-~-----~-----VOL. B No. 7, pp. 181-212 JANUAHY 1953

Philips Technical ReviewDEALING ~TH TEC~CAL PROBLE~

RELATING TO THE PRODUCTS, PROCESSES AND INVESTIGATIONS OFTHE PHll..IPS INDUSTRIES

EDITED BY THE RESEARCH lABORATORY OF N.V. PHILIPS' CLOEILAMPENFABl,'lIEKEN,EINDHOVEN,NETIlERLANDS

MAGNETIC SOUND RECORDING EQUIPMENT

by D. A. SNEL. 621.395.625.3

The recording of sound on magnetic material is an invention which dates back to the lastcentury. Then it was practically forgotten, left behind by other, more perfected methods ofrecording; electronic amplifier techniques developed in the meantime have revived interest init, however. In a relatively short time the magnetic system has now captured a large field ofapplications which were prClJÏollsly dependent on other methods, besides opening lip nelV

spheres in sound recording. .

Introduetion

In 1877 Edison surprised the world with hisphonograph, an instrument for the recording andreproducing of sound, in which the vibration set upin a diaphragm by the sound waves were mechanic-ally recorded on a steel cylinder covered with tin-.foil. The cylinder was played back by means of thesame apparatus. In Edison's phonograph a vertic-ally modulated groove. was produced on .thecylinder, i.e. a groove the depth of which varied inaccordance with the deflection of the diaphragm,in contrast with the gramophone disc introduced bythe American Berliner, which is still employedtoday and in which the groove is modulated laterally,i.e. in the form of a curved line 1). There is no needto dwell on the point that the direct, mechanicalrecording and reproduetion of sound was latersuperseded by indirect methods using an electricaldevice (an amplifier) between the receiving element(microphone) and the cutter, as also between theneedle and the reproducing diaphragm (loudspeaker).The invention of the sound film introduced an

optical method, again in conjunction with the neces-. sary electrical appliances, in which' the sound isreéorded: on the film photographically in the formof. a track, either of constant widt11 and v~riabledensity, or constant density and variable width; th~reprodtïction is obtained by means of a scanninglight slit and photo-electric cell.

I) Sec L. Alo,ns, Philips techno Rev. 13, 134" 1951 (No. 5).

In between the two systems of the gramophonerecord on the one hand and the sound film on theother, there is the Philips-Miller system whichemploys a sound track with variable-width modula-tion, cut mechanically in a celluloid tape coatedwith black lacquer and scanned by optical means 2).

Each of these methods has its own specific advan-tages and drawbacks, mid each is employed today'in its own particular sphere. In recent years, how-ever, another system of sound recording and repro-duction, basedon the use of magnetic materials,has found wide application. As long ago as 1898,Valdemar Po u l s en, a Dane, patented a magneticsystem of recording; the sound being recorded on asteel wire wound on a drum and passed betweenthe poles of an electromagnet. Naturally hisequipment was primitive seeing that, at that time,the amplifier had not yet been invented. Poulsen'ssystem was not developed and was accordinglyshelved. About in 1935, however, when moderntechniques of amplification were known, Po ul s en'sidea was taken up by Stille, Schüller and othersin Germany; Lorenz in that country and Marconiin England placed a machine that worked with asteel tape on the market, The speed of the tape was.about 1.5 m/sec, and, it "ras poásible to record fre-quencies up to 6000 c/s; the tape was wound on aspool about 70 cm in diameter and the playingtime was a good half-hour ..

. 2) Sec Philips techno Rev. 1, 107-114" 135·141, 211·214,231-236, 1936.

;

PHILlPS TECHNICAL REVIEW . VOL. H, No. 7"182

As early as 1927 F. Pfleumer in Germanyand Joseph O'Neill in the U.S.A. had developedpaper and plastic tapes coated with a layer ofmagnetic material. In Germany, in the'· period1940-194.5, magnetic recording systems using suchtapes were further developed on a large scale.

Magnetic recorders marketed nowadays are eithei·of the "wire" or "tape" variety. The wire recorder.will not be discussed hêre. Certainly, the equipmentis usually less complicated and therefore cheaperthan the tape recorder, but it has certain definitedisadvantages; if the wire breaks, or if it is inex-pertly handled, it will often become hopelesslytangled; the joining of two ends, either to repair abreak or for the "montage" of different selections,can only be achieved by knotting, which is amuch less reliable method tha~ the·splicing oftwoends of tape. Several other, more fundamental, dis-advantages of wire in contrast with tape are men-tioned in the following paragraphs.

General construction of a tape recorder; propertiesof the tape

The tape is magnetized by means of a "recordinghead" which is in effect an annular electromagnetwith small air-gap, along which the tape is passed(fig. I). The modulation current flows in the magnetcoil; this current may consist of the alternatingcurrent generated by a microphone as a result ofthe sound to be recorded. For the reproduetion thealternating magnetization produced in the tape,the "sound-track", is scanned by a "playback"head, which is almost identical with the recordinghead; it contains a coil, across the ends of which analternating voltage is produced by the passage ofthe tape, this voltage being amplified for passingto a headphone or loudspeaker.

