US2468224A - Magnetic reproducer using endless tape records - Google Patents

Description

April 26, 1949. w. A. MUNSON MAGNETIC REPRODUCER USING ENDLESS TAPE RECORDS Filed May 11, 1945 2 Sheets-Sheet l /N VE N TOR WAMUNSON GYHJMM A T TORNEV April 26, 1949. w. A. MUNSON MAGNETIC REPRODUCER USING ENDLESS TAPE RECORDS Filed May 11, 1945 2 Sheets-Sheet 2 FIG. 4.

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Patented Apr. 26, 1949 UNITED MAGNETIC anraonucan USING ENDLESS Y TAPE aacoaps Wilden Munson, Chatham, N. 1., asaignor to Bell Telephone-Laboratories, Incorporated, New

Yorlr, N. Y., a corporation of New York Application May 11, 1945, Serial No. 593,151

' 2 Claims. (Cl. 119-100.

This invention relates a magnetic recording and the principal object of the invention is a method of and apparatus for obtaining good quality reproduction with a single pole-piece.

In the present state of. the artot magnetic recording it is believed that for high quality reproduction best resultsare obtained-by transverse or perpendicular recording on thin tape by means oi a pair of opposed pole-pieces disposed on opposite sides of the tape with the pole tips properly adjusted to confine the recording flux at any instant to a very short segment oithe tape. With this method of recording the tape may be highly magnetized in a perpendicular'direction and due to the high concentration of the flux it is possible to record and reproducea given range of frequencies at lower tapespeeds than are practicable in systems of other known. types.

A-reproducing system or this type however, is not well suited for some applications such for example as multiple unit. machines-using loops'of tape, since the necessity for placing one of the pole tips within the loop. complicates the design and makes it difllcult to devise a compact structure.

point are well understood but they have never been used extensively, largely, it is'believed, because they ordinarily give a low level, poor quality reproduction. This is due primarily to the fact The advantages of single pole-piece reproducing systems from the mechanical design standwhere a once made, is used for repeated I reproductions.

For applications of this type the tape loops are made up to proper size and mounted on spaced rollers for the recording operation which is performed in the conventional manner. In the preierred reproducing apparatus each of a plurality of these prerecorded tapw is mounted on the periphery of a thin disc and-the discs are stacked and secured together on a rotatable base plate to form a cylindrical assembly. The reproducing magnets, one for each tape are disposed around the periphery oi-this assembly in staggered relation.

dinary motors some form of speed reduction mechanism must be used. According to further features of this invention the discs on which the tapes are mounted may be of annular form so that when stacked they deflne'a central, cylinthat the magnetic record'meinber has not been prerecorded to the optimum condition necessary tor satisfactory reproduction by a single pole-.

piecet i p v v The purpose of this invention is to. providev I means for obtaining single pole-piece reproduc ,tion of good quality, and satisfactory utilization oi the mechanical advantages 01- a single polepiece reproducing system.

drical space in which the necessary speed reducing mechanism may be mounted to further Fig. 2. is an elevation, partly in section, of the apparatus; a I Fig. 2A is an enlarged view in section of a'portion'oi' the reproducing apparatus shown in Fig.

2"jt'o .illustrate :the stack of record units and spacers; j

-Fig. 3 is a recordertorv prerecording the records used in'the apparatus; and

According to this invention, good quality re- 1 production is obtained witha single pole-piece by prerecording the tapesto a hlghdegree of. v

I perpendicular magnetization with opposed pole pieces, thereby obtaining-records of such high signal level and such good high frequency definition space within the ring assembly for the-speed.- reducing mechanism required between the tapes and driving motor. This procedure isoi course v primarily applicable to cases in which immediate Fig. 4 is a'perspective view of a single record Referring first to Figs. 3 and 4 the disc or ring lmay be of aluminum or any other suitable material and is of a diameter determined by the playing time required at the selected reproduc- 45- ing speed. The loop of tape 2 shown mounted on the disc is formed by cutting the tape to the exact length required to fit tightly around the periphery of the ring, butt-welding the ends, and

grinding-down the weld to smooth surfaces which will not interfere with the recording and reproducing operations.

The loop of tape so formed is then placed on r the rollers 3 and l which are positioned to hold the tape under sufllcient tension so that, it may reproduction of a recording is notrequired and are driven at any desired recording speedby the 3 motor 5. Tension may be produced in the tape by springs acting against the bearings of the roller 4 or by a conventional idler wheel I.

The recording operation may be performed in any of several known ways-but for purposes of illustration it is assumed that erasing and biasing are eifected by high frequency alternating current from an oscillator I. As explained in more detail in Patent 2,235,132 to Wooldridge. the erasing magnets 8, 8 may be energized by currents of the fundamental oscillator frequency such as 20,000 cycles per second through a suitable isolating and control network while the recording magnets III. III are biased either by currents of the same frequency or by harmonic frequency currents through the network II. The signals to be recorded from the source I2 are superposed on the biasing currents in the magnets I0 through an amplifier I3 which has an output level high enough to record the signals at the -maximum intensity permissible without overloading the tape.

In practice the system will also be provided with conventional reproducing facilities as shown for example, in the Wooldridge patent so that the record so produced may be monitored and rerecorded if necessary before being used in the reproducing mechanism.

The tape length and recording speed will of course be chosen with due regard to the length of message or signal-to be recorded and preferably the message should begin and end a short distance from the weld in the tape and the reproducing' circuit should be disabled momentarily while the weld is passing the reproducing magnet to prevent the welded section from introducing objectionable noise.

