Audio tape cassettes - Philips Bound... · Audio tape cassettes P. van derLely and G.Missriegler "Speech ... no need to ensure aconstant tape speed by using adrive capstan and pressure

1970, No. 3 77

Audio tape cassettes

P. van der Lely and G. Missriegler

"Speech (or singing) recorded on the cylinder can be reproduced as often as desired, withno weakening of the recording, and the timbre of the voice comes out well ... The repro-duced speech is of great purity and clarity, without annoying background noise. The laterinstruments reproduce with extraordinary fidelity not only what is spoken and sung, butalso what is whispered into the microphone; even the faint sound of breathing can be repro-duced."Since V. Poulsen described his recording of sound on steel wire in these enthusiastic termsin the Annalen der Physik for 1900 (the "cylinder" served for winding on the wire), a greatdeal has changed in the technique of magnetic recording. In the thirties steel tape was stillbeing used as the recording medium: this had to be played at a speed of two metres persecond. However, the development of iron-oxide coated tape and high-frequency biasingled to a rapid increase in the use of magnetic sound recording after the Second World War,and to its wider popularity among the general public. Growing requirements for convenientoperation and for effective proteetion of ever-thinner tapes .resulted in the introduetion oftape cartridges and cassettes. Among these the Compact Cassette developed by Philips,and now internationally standardized, is outstanding for its convenient shájik and smalldimensions, and has won a considerable and steadily growing popularity.

Why cassettes?

In the last ten years the use of tape recorders hasincreased to an extent that at one time would haveseemed almost impossible. The popularity of the do-it-yourself sound recording is perhaps only to be com-pared with the popularity achieved through the yearsby home movies with 8 mm film.The technical improvements that have accompanied

this development, and are reflected .in the products,have taken various directions. They have led not onlyto a higher quality of reproduetion but also, and noless significantly, to simpler operation of the recordersand to a reduction of their volume and weight. Thisdevelopment has also brought the tape recorder into aspecial field, that of the dictation machine.Like loading a camera with 8mm cine film, threading

the tape into a tape recorder or dictating machine is arelatively awkward operation for the inexperienceduser. And now, that recording equipment is so muchmore portable, tape often has to be loaded in difficultconditions, e.g. in a moving vehicle. The cartridge or

P. vall der Lely is with the Hasselt (Belgium) branch of the PhilipsRadio, Television and Record-playing Equipment Division; Dipt-Ing.G. Missriegler is with Oesterrelchische lPhilips Industrie, WerkWirag GmbH, Vi~llI1a. .

cassette provides the answer to this problem. It relievesthe user of the need to manipulate the tape himself andit offers effective protection.The proteetion of the tape is the second irriportant

function of the cassette. With the development ofthinner tapes, and with the lower tape speeds and nar-rower sound tracks made possible by improvement ofthe magnetic properties, it has become imperative toprotect the tape from dust and fingerprints. In fact, theuse of a cassette is almost essential if the full potential-ities of present-day' tapes are to be realized [11.

What type of cassette?

In recent years many and various types of audio tapecassettes (often referred to as "cartridges") have ap-peared on the market. They may be divided into twogroups: one-reel and two-reel cassettes.The one-reel cassette is shown infig. la; at the end

of the tape there is a catch-piece which is fed into asecond, empty cassette after the cassette has been

[1] In view of the two advantages noted here it is not surprisingthat tape cassettes are also being used for video recording. Avideo cassette and associated colour video recorder for use inthe home are at present under development.

78 PHILlPS TECHNICAL REVIEW VOLUME 31

Fig. 3. a) The record/playback head has a soft-iron core or yoke with a very short air gapThe tape is in contact with the head at the location of the gap. The magnetization of thetape sends its lines of force partly through the soft-iron core of the head and thus through thewinding round the head.b) Scale drawing, magnified about 1300 times, of a tape 18 f1.m thick moving at a speed vpast the gap, 2 f1.m long, of the record/playback head. The tape consists of a plastic basecoated with a magnetic layer. d thickness of the magnetic layer. a average distance betweentape and head. I length of the gap. The dimensions correspond to those in actual cassetteplayers.

g

Fig. I. Various types of audio tape cassette. a) One-reel cassette.b) Reel-to-reel cassette. c) Compact Cassette. cl) Cassette for thePocket Memo dictating machine.

loaded in the machine: the empty reel engages withthe catch-piece and begins to wind on the tape. Theone-reel cassettes are used in dictating machines. Inanother form of one-reel cassette the tape has an endlessloop; since the fast forward winding and rewindingneeded for finding a record-ing quickly is not possiblewith this kind of cassette,it is only suitable for someapplications (announce-ments, back-ground music).

Unlike the one-reel cas-settes the two-reel cassettescontain both a supply reeland a take-up reel. In theiroriginal form they were nomore than an encapsulationof the two reels of a taperecorder together with thelength of tape between them.At first, therefore, theywere rather bulky (fig. lb).The trend of development,however, was towards smal-ler dimensions. At Philips

Q

tills development led to the Compact Cassette (fig. le),which is now very widely used not only for makingrecordings but also as the "musicassette" with pre-recorded tape [2]. The compactness of the Philips cas-sette is partly due to the use of flangeless reels with tapewhich is narrower than usual - 3.81 mm (0.15 inch)instead of 6.25 mm (0.25 inch). But an even moreimportant factor is that the quality of the magnetictapes has advanced to such a stage that the design ofthe cassette could be based on a tape speed of only4.76 cm/s (1i inch/s). For general use the CompactCassette seems to have won the day from the one-reelcassettes, not j list because it is compact but also be-cause it can simply be taken from the machine withoutfust having to wind it back, and because the equipmentthat the cassette fits into does not have to be as com-plicated.

Even more advanced miniaturization than with theCompact Cassette was possible in the design of a cas-sette for a pocket dictation machine (fig. 1d). Here,since the quality did not have to be so high, there wasno need to ensure a constant tape speed by using a drivecapstan and pressure roller: the tape is transporteddirectly by the take-up reel, and runs faster as the dia-meter of the winding on the reel increases. The resultwas not only a very small cassette but also an extremelycompact transport mechanism.