The materialon which the sound is to be recordedmust be capable of permanent magnetization and

Fig. J. The recording head, over which the magnetic tapepasses. The modulation current flows in coil S. The tape Bpasses in a gentle curve over the gap A. Recording takesplace at a.

must therefore exhibit a certain hysteresis and rem-anence; for reasons which will be explained pre-sently, moreover, the coercive force must not be toolow, sayin the order of ItoHe=O.OI WbJm2 3). Lastly,the structure of the material must be very finelycrystalline. This requirement is related to the inherenttape noise, which must always be as little as possible.Magnetization ofthe tape does not vary continuous-ly from point to point, but is constan,t in magnitudeand direction within certain small zones (Weisszones) and changes suddenly between the bounda-ries of one zone and another. The ~oise arising fromthis discontinuity is so much the less according asthe Weiss zones are smaller, and, since one suchzone can at most extend to the confines of one whole.crystal, a finely crystalline structure is obviouslydesirable. The simplest course is to use as basis apowdered magnetic material. As a rule, iron oxide;either red (y-Fe2Ûa) or black (Fe304) in a grain sizeof less than 1 fL is employed.

The powder can be incorporated in the tape intwo different ways: it can be either mixed thorough-ly with a thermoplastic material such as polyvinylchloride, the tape being then extruded or rolled inthe hot condition; or it can be mixed with a lacquerwhich is subsequently applied as a very thin layer(e.g. 15 fL) to a paper or plastic tape about 35 fL inthickness.

The low noise level resulting from the use of finely crystallinematerial is also the reason why tape is usually preferred towire. Certainly a solid wire can be made that will exhibitvery.small crystals, but wire introduces another very importantcause of noise which is entirely absent in tape and which willbe explained in the following.

A. homogeneous tape, made in accordancewith the first of the two above-mentioned methods,is the most able to meet the numerous and, insome cases stringent, me eh an i c a I requirements im-posed by its use in magnetic recording instruments.The chief requirements, standardized in somecountries, are that the tape shall not' break underloads up to 2 kg, and that the elongation one minuteafter removal of a load of 1 kg shall not exceed 0.2%.Further, the tape must not pull out of straight,assume a curved cross-section or shrink noticeably;it must be easily cemented and sufficiently smoothto enable it to slide over the recording head andother components without causing undue wear. Thetape itself should not be subject to too muchwear either; in particular it must shed as-little dustas possible, must be flexible and maintain all these

3) lp C.G.S. units; n, 100 oersted.

JANUARY 1953 MAGNETIC RECORDERS 183

characteristics against the effects of humidity andtropical temperatures.

As far as weal' is concerned, it will be clear thateven the least, usually unavoidable, wearing of thetape would soon fill up the air-gap in both recordingand playback heads with magnetic powder and thusprovide a magnetic shunt for the tape or coil. Thisdifficulty is surmounted by filling the gap in advancewith a non-magnetic material. Such material mustbe harder than that of which the pole pieces of theheads are made, to prevent it from wearing awayand leaving an open gap. Beryllium copper is verysuitable for this purpose.

Standardized dimensions for the magnetic tapeare: thickness maximum 60 fL, width 6.20-6.35 mm.The length is unspecified, but is usually not morethan 1000 metres. The tape is magnetized eitheracross the full width, or only for a part of it, toenable two different tracks to be recorded next toeach other, or for stereophonic recordings, in whichcase two tracks differing only very slightly fromeach other are made simultaneously. In specialcases, as many as six tracks have been made on onetape.

Recording with high-frequency auxiliary field; theresponse curve

If a magnetic particle in the tape passes the alter-nating magnetic field of the recording head (i.e. thegap and its immediate vicinity) so rapidly that thetransit time is short compared with the periodicityof the field (i.e. of the sound to be recorded), it maybe assumed that this magnetic field is constantduring the time that the particle is passing throughit. The particle is magnetized to a certain degree,mainly in the longitudinal direction of the tape 4)and a part of this magnetization is retained in con-sequence of the remarience; the remanent magneti-zation at any point in the tape is of course morepronounced according as the magnetic field isstronger at the moment when that point is passed.

This is not enough, however. It is necessary forthe magnetization of the tape to be directly pro-portional to the field strength, since otherwise theweÜ-kn~wn disturbance due to non-linear distortion(overtones and combination tones) will be manifest-ed in the sound reproduced. ,Although the magneti-zation curve of most magnetic materials, includingthat of magnetic tape, is anything but linear, therequired linearity can be obtained by an artifice.

