-REF ID:A105054 

Sound Recording on Magnetic Tape . ' 

By C. N. IDCKMAN 

This paper describes an impro,•ed method of recording sound 

magnetically on a steel tape, similar in principle lo that of the 

Poulsen telegraphone. In the latter a longitudinal magnetic pattern of the Yoicc current is imprinted on a steel wire by drawing it 

rapidly past recording pole pieces. The high speed used by Poulsen 

and subsequent inyestigators has been directly and indirectly a limiting factor in the application of magnetic recording to commercial 

uses. The system here described makes use of pt'rpendicular 

magnetization. This method makes it possible, with suitable 

equalization, lo obtain a substantially uniform frequency-response 

characteristic up lo 8000 cycles per second with a tape spt't'd of 

only 16 inches per second. In many cases a speed of 8 inches per 

second is adequate for recording speech. At the same time the 

ratio of signal lo background noise has been substantially increased. 

The decrease in efficiency resulting from the use of perpendicular 

instead of longitudinal magnetization is offset to a great extent.by 

the use of a bctll'r design and construction of the pole-pieces and a 

more suitable recording medium. The recording medium is a 

steel tape having a thickness of about 1.0 to 2.0 mils (0.025 to 

0.051 mm.) and a width of about 50 mils (1.3 mm.). 

INTRODUCTION 

A SYSTEM of recording speech magnetically on a steel wire was inYcntcd by Poulsen almost forty years ago. The wire was drawn 

past a pair of pole-pieces surrounded by coils carrying a speech current. 

A magnetic pattern corresponding to the current was thus impressed on 

the wire. \:Vhen the wire thus magnetically treated was again drawn 

past the pole-pieces a current corresponding to the recording current 

was induced in the surrounding coils. It was common practice to 

place the pole-pieces on opposite sides of the wire and offset with 

respect to each other. The magnetic pattern in the wire thus consisted mainly of a Yariation in the intensity of magnetization, the 

direction of the magnetization being substantially parallel to the axis 

of the wire. This method of putting the record on the wire is known 

as longitudinal magnetization. With such a system the wire must 

travel at a very high speed if high frequencies are to be recorded and 

reproduced. It was customary to use speeds of from six to ten feet 

per second. By using tape instead of wire, the recording and re165 
REF ID:.5054 

166 BELL SYSTEM TECIINICAL JOURNAL 

producing pole-pieces may be placed directly opposite each other so 

that the magnetic pattern consists of variations in the intensity of 

niagnelizalion, the direction of the magnt!lization being substantially 

perpendicular to the surface of the tape. This type of magnetization 

will be called perpendicular magnetization. There js another method 

of recording in which the magnetization is in a direction perpendicular 

to an edge and parallel to the surface of the tape which has been called 

cross or transverse magnetization. 

In spite of the fact that the principle of magnetic recording has been 

known for a long time, there has been very little literature on the subject until recently. Several papers 1 which deal almost entirely with 

the longitudinal method of magnetization have been published abroad 

during the past two years. Cross magnetization is discussed briefly 

in one of the papers. Apparently, perpendicular magnetization has 

not been seriously considered. This paper will treat mainly the 

perpendicular method of magnetization with which a good frequencyresponse characteTistic may be obtained with a tape speed of only 16 

inches per second. 

FORMS OF RECORDING. MEDIA 

Sted wire has been used as a recording medium in most of the 

telegraphoncs. This was probably because it was easier to obtain. 

When wire is used it is necessary to make the longitudinal separation 

of the pole-pieces rather large. This is done in order to minimize the 

distortion caused by the continual rotation of the wire about its axis. 

Such rotations change the relation of the magnetic patterns in the wire 

with respect to the reproducing pole-pieces from that which existed 

at the time the record was made. 

