Recording Demagnetization in Magnetic Tape Recording

PROCEEDINGS OF THE I.R.E.

Recording Demagnetization inMagnetic Tape Recording*0. WILLIAM MUCKENHIRNt, MEMBER, IRE

Summary-An analysis of the magnetic tape recording processemploying supersonic excitation is presented by considering theeffect of the spatial distribution of the magnetic field around the re-cording head air gap on the magnetic history of an unmagnetizedelement of tape as it tracks across the recording head. This leads toan effect which is termed "recording demagnetization" and servesto explain certain performance characteristics. An experimentaltechnique developed for the measurement of this recording de-magnetization is described, as is the method of measuring the air-gapfield distribution. Finally, the correlation of the measurements of therecording demagnetization with normal recording performance char-acteristics is reported.

INTRODUCTION

I APID PROGRESS in the development of prac-tical magnetic sound recording has been achievedin recent years. The use of a supersonic excitation

or bias in the recording process has contributed sub-stantially to the high fidelity attained in present sys-tems, but has likewise increased the complexity of therecording process for which several theories have beenproposed in the literature. Noteworthy among these isthe work of such men as Toomin and Wildfeuer,IHolmes and Clark,2 Wetzel,3 and Camras.4 However,most of this work was restricted to a steel wire mediumand none included the effect of the spatial distributionof the magnetic field around the air gap of the record-ing head. The magnetic field measurements of Clarkand Merrill5 also were made on wire recording heads andwere used quantitatively in an analysis of the reproduc-tion process. The analysis proposed herewith pertainsprimarily to the effect of the magnetic field distributionof the recording head air gap on the recording processwhen the medium is a magnetic tape undergoing longi-tudinal magnetization.

In order that the analysis and corroborating meas-urements may have practical significance, the per-

* Decimal classification: R365.35. Original manuscript receivedby the Institute, June 14, 1950; revised manuscript received, March8, 1951.

t University of Minnesota Institute of Technology, Minneapolis,Minn.

I H. Toomin and D. Wildefeuer, "The mechanism of supersonicfrequencies as applied to magnetic recording," PROC. I.R.E., vol. 35,pp. 664-668; November, 1944.

2 L. C. Holmes and D. L. Clark, "Supersonic bias for magneticrecording," Electronics, vol. 18, pp. 126-136; July, 1945.

3 W. W. Wetzel, "Review of the present status of magnetic re-cording theory, Part II," Audio Eng., vol. 31, pp. 12-16; December,1947.

4 Marvin Camras, "Graphical analvsis of linear magnetic record-ing usinig high frequency excitation," PROC. I.R.E., vol. 37, pp. 569-573; May, 1949.

6 D. L. Clark, and L. L. Merrill, "Field measurements on mag-netic recording heads," PROC. I.R.E., vol. 35, pp. 1575-1579; Decem-ber, 1947.

formance characteristics of Figs. 1 and 2 are presented.Curves A, B, and C of Fig. 1 are the frequency-responsecurves for a constant recording current, and for record-ing and playback speeds of 4.4, 8.3, and 16.4 inches per

20 50 100FREQUE NCY (CPS)

500 1000 5000 10000

FREQUENCY RESPONSE

t0 '.

__C_ \ \ \20 0

4 0 ! 0 0 0

WAV E L E N GWTH (MILS)

Fig. 1-Frequency and wavelength response characteristics for con-stant current recordings. Curves A and A'; B and B'; C and C',are for tape speeds of 4.4; 8.3; and 16.4 inches per second, respec-tively.

second, respectively. Curves A', B', and C' show thesame data considering the wavelength of the recordedsignal on the tape as the independent variable. It fol-lows directly from curves A', B', and C' that, for con-stant current recording, the output varies directly with

50---- -00CP

4000 CPS

400 CPS

- - -- -. ~~~6000 CPS

- O~~~PS

20~- ~ID 2.0 3.0

EXCITATION CURRENT (MA)

Fig. 2-Experimental curves showing variation of signal outputvoltage as a function of the magnitude of the supersonic excita-tion with signal recording current and tape speed both held con-stant.

the tape speed, and the factors influencing the shape ofthe characteristics are independent of tape speed. Itwas considered advantageous therefore, in the analysisof the recording and reproduction processes, to use thephysical wavelength on the tape as the independentvariable, for then the tape speed factor was eliminated.

