.,..

RESEARCH DEPARTIVIENT

THE E.M.I. :MAGNETIC TAPE RECORDER

BTR/l

. /)

Re search Du))urtm§lllj:;

REPORT NO. c.o68 SERIAL N"0:-I9487.lQ

THE g.M .. I. hLAGNETIC Ti-\.PE RECOl-{DER

SUGnary

Nos. c.o68.l to c.o68.3

BTR/l

Tests have been dade on two E.I,I.I. magnetic tape recorder s? TY-.;:Je BTR/l. T.heir perfor dance VITaS COnpc.r2 bIe witn that of tile I\ll:~~enetophon K7 ~ but it is likely that SOiile 111Gdifications will be required for opE;rational use in the B.B.C.

101 General D~ri~tion

Each recorder is housed in a sheet-steel cabinet rather similar in appearance to that of a Type D disc recoro.el' but vJith a counterbalanced 5 ;la s s panelled? lid. Control is by deans of push-buttons which are ;l1ounted on a sloping panel at tne front of the cabinet together with an input potent iOLleter and a lileter indicat ing signal leveL. T11ere are four .push-buttons [,1arked Record'l Replay, Spool and Stop, and a safety catch is fitted to the Record button to I' educe the possibility of a wanted recording being aCCidentally erased. On the machines tested the push=buttons wers stiff and did net al\vays latch in cdter being pressed. This somethles resulted in pile-up and breakage of the tap~.

The "amplifiers, high frequency oscillatoi and power supply units are housed in tIle cabinet and are accessible through doors opening in the front. T;1G interconnections between the VElr ious units are uade by C.c:nnon type plugs and sockets but access to the EU;lplifiers wOLd .. d be easier and the viLole arrangement viJ01.11d be tidier if a smaller number of plugs and socket s Vii tl1 L1e safile total number of contacts were used. It would also be advantageous if? after tIle plugs vJerc 1Ni thdrawn? the amplifiers could be removed without the need for undoing four screVJS on each unit. A central meter can be plugged to the various w~its to measure valve feeds and to check bias and wipe currents.

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At the ba.c};: of the i;:iachine are three mains socket s for the supply of auxiliary equipment such as a soldering iron, a tone source or a cathode ray oscillograph.· These socketsJhoweve~ are of a two-pin type not used in the B.B.C.

The maximwn current consumption of El single recorder is about 1.5 amps.' at 230v. A.C.

The wiping? recording and reproducing heads are of the Magnetophon type~ and are mounted in an assembly similar to thos3 of tne H.T.S. and K7 mach~nes. The recording and reproducing heads both have a pair of sillsll set screws by which their alignment lllay be adjusted. The a)iGnl1lent of the recording head is normally set at the factory by means of Cl travelling Il1icroscope so that all machines fwve a similar alignment. This adjustment [llUst not be subsequently altered or l'Gcordings liill not replay satisfactorily on other 28chines.

The lilethod of dri vine; and revJinding the tape is similar to tnatl1sed in the K? ~c:ne t ophon \vhich ::la s been described in Research Re)ort C.O)J/3. In the E .111. I. machine s ~ hov/ever 1 t"18 illot or s are Tilounted at the bottom of the cabinet and drive_ through long vertical rods with a flexible coupling at each end. The motors are therefore remote from the head unit and 11lagnetic induction into the rep'roducing head 9 which ViaS a {{lajor cause of hum on tIle IVlagnetophon 9 has been reduced. Nevertholess 7 as shown later? the hum produced in this B.k.I. e~uipffient was still quite large.

Synchronisation of programme on consecutive reels of tape can be arranged in two ways:-

1. A brake, operated by a push-button on tne control panel, is fitted to the main driving :motor on each machine. If the second 111a chine is started a li t tle e8r ly it CD..n be slo1iwd dovm into synchronism with the first machine by operating the brake.