,'I) In principle it is also possible to -magnetize the tape in

the direction of the width or thickness. With the usuallongitudinal magnetization a certain amount of magnetiza-tion always occurs in the direction of the thickness as well.,

This consists in superimposing on the' modulationcurrent an alternating current ("auxiliary current"),the frequency of which is well beyond the audiblerange. The way in which the auxiliary high-frequen-cy field thus produced accomplishes its purpose willnot be discussed here; this will be explained in anarticle shortly to be published in this Review,dealing with the mechanism of magnetic recordingin greater detail. It is sufficient to say here that,in principle, the necessary linearity can also beensured by means of D.e. pre-magnetization; thisintroduces a considerable amount of backgroundnoise, however, which is absent when the high-fre-quency method is used ..The frequency of the auxiliary field must be so

high that it changes polarity several times duringthe passage of a magnetic particle through it. Onthe. other hand, it has just been pointed out thatthe field at the sound frequency may be regardedas constant during that time. Naturally this is nolonger the case either where the highest audio·frequencies are concerned. What we then recordat a given point on the tape represerrts, as it were,an average of the field strengths to which the par-ticle is subjected, not when passing the gap, asmight be supposed, but when traversing a smallzone at the edge of the gap. For a further explana-tion, reference must once more be made to thearticle anounced above, One result of this formationof an average is that the high frequency componentsof the sound are recorded relatively weakly.A similar effect is encountered in the playback.

The alternating voltage induced in the coil of theplayback head by the movement of the tape isproportional to the speed of variation of the fluxin the coil. This flux is dependent on some kind of"average" of the magnetization over a length-of thetape equal to the width of the air-gap. An average isthus formed in this case, too, again with detrimentto the higher frequencies in the sound. The smallerthe air-gap and the greater the tape speed (v), thehigher the frequency at which this effect will becomenoticeable; the air-gap must be smaller than theshortest wavelengthto be recorded and reproducedby the tape. The wavelength of a sinusoidal signalof frequency ! is v/f. At the much used speed' ofv = 76.2 cm/sec (30 inch/sec) and with !. 10000cis, the wavelength v/i = 76.2 fL. For the speedmentioned the playback head is' usually given a gapof 10 fL to 20 fL; the gap' in the recording head maybe made slightly larger, say 25 fL to 40 fL, this beingadvantageous fr01I1:the poiii,;of view of moreuniform magnetizatiou throughout the thickness ofthe tape. .

184. PIHLIPS TECHNICAL REVIEW VOL. 14., No. 7

Let us now consider the respons:e curve of themagnetic recorder as a whole, viz: the alternatingoutput voltage as a function of the frequency of atape recorded with constant curren.t amplitude.Since the voltage induced in the playback coil isproportional to the speed of variation of the magne-tization, it increases with frequency; in other wordsthe I'esponse curve exhibits a natural gradient to afactor of 2 (6 dB) per octave. In the higher frequen-cies the increase is not so marked, and at the upperend of the range there is even a drop. In part, thisis due to the above-mentioned influence of the finitedimension of the gap, but there' are other causes,'too. In the sound track the shortest wavelengthstend to be levelled out, seeing that closely spacedelements, magnetized with opposed polarity, havea demagnetizing effect on each other; in principlethis effect should be counteracted by the iron intheplayback head as soon as the relevant section of thetape approaches it, but, owing to the small inter-vening space that always remains between tape andhead, this counterbalancing effect is not complete.(In order to suppress the effect in point as much aspossible, the coercivity of the magnetic materialshould be high.) In the shortest of the wavelengthsconcerned this space between tape and head pro-vides an opportunity for the lines of force to closeup to a large extent without passing through themagnetic circuit of the playback coil. Lastly, theeffect of eddy current losses in the recordingand playback heads becomes more pronounced asthe frequency is increased.Whereas the response curve of the magnetic

recorder accordingly assumes the form shown infig. 2, a flat characteristic is our ultimate aim.

+1

+50 i-:-0 1/

:.- I\./'

v-a V5 ,/'0

5,,;

05

\\\

dB

t -5-1-1-2-2-3-3-4

50 100 200 500 1000 2 3 4 5 67 10000 cl'726.16

Fig. 2. Overall responsecurve of a magnetic recorder, showingon a relative scale the alternating voltageobtained by repro-duetion with constant amplitude in the recording coil, as afunction of the frequency. The fnll line shows the responsewithout any correction;the broken, for the greater part hori-zontal, line is obtained with an RC network in the playbackamplifier. The higher frequencies still require boosting.

Our first course in pursuing this aim is to correct therise in the middle frequencies. This can be achievedby means of a simple RC network in the playbackamplifier, the result being the largely horizontalbroken line in the figure. In the second place, thehigher frequencies need to be boosted. This can be -effected in both the recording and the playbackamplifier. In principle it is better to boost the highnotes at the recording stage, as they are then placedfurther beyond the noise level of the tape. Thisdoes involve the risk, however, that, in the louderpassages the saturation magnetization of the tapewill be reached in the higher frequencies, with con-sequent distortion. A oompromise is therefore made,the correction being introduced partly in the record-ing and partly in the playback. It is a point inour favour that in orchestral music - during therecording of which the interference due, to noise anddistortion is the most disturbing - the strength ofthose components of which the frequency is high isusually less than in the middle and lower frequencies.This circumstance makes it possible to shift thegreater part of the necessary correction in the higherfrequencies towards the recording side.