1951 891


The curves of Fig. 2 illustrate how the signal outputvoltage of different frequencies varies with the magni-tude of the supersonic excitation, all other factors re-maining constant. Two features of these characteristicsare to be noted. The maximum output occurs for dif-ferent excitation for different signal frequencies, mov-ing toward lower excitation currents as the frequency isincreased, and the output of the higher frequency signalsis substantially reduced for higher excitation currents.This latter effect has been reported by Holmes6 andWetzel,7 but no satisfactory explanation has beenpresented before this time.

In the analysis that follows, an effect which istermed "recording demagnetization" is predicted. Itserves to explain the performance characteristic whenthe magnitude of supersonic excitation is varied and toreduce considerably the discrepancy between the idealand observed short wavelength response for constantcurrent recording. By ideal characteristic is meant thatcharacteristic depending only upon Faraday's law ofmagnetic induction for which a drop in response voltageof six decibels per octave increase of wavelength wouldresult (curve D, Fig. 13).

THEORETICAL ANALYSIS

The recording process is best understood by focusingattention on an unmagnetized element of the tape andfollowing its magnetic history in detail as it tracksacross the recording head. On the assumption of sinu-soidal signals and excitations, this history is a functionof the speed of the tape, the amplitude, frequency andphase of the signal, the amplitude, frequency and phaseof the supersonic excitation, the nonlinear magneticcharacteristic of the tape, and the magnetic field dis-tribution around the recording head air gap. An im-mediate simplification is effected by combining thespeed and frequency into a wavelength function as waspreviously suggested. The functional variation of themagnetizing field that an element of tape experiences inmoving across the recording head then may be expressedmerely as a function of the distance along the path ofthe tape.

For the purposes of this paper, a long wavelengthsignal is defined as one whose wavelength is long com-pared to the distance over which the gap field has an ap-preciable value and a short wavelength signal as onewhose wavelength is less than the effective air-gap field.The composite variation of the magnetizing force withdistance across the recording head combining a longwavelength signal of fixed amplitude and phase, a super-sonic excitation of a given amplitude and phase, and the

6 Lynn C. Holmes, "Some factors influencing the choice of a me-dium for magnetic recording," Jour. Acous. Soc. Amer., vol. 19, pp.395-403; May, 1947.

7 W. W. Wetzel, "Review of the present status of magnetic re-cording theory, Part III," Audio Eng., vol. 32, pp. 26-30; January,1948.

gap field distribution is illustrated by curve oabc -* * n,in Fig. 3(a). The effect of the long wavelength signal issuch as to produce an asymmetry in a given (positive

DISTRIBUTION

(b) (c)Fig. 3-(a) Schematic representation of the magnetic field variation

across the recording head air gap during the recording process ofa long wave-length signal. (b) The magnetizing sequence (not toscale) experienced by an unmagnetized tape element as it movesto the center of the air gap. (c) The magnetic sequence as thetape element leaves the center of the air gap.

for the phase shown) direction. For a long wavelengththe signal amplitude varies little across the gap lengthand the effect is essentially that of a constant magnetiz-ing force, its magnitude varying only as determined by

(a)

(b) (c)Fig. 4-(a), (b), and (c). The corresponding representations

of Fig. 3 for a short wavelength signal.

the gap field distribution. A corresponding relationshipis shown in Fig. 4(a) for a short wavelength signal of thesame amplitude and phase and with the same supersonic

892 August

Muckenhirn: Magnetic Tape Recording

excitation. Several cycles of the short wavelength signalextend over the effective air-gap field region producingcorresponding reversals in the direction of the asym-metry, the peak values of which vary in accordancewith the gap field distribution. In both cases a phasinghas been assumed such that both the signal and super-sonic excitation have a positive maximum at the centerof the air gap.The effects of applying the resulting magnetic field

variations (curves oab ... m) of Figs. 3(a) and 4(a)) tothe magnetization curve of the tape are illustrated inFigs. 3(b) and (c), and 4(b) and (c), respectively. Anelement of unmagnetized tape then in moving to thecenter of the air gap experiences a magnetic historyshown by the lines oabcdefg and in moving from thecenter out of the air gap experiences a history shown bylines ghijklmr. Separate diagrams are used to illustratethe incoming and outgoing of the tape element so asto reduce confusion in following the curves. Theremanence of the element of tape at this point in therecording process is represented by Or in Fig. 3(c) fora long wavelength signal, and by the smaller value Or,Fig. 4(c), for a short wavelength signal, the smallervalue resulting from the asymmetry reversals. Both ofthese values are below the residual induction of a sym-metric cyclical hysteresis loop having the same peakvalue of induction. This reduction in remanence due tothe complex magnetic history of a tape element as ittraverses the air gap is designated herewith as "record-ing demagnetization."