When the pole-pieces have a wide separation, high linear speed must 

be used in order to record and reproduce high frequencies. The high 

speed required in this method of recording gh·es rise to a number of 

mechanical difficulties. )'he contacting pole-pieces 'vear away rapidly 

and it is difficult if not impossible to construct and hold them so that 

they will ride smoothly against the wire. These variations in contact 

with the wire change the magnetic reluctance of the flux path so 

that the signal strength varies. and an excessive amount of noise is 

introduced. 

Recording on steel discs has been investigated from time tu time 

but no practical results have yet been reported. 

l See list at end of this article of recently publishe<l papers dealing with magnetic 

recording. 
9REF ID:A105054 

SOUND RECORDING ON MAGNETIC TAPE 167 

Steel tape as a recording medium was suggested by V. Poulsen in 

his U. S. patent No. 661,619-1900. Its use eliminates many of the 

objectionable features of the wire recording system. The magnetic 

patterns in the tape pass the pole-pieces during reproducing in the same 

relative positions as at the time they were made. It is practical to 

wind the tape on reels of pancake shape>. Snarling difficulties encountered when using wire are thereby avoided. Thin tape permits 

the use of smaller pulleys without exceeding the bending fatigue limit 

of the metal. The use of tape permits the perpendicular method of 

magnetization to be employed. High frequencies may therefore be 

recorded and reproduced with a relatively low linear tape speed. 

METHODS OF MAGNETIZATION 

There are two methods of longitudinal magnetization in use, one 

and two pole-piece recording. A detailed description of these methods 

is given in two of the papers which have been mentioned. It will be 

sufficient here to consider them only briefly. 

p 

MOVEMENT OF TAPE 

~t-------

~ ti1------

~ 

LENGTH OF TAPE 

Fig. !-Longitudinal magnetization of a recording medium uy a single pole piece. 

SIG =Signal coil; DP = Depolarizing coil. 

Figure 1 shows the action taking place in recording with one polepiece. },[ is the recording medium and P is the recording pole-piece. 
.. ID:A105054 

168 BELL SYSTEM TECHNICAL JOURNAL 

It wiJl be assumed that the recording medium has been previously 

magnetized by drawing it past a pole-piece so that the residual magnetization in it has a direction as indicated by the upper arrow at the 

lef t.1 In this method of recording the magnetization is principally 

parallel to the axis of the medium but in order to simplify the drawing, 

the direction of magnetization in Fig. 1 is shown at a considerable 

angle. If the pole-piece P carries a steady flux in the direction indicated by the heavy lines, this flux will spread in the medium. At the 

point 2 in the middle of the pole-face, the flux. will be substantially 

perpendicular to the axis of the medium. On either side it will be 

approximately parallel to the axis of the medium but of opposite 

directions. As the elements of the recording medium approach the 

pole-piece P, they will first be subjected to the flux 1 which is in approximately the same direction as the residual magnetization in the 

medium so that no appreciable change will take place. When the 

elements are directly opposite the face of the pole-piece they will be 

acted on by a flux 2 which is nearly perpendicular to the residual 

magnetization of the medium. When the elements reach the position 

3, the flux will be in opposition to the original magnetization within 

the medium. If it were not for these changes in the direction of the 

flux while the elements are passing from the position 1 to the position 

3 a signal record without appreciable distortion could be left on the 

medium at the point 3 by superimposing a signal flux on the steady 

flux in the pole-piece P. It will be realized that the positions 1,. 2 and 3 

are not discrete points but that they cover an appreciable distance. 

The spreading of the flux at 3 w:ill be considerable so that it will be 

necessary for the medium to travel at high speed in order to get the 

recorded signals away from the recording flux before the record is 

distorted by subsequent signals. 

Figure 2 shows a similar diagram for two pole-piece recording. 

Where two pole-pieces are rel;ltively close to each other, the flux 

will not spread so much in the medium and the direction of magnetization will be approximately the same as that of the recording flux. 