This machine - the Philips Pocket Memo - isshown infig. 2 beside the smallest of the Philips rangeof cassette players. Later on we shalllook more closelyat some of the special features of both these machines,but first we shall look more generally at the magneticrecording process and the relationship between tapespeed and quality of reprod uction.

v---

--

1970, No. 3 AUDIO TAPE CASSETTES 79

Fig. 2. Left: Pocket Memo dictating machine; right: cassette player with microphone.

Recording and playback at low tape speed

The design of the Compact Cassette is based on atape speed of 4.76 cm/so This is low compared with thetape speeds of 9.53 cm/s and 19.05 cm/s commonlyused for sound recording, and this means that when ahigher frequency f is being recorded the wavelength Jeofthe periodic magnetization on the tape is small, sinceJe = v//, where v is the tape speed. The recording andreproduetion of such small wavelengths sets certainrequirements on the recording and playback process.The success achieved in meeting these requirements hasbeen such that in normaluse frequencies up to 10 kHzcan readily be recorded and reproduced at a tape speedof 4.76 cca]«; a performance that ensures satisfactoryreproduetion of music.

Within the limited scope of this article the treatmentof the recording and playback process which now fol-lows is necessarily highly simplified. There are someavantages in describing the playback process first.

Playback

The requirements to be met by the playback processrelate mainly to the geometry of tape and head. Fig. 3aillustrates the way in which a tape moves past a record-

ing or playback head whose soft-iron core has a gapat the location where the tape is in contact with it.Fig. 3b shows a scale drawing, at about 1300 times fullsize, of a head with a gap length of 2 [Lm [3] and a tape18 [Lm thick, "triple-play" tape. These dimensionsapply to the cassette player.

Let us assume that the magnetic layer in fig. 3b issinusoidally magnetized. The magnetization in andaround the gap sends its lines of force partly throughthe soft-iron core of the head, and hence through thewinding around it. The movement of the tape causesthe magnetic flux (/J enclosed by the coil to vary, thus

[2J The Compact Cassette has been internationally standardized:lEC Publication 94, Addition 1. In addition to its use forsound recording, the Compact Cassette is also beginning tobe used for recording digital signals; for program input insmaller computers and for use in cash registers it can be animprovement on the punched tape. These applications requirea somewhat modified construction, and sometimes cassettesare used that have been comprehensively inspected for "drop-outs" (momentary interruptions of the signal), which causemore of a nuisance in digital recording than in sound record-ing. Preparations for the standardization of digital cassetteshave reached an advanced stage.

[3J It is customary to call the dimension of the gap in the directionof tape travel the "length ", although it is much smaller thanthe dimension perpendicular to the plane of the drawing infig. 3, which is called the "width".

80 PHILIPS TECHNICAL REVIEW VOLUME 31

inducing in the coil a voltage E which is equal to-Nd(/Jjdt (N being the number of turns). If we repre-sent the flux (/J, which varies sinusoidally with time t,by (/J= (/J~ejw/, where t» = 2nJ, then we may write

d(/JE = -N -- = -Njw(/J.

dt

We see that the amplitude of E increases linearly withthe frequency f.Thus, each time the frequency doublesthe level of E increases by 6 dB. This 6 dB increase peroctave is represented by the straight line A in fig. 4.

(1)

is illustrated by curve B of fig. 4. If the wavelength oran integral multiple of it is approximately equal to thegap length, then there is no transfer at all. In practicethe minimum wavelength is taken to be 2/, at which thegap loss is 5 dB. In the cassette player of today the gaplength is 2 !Lm.The limiting wavelength is thus 4 !Lm,which corresponds to an upper cut-off frequency of12 kHz.In addition to the gap losses, attenuation occurs at

short wavelengths because the tape is not in completecontact with the head but is displaced from it by an

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-f 105HzFig. 4. The frequency characteristic of the playback process is determined by a number ofeffects. With a constant magnetic flux through the playback head the induced voltage at theterminals of the head increases linearly with the frequency f(curve A). The flux through theplayback head drops to zero, however, when the frequency is so high that the wavelength Äof the magnetization on the tape or an integral multiple of it is approximately equal to thegap length I (curve B). Furthermore, with rising frequency the loss increases, partly becausethere is a certain spacing a between head and tape (curve C) and partly because at higherfrequencies a decreasing part of the thickness d of the magnetic layer contributes to the out-put (curve D). The addition of all these losses results in the frequency characteristic E. Withcertain simplifying assumptions the curves were calculated for a tape speed of 4.76 cm/s,I = 2 !Lm,a = 0.2 !Lm,and d = 6 !Lm, i.e. values that are normally encountered with cassetterecorders. .

We shall use this straight line as the basis for calcula-ting the frequency characteristic of the playback pro-cess. In practice it is only at low frequencies that anyapproximation to this characteristic is found [4]: athigh frequencies there are losses because of certaingeometrical factors.

To begin with, there are the gap losses. Clearly, thedetailed pattern of the tape magnetization between thebeginning and end of the gap cannot be observed bythe playback head. The wavelength of the magnetiza-tion on the tape must therefore be large compared withthe gap length I. The nearer the wavelength approachesto the gap length the smaller the recorded signal; this

average .distance a. To calculate this attenuation westart with the flux which is induced in the head by athin layer of the tape of thickness dy and located at adistance y from the head; this flux is proportional toe-21tJ'/J. [5]. If the whole magnetic layer of the tape isuniformly magnetized - this is not in fact true but weshall permit ourselves the assumption to make thingseasier - then the total flux (j) through the head is pro-portional to the average value of this function over thethickness d of the tape, i.e. proportional to

d+a

1 j' Àd. e-21CY/), dy = 2nd (2)

a

1970, No. 3 AUDlO TAPE CASSETTES 81

The factor e-2nalÄ represents the attenuation due to thedistance a; this attenuation is 54.5 dB per wavelengthdistance between tape and head. In practice the distanceis 0.1 (Jomto 0.3 (Jom,which, at a wavelength of 4 (Jom,corresponds to an attenuation of 1.4 dB to 4.1 dB. This"spacing loss", calculated for a spacing a of 0.2 (Jom,isgiven by curve C of fig. 4. '