It follows from the above discussion that the record-ing demagnetization is a function of the amplitude,wavelength and phase of the signal current, the ampli-tude, wavelength and phase of the excitation current,the gap-field distribution, and the magnetic character-istics of the tape. The complexity of the function isfurther increased by the nonlinearity of the tape char-acteristics and its value therefore was determined on apurely experimental basis.The general influence of recording demagnetization

on the performance characteristics follows from theabove analysis plus an extended visualization of themagnetic cycle as certain factors vary. As the signalwavelength decreases, the remanent flux is reduced,and the tape output on reproduction is thereby reduced.This is in accordance with the wavelength response char-acteristic (Fig. 1). As the amplitude of the supersonicexcitation increases, the effective air-gap fringing fieldextends over a greater distance. For a signal of a givenwavelength then, the ratio of the signal wavelength tothe effective gap length will decrease and the recordingdemagnetization becomes greater. This correlates withthe output versus excitation amplitude curves of Fig. 2.The final remanence of a tape element at the conclu-

sion of the recording process is not necessarily a valuesuch as Or in Fig. 3(c) or 4(c), but rather this value Orless the effect of self-demagnetization, an effect which

has been discussed by Camras,8 Wetzel,9 and others. Acorrelation of the self-demagnetization effect with therecording demagnetization effect is illustrated in Fig. 5,in which the remanence Or of Fig. 3(c) or (4c) is repre-sented by OB3. The reduction in magnetic inductiondue to the self-demagnetization effect for a signal having

Fig. 5-A section of the magnetization curve illustrating the effect ofself-demagnetization.

a wavelength to which the shearing line OP applies isthe difference between OB3 and OB1. In the reproduc-tion process, however, a recovery of induction is madealong PB2 to give an over-all loss due to self-demag-netization represented by the difference between OB3and OB2. For long wavelength signals, e.g., those greaterthan 20 mils, the shearing line OP approaches thevertical so that the self-demagnetization loss is negli-gible and the value OB3 (or Or) represents the finalremanence.

EXPERIMENTAL PROCEDUREThe experimental validation of the proposed theory

leading to the concept of recording demagnetizationrequired, in very brief terms, a determination of thefringing air-gap field of the recording head, a method ofconducting a small section of the tape through the mag-netic sequence corresponding to a given set of recordingconditions, and finally the measurement of the resultingremanence of the tape section and its correlation withthe actual recording system performance characteristics.The gap-field measurements in this work were made

on a ring-type head having an average gap length of0.83 mil (0.00083 inch) as measured under a IOOXmicroscope. This small gap length imposed severe limi-tations on the dimensions and positioning of the ex-ploring coil to be used for the measurements. A finalunit using 0.3-mil tungsten wire constructed as shownin Fig. 6 proved satisfactory. Fine adjustment of align-ment of the 0.3-mil wire with respect to the air gapwas provided by the mounting assembly. Horizontalmotion of the coil across the gap and vertical motionabove it at any horizontal position were made by cali-

8 Marvin Camras, "Theoretical response from a magnetic-wirerecord," PROC. I.R.E., vol. 34, pp. 597-602; August, 1946.

9 W. W. Wetzel, "Review of the present status of magnetic re-cording theory, Part I," Audio Eng., vol. 31, pp. 14-17; November,1947.