It is again assumed that the residual magnetization within the medium 

is mainly in the opposite direction to the motion of the medium as 

indicated by the upper arrow at the left. The flux 1 will have no 

appreciable effect on the residual magnetization. The flux 3 is in the 

opposite direction to the residual magnetization and were it not for 

•In Figa. 1, 2, 3, 4 and 6, the heaV}' lines passing through the pole-pieces re~t 

the instantaneous recording flux. The density of the fine lines in the recording medium represents the intensity of magnetization. The arrows above and below 

the medium show the direction of this magnetization. The curve below represent& the nature of the signal that has been recoided, on the tape. 
9REF ID:A105054 

SOUND RECORDING ON MAGNETIC TAPE 169 

these changes in the direction of the flux while passing from 1 to 3, 

a modulation of the flux 3 might be expected to leaye an undistorted 

record on the medium as was the case with one pole piece recording. 

HoweYer, in the case of two pole-pieces, the elements still must pass 

the pole-piece P2 where the flux 4 is approximately perpendicular to 

the medium. After passing the pole-piece P2 the record is subjected 

to the flux 5 which is in the opposite direction to the recorded flux. 

The record which was made by the flux 3 is therefore distorted by 

fluxes 4 and 5. This distortion is greater than it is for one pole-piece 

M 

T· 

MOVEMENT OF TAPE 

~1------..... Ii 

N 

~1--------------3'....------+---.,,,C...---1-----1~ 

~ 

LENGTH OF TAPE 

Fig. 2-l.ongitudinal magnetization of a recording medium by two pole pieces. 

recording. In practice the stray fluxes 4 and 5 are sufficiently small 

so that the distortion introduced may be tolerated in exchange for the 

improved frequency response which is obtained with two pole-piece 

recording. 

Figure 3 shows the action taking place where cross-magnetization is 

used. It is here assumed that the recording medium has been previously magnetized so that the residual magneti~ation is in the direction indica~d by the uppc>r arrows at the left. W represents the 

width of the recording medium which in this case is a steel tape. It will 

readily be seen that if Tl' is very large there will be considerable spread-
REF ID:\1t05054 

170 BELL Sl'STEM TECIINICAL JOURNAL 

T 

w M 

l 

LENGTH OF TAPE 

Fig. 3-Transverse magnetization of a steel tape. 

_J_ __ 

T M 

T MOVEMENT OF TAPE 

I 

I i 

ii---~~' LENGTH OF TAPE 

Fig. 4-Perpcndicular magnetization of a steel tape. 
~REF ID:A105054 

SOUND RECORDING ON MAGNETIC TAPE 171 

ing of the recording flux within the tape. The-recording flux is always 

at substantially right angles to the axis of the tape and parallel to its 

surface and is in the uppoi>ite direction tu the residual magnetization. 

If Wis made quite small or in other words if the pole-pieces P 1· and 

P2 are directly opposite each other with the thin dimension of the 

tape between them, we have the conditions shown in Fig. 4. The 

tape is so thin that there is very little spreading of the flux so that the 

width of the flux path is not appreciably dependent on the strength 

of the signal. This type of recording is called perpendicular magnetization in order to distinguish it from cross-magnetization, where the 

width of the tape instead of the thickness determines the pole-piece 

separation. The perpendicular method of magnetization permits a 

relatively low tape speed. The thickness of the pole-piece tips 

determines the frequency response for a given tape speed. 

:Method of Recording with Perpendicular Magnetization 

If the tape is first subjected to a saturation flux which is at right 

·angles to the surface of the tapt:', it will be left with one side of north 

and the other of south polarity. If the tape in this condition is 

passed between recording pole-pieces carrying only AC flux, it is 

obvieus that only half cycles will be recorded. The record is therefore 

much distorted. The current reproduced from such a record is 

similar to the alternating current which may be obtained from a 

single wave rectifier. 

