US2268097A - Sound recording apparatus - Google Patents

Description

A 30, 1941. J. L. UNDERHILL ETAL 2,263,097

SOUND RECORDING APPARATUS Filed May 28, 1938 25 1 1G. Z. 28 flMPL/r/En SHUTTER 1 RECTIFIER 3 I'll? 'I'ER n/cRoPHoA/z: J 24 HF fiMPL/F/ER 1 26 I A GflLV/I/VOMETER mm 1 10 a lzwen tors Joseph L. Underhill Lenn .DL mic/B Patented Dec. 30, 1941 SOUND RECORDING APPARATUS Joseph L. Underhill, London, England, and Glenn L. Dimmick, Haddonfield, N. J., assignors to Radio Corporation of Delaware America, a corporation of Application, May 28, 1938, Serial No. 210,592

Claims. (01; 179-4003) This invention relates to an improved sound recording apparatus and more particularly to sound recording apparatus, for variable density recording with ground noise reduction.

In Dimmick Patents Nos. 2,095,317 and 2,095,-

318, patented October 12, 1937, there are disclosed methods and apparatus for variable density sound recording by what is known as the penumbra method, wherein an opaque mask produces a graduated intensity in a beam of light and the beam of light is then vibrated acrossa plate having a slit so positioned as to select anappropriate portion of the beam which is thereafter directed onto the sensitive film.

In both of the aforesaid patents, ground noise reduction may be applied, and specific apparatus for that purpose is shown in Fig. '7 of thelatter of the patents.

The present invention is an improvement on that of the said patents in that an improved method of and apparatus for ground noise reduction are provided.

Ithas been customary in the prior art to operate the noise reduction amplifier, both for variable width and variable density recording, so

that the bias or shutter current from the ground noise reduction amplifier decreases as the modulation increases, reaching zero at some predetermined modulation level. The reason for this is that the zero current shutter position may be definitely fixed mechanically and forms an interchangeable end stop at maximum modulation.

improved method of and noise reduction. Another object is to provide an improved method of variable density recording.

apparatus for ground Another object of the invention is to provide ground noise reduction an improved type of shutter.

Another object of the invention is to provide a recording system wherein groundnoisereduc- If the prior art systems were operated in the reverse direction, that is, if the noise reduction current were to rise with increased modulation, it would be difficult to rent at a predetermined The systems of the prior art had the disadvantage that the characteristic curve of the vacuum tube amplifiers usually becomes less and less linear as it approaches zero current, and it has heretofore been difiicult for that reason to establish a definite end value of the shutter opening with increased modulation andat the same time to secure a linear response of the ground noise: reductionmechanism in accordance with modulation.

In our improved apparatus for and method of ground noise reduction, we accomplish both of the foregoing desirable results simultaneously by increasing the bias current with increased modulation and providing optical means for determining the end position of the shutter movement.

suddenly limit the curand unchanging value.

One object of the invention is to provide an tion current increases creases. i Another objectcof the invention is to prov1de a recording system wherein the ground noise" 7 reduction shutter response has linear relation to the amplitude of modulation,

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of the specification and the accompanying drawing, in which Figure 1 is a perspective viewof the improved recording optical system,

Figure 2 is a schematic wiring diagram of the apparatus,

Figure 3 shows the relation between the, recording beam and the slit at the position of maximum modulation, and

Figure 4 shows the relation cording beam and the slit at. minimum modulation.

Referring first to Fig. 1, the usual exciter lamp [0 directs a beam of light past the penumbra mask I i-|2, as in Patent 2,095,317, through the condenser lens 13, the aperture IS in the between the re the position of rents passing through the coil 24 Light reflected from the mirror I! is directed by the, lens 18 to the slit plate IS, the lenses [6 and 18 to- 40 gether forming an image of the aperture IE on the slit plate IS in 20. Due to the eifect of the mask H, a beam of light graduated in intensity, as indicated at is formed around the slit 2B in the plate l9 as the modulation in proper relation to the slit tude of the sound impulses from the amplifier 21 increase, the output from rectifier, the filter, and the variable current through the coil 25 increase.

