt "hers given a B~rroH~ re:pr~dUcillg head andparal1el tracking aUI1 ,to ,the Post Office ,. e to eriabJ.,~ , ,: ~x~~o:r'lffients' to be car-ried outo

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TES-;.,;T;;.;;· S:.-.;;O..;;;n~E~vlo 1.: pr SCS

EoM;,.J:. ' qj,sc:,;. havebeerf used, for most bf the cuttoJ:" exper~~~'rit's becauso 0:' the,ir great 8:::<uniform1ty then ,JVioB.S., but they a180- ' are rt6 t ' 't'mtirely' sahsfactory ~ the main fa]llts being non-uniformity of the coating and 'norr~:' flatness of tho base. Occ~sionally surface noise i s troublosome, but the ; " ~~M~ 1. proc.1uc 't is mUCh better cO.Lltrolled thnn that of Mo S.S., and therefore mu~h more unif9rm. · The , manufacturing process is also much superior , so that grit , ono of the main CBuses of surfaco noise;. is excluded fairly suc-· GGssfully. Novor;the;l\c?S),h some recont tes ts on EQ MoL discs have Shovm that this type of dis~ also could bo improvedo

In making comparisons betW(.Jen E.Me 1. and nAdvance" di scs at Bagley Croft, it was obseI'vocltbat one de:.ivery of E ~M. Io discs shoWQd evidenc8 of the effect just described 'for MoS.S~ di s cs, but to a lesser degree. '1'he results of the comparison are shown in F'igol, both discs being cut vvi th the same cutter. Tho E~ MG l.) disc.: became n.oisY"but cOlldit iollswore restored t o normal, 8S vIas inchcatedbj cleanin'g the cutt0r a.'1d cutting 'another band e t the centl~e , of tl'e r;lisc. In Fig~2 a furthor test is shown in which the cutter was cleaned after each small band was cut, 'a.nll. this indicates that provided extraneouEl mc.tter 'can ' be excludt3d E.M. I. di scs , will be equivalent to the usual American 'type. t~Advanceu discs a;ce · typica l of the good 'grade' of American ~scs $ and a r e v ery"sini.i l g,r to Kleencut and :?resto. EXE'.JTl.ination of a cutter After cutt ing an E.M.1., disc exhibiting this phenomenon Showed 8 granular material which undor '~he microscope appeared to be silica~ and not thA sticky material which one f .i.n<ls on Mo[1QS. discso Some of this was washed. on to a microscope slide ,.and JIi .:> lVl. r. has been aCqUainted. with t he position. It is, however, only fair to stato that in gene ral' E.M.I. discs do not suffer from this defect, but t l1f::t cccas ionnl batches may <10 so. ]'urthermore, the e ffect when present is le,ss serio\.!s than on M.SoS . disci3~

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OONCLUSION

It is our opllllon that precautions ehould be taken to ~xclude dust .and other ex Graneous matter at all stages of the . manufacturing process, even if ; filtration is resorted to before dipping or spray ingQ The M.S.Se process is not as satisfactory as ·that used by EcM.I.

The subject of cellulose di scs is being actively discussed with the two manuf8cturers in thi s country - Messrs. M.S •. S~ and E.M.I. - ~d a specification is being drawn up to which it is hoped they will agree.. This specification will state' ~he va.rious requirements which the B.13~C. considers to be ~.ssential. So far the que·stion of static charge has not come into the discu~8i.ons ." but the MeS ... S'. ' No~12 lacquer produced a very high char'ge, Bnd E.M.L.', discs fell between thi s and American discs, which produce the lowest charge. Th~ electric charge makes the removal of swarf difficult · except by eJ,.ectro-static means, and for the normal user of discs is obviously a disadvantage. The charge appears to have some (lonnection with the hardness of t he disc material, but no doubt other factors also enter into the problem which is not so ,fa~ understood. Negotiations at the moment with the menufacturers are intended ' to produce discs of a more uniform natu.ro having a better coating which will not have so much surface noise, and will be supplied on a base flat enough ·to enable us to achieve the high standards of recording which we consider to be essential.

(H. L. Kirke)

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