If on the other hand the tape is passed through an alternating highfrequency field which is strong enough to erai;e the record, it is left 

in a substantially neutral condition. If it is then passed between the 

recording pole-pieces, both half cycles will be recorded but there will be 

amplitude distortion. Figure 5 shows a magnetization curve for iron 

which has previously been demagnetized with alternating current. 

The slope of the first part of the curve is small in either direction of 

magnetization and then increases with increase in the flux and finally 

becomes smaller again. Small signals will therefore be recorded 

weakly and strong signals will be recorded relatively higher. Both 

will have wave form distortion. The same effects would be obtained 

with longitudinal or cross magnetization. In the past, investigators 

have often utilized only one side of the magnetization curve. A direct 

current was used as a bias to bring the recording flux to the most 

suitable part of the cun·e such as at n, Fig. 5. 

The method employed here will be made clear from Figs. 6 and 7. 

As the tape elements enter the field of the polarizing pole-pieces P1, P2 

(Fig. 6) they are subjected to an increasing magnetizing force. The 
172 

T 

T 

p 

REF ID:eosOS4 

BELL Sl'STEAf TECIINICAL JOURNAL 

B 

Fig. 5-Typical magnetization curve for steel. 

POLARIZING 

POLES 

P1 

RECORDING 

POLES 

P3 

MOVEMENT OF TAPE 

I 

' 

M 

P4 I I 

Ill----~-------~--r -+--.-• ~ .. 

LENGTH OF TAPE 

Fig. 6-Perpcndirular magnetization method of recording on steel tape. 

SIG =Signal coil; l' = Polarizing coil; DP= Depolarizing coil. 
~REF ID:A105054 

SOUND RECORDING ON Af..1GNETIC TAPE 173 

elements are magnetized to the saturation point Pas shown by curve a, 

Fig. 7. As the clements leave the polarizing field they are subjected to 

a field of decreasing strength so that the magnetic induction drops 

along the curve b to R, this point being reached when the applied 

field is zero. In Fig. 7, the magnetizing force II refers to the externally 

applied field. The tape clements then pass between the recording 

pole-pieces which carry a flux in opposite direction to that of the 

polarizing pole-pieces. If there is no signal current present, the 

magnetic induction will be brought down to the point N Ly the Liasing 

field. As the elements pass out from between the pole-pieces, the 

field will decrease to zero and the magnetic induction will change from 

+B 

-B 

Fig. 7-Diagram showing the cycles of magnetization through whkh the elements of a steel tape may pass during the process of recording. 

N to 0, which is a substantially neutral condition. However, if there 

is a signal current present al the time the tape element::; are passing 

between the recording pole-piE'ces, the magnetization will be reduced 

to a point A higher than N if the cycle is in opposition to the bias flux 

or to the point Blower than N if the signal flux is in the same direction 

as the bias flux. In either case the elements will retain a magnetization value corresponding to A' or B' respectively. This system makes 

it possible to record over a longer portion of the magnetization curve 

without appreciable distortion. 

Unless the proper value of biasing field is used to bring the magnetization approximately to the point N when no voice current is present, 

the mmdmum recording range cannot be obtained without excessiYc 

amplitude distortion. For example if no bia::; is used, it ha::; been found 

.. 
REF ID:eosOS4 

174 BELL SYSTEM TECIINICAL JOURNAL 

that t.h<:: output. plotted against input will be too steep as shown in the 

curve 1, Fig. 8. If the bias is too great, the output input curve will be 

inclined too much as ~howu in l:Urve 2 of the same figure. When the 

...J "' Ill 

Ill 

50 

40 

i3 30 Q 

I-

:> 20 11. 

I-

:> 

0 

10 

0 

I 

I/ 

,, / 

!~ ,,,,. 

10 

/ 

v 

j 

I / 

/ 

./ 

/ 

20 

---- '-'v '.'!. 

/ !/ 

I/ 2 v 

/ V" 

/ v 

/ 

30 40 50 

INPUT IN DECIBELS 

-

.. 