Erasing

Recordings on magnetic tape exist by reason ofthe magnetized condition of the particles in the tape,and they can, therefore, be modified by a strongmagnetic field, in contrast with other types of re-cording which, apart from the effects of physicaldamage, are permanent. For this reason magnetictape recording should not be stored in places wherestrong magnetic fields set up by permanent magnets,transformers, motors etc. occur. The requirementthat magnetic tapes should not be too sensitive tomagnetic fields with which they are accidentallybrought into contact is yet another reason why thecoercivity of the material should not be too low(see above).At the same time, however, the susceptibility of

magnetic recording to magnetic influences is pu t togood use in that used tapes can be rendered magnet-ically neutral and thus made serviceable for freshrecordings. This erasing is effected by a separate"erasing" head, this being in the main identicalwith the recording and playback heads. A high-fre-quency magnetic field is produced in the gap of theerasing head, such that the materialof which thetape is made, when passing the head, is subjected toa number of hysteresis loops which first increaseand then decrease to zero on the B-H CUl'Ve.After passing the erasing head, then, the tape isonce more in the demagnetized condition.

.JANlJAH,Y 1')53 .vIAGNETIC HECORDEHS IHS

Fig. 3. The magnetic heads of a Philips tape recorder, arranged as a singtc uni t (sec fig. 6);from right to left the erasing, recording and playback heads. JII fro n t 0(' the two last-mentioned heads a mu-metal screen is provided, this being placed o vcr the tape to clirnina u-iu terfcrcuco due to cross-talk .md possible al t.cruu ting Iiclds in lhe vicini t y.

In most recorders the sequence of operations IS

such that, for recording purposes, the tape firstpasses through the alternating field of the erasinghead, thus ensuring a perfectly "clean" basis for therecording.

In the case of w ire recorders it is much more difficul t toerase a recording completely. This is due lO the fact that thewire is drawn through slots in the recording and erasingheads, and that side 0(' thc circular cross-section of 1hc wirewhich faces the open cnd of the slot is only sligh tly exposedLu the recording and craRing fields. Since wire is always mure

or less subject to twisting, after erasing, certain parts of thewire will, generally speaking, retain traces of the previousrecordiug. vftor repeated recordings and erasures all thesercsiducs tend to produce a background babble, which consti-tu t.cs extra noisr-, no t occurring in tapes.

The frequency of the erasing field is normally:35 or 40 kch and the auxiliary recording fieldgenerally soruewhat higher, e.g. 100 kc/s. The largermagnetic reeorders will usually incorporatc twoseparate oscillators to provide these frequencies,but other equipment may have only one oscillatorgenerating the two frequencies required, asharmonics of a fundamental frequency.

In the larger equipment the erasing, recordingand playback heads are mounted in a common,detachable, hol der. Fig. 3 shows the unit as fittedto one of Philips ' tape recorders, this assembly beingfitted with plugs for direct connection to the ampli-fiers. The playback and recording heads are housedill cornp a rtrnen ts of high permeability material suchas mu-metal, in order to limit cross-talk between the

heads and induction from external magnetic fieldsat mains frequency (hum). The gaps in the recordingand playback heads must be precisely at right anglesto the tape; any obliqueness would again affectthe higher frequencies 5).

The tape driving mechanism

For a satisfactory quality of repruduc tiou thespeed of the tape Jla~sillg the recording andplayback heads must be highly constant. Themechanical problems to be solved in order to meetthis and certain other requirements connected withthe tape drive are worthy of beilIg discussed Ij). IIIthis we shall base our considerations mainly 011

the design of Philips" tape recorders.

In principle, uniform mot.ion of the tape is uh-tained hy means of all accurately machined sp indlewhich is driven hy a synchronous motor and againstwhich the tape is pressed by a rubber roller (fig. 4).

") Actually, it is only necessary for the gaps in the recordingand playback heads to be parallel to each other. Inorder to ensure proper reproduction of a recorded tape onany suitable instrument, perpendicular gaps should ofcoursebe used. - It may be added here that, in the caseof "amplitude" recordings, as ill the P hili ps-Mille r systemas well as in one of the photographic methods, obliquepositioning of the gaps results in non-linear distortion ofthe sound: see J. F. Schon ten, Philips teehn. Rev. 6,110-119, 1941. In our own case (intensity recording) thereis only linear distortion (variation ill the response curve,as mentionecl in the text).

6) See also J. J, C. H a I'de nb erg, The transport of soundfilm in apparatus for recording and reproduction, Philipstechn. Rev. 5, 71.. fll, 1940, in which article similar prob-bierus concerning the drive of t.hc Ph il ip svvl il l e r systemare discussed.