The factor ().j2nd)(1 - e-2ndIÄ) indicates that as thewavelength becomes shorter the contribution to theoutput signal comes more and more from the magneti-zation close to the tape surface. There is a "thicknessloss", and its magnitude appears in fig. 4 from curve Dfor the tape-head configuration of fig. 3b, i.e. for a layerthickness of 6 (Jom.If at each frequency we add up the three different

sorts of losses mentioned above and then subtract thesum from the theoretical 6 dB/octave characteristicA,we obtain the curve E of fig. 4. This curve gives thefrequency response ofthe playback process, and is validfor the assumption of homogeneous magnetizationover the whole thickness of the tape. It is evident thatthe cut-off frequency is determined primarily by thegap losses (curve B). One of the related problems witha mass-produced product like the cassette player is that'of making the very narrow gaps in the heads accuratelyto size and with properly finished edges; an increase of1 micron in the effective gap length could bring thetheoretical cut-off frequency mentioned above from12 kHz to 8 kHz.There is another possible cause of losses at high fre-

quencies, and this is also significant because the cassetterecorder is a mass-produced product. We have tacitlyassumed in our analysis that the tape is magnetized bya recording head in which the width of the gap, likethat of the playback head, is perpendicular to thedirection of travel of the tape, i.e. perpendicular to theplane of fig. 3. The correct setting is made as accuratelyas possible by individual adjustment of what is usuallycalled the "azimuth" of the head. A difference inangle a between the recording and reproducing gapsgives an effective lengthening of the gap and additionallosses at short wavelengths. These losses, again ex- ,pressed in dB, are given by

sin(nw tan a/À)20 10gIO À dB,

nw tan a/

where w is the width of the sound track. A plot of (3)as a function of frequency gives a curve similar tocurve B of fig. 4. In monaural recording on a cassettetape the track width is 1.5 mm; a 3 dB attenuation ofthe cut-off wavelength of 4 (Jomoccurs here at an azi-muth error a of only 4'. Regular adjustment of theazimuth ofthe head is obviouslynot possible in cassettemachines, and therefore azimuth errors may cause sig-

nificant deterioration of the frequency characteristic.They are mainly significant when a tape is played backon a different machine from the one on which therecording was made.

Recording

To obtain a linear relationship between magnetiza-tion and signal current during the recording process,a high-frequency a.c. current is passed through the"recording head together with the signal currentIvl. Theamplitude of this bias current is so high as to cause themagnetic field at the gap to periodically exceed thecoercivity of the magnetic material in the tape; its fre-quency is several times higher than the audio frequenciesto be recorded. The tape acquires its remanent magne-tization when the tape has passed the gap and traversesa zone in which the peak value of the magnetic fieldhas decreased to a value exactly equal to the coercivity(fig. 5). This recording zone has a certain extent, partlybecause all the magnetic particles in the tape do nothave exactly the same coercivity. It is desirable that theinstantaneous value of the signal should not change'much during the time that a point of the tape passesthrough the recording zone, in other words that the

-Fig. 5. The magnetization of the tape takes place in a zone Z, inwhich the magnetic field strength has decreased to the coercivityof the tape. The path through this zone is longer at a high biascurrent (left) than at a small bias current (right).

extent of the recording zone should be small withrespect to the wavelength to be recorded. The width ofthe recording zone increases with the magnitude of thebias current (fig. 5). For short wavelengths it is there-fore best to use a smaller bias current. The recordingzone then contracts around the gap and no longercovers the full thickness of the magnetic layer. This isno disadvantagè, however, since as we saw above at

(3) short wavelengths the contribution to the output signal

[4] Unless the wavelength on the tape is much larger than thelength of tape in contactwith the head; this is not the case forthe cassette player, where the wavelength at 50 Hz is onlyabout I mm.

[5] R. L. Wallace, Jr., The reproduetion of magnetically recordedsignals, Bell Syst. tech. J. 30, 1145-1173, 1951.

[6] The idea of the high-frequency bias current seems to have hadthree different originators; see W. K. Westmijze, Studies onmagnetic recording, Thesis, Leyden 1953. Its linearizing actionis dealt with by the same 'author in: The principle of themagnetic recording and reproduetion of sound, Philips tech.Rev. 15, 84-96, 1953/54.

82 PHILlPS TECHNICAL REVIEW

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-2

VOLUME 31

a

Fig. 6. The magnetization inside amagnetic tape, measured on a 5000 : 1scale model. The magnitude of themagnetization is expressed by the lengthof the arrows; for comparison theremanence M; of the magnetic materialis also given. The gap in the head isshown beneath the longitudinal sectionof the magnetic layer in the positionwhich it took up at one of the instantswhen the signal current passed throughzero. a) Short wavelength and low biascurrent h. b) Short wavelength andhigh bias current; although the signalcurrent Is is three times greater than in(a), the remanent magnetization is lower.c) Long wavelength and low bias cur-rent; only part of the layer thickness ismagnetized. d) Long wavelength andhigh bias current; almost the entirelayer is magnetized.


comes only from the surface of the magnetic layer. Onthe other hand at low frequencies, i.e. at long wave-lengths it is useful to magnetize the layer over its wholethickness, and for this purpose a higher bias current isdesirable.The effect of the bias current on the recorded rema-

nent magnetization at long and short wavelengths isillustrated in fig. 6, obtained from measurements on ascale model of the magnetic recording process magni-fied 5000 times [7l. For a short wavelength a compari-son of fig. 6b and fig. 6a shows that doubling the biascurrent Ib gives a smaller remanent magnetization eventhough the signal current Is is made three times aslarge. In this case the magnitude of the magnetizationfinally impressed on the tape is determined not only bythe signal current but also by the bias current. At longwavelengths the situation is different; in fig. 6c,d it canbe seen that when the wavelength is increased by afactor offour the remanent magnetism is the same bothfor high and low bias current, though with the highbias current a greater depth of the layer is magnetized.At this wavelength the result is an increased outputvoltage on playback.The relative levels at which high and low frequencies

are recorded on the tape therefore depend partlyon themagnitude of the bias current. This has to be takeninto account when devising the éorrections to the fre-quency response that will give the correct relative levelson playback; more will be said about this when wedescribe the cassette player on page 88. In any casethe choice of the bias current has in practice to be acompromise between what is desirable for high fre-quencies and what is desirable for low frequencies rei,The compromise adopted for the cassette player ratherfavours the high frequencies; the bias current is lowerthan in normal sound recording.When the magnetic coating of the tape is saturated,