110 70 eo 

Fig. 8-The effect of lhe uiasing current on the slope of the output-input cUJ"\·es. 

level 

proper value of bias is used the curve 3 is obtained which is inclined 

at 45 degrees. Measurements of output versus input may therefore 

be used to det.cnnine the proper bias current. 

Another method of determining t.he proper amount of bias is to plot 

the output obt.aincd from records of very weak signals as a function of 

-1 0 1 2 3 4 

BIAS CURRENT IN MILLIAMPERES 

Fig. 9-Effect of the biasing current on the intensity of weak signals. 

the bias current used during recording. Figure 9 shows such a curve 

for a 1000-cycle record. In selecting the proper bias current from the 

curve, the point. at t.he crest gives the most efficient value but it is 
~REF ID:A105054 

SOUND RECORDING ON MAGN-ETIC TAPE 175 

better to favor some point slightly to the right of the crest in order to 

avoid the danger of very strong signals operating too far down on the 

left side of the curve. The recording current for Lh~ 1000 cycles had a 

value 30 db below the overload so that only a small amplitude was 

used in obtaining the data for this curve. 

If the same set of pole-pieces is used bolh for recording and reproducing, there is no question of getting the reproducing pole-pieces in 

the same alignment as the recording pole-pieces. If different sets arc 

used care must be taken to get the same alignment. 

In order to keep the signal high above the tape noise, it is desirable 

to record so that the flux or current amplitude is independent of the 

frequency. Since the impedance of the recording coils rises rather 

sharply with frequency, it is necessary either to place a high resistance 

in series with the recording coils or to connect them to a high impedance. 

The later method is of course the more efficient. Since the energy 

present in the higher frequencies of voice and music is usually less 

than in the 1000-cycle region, an amplifier having a rising characteristic may be used in order to record these frequencies at a higher level. 

A corrective network may be used in reproducing to obtain the desired frequency response. Such a procedure increases the apparent 

ratio of signal to the tape noise. 

REP~ODl]CJNG 

If the thickness of the pole-piece tips is small with respect to the 

wave length of the signal on the tape, the voltage generated in the 

reproducing coils is proportional to frequency, so that the coils may 

be matched to favor the lower frequencies. If a straight line frequency characteristic is desired, a corrective network may be used. 

40 

Ill 

ul 30 Ill 

~ ~ 20 

ILi 

Ill 

z 

0 

:Ji 10 

~ 

0 

20 

/ v 

y --- i,..~ ~ 

~ , "" 

I/I"" " 

:,.. 

.- 3 

1 .... ~~~ 

I,;'~ 

50 100 500 1000 

FREQUENCY IN CYCLES PER SECOND 

r-... 

I\ 

' \ 11 

\ 

5000 10000 

Fig. IO-Frequency response cunres obtained with the perpendicular method of 

magnetization. 
• 

REF ID:.5054 

176 BELL Sl'STEM TECIINICAL JOURNAL 

Figure 10 shows several frequency response curves. Curve 1 

shows a response without the use of equalization for a tape speed of 

8 inches per second. Curve 2 shows the response under the same conditions for a tape speed of 16 inches per second. Curve 3 shows the 

same signals of curve 2 reproduced through a suitaLle equalizer. 

The ratio of the maximum reasonably undistorted 1000-cycle signal 

to the noise with typical good tape is about 38 db. The transfer loss 

is approximately 60 to 70 db. The maximum power required in 

recording is about 0.3 milliwatt. 

CII.\RACTERISTICS OF MAGNETIC RECORDING 

Magnetic recording differs from other methods in several respects. 

Since no processing is required, the record may be reproduced without 

a long delay. The recording medium may be used over and over again 

for new records. It is only necessary to subject the tape to a strong 

magnetic field in order to obliterate a record. The obliteration is 

conveniently done at the same time that the new record is being made. 

Where temporary records are desired, magnetic recording therefore 

has some advantages over other methods. On the other hand it should 

be fully appreciated that the records may be kept, filed away, or 

reproduced thousands of times with no appreciable deterioration in the 

quality. 

