US2230913A - Magnetic sound recorder - Google Patents

Description

'Feb. 4, 1941. E, C LER 2,230,913 MAGNE TIC SOUND RECORDER Patented Feb. 4, 1941 UNITED STATES MAGNETIC SOUND RECORDER Eduard Schiller, Berlin-Johannisthal, Germany,

assignor to Licentia Patent-Verwaltungs-Gesellschaft, Berlin, Germany Application August '17, 1939, Serial No. 290,389 In Germany March 3, 1936 6 Claims. (01479-1002) This invention relates to magnetic sound recorders, and. particularly to telegraphones or devices of the type in which the sound is recorded by selectively magnetizing successive portions 5 of a traveling wire or band in accordance with the magnitude and frequency of the sound waves;

The practice has been to pass the sound track or magnetic hand through the field of a strong magnet as it approaches the recording electro- 1 magnets, thereby erasing any previous history before a record is entered on the moving band. The magnets and electromagnets previously employed for this purpose have not been entirely satisfactory as the magnetic field between the two poles of the magnet varied greatly both as to intensity and direction. A part of the magnetic field would extend longitudinally of the sound carrier but the adjoining leakage field extended in various directions and was of such strength that the magnetization that ultimately remained on the sound carrier deviated considerably from I the desired longitudinal direction.

An object of this invention is to provide a magnetic sound recorder having erasing magnets with pole pieces of such design that the active stray field extends in the longitudinal direction of the sound carrier. An object is to provide an electromagnet for removing any prior records from a magnetic sound carrier as it approaches the recording head; the core of the electromagnet being approximately circular with a small radial gap across which the sound carrier travels in a direction tangential to the core, the pole piece at the departure side of the core having a longer active surface than the pole piece at the advance side of the core. A further object is to provide a magnetic system for conditioning a magnetic sound carrier as it approaches the recording head, the magnetic system including an approximately circular magnet having a short gap across which the sound carrier passes and a trailing pole piece at which the leakage fiux extends longitudinally of the sound carrier and in a direction opposite that of the fiux at the gap of the magnet Another object 55 invention will be apparent from the following specification when taken with the accompanying drawing in which: i

Fig. 1 is a schematic view of an erasing electromagnet that constitutes one embodiment of the present invention; 5

Fig. 2 is a schematic side elevation of another construction;

Fig. 3 is a fragmentary plan view of the same; and

Fig. 4 is a curve sheet showing the magnetiza- 10 tion of the sound carrier at difierent points along its path of travel across the erasing magnet and the recording head.

In Fig. 1, the core I is approximately circular and carries a direct-current magnetizing wind- 15 ing 2 that establishes a strong magnetic field at the gap 3 across which the magnetizable sound carrier 4' travels in a tangential direction. The sound carrier moves from left to right as indicated by the arrow :1: and the pole piece at the 20 right or departure side of the core I is spaced from the sound carrier 4 by a greater distance than is the pole piece at the left or approach side of the core, the terms departure and approach having reference to the travel of the 25 hand t. The modification of the core I from a true circular shape, such as was previously used, has been greatly exaggerated in Fig. 1 as the desired distribution of the magnetic field can be obtained when one-pole piece is bent inwardly 30 of the other by as little as'about 1 mm. A block or strip of non-magnetic material-5 may be located in the space between the inturned pole piece and the band 4 to provide a supporting table for the moving band.- 35

The broken line vectors a indicate the magnetic field that extends longitudinally of the band t at the gap 3. The particular shape of the magnetic core extends the field along the record carrier and the active polar surface of an the pole piece at the departure side of the core is substantially longer than the active polar surface at the approach side.

In the construction shown in Figs. 2 and 3, the core l' carries a direct-current winding 2 for 4,5 establishing a strong magnetic field across the gap 3 between the pole pieces that are of difi'erent effective, lengths, as measured in the direction of travel of the record carrier 5. Both pole pieces have plane surfaces along which the record carrier travels, and the pole piece at the departure side of the core is extended beyond the adjacent leg of the core to form a polar tip 8 at which the leakage fiux, indicated by the dot-' ted line vectors 6, is approximately longitudiml of the record carrier but in a direction oppositegto thatof the field a at the gap 3. MA polsrizillng field c, in the same direction as the satura as magnetic field a. is established at the narrow slot I oi the recording magnet I by the direct-current winding 9. The alternating field corresponding to the sound or signal to be recorded is produced by current flow in winding Ilionthe core 8.