the maximum output level of the tape has been reached.Exceeding this level will cause non-linear distortion.When making a recording the aim is to magnetize thetape to a level at which the loudest passages do notquite reach the maximum output level; this is oftendefined as the signal amplitude at which the third har-monic caused by non-linear.distortion of the fundamen-tal reaches 5% of the amplitude of the fundamental.This definition applies for low frequencies. At high fre-quencies it is not usually possible to drive the tape intosaturation. This is because an increase ofthe signal cur-rent, like an increase of the bias current, makes therecording zone broader, so that the recorded signalsare smaller for the short wavelengths, which are of thesame order of magnitude as the width of the recordingzone or less. At short wavelengths, therefore, the im-pressed magnetization does not increase linearly with

the signal current but reaches a maximum. This devia-tion from linearity at high frequencies is the cause ofnon-linear distortion. At high frequencies, also, thepermissible distortion determines the maximum signallevel that can be recorded on the tape, but 'in this caseit is not connected with magnetic saturation. At a higherbias current the deviation from linearity will start atlower signal currents, which means that the maximumundistorted output level of the tape is lower.

The tape

Audio tapes are made by coating a plastic base witha thin layer consisting of a suspension of magneticmaterial in a volatile solvent, mixed with an organicbinder. After drying, the magnetic material- particlesof iron oxide or nowadays chromium dioxide - remainon the base embedded in the binder. The following fourfactors have contributed significantly to the improve-ment in tape quality.1. Improvement in the shape of the particles, giving the

material better magnetic properties;2. A more uniform distribution of the particles in the

binder;3. The use of binders with better resistance to wear;4. Finishing treatment of the surface to make it

smoother.Originally cubic iron-oxide particles were used (fig.

7a). An advance came with the use of smaller, needle-shaped particles, about 1 (Lmlong and 0.2 (Lmthick(fig. 7b). Smaller particles are important because theaverage size of a particle should preferably be less thanhalf the shortest wavelength to be recorded. The shapeanisotropy of the needles gives a greater coercivity;moreover, needle-shaped particles can be orientedparallel to the direction of travel of the tape while thecoating is still wet. This treatment increases the rema-nent magnetization of the tape and thus gives a higheroutput voltage on playback.

Since the particles are magnetic, they tend to clustertogether in the binder while it is still wet. This effect("clumping") makes it difficult to obtain a homoge-neous distribution. During the preparation, which takesplace at high temperature (about 350°C), the iron-oxide particles are sometimes found to be sinteredtogether. This leads to a high noise level and a roughsurface, which increases the average spacing betweentape and head and consequently increases the spacing

[7] D. L. A. Tjaden and J. Leyten, A 5000: 1 scale model of themagnetic recording process, Philips tech. Rev.25, 319-329,1963/64.

[S] There is no need to be content with a compromise if low andhigh frequencies are recorded one after the other, each underits own optimum conditions, on the same sound track: E. deNiet, K. Teer and D. L. A. Tjaden, Magnetic recording ofaudio signals at low tape speeds, 4th Int. Congress on Acous-tics, Copenhagen 1962, paper No. NIl.


a

loss. Clumping can be countered by adding dispersionstabilizers to the suspension. These are organic sub-stances which adhere to the iron-oxide particles. Nowa-days more effective dispersion stabilizers are available,and in addition there are other effective methods ofdispersion. Modern tapes are therefore more homo-geneous and have a smoother surface.

A smoother surface gives a lower spacing loss. An-other improvement in this respect comes from the useof binders which have better resistance to wear, so thatthere is less contamination of the head from abrasionof the magnetic layer. Moreover most tapes are nowgiven a finishing treatment to make the surfacesmoother. This is a calendering process, in which thetape is passed between heated rollers under pressure.

Tapes with chromium dioxide have particularly goodproperties. This is mainly because the chromium-dioxide needles are more uniform in size than the iron-oxide needles and also because they do not sintertogether during preparation (fig.7c). There is no sin-tering since, unlike the iron oxide, the chromium-dioxide particles are prepared in a wet chemical pro-cess at ordinary temperatures. As they do not formclusters to the same extent, the chromium-dioxideneedles can also be better oriented, giving a higherremanence.

Apart from the improvements in the magnetic coat-

b

ing, there have also been important improvements inthe base. The oldest tapes, intended for a tape speedof 76.2 ctn]s, were su bjected to considerable mechanicalstress and the paper base had therefore to be verythick; the thickness of these "standard" tapes was54 fLm. The availability of stronger base materials (inthe order of their appearance: cellulose acetate, poly-vinyl chloride and polyester) and the reduction of tapespeeds made it possible to introduce thinner tapes (seeTable I). Tapes with thicknesses of 12.5 urn and 9 fLm,which are the latest development in this field, are ex-clusively for use in cassettes; since the bias current incassette recorders is lower, the magnetic coating of thesetapes is thinner. The greater flexibility of thin tapesgives better contact between tape and head.

An important aspect of the improvement in tapes is

Table J. Stages in the development of thinner audio tapes.

Total Thickness of Type designationthickness magnetic layer Name of cassette

(fl-m) (urn)- -

54 15 standard tape -35 10-11 longplay tape -25 10-11 double-play tape -18 6 triple-play tape C6012.5 3-4 - C909 3-4 - C 120


c

that it has led to an increase in the density of the infor-mation that can be recorded on magnetic tape. Con-tributory factors here have been the introduetion ofthinner tapes, improved characteristics at short wave-lengths, higher remanence and improved noise charac-teristics; the last two factors have made narrower soundtracks possible. A reel with a diameter of 18 cm, forexample, can hold 360 metres of standard tape. Inthe old days, recording over the full width of the tapeand using a tape speed of 38.1 cm/s, which was thennecessary for high quality, the playing time was 15 min-utes. The same sound quality can now be obtainedwith a four-track recording and a tape speed of9.5 cm/soThe same 18 cm reel can now take 1080 metres oftriple-play tape and thus at this speed gives a playingtime of 4 X 180 minutes. This means a 48-fold increaseof the information per unit volume.