The magnetic system is very convenient for use where short delays 

are desired. A short loop of tape in conjunction with recording, 

reproducing, and obliterating pole-pieces is all that is required. Instead of a loop of tape, a disc or cylinder rotating at high speed may Le 

used to carry tht: recording medium. The latter method makes it 

possible to obtain very short delays. \Vhere perpendicular magnetization is used, very long records may be obtained from a medium which 

occupies a relatively small amount of space. For example, a thin 

coil of 2 mil tape 9 inches in diameter will give a playing time of 1/2 

hour with a tape speed of 16" per second. 

There are no moving parts in the modulating unit. The difficulties 

of obtaining high frequencies due to the inertia of the cutting stylus in 

mechanical recording are therefore not present. The system is 

subject to the same difficultiei; of eliminating flutter that we find in 

other methods of recording; however, mechanical vibrations due to 

the motor and other moving parts of the recording system do not have 

to be filtered out as is the case with mechanical recording. There are 

of course no shavings. The recording medium cannot be easily 

scratched and may be handled in any kind of light and subjected to 

large variations in temperature. When properly wound on reels it is 

not liable to breakage or damage during transportation. 
-REF ID:A105054 

SOUND RECORDING ON MAGNETIC TAPE 177 

LITER.\ TURE 

Stille, K., "Die Electroma~netische Schallaufzeichnung," E. T. Z., 1930, p. 449. 

Hormann, E., "Zur Theorie der Magnetischen Tonaufzeichnung," "On The Theory of Magnetic Sound Recording," E. N. T., Vol. 9, pp. 3!!8-403, Oct., 1932. 

Meyer, E. and Schuller, E., "Magnetische Schallaufze1chnung auf Stiihlbander," 

"Magnetic Sound Recording on Steel Ribbons," Zeits. j. Techn. Plzysik, Vol. 

13, No. 12, pp. 593-599, 1932. 

Begun, S., "Die Diktiermaschine im Grossbetrieb," E. T. Z., 1932, p. 204. 

Hickman, C. N., "Delayed Speech," Bell Labs. Record, Vol. 11, pp. 308-310, June, 

1933. 

"l\fagnetk Recording and Reproducing," The l\Iarconi-Stille apparal us described, 

Wireless World, Vol. 34, [lp. 8-10, Jan. 5, 1934. 

Rust, N. M., "Marconi-Stille Recording and Reproducing Equipment," Marconi 

Ret•., No. 46,_pp. 1-11, Jan.-Feb., 1934. 

Frederick, H. A., "Recording and Reproducing Sound," Rev. Sci. lnstriime11ts, Vol. 

5, pp. 177-182, May, 1934. 

Braunmiihl, H. J. von, "Magnetische Schallaufzeichnung im Rundfunkbetrieb," 

"Magnetic Sound Recording in Broadcasting Service," Ftmkteclmische 

Monatshefte, No. 12, pp. 483-486, Dec., 1934. 

Mallina, R. F., "A Mirror for the Voice," Bell Labs. Record, Vol. 13, pp. 200-202, 

Mar., 1935. 

Volk, T., "Magnetophon, ein Neues Tonaufzeichnungsgerllt," "Magnetophone, a New Sound Recording Apparatus," Filmtech11ik, Vol. 11, pp. 229-231, Oct. 

26, 1935. 

Hansen, \V. If., "Das Magnetophon," "The Magnetophone," E. T. Z., Vol. 56, p. 1232, Nov. 7, 1935. 

Schuller, E., "l\'Iagnetische Schallaufzeichnung," "Magnetic Sound Recording," E. T. Z., Vol. 56, pp. 1219·-1221, Nov. 7, 1935. 

Schrage, \V. F., "Sound Recording on Magnetic Materials," Radio Craft, Vol. 7, 

pp. 537-562, March, 1936. 

•