The method of operation may be best understood irom a consideration of the typical hysteresis curve of Fig. 4. Each magnetic particle of the record carrier passes the gap 3 for which the magnetic field intensity is represented in Fig. 4

by the vector quantity a. The flux density in the ferromagnetic band 4 rises to the value P1 along some line such as P1 as the magnetic particle approaches the gap 3, the exact shape oi this part of the curve depending upon the prior magnetic condition of the particle. The intensity of field a. is sumciently great to produce saturation and therewith the extinction of the prior magnetic history ofthe particle. The field intensity is zero at some point along the pole piece beyond the gap 3 and the induction falls to a value P: that is determined by the retentivity of the carrier 4. The particle then enters the field b at the polar tip 6 where the flux density of the carrier is represented by point P3 of the curve. The flux density has a value corresponding to point P4 after it leaves the field b, and then rises to a value P under the influence of the unidirectional polarizingfleld c that is superimposed upon the alternating field d of the signal at the recording head. The point P5 is the operating point on the magnetization curve about which the induction is varied by the alternating field d.

, It has been demonstrated that telegraphone I record carriers treated by the described erasing magnetic systems give a higher sound output at the reproducer, for equal intensity input signal currents, than the record carriers from which the prior history was removed by the previously known erasing magnets. The record carriers exhibited a lower noise level when pretreated by the new erasing magnets and thus permit. higher amplification of the reproduced 1 signals. t

ing a longer active suriace than the pole piece at the advance side 01 the core.

2. In magnetic sound recording apparatus, a magnet as claimed in claim 1, wherein the pole piece at thedeparture side of the core is more distant from the path of travel oi the record strip than is the other pole piece, and a strip of non- 'magnetic material is located between the path of the record strip and said pole piece at the departure side of the core. 7

3. In magnetic sound recording apparatus, the invention as claimed in claim 1, wherein the pole piece at the departure side of the core extends along the path of travel of the record strip and terminates in a polar tip at which the direction of the leakage flux is opposite that of the flux at said radial gap.

4. In apparatus for the recording of signals on strips of magnetic material, a magnet for eliminating'prior recordings from a strip as it approaches a recording unit; saidmagnet comprising an approximately annular core and a direct current winding on said core, said annular core having an air gap over which the record strip travels, the core section at the departure side of the gap being spaced from the record strip by a distance substantially greater than that between the core and record strip at the advance side of the air gap.

5. In magnetic sound recording apparatus, an erasing head comprising a core having pole pieces spaced apart by a gap across which the record carrier travels, and a direct current windingon the core, the pole piece at the departure side of the core being extended along the record carrier path to form a leakage polar tip at which the direction of the leakage flux is opposite to the direction of the flux at the air gap.

6. In magnetic sound recording apparatus, th combination with means creating a strong magnetic field substantially longitudinally of a moving record strip, means acting upon a record strip after the strip passes beyond said .first means to establish a magnetic field of less intensity and extending longitudinally of the strip in 9. direction opposite that of the first field, and a recording head for magnetizing said record strip after it passes from the second means, said recording head including a magnetic core and a winding on said core for connection to a source of audio frequency energy, of polarizing means for establishing a magnetic field longitudinally of the record strip and in the same direction as the field of said first means, said polarizing means comprising a direct current winding on the magnetic core of the recording head.

EDUARD scHiiLLEn.

CERTIFICATE OF CORRECTION. Patent No. 2,250,915. February L 19in.

EDUARD SCHULLER.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 55, claim 1, after the word "pieces" insert --spaced--; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 25th day of February, A. D. 19141.

7 i I Henry Van Arsdale, (Seal) y Y I Acting Commissioner of Patents.

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