When the current through the coil 25 increases,

the shutter ll, Fig. 1, is moved downwardly, thereby causing light of greaterintensity to strike the slit 20 at the same position of the beam. As shown in Fig. 4, when the mask H is in the zero current position with no modulation, the portion of the beam 2! of least intensity coincides with the slit producing a minimum density on the record 23 and a correspondingly great density on a print thereof, with correspondingly low ground noise. When the shutter H is withdrawn, a portion of the beam 2| of intermediate intensity, i. e., 50% of maximum intensity, falls on the slit 20, as shown in Fig. 3, when the galvanometer mirror I! is in the same position it would occupy if there were no modulation. Due to the fact that no current may flow through the coil 25 from the amplifienrectifier and filter 28 over this range, the current may be so chosen as to fall on the linear portion of the characteristiccurve of the vacuum tubes used, and a correspondingly linear response of the shutter ll may be secured. Likewise, if the shutter II is so constructed mechanically or the magnet 25 is so constructed that the response of the shutter II is not linearv with current through the magnet 25, then an appropriate portion of the characteristiccurve of the output tubes of the unit 28 maybe chosen to compensate for this lack of linearity and a response of the shutter II which is linear with input from the unit 21 to the unit 28 may bethereby secured.

In order to establish a definite end point to the effect of movement of the shutter l I, anauxiliary'stop l2 may be provided. It will be apparent thatwhen the upper edge of the shutter l I is past the upper edge of the-stop l2, no furthat our invention is not limited to the form shown in Fig. 1, but that, if desired, duplicate shutters may be used connected as shown and described and operated in opposition in the manner shown in Fig. 7 of Dimmick Patent No. 2,095,318, each of these shutters being provided with an appropriate stop either optical or mechanical.

It will also be apparentthat the stops 1 I and I2 do not need to be located between the lamp Ill and the lens 13. Either or both of these stops may be located, if desired, between lens l3 and lens 16, as shown in the aforesaid Patent No. 2,095,318. If desired, one of the stops may be located; where shown and the other located between the lenses l3 and I6 so that the two stops are optically coincident, or the penumbras produced by the two stops at the slit plate l9 are of the same height.

We claim as our invention:

1. Sound recording apparatus including means for directing a beam of light in the form of a fine line. upon a sensitive film, means for modulating said beam in accordance with sound impulses to be recorded, and means for modulating said beam substantially entirely in a linear relation to the envelope of the sound impulses to be recorded.

2. Sound recording apparatus including means for directing a beam of light to a slit, means for imaging light passing through said slit in the form of a fine line upon a sensitive surface, means for modulating said beam of light in accordance with sound impulses to be recorded, and means for further and simultaneously modulating said beam of light substantially entirely in a linear ther effect is produced by further movement of the shutter II. The variable intensity light beam remains fixed in the position shown in Fig. 3. Alternatively, a positive mechanical stop may be provided for the shutter .H precluding its movement beyond a position corresponding to the position of the light stop 12. Further increase of current through the magnet does not produce any further deflection of the shutterll. It will be apparent that the alternative constructions referredto above provide a stop for the movement of this shutter, which stop is either optical, as shown in Fig. 1, or mechanical, both producing identical results in that the effective withdrawal of the penumbra mask is positively limited and cannot'exceed a predetermined value on increase in ground noise reduction current.

It will be apparent to those skilled in the art relation to the envelope of said sound impulses. 3. Sound recording apparatus including means for directing a beam of light to a slit, means for imaging light passing through said slit in the form of a fine line upon a sensitive surface,

means for modulating said beam of light in ac cordance with sound impulses to be recorded, and

shutter means for further and simultaneously modulating saidbeamof light substantially entirely in a linear relation to the envelope of said sound impulses.

4. Sound recording apparatus of the penumbra type, including a penumbra mask movable in accordance with the envelope of the sound waves to be recorded, a fixed stop located at a position corresponding to the maximum amplitude of the sound waves to be recorded, and means for moving said movable mask substantially entirely in direct linearrelation to the envelope of sound impulses.

5. In sound recording apparatus of the penumbra type, a movable penumbra mask, means for moving said mask substantially entirely in linear accordance with the envelope of the sound waves to be recorded, and means for limiting the effective movement of saidmask to correspond with the maximum amplitude of sound waves'to be recorded.

JOSEPH L. UNDERHILL.

GLENN L. DIMMICK.

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