With such a long playing time on one reel of tape itbecomes difficult to find a particular passage or pro-gramme; the reel with four-track tape has become arelatively inaccessi bie carrier of information. Hereagain the cassette provides a welcome solution, sinceit can be taken out of the machine without first havingto wind the tape back, thus offering increased informa-tion density in a smaller package.

[9J L. F. Ottens, The Compact Cassette for audio tape recorders,J. Audio Engng. Soc. IS, 26-28, 1967.

Fig. 7. Electron photomicrographs of the magnetic layer of a) atape with cubic iron-oxide particles, in common use about 20 yearsago; b) a tape with needle-shaped iron-oxide particles as com-monly used today, and c) a recently marketed tape with chro-mium-dioxide particles. The magnification of all three photo-micrographs is 14500 X. Those in Cb) and Cc) were made by thereplica technique; in Cc) the white bars are chromium-dioxideneedles that have stuck to the replica, while other chromium-dioxide needles have only left irnpressions in the replica.

The Compact Cassette

We shall now deal in somewhat more detail with thedesign of the Compact Cassette [91. We have alreadymentioned that it was designed for a tape width of3.81 mm and for a tape speed of 4.76 cm/so Two soundtracks are recorded on the tape, one in the forwarddirection and one in the reverse direction. Dependingon the thickness of the tape used, the cassette gives aplaying time of 2 X 30 minutes, 2 X 45 minutes or 2 X 60minutes; the three types are designated by the num-bers C 60, C 90 or C 120 (see Table I). In stereophonicequipment the 1.5 mm wide sound tracks are each sub-divided into two tracks ofO.6 mm, with a separation of0.3 mm, for the left-hand and right-hand channels (fig.8). This division of the tracks makes monaural andstereo recordings fully compatible. Crosstalk from onechannel to the other is minimal; the channel separation

1.510.61 -L

061 -R3.8

1.51R-

L-MONO STEREO

Fig. 8. Arrangement of the sound tracks on a cassette tape.L left-hand channel, R right-hand channel.

wound on to hubs I and fastened to them with aclamping-piece 2, shaped in such a way as to preserveas far as possible the circular shape of the hub. Alining 3 inside the cassette holds the tape in place atboth sides. To keep the power required for fast windingas low as possible, the tape is not threaded over fixedguide spindles but over rollers 4. When the cassette isloaded into the device and tape transport is switchedon, the erase head 5 and the record/playback head 6are brought into contact with the tape through openingsin the cassette; at the same time the pressure roller 7 ispressed against the driving spindle or capstan 8. Eachcassette contains high-permeability screening 9, which

86 PHILIPS TECHNICAL REVIEW

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5 100 2 5 1000 2 5 10000HzFig. 9. Separation between the two channels for playback of astereophonic programme on a Compact Cassette.

Fig. 10. View inside a cassette in a cassette player. 1 hubs. 2 clamping piece. 3lining. 4 rollers.5 erase head. 6 record/playback head. 7 pressure roller. 8 capstan. 9 high-permeabilityscreening. 10 felt pressure pad. // reference holes and pins. 12 recording lock. 13 supportpoints for tape transport. /4 tape guides. 15 tape guides. 16 screw for adjusting azimuth.

VOLUME 31

is presented in fig. 9 as a function of frequency. The connects up with the screening in the cassette player tochannel separation is more than sufficient to give an protect the head against stray fields, and a felt pressureunimpaired stereo effect and is in fact greater than the pad 10, which presses the tape against the head with aseparation on stereo gramophone records. force of 0.1 N to 0.2 N. Two reference holes 11 corre-

Fig. JO shows the interior of a cassette. The tape is spond to the two locating pins in the cassette player; a


spring pushes the cassette towards the heads. A record-ing can be protected against accidental erasure bybreaking the lip of the recording lock 12; there is onelock for each track. A pawl in the cassette equipmentthen falls into the hole vacated by the lip and preventsrecording. At both ends of the tape there is usually athicker polyterephthalate strip which takes the strainwhen the tape suddenly stops at the end of fast forwardwinding or rewinding.A few support points 13 help to guide the tape in the

cassette. To give more accurate guidance of the tapepast the head, the capstan draws the tape between twoaccurately aligned guides 14 fitted to the housing of therecord/playback head. There are two similar guides 15on the .erase head. We have already mentioned the'importance of accurate guidance ofthe tape in avoidingazimuth errors. The azimuth of the record/playbackhead is individually adjusted in every cassette player bymeans of an adjusting screw 16.

Philips cassette equipment

Transport mechanism

The transport mechanism of a cassette recorder orplayer has to feed the tape over the record/playback headat an accurately constant speed. Fluctuations in thisspeed, due for example to non-circularity of rotatingparts, result in fluctuations of pitch in the reproducedmusic, which can be very annoying and are referred toas "wow" or "flutter". Apart from feeding the tape ata constant speed, the transport mechanism for batteryequipment also has to take very little current. In addi-tion the mechanism must be accommodated in a smallvolume of prescribed shape.

The tape is transported by the capstan, with a rubberroller pressing the tape against the capstan. For uniformtape feed it is of prime importance that the motordriving the capstan should run at a constant speed,which does not decrease when the battery voltage dropsor when the mechanical load increases. During play-back the load does increase since the radius of the rollof tape becomes smaller as it unwinds while the brakingtorque acting on it remains constant; this causes anincrease in the tension on the tape. To keep the motorspeed constant, Philips cassette machines contain anelectronic control circuit (fig. 11). This circuit keepsthe tape speed constant to within 0.5 % during varyingload, and continues to operate as long as the batteryvoltage, which is 7.5 V nominal, has not dropped below5 V.

In the control circuit the motor M forms one of the branchesof a bridge circuit whose out-of-balance voltage is applied acrossthe emitter-base junction of a transistor Tr 1. This transistor drivesa second one, Tr 2, which determines the current through the

whole bridge. If the motor speed changes, the opposing voltageinduced in the motor coil changes in proportion to the speed, andthis change reacts on the balance of the bridge in such a way asto oppose the speed variation. This is accompanied by a changein the current through the motor and hence bya change in thevoltage drop across the ohmic resistance in the motor. The resis-torRl is given a value such that the voltage drop across it balancesthe drop across the ohmic resistance of the motor. The copper-wound resistor Reu has the correct temperature dependence tocompensate various temperature effects in the motor and semi-conductors.