186 PHILIPS TECHNICAL REVIEW VOL. 1<1" No. 7

This driving spindle must of course he made of non-magnetic material and should he as little as possiblesubject to wear.

KI K2 K3

Fig. 4,. Principle of the tape drive. A transport spindle;B = magnetic tape; R = rubber roller holding the tapeagainst the transport spindle. Erasing, recording and playbackheads Kl' K2 and ](2 are assembled in a common unit. M3 =winding spool; ./1'[1= supply spool. Gl and G2 = guide rollers.

Eccentricity is reduced to a point below the toler-ance limit of about 4 fJ. by grinding the spindle in itsown hall-hearings. The rubber roller must he highlyuniform in composition and the pressure such thatthe tape will just he prevented from slipping whena certain tensile force (about 800 gr) is applied.

Synchronous motors ensure the roller to he ro-tated at a perfectly constant speed as an average,hut there is always a certain slight oscillation super-imposed on this rotation, owing partly to the finitenumber of poles in the motor and partly to a naturaltendency to "hunt"; in accordance with the loadon the motor, the ro~or lags in phase with respectto the stat or field, and this phase angle, generallyspeaking, does not adjust itself aperiodically, butin an oscillatory manner. This tendency is suppress-ed as much as possible in the design of the motor,a discussion of which would take us too far afield,however. Any residual effects, including irregulari-ties in running due to the poles, are further reducedby placing a flywheel on the transport spindleand by transferring the motion from the motor bymeans of a flexible coupling ("mechanical filter").

When recording or reproduetion is to be com-menced, the transport spindle must quickly assumeits correct working speed, viz. within a matter ofseconds. In order to ensure this, and to permit themotor to come easily into synchronism with themains, the motor must have a certain starting torque;this will have to be so much the greater accord-ing as the inertia of the flywheel is increased. It istherefore not advisable to use a heavier flywheel thanis necessary to keep fluctuations in speed just withinthe tolerance, for which limits have been standard-ized internationnally; the motor can then be suit-

ably proportioned to ensure that the correct speedis obtained within the desired space of time (inPhilips magnetic recorders about 2 seconds).

The Comité Consultatif Interna tional de Radiodiff usion havelaid down the following limits for fluctuations in the speedof magnetic recorders intended for broadcoasting purposes:max. 1 per thousand at 30-inch/sec tapcspced,max. 1.5 per thousand at 15 inch/sec,max. 5 per thousand at 7.5 inch/sec. ,Special equipment has been developed by means of which,when reproducing a recorded frequency of 3000 cis, a directreading as a percentage can be obtained of variations in speeds,if desired split into fluctuations of frequency < 20 cis(wow) and >20 cis (flutter).

The synchronous motor 'driving the transportspindle must not fall out' of synchronism when themains voltage fluctuates within the extremes ofthe prevailing values. In some equipment, moreover,it must he suitable for two taplil speeds, in which casethe dimensions of the flywheel should he such thatspeed fluctuations willnot he too great at the lowerspeed, the motor being such that, with this (relativelyheavy) flywheel, it will develop the proper speedquickly enough for the higher of the two tapespeeds as well 7).During recording and playback, not only the trans-

port spindle has to be driven, but also the spool onwhich the tape is wound up. Simultaneously thesupply spool from which the tape is being unwoundmust he braked in order to ensure that the taperemains taut; this is esserrtial in order to keep thespace hetween the tape and the heads, as mentionedabove, sufficiently small (fig. 4). For rewindingpurposes the supply spool has to be driven, and, toprevent looping of the tape, the other spool is thenbraked. As a rule, higher speeds are required forrewinding than for recording of playing; it is alsoconvenient to be able to run the tape throughfaster in the forward direction when collectingparticular passages of recordings for montage pur-poses.

In the larger magnetic recorders all these condi-tions are met by providing separate motors for thetwo spools. FOl: recording and playing both are ener-gized with a fraction of the mains voltage; the work-ing spool then slowly winds up the tape and theemptying spool receives a slight torque in the oppo-

7) In recorders that work with a slow tape spccd the motorcan be run with the tape disengaged from the transportroller; the tape can then bc started up immediately atany given moment by springing it suddenly against thcroller. If the tape speed is on the fast side, however, thisinvolves too mueh risk of snapping the tape; for thatmatter, even in slow-speed instrumcnts suitable meanshave to he provided for absorbing such jerks.

JANUARY 1953 MAGNETIC RECORDERS 1117

site direction, so that the tape is held taut. For rapidrunning or rewinding the motor is fully energizedto give a tape speed some 10 or 20 times faster thannormal.

The tensile forces exerted on the tape by themotors and the inevitable slight fluctuations in theseforces must obviously affect the uniformity of thetape movement past the recording and playbackheads. The outcome is a complex system of forces,the effect of which can best be studied with theaid of an equivalent electrical circuit. This is showninfig. 5, a brief explanation being given in the cap-tion.