2. The second luachine may be started frolll ti:lG first through a relay which may be operated either by pressing a button on the first machine, or by the passaGe of a piece of 41etallic foil on the tape pa.st a pair of contacts

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on the first machine. For this purpose~ reels of tape are supplied having metal foil attached at given positions near the beginning and end of the reel.

1.3. OsciJ).at.or and A.mplif.i.ers

The o~cillator is of the Hartley type, and supplies boLl the bias and the viipe current sat a freqv.ency of 75 ~c/so The recording head is transformer fed from the oscillator, and the wiping head is fed through a further power stage.' A potentiometer in the bias circuit allows the bias current to be varied from atout 10 nlA to 30 mA ~ the value of current normally used being about 15 mA. .

The reco~ding amplifier has a high impedance cathode follower input? followed by two stages of amplification. Variable equalisation is provided between the second and third stages, and the frequency characteristics obtainable are shown in Fig. 1. The upper curve, Fig. la, represents the recording characteristic normally used with new heads? but apparently it is usual practice to increase the high frequency re spo.nse somevJhat a s the heads become vrorno To provide the norma:;' 1 Kc/s line-up level, with the input potentioEleter turned fully up, reCluires an input level of -30 db absolute.

The r~producing amplifier consists of two stages, p'iving an output level of about - 35 db absolute into ~OOO for 1 Kc/s line-up level into the recording head. The input impedance is low compared with the impedance of the rep~oducing head, and this gives the 6 db/octave slope in the frequency response necessary to compensate for the rise with frequency of the e.mof. in the reproducing head. Continuously variable high frequency correcti00 is provided by two equalisers connected id series in the anode circuit of the 1st stage. One is an RC network (EQ.2) givinb a slight top cut; the other is an LC network (EQ.l) giving a top-tip of up to about 10 db at 10 Kc/so Unfortunately the two equalisers interact as shown by the response curves given in Fig. 2. The dip that may occur in the response Rt about 2.5 Kc/s is caused by the resonance of the capacity of the RC network with the inductance of the

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LC network. The value of the variable resistance in the LC equaliser 'appears to be too high, for the complete range of high frequency correction is covered by turning the adjusting screw through a fevl degrees from the miniYllUlTI ,position. For operational use it might be better to provide equaliser adjustment in fixed steps as this would facilitate compensation for different tapes, and.possibly each tape could .have a particular equaliser setting specified for it and the reel labelled actiordingly.

The output from the reproducin,i': amplifier is normally fed through a potential divider to the loudspeaker amplifier. A switch enables this circuit to be broken, and the loudspeake~ amplifier to be fed from th8 first stage of the recording amplifier for listening to the incoming progralllille. This is qUite satisfactory for a single machine but vvith a pair of machines forming a channel, in tbe usual B.B.C. fashion, "comprehensive check" facilities vJOuld require switching of the loudspeaker or else the use 6f two speakers. For this type of operation it v[ou10. be better to modify the existing cirCUits to allow'of the switching o~ incoming or reproduced programme from either machine to the same amplifie.r and loudspeaker.

Several reels of E.U.I. tape were supplied with the machines. This tape appears to be of the same type as Magnetophon C tape, but difficulty has been experienced in spooling it in the norr;181 vray Vii th the coated surface outside. The makers therefore recol1ullended that the tape is twisted between each spool and the guide pulleys~ so that it is wound with the coated side innermost. Under these conditions? spooling appe8rs to be satisfactory, but trouble often occur S vtThen start ing a full spool as the twist may become wound on to the reel o'r the tape may break.

Tested on a K7 Ha{;netophon machine? the E .1\'[.1. tape gave a rather poorer sign81-to-noise ratio than the German C tape, but as will be shown in the next section? the results for the t'ilO types of tape were practically identical viThen measurements were made on the E.M .. I. machines.