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046 Formvarl5.7 mi is kJ Tungsten

T 0Elevation

Tension (::, Xadjust Polystyrene

Acetatestrip

Coppertubing

-Fibrel

Fig. 6-Sketch of exploring coil used to measure

the gap-field distribution.

brated screw adjustments. The sensitivity of these ad-justments was such that horizontal displacements downto 0.05 mil and vertical displacements to 0.1 mil couldbe made and read on calibrated dials.The exploring coil was connected to an amplifier the

output of which supplied a wave analyzer unit inparallel with a vacuum-tube voltmeter and a cathode-ray oscilloscope, as shown schematically in Fig. 7. The

OSCILLOSCOPE

_Jn | VACUUM TUBE

} |DECADE VOLTMETER

x AMPLIFIER

HARMONIC WAVE

ANALYZER

Fig. 7-Block diagram of the equipment used in measuringthe air-gap field.

wave analyzer unit consisted of a cathode follower, a

low-pass m-derived filter, and an amplifier stage feed-ing a Hewlett Packard Model 300A Wave Analyzer.The cathode follower was used to match impedanceand the filter to attenuate the supersonic voltage whenmeasurements were made with both the supersonic ex-

citation and signal applied to the recording head.The measurements made with the exploring coil were

voltage measurements representing the voltage inducedin the coil by the time rate of change of the magneticflux linkages. Since the coil dimensions were large com-

pared to the dimensions of the region for which the fielddistribution was to be measured, the change in the

voltmeter reading with change in coil position ratherthan the actual voltage was of significance. If the coilwere moved vertically the change in reading would bedue to the change in the horizontal component of fluslinkages, while if the coil were moved horizontally thechange in voltage would be due to the change in the

50

cc

3t

u

zN_:

z93

Obtained for lower edge of0 oexploring coil at the values:

curve ySa 0

'-,b b OlecSic 0OUmild 0O4mit

0 e 0.6mit

A Af 11.0~~mitl

0 2 4

HORIZONTAL DISPLACEMENT (MILS)

Fig. 8-The horizontal component of magnetizing force as a functionof distance from the center of the recording head air gap, as meas-ured with the lower edge of the exploring coil at different dis-tances above the gap.

vertical component of flux linkages. Since the No. 46Formvar return wire was 15.7 mils above the 0.3-milwire (Fig. 6), the change in flux linkages due to theNo. 46 wire was considered negligible for small move-ments of the coil, and the entire flux linkage change wasassumed to be due to the movement of the 0.3-mil wire.The results of these measurements are shown in finalform in Fig. 8.With a knowledge of the variation of the horizontal

component of the magnetizing force across the gap itwas possible to combine the signal, supersonic excita-tion, and field distribution effects into a single functionwhich represented the magnetic cycle of an element oftape as it passed across the gap. Examples of this areshown in Figs. 9 and 10 for signal wavelengths of 8 mils

Fig. 9-The variations of the magnetizing force over one-half the airgap during recording with supersonic excitation and a long wavelength signal.

August894

Muckenhirn: Magnetic Tape Recording

and 1 mil, respectively, and for a signal current of 0.18milliamperes rms, and an excitation current of 1.57milliamperes rms. Throughout the work an excitationhaving a wavelength of 0.2 mil (corresponding to 40kilocycles per second at a tape speed of 8 inches per

second) was used. Since symmetry about the centerline of the gap is assumed, the complete magnetic his-tory of a tape element is illustrated by considering itas entering the gap region from right to left in Figs. 9and 10, and leaving from left to right. Initially it was

400

300

4200 _0.2 MIL WAVE LENGTH

00

00

100

Fig. 10-The variation of the magnetizing force across one-half the

air gap for the same magnitudes and phasing as in Fig. 9, hut for

a short wavelength signal.

thought that a maximum signal would be recordedwhen the signal and excitation reached a maximumsimultaneously at the gap center, for under these con-

ditions a maximum magnetizing force would be estab-lished. Accordingly, the curves of Figs. 9 and 10 were

drawn for such a phase relationship.With the magnetic cycle known, the problem then

resolved itself into devising a method of magnetizingan element of tape with such a cycle and measuring itsremanence. In order to do this, the tape sample was

held in a fixed position with respect to the gap insteadof moving as in actual recording. The current throughthe head was then varied smoothly between maxima

and minima values similar to those on Figs. 9 and 10,which gave a field variation at the center of the gap

corresponding to a given recording condition. In otherwords, the spatial variation of the field, such as isshown in Figs. 9 and 10, was transformed into a timevariation of the field for the element of the nonmovingtape at the center of the gap. For each recording con-

dition it was necessary, therefore, first to tabulate fromthe gap field distribution data the values of currentthat would produce the proper successive positive andnegative peak values of magnetizing force.