+.

Fig. 11. Electronic control circuit which keeps the speed ofmotor M constant with decreasing battery voltage and varyingload. A decrease in motor speed causes a decrease in theinducedvoltage generated in the motor; this puts the bridge circuit out ofbalance, causing transistor Tr 1 to drive transistor Tr 2 in adirection such that the current through the bridge circuit in-creases, and with it the motor speed. The temperature dependenceof the copper-wound resistor Reu compensates the sum of anumber of temperature effects in the motor and semiconductors.

, A motor speed control system prevents slow varia-tions in tape speed, but not the faster variations thatcause wow and flutter.To suppress these faster variations a flywheel is fitted

to the capstan of a tape device. Because of the limitedspac~ available in the cassette transport mechanism,the flywheel can only be fairly small in diameter (seefig. 12), but this is compensated by using a thin, fast-running capstan. This has the disadvantages that thecapstan has to be machined even ,more accurately and

88 PHILIPS TECHNICAL REVIEW VOLUME 31

(.:\.ÖI

Fig. 12. View of the cassette transport mechanism from below.

that it may bend under the lateral force exerted by thepressure roller. Misalignment of the capstan affects thetape feed, causing effects such as azimuth error. InPhilips cassette equipment the compromise taken is tomake the capstan diameter 2 mm and the speed about7.5 revolutions per second.

The capstan with flywheel is driven by a rubber beltand pulley system. This gives better damping of any fastripple in the speed of the motor than a system of inter-mediate pulley wheels. Ripple of this kind can occurbecause the rotating rotor tends to "cling" at certainangular positions. Another advantage of such a drivesystem is that it gives greater freedom in the layout ofa tape-transport mechanism. For these advantages weare prepared to accept the disadvantages of possibleflutter due to variations of belt thickness or slipping ofa stretched belt. Precision-ground belts are used thathave thickness variations of less than 2%. The samebelt drives both the flywheel and the take-up reel, butthe take-up reel is driven via a slipping clutch which isneeded because the speed of revolution of the take-upreel decreases as it fills up with tape.

To measure the magnitude of the flutter, which con-sists of frequency modulation of the tones recorded onthe tape, these tones are demodulated. The modulatingsignal can then be analysed into its constituent frequen-cies. A frequency analysis carried out in this way for theflutter of a cassette player is shown infig. 13. The fourprincipal freq uency components are seen to lie at therotational frequency of the belt (about 3 Hz), the slip-

~- ....... - . .-. . ....- ........... _ .. -.;fiV.. ..i!1..I. .I.f'\r r~~ .r:~ -/

\1 f 1\ '{ Ir -ti ..,-- - r- 1-', 1---I \j

~ ~-L' I I ..~0 i

0".I .....;. -"j1 _I ..

~la; ~bf c I 10'of--..'- l-

10 I, 100HzL :. - . . ..

"",*OI.lItNll" IN C'I'CUlS Pf:,,''$i(:OND

Fig. i3. Frequency analysis of the flutter in a cassette player. Theamplitude is marked on the scale from top to bottom. Four fre-quency components may be observed, originating from the rubberbelt (a), the slipping clutch (b), the capstan (c) and the motor( d).

ping clutch (4.6 Hz), the capstan (7.5 Hz) and themotor (32 Hz).

The magnitude of the flutter is expressed as the per-centage frequency variation of a tone. The annoyancecaused by flutter depends on the rapidity of the varia-tions; the most annoying are variations with a frequen-cy in the region of 4 Hz. All frequency components inthe flutter are thus not eq ually troublesome. It is custom-ary to "weight" them against a certain specification,such as the widely used weighting curve (fig. 74) laiddown in the German standard DIN 45507. For thesimple Philips cassette players the flutter, when weight-ed from this curve, must be no more than 0.4 %; moreexpensive equipment has to meet a stiffer specification.

Fig. 14. Weighting curve after DIN 45507; which shows the weight-ing that the various frequency components of the flutter in a soundrecording have to be given in order to express the flutter in a singlenumerical value corresponding to the degree of annoyance ex-perienced.

Current consumption of the transport mechanism

The tape-transport mechanism in the Philips cassetteplayer takes a current of 75 mA. When the batteryvoltage has dropped to 5 V, the electrical power sup-plied to the player is distributed among the componentsof the mechanism as shown in fig. IS. The batteryvoltage is usually higher than 5 V; the speed controlthen has an appreciably larger share, because it dissi-pates the surplus power. The friction in the bearings of


Slipping clutch

Residualfriction

Fig. IS. Distribution of electrical power among the components ofthe transport mechanism when a tape cassette is played on aPhilips cassette player. The fractions of the bearing friction dueto the lateral forces originating from the rubber belt and pressureroller are presented separately. The distribution shown here relatesto the situation when the battery voltage has decreased to 5 V.At a higher battery voltage the share of the control system is muchlarger.

motor and flywheel is considerably increased by thelateral forces exerted by the drive belt and the pressureroller; the flywheel bearing friction is in fact almostentirely due to these lateral forces.

Electroacoustic characteristics

The electroacoustic characteristics of a tape player,such as frequency response and signal-to-noise ratio,are determined not only by the characteristics of thetape and the tape geometry (see fig. 4) but also by theprocessing which the signals presented for recordingand reproduetion receive in the electronic circuits ofthedevice. The frequency characteristic resulting from therecording and playback process is not usable until ithas been corrected (or "equalized") in these circuits.The purpose of the equalization may be given a some-what wider formulation: to obtain the best signal-to-noise ratio, maximum use must be made at all frequen-cies of the dynamic range of the tape, and the replayoutput characteristic must be flat within the widestpossible range of frequencies.The dynamic range of the tape is not identical for all

tapes and at higher frequencies it also varies with thebias current (see page 83). To avoid a situation inwhich every manufacturer introduced his own correc-tions, and tapes would not necessarily be interchange-able, an international standard has been laid down for

[10) IEC Publication 94.

the variation of the .magnetization on' the tape withfrequency, fo~' a, constant signal at the input of therecording amplifier [19]. The magnetization is definedin this standard as the magnitude of the "surface induc-tion", 'whiëh is the flux density at right angles to thesurface ofthe tape whim the tape is ~oved along againstan ideal reproducing head. Different surface-inductionfrequency curves have been laid down for the variousstandard tape speeds, and curve M in fig. 16 is anexample for the tape speed of 4.76 cm/so In practicethe playback amplifier is equalized with the aid of a'reference tape, taken to be magnetized in accordancewith this standard ; the amplifier is given a frequency.characteristic such that when the reference tape isplayed, a virtually frequency-independent signal ampli-tude appears at the output. The replay characteristic, ofthe Phili ps cassette players is given by curve P of fig. 16.