Fig. 5. Equivalent electrical circuit of the mechanical tapedrive, by means of which the influence of the various compo-nents on the speed of the tape can be investigated. A rotatingmass is represented by a choke (L), a constant force by ane.m.f. (E), fluctuations in the force by an alternating voltage(c), a friction or a constant braking force by a resistor (R) anda spring force by a capacitor (C). In particular, JW2 = synchron-ous motor (represented by E2 and c2); L2 = flywheel withrotor and transport spindle; C2 and R2 = spring force andinternal friction of the flexible coupling; Ala = motor forwinding spool (with Ea and ca); La mass of rotor and windingspool; 1\fl motor of supply spool (with braking forces RI andel); Ll = mass of rotor and supply spool; Rk = friction oftape against the heads; Lgl and Lg2 mass of guide rollers;Cbi' Cb2' Cba, Cbi and Cbs = resilient sections of the tape.The tape speed at the head is represented by current flowing

in Rk; in the ideal case tins would be a direct current withoutany ripple. To this end the ripple voltage from C2 must befully smoothed by L2' C2, R2 and Cba; Cl is largely smoothed.hy Ll' the remainder is smoothed by the "filter" Cbi' Lgl'Cb2; similar conditions apply to Ca.

. The forces applied by the spooling motors (regard-ed as constant for the purposes of the diagram andrepresented by the e.m.f. E3 and resistance RI) willin general depend on the diameter of the roll oftape wound, or left, on the respective spools. Inprinciple, this could result in a variation in the tapespeed 'from the beginning to the end of the tape.It is possible to design the motors so that the forceswill be independent of the diameter; experience hasshown, however, that ,a certain amount of variationin the force, to the extent of say 1 :4, is quite per-missible. For such a limitation of the variation itsuffices to make the spool core? large enough.

It should he noted herc that the type of drive just described,in which constant speed is ensured mainly by the transportspindle, can be used only in tape recorders, not with wire.In wire-recording, variations in speed are therefore usuallyappreciably greater, ~nd a gradual change in the speed fromthe beginning to the end of the recording is by no means soeasily prevented. This is particularly troublesome whendifferent parts of a recording are extracted and assembled;during the playback a sudden change in the pitch of the soundmay then be audible.

Rapid spooling and rewinding of the tape in-troduces another problem of its own, namely thepossibility of stopping the tape suddenly when awanted pàssage is found. Both the spools mustthen be capable of stopping quickly and simul-taneously if the tape is not to be allowed to runslack, 'or to' be stretched possibly to the breakingpoint. Numerous devices have been tried out. InPhilips' recorders this problem is solved by meansof carefully baianced brake-bands on the spools,act~at.ed by springs which are released by Bow dencables whe,n a push-button is depressed to stop themotors, This system is very reliable, besides beingsuperior to, and more flexible than the electro-magnetic braking system often employed, whichnecessitates special measures to avoid audible inter-ference due to induction effect in the tape. Toprevent unnecessary wear, a slide is pushed up whenthe tape is to be run through quickly, thus holdingthe latter clear of the erasing and recording heads(this can be seen between the heads in fig. 3); thetape, however, maintains light contact with theplayback head, so that the sound can be more or .less followed and a wanted passage rapidly located.

Some current models of tape recorder'

Of the different models of magnetic recorder madeby Philips let us now consider two with a view toexplaining some of the special features, viz. a largemodel for professional use in broadcasting studiosetc. (type No. 10028/03) and a small portable"semi-professional" set, which is at the same timesuitable for amateur use (type No. EL 3540).

The first of these is depicted in fig. 6. There aretwo tape speeds, viz. 76.2 and 38.1 cm/sec which,with a standard tape length of 970 m correspondsto a playing time of 21 and 42 minutes respectively.The response curve is flat to within plus or minus2 dB between 30 and 15 000 cis and the noise level(background, hum etc.) is 54 dB below signal levelon full output, i.e. the level at which distortion is2%. Variations in the tape speed, both transientand throughout the length of the tape, are within'0.1% at the speed of76.2 mn/sec and within 0.15%at 38.1 cm/sec.

IIHl PlllLlPS TECHNICAL REVIEW rUL. I,J, No. 7

Li'ig .. o. largc IIIHg:Ilt't.ic ruco rdr-r for j,ruaclca~lillg-. lIlld lillilstu d ios, etc., type \0. 100~U 03. Uil t.he top pallel \\'111 he seenthe tape spools, t l«- h~ad a"'('I1I"" alld till' «ou t rol k uob-.'The oscillu t ors <lIHI \U1ta1-!-T :-:lIppl~- 1;llit:-: ;11'1' IlUu~cd ill;-;idl' t lu-ca hi ru-t .