Experimentally the sequence of current values cor-

responding to the successive positive and negative peakvalues of magnetizing force were obtained manually bymeans of the potentiometer control of the bridge circuit

shown in Fig. 11. The remanence of the tape elementdirectly over the gap center after it had been sub-jected to a magnetic cycle corresponding to a givenrecording condition would be a value such as Or, as

shown in Figs. 3(c) and 4(c), and therefore would in-

Fig. 11.-Manual control circuit by means of which a tape elementwas carried through a predetermined magnetic cycle correspond-ing to a given recording condition.

clude the recording demagnetization but not the self-demagnetization. Measurement of this remanence wasmade by a remanent pulse playback system which con-sisted of moving the tape across a play-back head, theoutput of which was amplified, integrated, and appliedto a cathode-ray oscilloscope as shown schematically inFig. 12. Because of the integration, the peak oscilloscopedeflection was proportional to the remanence. Althoughmost of the correlation was concerned with a qualitativeverification of characteristic curve shapes, some quan-titative measurements were made and therefore a cali-bration of the remanence measuring system was made.

PLAYBACK DECADEHEAD AMPLIFIER

3AMPLIFIER

Fig. 12-Block diagram showing the method by which the remanentmagnetic flux of a tape element was measured.

CORRELATION OF EXPERIMENTAL MEASUREMENTSWITH THEORETICAL ANALYSIS

A threefold correlation of experimental measurementsbased on the proposed theory of the recording processand the measured results of normal recording wasmade. These included a quantitative correlation of theremanent flux for a long wavelength signal, a correlationof the wavelength response characteristic including thegap-length effect, and a correlation of the signal outputversus supersonic excitation characteristic.The quantitative correlation was made for a signal

wavelength of 20 mils (400 cps at 8 inches per second)and an excitation wavelength of 0.2 mil. The signalrecording current was 0.18 milliamperes and the ex-citation current 1.57 milliamperes. For these conditionsthe sequence of peak recording head current values weredetermined, and a tape element was carried through thissequence with the aid of the circuit of Fig. 11. The tapewas then played back through the calibrated remanentpulse playback system shown in Fig. 12 to give a meas-ured remanent flux of 0.041 maxwells. The measuredoutput of a normal recording for equivalent conditionsgave a peak remanent flux of 0.047 maxwells. The dis-

8951951


crepancy between these is less than 12 per cent, thedirect remanent measurement flux being lower thanthat of the actual recording. This represents approxi-mately 0.9 decibel in the output, a difference which iswell within the experimental precision.

In order to establish the correlation with the wave-length response characteristic, elements of the tapewere magnetized along a magnetic sequence establishedfor a recording current of 0.18 milliamperes. Initialmeasurements were made with a phasing guch that boththe signal and excitation had a positive maximum valueat the center line of the air gap. Figs. 9 and 10 showsuch a cyclic variation. The results were somewhatsurprising in that the 2-mil wavelength signal gave anoutput of virtually zero and shorter wavelengths downto one mil produced output pulses of reversed polarity.This indicated that for the shorter wavelengths the

2 * 5 10 2

WAVE LENGTH (MILS)

Fig. 13-Wavelength response characteristics. Curve A shows themeasured effect of the recording demagnetization as a function ofthe signal wavelength.

negative part of the cycle was sufficient to counteractthe positive magnetization and leave a reversed rema-

nence. It was necessary, therefore, to rephase for suchwavelengths and determine the magnetic cycle for thenew phase. It was interesting that for a certain phasingthe pulse became saddle-shaped, indicating it was onlyat the center of the gap that the reversed field was suffi-cient to produce appreciable demagnetization.The results of the output measurements for a phase

that would give maximum output are shown as curve A,Fig. 13, in which the 20-mil wavelength is taken as a

reference and plotted arbitrarily as 20 decibels. Forwavelengths greater than 20 mils, the signal magnetiz-ing force is attenuated along the gap-field distributionline and therefore would suffer the same recording de-magnetization as would the 20-mil wavelength signal.The droop in the curve with reduced wavelength repre-

sents the loss in decibels due to increased recordingdemagnetization above the 20-mil wavelength value.