3. Overall Perfo.r..mance

Before making measu.::ement s of distort ion and signal .... to-noise ratio on any recording system? certain recording

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levels must be chosen as reference levels for the various tests" The normal procedure is to select the highest recording' level at "l;vhich the distortion produced is reasonably small~ and to make signal-to­noise ratio measurements relative to t his level. The f:igure obtained naturally depends upon the amount of distortion that is tolerated at peak level. In magnetic recording? hml8ver? it is impossible to measure the level recorded on tll.e tape directly and independently of the reproducing muchanismand the results also depend? to some extent? on the recording and reproducing equipmerrt used.

In all the te st s made on the E-.H. I • equipment the 1 Kc/s line-up level used was that recoil1mended by the makers. This level gave zero reading on the programme level meter fitted to the eqUipment and resulted in a current of approximately 1 mA in the recording head. Peak level was taken to be 8 db above this.

3.1 Frequ§Pcy Respoqse

As received q the overall frequency responses of both recorders were about 6 db dovm at 10 Kc/s. The ' reproducing a.t:ilplifier e c;ualisers vvere therefore adjusted until the responses were approximately flat. The final curve s obtained are shown in Fig. 3, for E.M. I. tape and for a typical Magnetophon C tape.

3.2 ~lgnal-to~Noise Ratio

Measurements were made with a B.B.C. noise-meter and with an unweighted valve-voltmeter. A high-pass filter cutting below 150 c/s could be added to discriminate between hum and other noise.

The results are givsn in Table 1, relative to peak level at :;.. Kc/s. Usin:" the noise-meter? signc:ll-to-noise ratios of the order of 50 db were obtained for both E.M.I. and NIagnetophon tapes. With the 150 c/s filter inserted the valve-voltmeter gave approxirnately the same re suIt s, but when the filter ViaS removed signal-t o-noise

· - 6 -

.,ratios were only about 38 db. L'l.. considerable amount of hum was present5 the main component of which was found to be 50 c/s.

303 pistortion

Table 2 gives the results of harillonic and intcrsodulation tests at different recording levels. It can be seen that, except at the higher test frequencies, the distortion at the levels used ViaS less than 2%. 'rIle larGe distortion present at high frequencies was-primarily due tO,the higher value of recording head current used; as shown in Fig. 1 the reco'rding; characteristic rises over 10 db betvveen' 1 Kc/sand 10 Kc/ s? and most of thi s rise occur s between 5 Kc/s 2nd 10 Kc/so The internlOdulation betwGcn 8 Kc/s and 10 Kc/s could be reduced by using a flatter recording characteristic thOU[:;l'l probably at the expense of SOl:18

inprease in noise level.

304 Wm! and Flutter

Wow and flutter were lileasured as described in R,,'search Report NG. c.065, the variations of frequency in a test tone being recorded on a Henry HUf;hes Pen R;}corc1er., The re cords ShO'll that tvJO dm:li.nant per iodic var in tioilS were present in each ilIa chine ~ the EtIilplitude of each varying in a rando;,l mannor about a most probable va,lue. The per iodic var ia t ions in each "lachine I,vere a, cyclic WO'if occurring about four tiLlf.'S per second 1J.pbn vrhich ',VD,S superposed a cyclic flutter occurring about 25 tides per second, the wow and flutter arising from irregularities in the idler wheel and driving Dotor shaft respectively.

I lLchine 1 ~ the L~ c/s WOVl had a 1l10St probablG (~}eak to peak) value of 0.10 per cent but values up to C.17 . per cent were recorded. The 25 c/s flutter had a BOSt probable value of 0.25 per cent with values up to 0.45 per cent being recorded.

"

In Machine 2, the 4 c/s wow [lad again a [lOst probable value of 0.10 per cent but lllagnitudes up to 0.25 per cent were observed. Tile 25' c/s flutter in this Llachine was sl~laller ~ ha ving a LloSt pr oba ble value of 0.15 per cent with a rangG of variations up to 0.27 per cent.

3.5 Listening Tcst~

Listening Test s were Dlade to CODp3re direct programne' and prograCll-'1e reproduced from a K7 M,"..gnetophon? a Type D, disc recorder, and each of the EoM.I. recorders.