When the surface induction is the same at different frequencies,then - neglecting for a moment the losses indicated in fig. 4 -the induced voltage at the terminals of the playback head is alsothe same at these different frequencies. This explains why theplayback characteristic P in fig. 16 isvery like the mirror image ofthe magnetization curveM. The standard curve M thus determinesto a large extent the playback characteristic of a cassette player;at high frequencies, however, P rises steeply to offset the gaplosses occurring on playback (fig. 4, curve B), and this high-frequency equalization may differ from one type of cassette playerto another.

What frequency characteristic should we now give tothe recording amplifier to ensure that the overall fre-quency characteristic is flat when a tape is played backthrough the playback amplifier corrected in the mannerdescribed? The answer to this question depends on howlarge we decide to make the bias current. We have ex-plained on page 83 that the relative levels at which lowand high frequencies are recorded depend on the magni-tude of the bias current, and that the maximum outputlevel of the tape at high frequencies also depends on thebias current. With a higher bias current the high fre-

. 2 5 100 2 5 1000 2 5 70000Hz

Fig. 16. M internationally standardized curve [10) showing thevariation of tape magnetization with frequency for constantvoltage at the input of the recording amplifier; the curve relatesto a tape speed of 4.76 cm/so P frequency characteristic of theplayback amplifier in the Philips cassette player.


quencies give a smaller recorded signal, and at the same. time the maximum undistorted output level of the tapedecreases at high frequencies. On the other hand, wealso saw that good recording of the low frequenciesrequires a relatively high bias current. If we are to meetthis requirement as well we shall have to compensatethe attenuation of the high frequencies by an appro-priate high-frequency "lift" upon recording. This canin fact be done without exceeding the maximum outputlevel of the tape, since the energy content of the higherfrequencies (higher than 2 kHz) in an average musicsignal is smaller than the energy content of the lowerfrequencies. The aim, however, is not only to obtainan optimum frequency response but also to achieve anoptimum distortion-free modulation of the tape. Wetry to achieve this by setting the bias current to a valueat which the low-frequency and high-frequency signalsin an average music signal both approach the limit fordistortion-free modulation on the tape to about thesame extent after the high-frequency compensation inthe recording amplifier. If a signal exceeds the per-missible amplitude, then there will be simultaneous dis-tortion .of both low and high frequencies. If the biascurrent is higher than this optimum value, more high-frequency compensation will be needed; this conflicts,however, with the reduced modulation limit ofthe tapeand the high frequencies are the first to show distor-tion. At too low a bias current the low frequenciesbecome distorted first.

The term "average music signal" used here suggeststhat practical experience must also have a say in thechoice of bias current and high-frequency compensa-tion. In the Philips cassette player the recording ampli-fier has been given the frequency characteristic shownby curve R of fig. 17. This lifts not only the high fre-quencies but also the frequencies below 200 Hz. Thiscorresponds with the shape of curve M in this region,and the object is to improve the ratio of the signal to

T VP<r--.... .>~

2. 5 100 2 5 1000 2 5 10000Hz

Fig. 17. R frequency characteristic of the recording amplifier inthe Philips cassette player. T overall frequency characteristic,measured via recording and playback (not including microphoneand loudspeaker).

any hum in equipment supplied from the mains. Thefrequency characteristic resulting from all the correc-tions applied in recording and playback is representedby curve T of fig. 17. The corresponding signal-to-noiseratio for present-day players and tapes is about 45 dB:

"Musicassettes"

All musicassettes are prerecorded with stereo pro-grammes. To speed up production the music pro-grammes on the master tape are not transferred to thecassette tapes at the nominal tape speed but at a con-siderably higher speed, 32 times higher in the latestproduction machines. This means that the master tape,which is modulated at a speed of 19.05 cm/s, is playedat the rather astonishing speed of slightly more than6 metres per second. The cassette tapes, four of whichare modulated simultaneously with the signal from themaster tape, are run at a speed of more than 1.5 mis.All four tracks are prerecorded simultaneously, onepair from back to front.

During the copying process all frequencies are multi-plied by a factor of 32; the amplifiers cover a frequencyrange of 200 Hz to 500 kHz and the playback andrecording characteristics are correspondingly trans-formed. The high-frequency bias current has a fre-quency of 2.4 MHz. Ferrite heads are used to avoideddy current losses and rapid wear; the gap length ofthe recording heads is 4 fJ-m.

Large numbers of programmes are successivelyplayed on to a single cassette tape 1500 metres long,the programmes being punctuated by signal toneswhich act as "cues" in the semi-automated assemblyprocess for the cassettes. It would exceed the scope ofthis article to go into the details of this process [11l.

During recording great care is taken to minimize flutter,which is less than 0.06 % when properly weighted bythe curve of fig. 14. The perpendicular alignment of therecording gap is also carried out very accurately, theangular errors being less than 2'. This is very impor-tant, because azimuth errors in playback equipment canseriously impair the quality ofreproduction when musi-cassettes are played; everything possible is done toensure that the recording process contributes as littleas possible to azimuth errors that may ultimately exist.

Pocket Memo

The development ofthe Philips Pocket Memo (fig. 2)was prompted bythe desire to market an extremelysmalldictation machine that could be carried in the pocketand used for making quick verbal notes. Small 'size wasthe principal requirement which the design had to meet;a playing time of 2 X 10 minutes (or 2 X 15 minutes with

[11] For further information see J. L. Ooms, Multiple speed tapeduplicating, J. Audio Engng. Soc. 14, 343-355, 1966.