The en t.ire eq uipmen t I~ cun t.ai n cd III a st.eelcabinct . The dri\ illg mech an ism i~ muu n t.ed lI11t1('1'

the Lop panel, Oil iho upper "iele of \\ hiel! wi ll Jj('seen Ihe t \\'0 "poob, the recording, ['la)'back a n derasing unit a n d lite p ress-but.t.ou con t.rols (see also.Jig. 7). The cabinet is fitted with two drawers whichslide 0 ut to the rear and COil t aiu the record ing am-pl ifier, the playback amplifier wi t.h corrcct in g net-work, the two high-frequency oscillators (100 k cvs

for recording a n d 4·0 kc/s for erasing) and a curnrnonvoltage supply un it , The drawers are equippedwith built-in ['lug p in s Lo Iacilit a t e t.hei r rr-ruo v aland replacement fur scrviceillg purposes.

To ensure that the whole unit shall be as versatileas possible, the high-frequency and speech currentsare both variable; by means of two switches (lowerpanel, fig. 6), each of these can be adjusted to allyone of 11 stages. A control is also provided for

buosting the high tones to a variable extent duringrecording, so that optimum playback quality canhe obtained whatever the characteristics of themagnetic tape used.

The upper front panel cou tains a meter fro mwhich can he read the values of the recording andplayback currents, as well as the high frequency an derasing currents. The recording and playbackamplifiers are designed to receive an d deliver the~igllal al "line" level, i.e. a level that is suitable forIransmission by telephone line to a broadcasting-t.at ion. A jack ill parallel with the meter serves for«o nn ect ion of headphones for monitoring the re-

cording or playback.Owing to rhe very fine tolerances employed in the

workmanship, the seusit.ivitv of the recording andplayback heads - as well a,; their o t hcr cha ruc-rcri-Lics - vary very lit t lc. Defectivo heads call,therefore, be replaced wit.ho u t ally readjustment, togive the same uutput level within about 1 dB.

Fig. 8 depicts the other model under review. Thisi,; of much sirupler design and has only two heads,une fur crasing aud o n e fur recurding an d playback:a ~ingle oscillator (pre-magnel iz at io n aru] erasingare ho rh cflcct ed al 4.;) k c/s}: oul v one rn o lo r, which

Fig. I. C1o"'-lIp of the tup p an cl "I' the IIlag:lletic rccurdvrdepicted in fig. 6. The co mpo ru-uts of the driving mecha nisrnsk ctchcd ill fig. I· "re casi lv rvcog nizvd . _\t the front of thispanel, on thc left, \I ill bc SCCII thc plH,h-I)IIttOIlS for the control.The first of these (left to right) is fur the rewind; rotation ofthe knob on the extreme left controls the speed, both forward"lid reverse. The second button starts t lu- recorder at normalspeed forward; this is the ~'playback" set ting , seeing thatt he playback lu-ad is permnucntlv ("""H'('l('d to the amplifier(all amplifier, arc of course prcviou-Iv '" il"h('d Oil). The thirdpush-button brillgs in the erasing' Iwad alld recording head.T" avoid the possibilit.y of erasillg part. of a recording bydepressing the button accide-utallv, t.his hut t o n i, iuterlock cdill such a way that it fu nc tiou- o nlv when d~pn·.,,,·" t o g c t h c rwith the second button (whic h merely start" the machine).The fourth button returns all t he others Lo 11(·,,1 ral and stop"the instrument by braking the mo to rs,

JANUARY ]953 MAGNETIC RECORDERS 189

Fi~. R. Small portahle magnetic. tape recorder type '\To. EL 3540 for professional or am a tenruse. At the left the microphone; at t.he right the loudspeaker. The recorder itself is housedin onr :..mall carrying r-asr-.

by mechanical means fulfils all three of the functionsof normal drive, fast forwards and fast rewind;finally, recording and reproduetion are both carriedout by means of the same amplifier: one outputpentode is used for both functions by means of aswitching manipulation, however, on the under-standing that for the playback the pentode of theoscillator, which would otherwise be inoperative,is connected in push-pull with the first-mentionedvalve ill order to further improve output quality.

By thus making the most of the limited number ofcomponen t s, all instrument is obtained that ensuresquite good reproduetion and, besides, offers all theusual operational features. As the amplifier includesan ou tput stage, thc insrrumen t is suitable for usewith much lower input voltages and at higheroutput voltages than in the large model; hence, ifdesired, the speech current call be recorded directho-m a microphone ann the tape can be playedback direct through a loudspeaker. This tape re-corder works wit.h a speed of 19 cm/sec and thespools hold about 515 m of tape, giving an unin-t.errup ted playing time of 45 minutes. The responsecurve is flat to within plus or minus 3 dB between60 and 6000 cis and thc noise level is 50 dB belowsignal level on full drive. Transien t variations intape speed do not exceed 0.15% and, although theover-all variation is rather more than in the largerunit, it is still on the low side, viz. about 0.5%.