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The performance of tlle E.LI.I. machines VIas judged to be similar to that of the Lllcc,c;net o~ohon the background noise being perhaps slightly better. The hWll was mainly 50 cl s and was markedly audible only ,dlen listening at fairly high levels. The 4 cls V/OIN was not very noticeable on pl'ogramme but was 1 of cour se 1 very ob jecti onable vfhen listening to tone 0

A.lthough the general quality produced by both the lvIagnetophon and the E.M.I. lilachines vms high1 the reproduction iiIaS less clean than that obtained from a freshly cut disc, and a certain amount of modulation noise was observed.

4. Conclusions

The overall performance of the E.l\I.I. magnetic recorders was similar to that· of the K7 N1agnetophon. The signal-to;..noise ratio obtained wit:n the line-up level reCOGLlended by the manufacturers was not particu18rly good but the distortions were suall except at high frequencies v;Jhere considerable iiltermodulation occurred. It is therefore possible that the oVerall performance might be sonlewhat iaproved by increasing thG recording level and. by 1110difying tilG recording ciloractel"istic.

An QPpreciable e.L.1ount of wow was produced by , both lil8chiries 8 t the rotational frequency of the idler wheel? and eccentricity of the drive caused flutter? particularly on one mac11ine 0 The :presence of tIlese variations? together with'the tendency to produce high frequency distortion and a certain amount of Dodulation noise 1 were noticeable ii~len listening te st s were uade? and gave to the re produced pr ograul:,.le a certain quality which has so far been found to be pecu~iur to this type of recording.

Some LlOdifications to the present E 011.1. eqUipment would probab~y be desirable for BoB.C. u~e~ For instance, it would be difficult to provide comprehensive check facilities for tvw uachines in a normal recording channel, and it would probably be better to add El central control desk, si~ilar to that used for a Ty)e D disc recording channel. This could prbvide suitable line terIllinations, a B.B.C. peak progra_,i.:,le Lleter to raonitor

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input and output levels? amplifiers to raise t:1e level from the reproducing amplifier to normal line level~ and l~'1eanS of swi tclling the· same loudspeaker amplifier either to tile incoming prograrllrne or to the reproduced pr ogrammes.

The equalisers in the reproducing a~plifiers wore found to be rather unsatisfactory in that the vlhole range of one equaliser was covered by a very small rotation of the adjusting screws. There was interaction between the pair of equalisers in each a~plifier, the effect of v;hich ViaS sometimes to cause u.ndesired dips , in the middle of the high frequency range.

The push-buttons on the equipment tested ViGre found to be stiff and unreliable, YJhich might be cl. source of trouble in operational use. The necessity of putting twists in the tape may also lead to a nU.mbJr of practical difficulties.

The ma ins socket s mounted at the back of t~(le equipment are of a two pin type not used in the B.B.C.

GEA.

In V3 s t ig...;l·~ i Oil bJ::

Report by'

NL.~.~ (HoL. Kirke)

E.D. Daniel Eo Friend WeK.E. Geddes

E .D • . Du.;;liel Ho Davies

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TABLE 1

Signal-to-Noise Ratios in db. Measured Relative to Peak Level at I Kc/so

Machine No. 1.

l1ilotors running ~ but tape not in contact with 11ea d S 0 0 • 0 coo 0 0 Cl (I 0 () 0 (I 0 0 0 •

Using E.HeI. tape No. 1 000

11 11 iI Noo 2 000

Machine No. 2. I I

Motors running? but tape I

Valve voltmeter

umveighted

41

36 41

not in contact with I hea ds et 0 0 0 0 0 " ·0 0 0 0 0 0 0 / 0 0 Cl 0 0 41

i

Using E.M.I. tape No. 1 ••• 39 11 it 11 N' 3 36 o. 0 0 •

IJ ~agnetophon C tape o. . 36

Valve voltmeter

I with' iH of. filter

56

50 49

59

Noise meter

'55 51 51

59 54 50 I 52

1. Total Harmonic Distortion

250 cls JYi/C

IvVC

1 Kc/s ][l/C

MIc

4 Kc/s IvVC

IvVC

2. Intermodulation Distortion

1 + 1.25 Kc/s Ivi/C

MIc

5 + 6.25 Kc/s IvVC

.1vVC

8 + 10 Kc/s IvVC

MiC

... 10 -

!ABLE. 2

Results Qf Distortion Tests

1

2

1

2

1

2

1

2

1

2

1

2

Tape

I I E.MoI.