Curve B, Fig. 13, is the wavelength response character-istic for normal recording under the same conditions.To this has been added the measured recording de-magnetization losses (curve A) to give curve C whichwould represent the wavelength response characteristicincluding the effect of recording demagnetization. CurveD represents the idealized response in which there areno losses and which is closely approximated for thelonger wavelength signals. The playback losses due tothe gap length of the playback head neglecting theeffect of the thickness of the magnetic coating on thetape were calculated from the expression2'10

rigsin-x

loss = 20 log db,wig

x

where 1i is the gap-length and X is the signal wavelength.These losses were subtracted from curve D to give curve

E. In other words, the two upper curves of Fig. 13 are

calculated, while curves B and C are plotted frommeasured data.The author gratefully acknowledges the information

communicated to him by D. G. C. Hare1' concerningthe effect of the thickness of the magnetic coating of thetape on the frequency response of the playback system.The factor to account for this effect was given as

E-2Td/X where d is the distance from the surface of theplayback head to the center of the magnetic coating,and X is the recorded signal wavelength. The playbackloss function including the effect of the coating thicknessthen becomes

playback loss = 20 log (-2ird/N)

wilgsin

x

db. (2)

When the playback losses for the tape used are calcu-lated from (2) and subtracted from curve D of Fig. 13,there results curve F which correlates well with themeasured output of the actual recording, to which hasbeen added the effect of recording demagnetization(curve C). The remaining discrepancy is attributed tothe redistribution of the tape field in the presence ofthe play-back head, nonparallelism of the gap polefaces, and self-demagnetization effects.The reduction of output with increased excitation

(Fig. 2) was predicated on the basis of increased record-ing demagnetization with increased excitation for theshorter wavelengths. The experimental validation of

10 Otto Konei, "Frequency response of magnetic recording,"Electronics, vol. 20, pp. 124-128; August, 1947.

11 The DGC Hare Company, New Canaan, Conn.

(1)

896 A ugust


0m

-C

o-

>i

NORMAL RECORDING

- -x-PULSE MEASUREMENTS

EXCITATION CURRENT (MILLIAMPERES)

Fig. 14.-Recorded voltage output as a function of the excitationcurrent as measured from a normal recording and from tape ele-ments which had been magnetized by a magnetic cycle cor-responding to the conditions of the normal recording.

this is shown in Fig. 14 for signal wavelengths of oneand eight mils. The continuous lines pertain to resultsof normal recording, and the broken lines to the outputpulse measurements of tape elements which had beenmagnetized by the appropriate magnetic cycle and for aphase giving maximum output. It was discovered thatthe phasing for maximum output was a function of theexcitation current and had to be determined for eachexcitation. Since the self-demagnetization, gap-lengtheffects, and the like, were not measured, the signifi-cance is in the shape of the curves and not in the rela-tive outputs. Consequently, for comparison the curveswere superposed for an excitation of 1.6 milliamperes.The correlation of the curves is within 2.5 db over themeasured range, and the fact that the eight-mil curvesrise and the one-mil curves fall effectively substantiatesthe proposed theory of recording demagnetizat on.

Standards on Transducers: Definitions of Terms, 195 1*

COMMITTEE PERSONNEL

Standards Committee, 1950-1951

J. G. BRAINERD, ChairmanL. G. CUMMING, Vice-ChairmanA. G. JENSEN, Vice-Chairman

M. W. Baldwin, Jr.R. R. BatcherH. G. BookerF. T. BudelmanW. G. CadyP. S. CarterJ. W. Forrester

A. G. FoxR. A. HackbuschJ. V. L. HoganJ. E. KeisterE. A. LaportWayne MasonR. A. MillerL. S. Nergaard

A. F. PomeroyGeorge RappaportH. E. RoysP. C. SandrettoE. S. SeeleyR. F. SheaJ. R. SteenHenry Tanck

Bertram TrevorW. N. TuttleL. C. Van AttaK. S. Van DykeD. E. WattsErnst WeberL. E. Whittemore

Definitions Co-ordinating Subcommittee, 1950-1951A. G. JENSEN, ChairmanP. S. CARTERE. A. LAPORTM. W. BALDWIN, JR.

Task Group on Transducer Terms

A. R. D'heedeneH. W. AugustadtM. S. Richardson

J. G. KREER, JR., ChairmanJ. A. MORTON, Past Chairman

C. J. Wiebusch

R. A. LynnW. P. MasonJ. C. Shelleng

* Reprints of this Standard, 51IRE 20 S2, may be purchased while available from The Institute of Radio Engineers, 1 East 79 Street,New York 21, N. Y., at $0.50 per copy. A discount of 20 per cent will be allowed for 100 or more copies mailed to one address.

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Recording Demagnetization in Magnetic Tape Recording