US2675429A - Magnetic sound recording method - Google Patents

Description

April 13, 1954 H. ROHLING MAGNETIC SOUND RECORDING METHOD 2 Sheets-Sheet 1 Filed July 3, 1950 In venfor:

April 13, 1954 H. ROHLING MAGNETIC SOUND RECORDING METHOD 2 Sheets-Sheet 2 Filed July 3, 1950 Inven for:

Patented Apr. 13, 1954 JOTFFI'CE Hermann Rojhling, "Oberdorf, near Immenstadt,

. Germany (llaims. :11

In*tlre-"magneticrecording of sound a -so-"oalled recording head "is used *which contains -a ring magnet having an air gap which-faces*thea-ecord "carriertravelling past the recordinghead. The recording head is supplied -with-speech frequency alternating voltage which "correspondingly mergnetisesthe carrier moving-past it. "The magnetic "field ';produced in "the air gap "is not however homogeneous but is subject to "considerable leakage.

The latter "could be minimised by the use -of low speech voltages but this is not possible-owing to the high permeability of the record carrier fanditsgreat susceptibility to-residual magnetism. The recording head and the recordcarr-ier have two-entirely different'magnetisation curves. For these' reasons speech frequency magnetisation is basedupon appoint on -'the'straightportion of the hysteresis 100p of the "record carrier, this point being calledthe working point. This is achieved by bias 'magn'etisation, *the 'speech frequency alternatingcurrent having a bias magnetisation current superimposed thereon.

However, bias 'magnetisation considerably impairs the high frequencies of the-speech current inthe recording head-and this gives rise *to non linear distortion. The reason forthis is that "large leakage regularlypccurs in *the recording head, particularly-if direct-current is used for bias magnetisation. Attempts have been made to overcome these di'fiiculties by the -use of high frequency bias, but this has the disadvantage of necessitating theme of expensive amplifiers and generators in the electrical equipment of the re- *oordingapparatus.

The large lea'kage associated "with bias "magnetisation and the disturbance of the-useful'magnetisation'caused-thereby' has been reducedin" the past by making the magnetising" recording head "of annular form. In 'this annular "head there .is out an air-gap at the "point where it touches the record carrier. In this way however the object aimedatiis partly defeated again, because leakage current once'more occurs at-the'airgap and causes arconsiderable amount of-distortion, the removal of which isthe object ofthe present invention.

The invention based on the notion that what matters to avoid as completely as possible the occurrence at thepoint o'f contactibetween recording head-and record carrier of any leakage lines which are opposed to the direction ofispeech magnetisation. To .this end .it proposed in accordance with the invention that the speech new shouldbe arranged separately'from'the recording head insucha manner that part-of the flux of the control fieldis induced inthe recordinghea'd or its recording *gap. The building i up tof-the speech field is therefore independent of {the recording headwhichh'owever may carry the bias magnetising' coil a manner *known perse. With "advantage the bias magnetisation is produced by a -magnetic field of constant *field strength whereas speech excitation is brought about by a controlling field (hereinafter-*calleda speech frequency field) which-overlaps the bias magnetisation field only '-within the range of excitation-of the record carrier.

"Anairmored coiIis pr-eferabI-y'used for exciting the*speechdrequency field. For example, a-coil (of 400 turns of 0 .45 mm. diameter enamelled copper wiremay-be-usedj-its length heing 30 mm.

and its diameter '10 mm. It"has *been found that such-a "coil placed at "a distance of a "few m'illimeters from'the -recordcarrier gives perfect control of the record*carr-ier when a recording power of about 1 watt is used. The record gives ta reproduction with good freedom from distortion and pronounced sensitivityto frequencyzchanges as wellas large band width.

Abomparison of=this resiilt with a knownarrangem'ent of the same dimensions, using the same record carrier material and the same tre- *cording gapshows thatin the known recording 'methodsthere-are present large-scale detrimental influences the"cause'of which are the leakage ilinesproceedingfromthe recording gap whereas in "the arrangement I according to the invention :there is "only a very small quantity of *ferromagnetic medium'inthe totalpath of the-lines of force-of the speech frequency field where the "latter-is produced separately from the recording gap, and'in this way distortions are'suppressed.

"Thisseparation of the speech frequency ex- .citerfrom the recording head may be brought "about for example by placing the speech field exciter opposite the recording head which is in contact with therecord carrier andon the other s'ide'of the latter. The coil of the speech freiquency iexcit'er may bearranged at anangle to the record carrier "for example perpendicularly thereto. In that case the speech frequency field should be arrangedstaggered in such a manner with respect to the "placewhere the recording head'is adjacent the record carrier that only one half of the speech'frequency field comes'within the range of the recording head whereas the other half faces .away from the. recording head, preferably towards the direction'from which :the record carrier is moving.

The speech frequency field is only induced in the recording head at or adjacent the record carrier and in the portion of the record carrier adjacent thereto. The recording head itself may be of any desired dimensions, the only essential point being that the lines of force leaving the speech coil should excite the iron of the recording head only where it lies adjacent to the record carrier.

The drawing shows several embodiments of the invention;

Figs. 1 and 2 show two known forms of recording heads,

Figs. 3 to 7 show difierent arrangements in accordance with the invention.

In all figures the lines of force are shown in broken lines. In order to be able to show more clearly the lines of force within the carrier I which is extremely thin, the carrier and the recording gap are shown in the drawing with a purposely exaggerated cross-section.

