GERMAl\f Iv1AGNb"T re REC ORDERS FIRST REPORT: TONSCb'REIBER B

Report No , C. 053 Serial No . 1945/28

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Research Depart ment

Work ca rri e d cut by : W. J . Lloyd

SUNIMARY

REPORT NO. C. 053

Seria l No . 1945/ 28

GERI,lAl."'J 1'fAGNEI'1O RECORDERS, IT IRST REP::-"O::":P::':T=--: -TO~1:-::JS~'C-HR-EI IBER B

Mar ch 1 946

Fig s~ Nos . C.053. 1 to C. 053 . 11

This r oport de sc r ibe s the tests carr ie d out on a transportable magnetic r e c order us ed by the G0r rnan Army . It 1..l.s e s tl plastic tape , and has nine sync hr onous tape s1Je eds , ,n th,a novel device to enable r e production t o be ca r rie d out at a lowe r spee d than the recording bu t a t t h e same pit ch .

Amonc captured Ge rman e quipment the r e a re various t yp,es of magnetic recor ders , ':f j. th p l a st ic tapes in pl a ce of ' t he bulky and. heavy steel tape used , foI' examp le , in t he ' l\!Iarconi ~Stille syst em . Before the war the Ger mans [t'ld carr i ':)d out expe riments with p ap er t apes coa t et'!, wi t h fine l y divided iron , but they have s inc e deve l ope d tape s Irade of pol yvinyl chlor i de impregnated or coated with ivlagneti te (FE304), a magnetic oxide of iron . The tape is light i n we i ght , and as a re sult it has been possib l e to make r ecorders of

~' much smaller dimensions . There is t he add j,t i ona~. a dvantage that for edi t i ng purp oses t b,e tape cah be cut and joined as easi l y a s a Phi lips - Mi ller tape , e ' whGr0&S a stee l tape must be wel ded .

I n t hj,L1 r ,J ) Ol't H C',,-:; r man Array r e corde r , Tonschr e i b er B, will be desc r i bed . It Civeo no 1". t,::l r o c ol' cling facil H i e s , bu t in addit ion appear s to have been des:i{!;rie d to .simplify "ch:) deciphering of mes sage s transmi tted at hi gh speed i n morse or spe ech , for i t can rep lay recordings slO'Nly but with normal pitch, an d i nstoa d. ef a vari ab l e spee d drive it h:1 S nine fi xe d spee ds accUrately sync brorri. s ed by an os cillat or.

For conveni enco in tran sport the apparatus i f? .conta ined i n three units each approxinn t e ly 18 " lons , 13:" h~gh l and 10" wide . The first unit contai ns the r e c or der , the se cond u1:.i t t he amplifie rs .and pOl'rer supplies , and the t h i r d uni t spare s a nd spools of , tape . The firs t t wo units assemb l e as shown in F i C. 1 , beine claJnpe d togethe r and e l ectricall y connected by s pec'i al pl ugs and socket:i . The me cllani cal de Sign has ,been cleverly execute d , and l a r ge scal e production has jus tifie d the use of die - ca stings i n light a lloys t o keep dovm wei ght .

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MAGNETIC TAPE

Two kinds of tape are use d in these recorders . The first consists of polyvinyl chloride mixed wi th 41~~ (by weight) of Magnetite , and the tape is 6 nun. wide and 0 .05 Illl11 . thick . The second .tape, developed to improve the mechanical and electrical properties, Ins the same dimensions and consists of a polyvinyl chloride base stretched by a special process to increase its strength, and coated on one side OI1~y vTith a mixture of polyvinyl chloride and MaGnetite . It is claDned that a lower· background noise is obtained, but this point has still to be confirmed. ' All resulta given in this report are based on the use of the original tape . • The tape is .carried on spools containing lengths of about 1 km, which at the highest speed will run for 14 minutes .

Tests to deterr~ine the signal to nois e rati 0 were carried out, and for this purpose BBC amplifiers had to be used because the German amplifiers are rather poor electrically and produce distortion . The recording levels were adjust ed at various frequencies unti l the rraximum level vli th acceptable distortion w.~s obtained, the result being judged by observing the reproduced Signal on a cathode ray oscilloscope . Under these conditions the ratio of peak signal to noise was about 30 db on a valve Voltmeter .

RECORDER ' UNIT .

The general front view of this unit is sh01,11n in Fig. l, and in Fig. 2 the d':ltails are more clearly visible. The tape is drawn from reel (6 ) over the wiping , recording, and reproducing heads by the drive roller (1) against which it is pressed by idler (2 ), and is wound up on reel (5). The operation of recording , re-winding and reproduction is controlled by levers (7 and 8), e

. which also perform the necessary switching . Nine re C ording speeds are available, sel ectj.on being accomplished by contr ol (20 ), the speed being ' indicated on tachometer (4), friction driven from the tape driving shaft . The rear view of the recorder is she»Tn in Fig . 3. A synchronous mot or, M/O I, is operated from an oscillator which produces nine frequencie s corresponding to the nine recording speeds required, and this motor is directly connected to the drive roller . To provi de all the 'driving torque by this mea...l1S would require an oscUlator of considerable output , and there. is therefore a series vlOund A.C. motor , M/O 4, which transmits power through an idler wheel to drum (3 ) on the Same shaft as M/o 1. With this arrangement M/O 4 provides most of the torque, and M/O 1 has only to provide enough torque to synchronise the drive shaft . When the oscillator frequency is changed the voltage applied to M/O 4' is adjusted at the same time by control (20) to give approximately the correct speed . The nine speeds avail able are 120 cm/ sec ., 104 cm/sec ., '72 cm/sec ., · 52 cm/sec., 36 cm/sec ., 26 cm/sec ., 18 cm/sec., 13 cm/sec . ~ and 9 cm/sec .