Magnetophon EI.M.I.

Percentage Dtstortion I Recordinf~ level (db) 1

. I +4 _I +S +12 I o

! I

I I I 1.1 '1.0 11.2 ,10sl

11.2 107 I 0.9 0.9 I 1.2 2.0

1 E.nI.I. 1.0 1100 11.3 2.1

"

Maf?etophon ! J 11'.32 21,.09

jl'

, E.l'floI. O.S 10.9 !

I I i

1\ E.M.I. 0.9 0.9 11.0 0.9! M'::'i~netophon I 0.8 I 0. 9 1

! E.1vl.I. 0.9 10 •9 1101 I 102. ij_ ... ____ • __ ...l~. ___ ~_" jj .......... --.....!.

10.Q 0.8 -"j

E.lIIoI. 1.0 1.0 IvIagnetophon lo.s 0.8 1.0 E.M.I. 1 1.0

10

•9 0.8 1.0

I o. 7

f I I

E.lvI.I. 1.0 1.5 I 10.7 i Magnetophon I 11.0 1.0 1·5 E.M.I. 1.0 1.1 1.4 2.1 I

I I E .M. I. 1.5 2.4 6.5 14.0 I I

I M<.; bX1etophon <2·5 4.9 10 .. 6 I I I

j

1508 J I""YI 4.2 6.4 11 0,.0 ~ .,t:." • t~.!. 0 •

J 1

o

This d raw,ngtspecoficatlon IS the property of th~ Brttlsh Broadcasting Corporation and may not be reproduced or d isclosed to a third party on any form Without the wrttten permission of the Corporation

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Frequency m cycles per second

FIG. I.

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w o ° o

"" 01

" " o 0 o 0

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o <:> o o

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FREQUENCY RESPONSES OF RECORDING AMPL\F1ER.

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[OR'N \I~ R.EPO~T

ICH'D. 'N. C. 0 G 8·' IAP'D. 'f 4,,: '3 SHEETS I

BBC f\ESE~RCH DEPT

f..M.\. · M·P\GNEllC TAPE RECORDERS .

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W !> 0 0 0 q

Frp.que"l-::y

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VI Cl '4 (J) <D.- N 0 0 0 000 Cl 0 0 0 000

0 8 . In cycfes per second

FIG. 2..

W J' Y' :-J 0 0 0 0 0 0 0 0 0 0 0 0

N c c 0 0

(il u, (J) I .- l/')

~ C ID m

1+10

I+S i

- +4~

-8 W 0 0 0 0

FREQUENCY RESPONSES OF REPRODUCING AMPL\FIER. ------------ ---- ---r--- ---

BBC RESEAR.CH DEPT

E.M.L M~GNET\C TA-PE RECORDERS f\-EPORT

C.06S-2. OS 41 P ~ SHE£TS

This drawing/specIfication is the property o( the Brotish Broadcasting Corporation and may not be reproduced or dIsclosed to a third party In any (orm wIthout the wrotten permission o~ the CorporatIon

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VlOl ..... Cl)W-o 0 0008

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w o o b

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S g g 8 8 g • 0

Freq(ie.rrcy /1'1 cycles per second

FIG. "3.

N

§ w o o o

.~ o o o

'" o o o ::I

Cl! Vl W:-""Vl • C ~ m ro

w o o o o

OVERALL FREQUENCY RESPONSES ~FTER ADJUST1NG THE REPRODUCING AMPUFlER .EQUALISERS.

B~C ... E.M.I : M~GNET'C . TAPE RECORDERS P-EPOF\T

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