.In the known arrangement according to Fig. l a record carrier l is shown with a very simple recording head consisting of an iron core 2 and a coil 3. With this arrangement the record carrier slides over the iron core and is magnetised at that point. So-called bias magnetisation can also be effected in known manner either by means of the coil 3 or by means of a stationary magnetic field.

Lines of force leave the recording pole of the core 2 as indicated in dotted lines, the direction of the lines of force at the departure side a being the reverse of that at the arrival side b with respect to the direction of movement of the carrier. The resulting magnetisation record on the record carrier can therefore only consist of the difference from the arrival to the departure of the record carrier. This fact is known from prior literature and has led to different attempts at providing a solution.

Fig. 2 shows what happens when a ring recording head 2 is used. Depending on the dimension of the gap width 4, leakage fields occur in this case also to a greater or lesser degree, which likewise influence the recordcarrier at the departure side of the recording head 2 by their opposite direction.

In the embodiment of the invention according to Fig. 3 there is disposed on one side of the record carrier I, which may consist for example of a steel wire of 0.06 mm. diameter, a bias magnetisation ring 2 whose coil 3 is supplied with direct current, whereas on the other side of the carrier there is disposed an air-cored coil 4 for the speech frequency voltage, and it is staggered with respect to the head 2. The ring 2 is slightly rotated so that the air gap 5 is not pointed straight at the carrier and the gap is closed by a magnetic conductor 6, for example of transformer sheet metal, the gap 7 of which is in contact with the carrier.

In the embodiment according to Fig. 4 there is disposed on one side of the carrier a recording head comprising a simple iron ring 8 with its air gap 9 facing the carrier. On the other side there are a bar magnet it (which may be excited by direct current or may be a permanent magnet) and a speech frequency control coil H. Both coils may be disposed in the same plane or they may be in different planes. They are however so arranged with respect to each other and to thering 8 that the latter draws the lines of force both of the bias magnetisation and of the speech frequency field towards the gap 9.

Patent is:

4- In some cases it may be possible to use a simple iron pole piece instead of the ring. Also the coil ll could be provided with a core 12 of the lowest possible remanence.

Fig. 5 shows the combination of bias magnetisation and speech frequency field in a single coil l3, an iron ring 8 with an air gap 9 staggered with respect to the coil 13 being provided as in Fig. 4. Direct current voltage and speech frequency voltage are in this case applied to the same coil l3 from sources A l and [5 respectively, the coil l3 being an air-cored coil. The lines of force of the combined bias magnetisation and speech frequency field are induced in the gap 9.

-Fig. 6 shows an embodiment with several speech frequency coils. In this case a bias magnetisation recording head 2 with a direct current coil 3 is arranged as in Fig. 3 on one side of the carrier, whereas on the opposite side there are coils I 6 and l'! which may be connected in parallel. One of the coils, for example l6, may be especially adapted for high tone frequencies and the other, for example 11, for low .tone frequencies, anti-distortion means being connected in the circuit. The direction of the lines of force is arranged to be such that the field lines reinforce each other. If desired both coils can be provided with a U-shaped iron core. This has for effect that the control field produced is stronger though dependent-on fre- 111161103 Fig. 7 shows an arrangement for tone recording with echo effect. A recording head in the form of a ring l9 with a direct current coil 20 serves for bias magnetisation and has; two air gaps 2| and 22. Staggered with respect to the latter, and on the opposite side of the carrier, are two air-cored coils 23 and 24 which are energised by two separate speech frequency cur;- rents from two diiferentmicrophones via diiferent amplifiers. The distance between thegaps is calculated for a time interval ofsayH/ o of a second, taking into account the speedof movement of the carrier I. Similar arrangements are also possible with three gaps in the recording head and three speech coils.

In all embodiments shown, high frequency may be used for bias magnetisation instead of direct current, if desired. 7 I What I claim and desire to secure by Letters 1. Magnetic sound recording apparatus com;

.prising an electromagnetic transducer head, a

magnetic record member adapted to move past said head, said head being positioned on one side of said member, a signal coil for producing a magnetic field proportional to the signal impressed thereon, said coil being positioned on the opposite side of said member to said head, the axis of said coil being laterally spaced from a plane passing through the center of said head and through said member at right angles thereto whereby a part of said signal coil magnetic field is induced in said record member adjacent said head and in said head adjacent said record mem ber.

2. Apparatus as defined by claim .1 further including ameans to bias magneticallysaid rec:

0rd member'to a point on'the straight'line portion of the hysteresis loop thereof.

3. Apparatus as defined in claim 1 wherein said electromagnetic transducer head includes a C-shaped iron core having a gap definedthereby, said gap being positioned adjacent said the extremities of said iron core defining said gap.

4. A magnetic sound recording apparatus as defined by claim 2 further characterized in that the bias magnetization field is produced by direct current and is of constant field strength.

5. Magnetic sound recording apparatus as defined by claim 1 further characterized in that said coil is an air-cored coil for producing the signal coil magnetic field.

6. Magnetic sound recording apparatus as defined by claim 1 further characterized in that only those field lines of the signal coil magnetic field which are parallel with the direction of movement of the record member are induced in the record member adjacent the transducer head.

7. Magnetic sound recording apparatus as defined by claim 1 further characterized in that said coil is offset in the direction from which the record member is moving.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,484,568 Howell Oct. 11, 1949 2,539,400 Camras Jan. 30, 1951

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