Amplifier distortion at 1 Woutput is below 2%;at 6 Woutput below 5%. This set is equipped withan electronic indicator for checking modulationdepth of modulation, and the noise level in the repro-duced sound can, if desired, be further reduced atthe expense of the higher tones, by means of a tonecontrol. With a view to the use ofthis instrument byamateurs, its eperation (loarling and unlo arling etc.)has been kept as simple as possible.A special feature which we should mention in

closing this article is the device for demagnetizingIlH' recording/playback head. Each lime the set isswitched over from recording to playing, a CU1'1'enlimpulse is produced across the coil in the head,which results in a certain, albeit weak, rern anen t

magnetization. The same thing happens iftoo stronga signal overloads the head du ring recording, or ifa steel object, say a screw-driver, is brought 100

close to the head. As remanent magnetization of thehead introduces distor-tion and noise in the recording,it is essential to demagnetize the bead. In thesemi-professional tape recorder under review this iseffected all tomatically each time the change-overswitch is operated; when the button is depressed therecording/playback head is temporarily disconnectedand placed in contact with a charged capacitor.This capacitor discharges a damped oscillationthrough the head, thus fully demagnetizing it, re-gardless of the exten I of the spurious magnetization.

190 PHILIPS TECHNICAL REVIEW VOL. 14" _No. 7

A STABILISED EXTRA-HIGH TENSION RECTIFIER FOR 5000 V, se mA

In professional tape recorders with separate re-oording and playback heads, an impulse of this kindis not produced; so that there is less need to guardagainst other sources of magnetization, occasionaldemagnetization in the u?ual way by mean~ of ade~aying alternating field being sufficient.

Summary. A description of the principle of modern magnetic-tape sound recording. The composition of the tape is dis-cussed from the point of view of the magnetic and mechanical

requirements to which the tape has to conform; this is followed _by a general review of the manner in which the recording ismade, using an auxiliary high frequency field, the 'overall'response curve and its correction and the erasing of the record-ed sound track. Details of the tape drive are given, in par-ticular the means of ensuring very constant speed for bothrecording and playing, as also for rapid running and rewind-ing for "montage", checking and so on. In conclusion two ofthe tape recorders made by Philips are reviewed, viz. a largeunit for' use in broadcasting studios etc., and a small portableinstrument suitable for bothprofessional purposes and amateuruse. The more important features of these two apparatusare mentioned, together with some special details of the design.

I

by P. PERILHOU t *) and J. CAYZAC *).

A direct voltage that at the maximum variations in load varies only to the extent of roug/Zly0,01%, that is subjet to flucuuuions equal to 1I0tmore than 1f3000th part of the maills voltagevariations and that is variable within wide limits; these are the outstanding features of a sta-bilised rectifier designed specially.for laboratory work in the field of decimetrio and centimetl'icwaves.

There are many instances in which a rectifier isfound preferable to a battery (with chargingequipment), since it demands no regular mainte-nance. Moreover, especially when high voltages arerequired, a rectifier is usually very much less volu-minous and cumbersome and is therefore moreeasily transportable. Nevertheless, if a particularlyconstant voltage is needed, rectifiers have thisdisadvantage that the direct 'voltage is subject torelative variations equal to those inherent in themains supplies to which they are connected, unlessspecial precautions are taken to prevent this.Added to this is the fact that the internal resistanceof the conventional type of rectifier is üsually higherthan that of a battery of comparable output. Theoutput voltage of a rectifier is thus generallymore dependent on the load than is the case withbatteries.

Stabilised rectifiers which are capable of meetingthis difficulty were introduced considerable timeago; these 'include a control circuit that will main-tain a sufficiently constant output voltage on afluctuating mains supply and/or load to an even

*) Laboratoircs d'Eleétronique et de Physique appliquêcs,Paris. 1\1. Pierre Périlhou, head of the Philips Labera-toires Hyperfrequences at Paris, died on 11th April, 1950, asthe result of an accident. It was on his initiative that thework that forms the basis of the present article was under-taken, and the greater part of this work was carried' "Outunder his guidance. The article was written after thedeath of M. Périlhou.

621.314.67: 621.316.722.1

greater extent than a battery working on varyingloads. More than 10 years ago this Review includedan article on the subject of circuits suitable forthis purpose 1), the principle of which is illustratedwith reference to fig. 1.

R

10853

Fig. 1. Circuit for producing a stahilised direct voltage VoR = rectifier with _smoothing filter, delivering non-stabilisedvoltage Vi.' T = control valve. I = direct current. VB ("""Vo)= constant reference voltage supplied hy a battery.

In this figure R represents a rectifier with smooth-ing circuit. The output voltage Vi is dependent onthe mains voltage and the load current J. Thecontrol circuit comprises a triode T, known as thecontrol valve, which functions as a variableresistor in series with the load, in the positiveD.C. line. A battery of which the voltage VB isroughly equal to the desired output voltage Vo, isconnected between the grid of the valve and thenegative line, the polarity being such that thedifference VB - Vo serves as grid bias.

1) H. J. Lindenhovius and H. Rinia, A direct currentsupply apparatus with stabilised voltage, Philips technoRev. 6, 54-61, 194,1.