1970, No 3.· AUDIO TAPE CASSEITES 91

Fig. 18. Transport mechanism of Pocket Memo. 1 motor, la, bpinions on motor shaft. 2 turntable of take-up reel. 3 driving wheelfor supply reel with 4 pulley wheel. 5 rubber belt.6 turntable ofsupply reel.

9 [Lmtape) was desired, and also a rewind facility. Acassette was here the obvious means of making theaudio tape manageable. We have already mentionedabove that the Pocket Memo does not use a capstanfor tape transport but uses instead the mechanicallymuch simpler system of transporting the tape directlyby means of the take-up reel. Apart from saving spaceand simplifying the transport mechanism, this has theadvantage that less current is required. This is becausethere is no need of the slipping clutch used in capstandrive ofthe take-up reel, and this slipping clutch usuallytakes a fairly large amount of power (see fig. 15 whichshows the distribution of the power taken by the trans-port mechanism of the cassette).

Transport mechanism

The simplicity of the transport mechanism in thePocket Memo can clearly be seen in fig. 18. Attachedto the shaft of the motor I is a double pinion la, b.In the "record/playback" position the motor shaftis tilted upwards and presses pinion la, which has adiameter of 1 mm, against a rubber rim on the turn-table 2 of the take-up reel. In the "rewind" positionthe motor shaft is tilted downwards, and pinion 1bruns on the rubber rim of wheel 3. Mounted on thesame spindle as wheel3 is a pulley wheel 4 which drivesthe supply reel via belt 5 in the direction to wind thetape back on to the reel.

The rotating parts are given rotational frequencies that are asfar removed as possible from the value of 4 tev]« (fig. 19), sincethe human ear is most sensitive to flutter at a frequency of 4 Hz(page 88). A frequency analysis of the flutter is shown in fig. 20,in which the motor frequency fm and some of its harmonics canbe seen.The Pocket Memo is provided with the same electronic motor

speed control as the cassette player. The circuit is shown in fig. 11.Jn the Pocket Memo a signal is derived from the control circuitto give an indication of the battery voltage. Superimposed on thecollector d.c. voltage of Tr 2 is an a.c. voltage due to the com-mutation ofthe d.c. motor. By means of 'a battery-check switchthis voltage can be applied to the playback amplifier. If the batteryvoltage falls too low, there is no longer any voltage across Tr 2,which then becomes effectively a short-circuit to earth, and con-sequently the a.c. voltage will no longer give an audible signal.

Fig. 19. The motor 1 and the take-up reel2 (see fig. 18) are givenrotational frequencies which are as far removed as possible fromthe frequency 4 Hz, which is the most annoying flutter frequency.An indication is given of where they lie in relation to theweightingcurve of fig. 14. Unbalance of the rotor windings, magnetic "cling"at certain angular positions and commutator effects can lead tofluctuations with frequencies of 3 and 6 times the motor speed.

Tape modulation, frequency characteristic

The Pocket Memo incorporates a moving-coil micro-phone, which also acts as loudspeaker when the record-ing is played back. To ensure a good signal-to-noiseratio it is desirable that the tape should be modulatedto a reasonable output level, irrespective of the distancefrom which the microphone is spoken into. In most taperecorders, and in the cassette players as well, the record-ing level is adjusted by hand; in the Pocket Memo,however, this would make it unacceptably complicatedto use. The Pocket Memo is therefore given a fixedrecording level, which is set fairly high; signal limita-tion is provided by the natural saturation of the tape.Steps have been taken to ensure that the recording am-plifier remains well within the limits of its range oflinear operation even when the tape is modulated intosaturation, so that the tape is in fact the only limitingelement. The manner in which the tape limits the signalgives less annoying non-linear distortion than over-driving an amplifier, and the intelligibility of speech isnot so greatly affected.


2~ 3~~ !

6~!

12~!

300HzFig: 20. Frequency analysis of the flutter in the transport mechanism of a Pocket Memo.In the frequency band from 2.5 Hz to 300 Hz shown here the flutter originates entirely fromthe motor. This turns at a speed of fm revolutions per second; the component at the fre-quency 3 fm is due to magnetic unbalance of the rotor windings, the 6 fm frequency com-ponent is due to magnetic "cling" and commutator effects, and the 12fm component is dueto the commutator alone. "

Intelligibility of speech is again the significant factorthat should be borne in mind when examiningfig. 21,which shows the frequency characteristic of the PocketMemo, measured at the output terminals with an

ohmic load. The frequency characteristic does nottherefore comprise the microphone/loudspeaker; it canbe seen that it favours the frequencies which are ofmain importance for the intelligibility of speech.

-20 2100

Summary. Because they are easy to manipulate and provide effec-tive proteetion of the tape, audio tape cassettes are coming intoever-increasing use. Largely because of the continued improve-ment of tapes, it is now possible to use simple mass-producedcassette equipment to record frequencies up to 10 kHz at a tapespeed of 4.76 cm/s, and to obtain a signal-to-noise ratio of 45 dBon a track width of 1.5 mm. This has led to the development ofthe Philips Compact Cassette with a playing time of 2 x 60 minutes(on a tape 9 (.I.mthick); it is also marketed with a stereophonicallyprerecorded tape (Musicassette, maximum playing time 1x45min). Cassette recorders and players are mostly portable and canwork from batteries; an electronic control circuit keeps the motorspeed constant as the battery voltage decreases.

The Philips miniature dictating machine, the Pocket Memo, isequipped with an even smaller cassette, specially developed forthis machine, which gives a maximum playing time of 2 x 15 min.The tape is transported directly by the take-up reel; with thissystem the cassette and machine can be kept small and simple,and the current required is small.

5 1000 2 5 10000Hz

Fig. 21. Frequency characteristic of the Pocket Memo for record-ing and playback, excluding the frequency response of the micro-phone/loudspeaker. The characteristic favours the frequenciesimportant for intelligibility of speech.

Documents

Audio tape cassettes - Philips Bound... · Audio tape cassettes P. van derLely and G.Missriegler "Speech ... no need to ensure aconstant tape speed by using adrive capstan and pressure