Since the recordings are to be reproduced with only a single pole-piece it is important that the recording magnets be designed and disposed to give as nearly as possible pure transverse or perpendicular magnetization in the tape. The tips applicable to cases where only very short playin time is required. In one instance where the reproduced signal was to be of about one second duration, the discs I were about six inches in diameter and were driven at revolutions per minute to give a. tape speed of about 19 inches per second. In this case the discs were .050 inch thick and the tapes were .002 inch thick and of a width equal to the thickness of the discs.

The driving motor 25 is therefore coupled to the disc assembly through speed reducing mechanism which is conveniently mounted in the housing 28 within the central opening 21 of the discs. The housing 26 is secured to the base plate 20 and serves as the driving member for the disc assembly. While speed reduction between the shaft 28 and the housing 26 may be obtained with known mechanisms of various types, many of these known devices are not well suited to the purposes of this invention. With ordinary commercial motors a high degree of speed reduction is required to drive the record discs at speeds ofthe order of 60 revolutions per minute and if the reduction mechanism is to be largely or entirely housed within the disc assembly it must be very small and of such configuration as to fit readily into this limited space, Belt drives are obviously impracticable and gears introduce flutter which must be damped out by suitable mechanical filters.

of the pole-pieces I I, II should therefore be very thin in the direction of the tape motion and their effective thickness may be further decreased by aligning them with a very slight stagger in accordance with well-known practice in the art.

With such a system the spreading of the signal flux lengthwise of the tape as in a longitudinal system, is minimized and the tape is left in optimum condition for reproduction by a single polepiece at satisfactory signal level and without serious degradation in the frequency response characteristic.

The discs I are provided with a keying hole I! for properly orienting the starting points in the tapes of the recorded matter when mounting the tapes on the discs as shownin Fig. 4. These discs to the number required, and in this case twentyfour, are then stacked on the rotatable base member 20 with the stud 2I etxending through the keying holes in the discs which are then clamped together by the annular top plate 22 and the bolts 23 extending through some of the holes 24 of the discs. As a further precaution against possible slipping of the tapes on the discs it may be desirable in some cases touse a thin spacer between each pair of discs as shown in Figs. 2 and 2A. Such spacers may be of thin sheet cellulose acetate or other resilient material stamped out to the same general configuration as the discs I except that the spacers are of. slightly larger diameter so as to engage the side of the tapes when the assembly is secured together by the bolts 23.

This type of structure obviously is primarily One very satisfactory device for this purpose is the ball-bearing speed reducing mechanism disclosed in my copending application with J. B. Little Serial No. 554,894, filed September 20, 1944, U. S. Patent No..2,458,l81, granted January 4, 1949. In this mechanism speed reduction is obtained by means of a series of interconnected ballbearing units each of which comprises balls in a retaining ring in rolling contact with a fixed outer race ring and a rotatable inner race ring. The motor shaft drives one inner ring and the resulting planetary motion of the retaining ring is transmitted to the inner ring of the next unit. The several units are similarly connected together in tandem and the retaining ring 42 of the last unit drives the disc assembly through the clutch member ll which is secured to the top plate 44 of the housing 26.

By proper choice of the ratio between the diam-, eters of the balls and race rings and of the number of'units used any required speed reduction may be obtained in a relatively small space as explained in detail in the application referred to above. This mechanism also has the important advantage that it operates so smoothly that the usual flutter damping means is not required.

The reproducing magnets 30 are disposed around the periphery of the disc assembly on mounting blocks such as 3|, 32 and 33 each of which is preferably shaped to support several magnets as shown. To simplify the manufacture of these blocks and provide an orderly arrangement of the magnets, the spacings between the magnet supporting surfaces 34, 35 and 46 are made the same in all the blocks, in this case eight times the thickness of one disc unit, and the total heights of the several blocks are varied as required to align the pole-pieces of the magnets with the proper group of tapes in each case.

Each of the magnets 30 comprises a signal coil 38 supported on the arm '31 of the L-shaped brass plate 30 and a single thin lamination 39 of magnetic material extending through the coil to en-.

its tip portion extending slightly beyond the end of the arm and tapered to a very thin pole face in contact with the tape. The brass plates 38 are pivotally secured on'the mounting blocks by set screws 40 and the pole faces are held in light pressure contact with the tapes by coil springs 4| on the set screws as shown.

The multiple unit machine illustrated and described was intended to reproduce a frequency range suflicient for good speech quality at the relatively low tape speed of 19 inches per second and it was found to be capable of giving an output level of the order of 15 decibels greater at the higher frequencies than could be obtained when the tapes were recorded with a single polepiece.

What is claimed is: I

1. Magnetic tape reproducing apparatus comprising a plurality of thin rings, a loop of tape prerecorded to a high degree of perpendicular magnetization fitted around the periphery of eachring in tight frictional engagement therewith, means for holding the rings in a closely stacked cylindrical assembly, resilient spacers interposed between said rings and extending slightly beyond the peripheries thereof for engaging the edges of the tapes to prevent them from slipping on said rings, a reproducing magnet cooperating with each tape, a motor for driving the assembly and speed reducing mechanism disposed within the assembly and connecting it to said driving motor.

2. Magnetic tape reproducing apparatus comprising a plurality of thin rings, a loop of tape prerecorded to a high degree of perpendicular magnetization fitted around the periphery of each ring in tight frictional engagement there- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number I Name Date 873,042 Hagemann Dec. 10, 1907 873,541 Hagemann Dec. 10, 1907 1,416,063 .Reddig May 16, 1922 2,079,959 Fear May 11, 1937 2,229,293 Huntley Jan. 21, 1941

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