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· - 3 -

Motor 1111/0 3 drives the take- up spool t h rough gears , or re - winds the t ape at h i gh spe ed , the changeov er being ef fe ct ed by the operating 1ever s in front of the ma chi ne . Details of t he wi p i ng , r ecording and reproducing heads ar e shown i n Fi C-: . 4 . The t h ree heads are mounted i n a detachable ulli t , cO lULctions be i ng rra de thron~hthe co ntac t s (18 ) , and the unit is held in p lace by scrCV'TS (17) . The. remo val of previous recordings is carried out . by saturat i ng the t ape by passine direct current through t he wiping head (14) '. More deta ils of the recor ding head (15 ) and the reproducing head (16 ), which al' e s imila r , are given in Fig . 5b . The gap 'Qetween the chamfered ends over which t he tape runs is approximatel y 0 . 0005", and the gap at the other side i s ab oLl t 1 / 16 " . Magnetisation of the t ap e takes p l ace longitudinally. 'I'he t ape i s b i ass ed back from the sa turateq. condition in which it l~aves the wi pi ng head to t he op erating point by app l y i ng direct current to the recording head.

The mos t nove l feature of this r ecorder is the means provided for restoring t he p i tell to nor mal ,4Jhen rep roduc ing at a l ower speed than the record ing spe e d . The tape is t aken over a pit ch restoring head (11 in Fig . 2 ) whi ch c onsis ts of four rep~ducing hoads equal ly spaced round the pe riphery of a dr wn, and with a commutator to connect each head to the r epro ducing amplifier for a quarter of e a ch r e Volut i on . Details are shown i n Fig . 5a . The he ad is driven by a motor (M/ O 2, Fig . l ) , and the speed of rotation can be adjusted by control (12 , Fig . 2 ) so that its speed relative t o the tape i s s imilar t o that when recording .

The rotating dr um moves the active head in t h e same direction as the t ap e , a nd tho r ef ore -du rinz the t i oo it i s connect ed to t he reproducing ampl ifi er it scans a small l ength of t ape backwards . Thi s arrangement is probably adopted t o increa s e the number of scans per second , for -if repr oduct i on is at half t he r ecordinr; speed normal pi t ch i s obtained either by ruILl1i:n.z the r epl ay head at half t he recording speed in t h e opposite direction t o t he t3.!)G , or at one a nd a ha lf times the recording speed in the same direct ion a s the t ape . . The latt er arrange ment g ives three time s as many commu t a t ions , and t her efore three times aS Illc'1.ny s cans per second, and even thOUGh each se ction of about one hundredth of a second is reproduced ba ckwards t he scheme ;Ls quit e ef f ectiv e for rnorse and for speech if intelligibility is all that is required .

AMPLI FIER U1'JI'r

- Beneat h. the recording unit in Fig. l is shown the amplifier unit , which cont a:i. ns t he amp lif i ers for r e cording and reproduction , the oscillator for synch ro n i s i ng the r ocorder drive , and all the p'ower supplies .

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SYNCHRONISING OSCILLATOR - .

The upper half of the unit houses the synchronisine oscillator and its mains units . This section is shown in Figs . 6 and '1 . Connection between this sectioh and the one below it is made by the spring contacts (19). The swi tch . ( 20 ) is provided for selection of the nine frequenc ies corresponding to the nine tape speeds required, and a fine adjustment is given by control (21) •

Details of the f requencie sand t ape speeds appear in the Table in Fig. 8 , the vliril1£ diagram of the co ;nplete equipment . The electrical design of the oscillator i s not very good , and harmonics are produced on which it e is possible to synchronise if great care is not taken . The leakage field from the syncm.·onous motor is high, and hum is induced in the reproducing head , part ic ular l y at high t ape speeds, although the head is screened with mWlletal. In addition some induction takes place in the wiring .

RECORDnn AMPLIll'IER

. This is accommodated in the low-er section shown in FiBS . 9 and 10. It is a two- stage amplifier designed with a high :i,mpc danc e input to work from a radio receiver . . The maximum useful output is "'10 db, and the corresponding input level at full gain is -44 db .

RBJ?RODUCUJG Al1PLmH"R

This 3-stage amplifier is also accommodated in the lower section with the m,1.ins unit for both a mplifier s . The maximwn input level without dis tortion 18 - 55 db at full gain , the cOITesponding output level be ing e ""'22 db .

CONCLUSIONS

The design of the mechanical details of this recording equipment has been carried out very thoroughly, but the electrical design i s not so go od, and a hig h level of hum can be att...~ibuted to the electr ical layout . The freque ncy clmI'acteri :1tic (Fig . ll) and speed constancy are good enough for the p l.·~rpose for which t he apparatus was deSigned , but not Good enough for m cordine; mwo ic, whi le the noise level of the complete system is t eo high for normal BBC servic e .

. The pi tell re storing device is very ine;enious, but woulcl not be of general interest to a broadcastifig organisation, a lthough it mi ght be of use for s pecial effects . The use of' plastic tapes has great possibilities,

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and it has enab l ed the C-BrnBl1'3 to }?rodu.ce a recordinG equipment with a fair performance un0. a IIo j_t;:;ht cO~arable v!j.th our Ty-pe C di s c r ecorder , but some development woul d be ne cGssayy to satisfy BBC r eguirements . Other models for service purp ose--- and for broadr::ust u s a havo a lso been produc ed , for which a vory good perfOrIl1UnCe is claimed , and a further report will be made when it las been possible to carry out tests wit h t.tese oqu i pments .

RS

fll/~~~ (H. L. Kirke )

c.7· 9 '45.

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