US2268438A - Sound recording system - Google Patents

Description

Dec. 30, 1941. c. N. BATSEL 2,268,438

SOUND RECORDING SYSTEM Filed Dec. 29, 1959 45@ummmmmmnnwnmnm@ w i INVENTOR.

v 6504 N BA r551.

BY VA/KW ATTORNEY.

Patented Dec. 30, 1941 SOUND RECORDING SYSTEM Cecil N. Batsel, North Hollywood, Calif., assignor to Radio Corporation of America, a corporation of Delaware Application December 29, 1939, Serial No. 311,638

15 Claims.

other of which produces a track of constant width with variable density striations in accordance with the frequency and amplitude of the signal being recorded. Sound recording engineers are also familiar with sound systems and tracks wherein both variable density and variable area components are combined. It is also known that noise reduction can be applied to variable area recording by making opaque the unexposed and unmodulated portion of the sound track, and to variable density recording by shifting the point of modulation between a central point and one end of the film emulsion curve.

Each type of recording has certain advantages, and the combinations have been employed in order to realize all of the advantages. plication of noise reduction to either system generally increases the film range and provides the usual quiet track at times of no signal and during the reproduction of low amplitude sounds.

The present invention not only realizes to full advantage the features of each type of sound recording, but also combines therewith the feature of noise reduction which produces a still higher increase in signal to noise ratio over the entire film range. methods of obtaining noise reduction in variable density recording systems.

The principal object of the invention, therefore, is to improve sound recording.

Another object of the invention is to obtain a higher signal to noise ratio over the range of recorded amplitudes.

A further object of the invention is to increase the film range of a predetermined sound track area.

A further object of the invention is to simplify the noise reduction portion of a variable density recording system and to utilize existing portions of a variable area system to produce a combined variable density and variable area sound track.

A further object of the invention is to combine variable area and variable density recording with noise reduction.

Although the novel features which are believed to be characteristic of this invention are pointed The ap- It also simplifies certain present til out with particularity in the claims appended herewith, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawing forming a part thereof, in which Figure 1 is a diagrammatic arrangement embodying the invention; and

Figure 2 is a fragmentary portion of a sound film showing the general character of the sound track obtainable with the system of Fig. 1.

Referring now to Fig. 1, the optical portion of the sound recording system comprises a lamp 5, the light from which is gathered by a lens 6 and projected upon an aperture mask I having a rectangular aperture 8 therein. Adjustable longitudinally of the rectangular aperture 8 is a pair of shutters l0 operated magnetically by a coil l2 shunted by a resistance l3. Light passing through the portion of the aperture 8 not covered by the shutters I0 is projected past a penumbra stop 1-5 and then projected by a lens I6 to a mirror I! of a galvanometer IS. The light impinging upon the mirror I! is reflected to a slit mask 20 having a slit 2| therein, the light passing therethrough being projected by a lens 22 to a track portion 23 of a film 24.

The electrical portion of the system includes a microphone 26 connected to a standard voltage amplifier 21. The output of the voltage amplifier is divided, one portion being impressed upon a noise reduction unit 28 comprising the usual rectifier and direct current amplifier combination, and the other portion being impressed upon a variable gain amplifier 29 of any well known type but preferably one using variable mu tubes. The output of the variable gain amplifier is impressed over conductors 3| upon a coil 32 of the galvanometer IS. The output of the noise reduction unit 28 is impressed in a series relationship upon the variable gain amplifier 29, the shutter coil l2 and a coil 34 of a galvanometer l8, these units being connected over conductors 35, 36 and 31. The coil 34 is shunted by an adjustable resistance M. The output of noise reduction unit 28 is impressed on the control grids of the variable gain amplifier 29 to vary the gain of this amplifier and, consequently, the amount of amplification of the portion of the output of voltage amplifier 2'! passing therethrough. The connections are so arranged so that an increase in the output of voltage amplifier 21 increases the direct current output from the noise reduction amplifier 28 and decreases the output from the .and the no-signal point.

variable gain amplifier, as will be explained hereinafter.

It will be noted that the galvanometer I8 is provided with a mechanical tilting means comprising a screw 39 threadably mounted in a block 40 so that the turning of the screw 39 will tilt the mirror I! to move the light beam normal to the slit 2| of the mask 20. The position of the shutters l and galvanometer I8 is shown for a zero or no-signal condition. In this position, the shutters I I] are at their minimum separation, this distance being controlled by a block 42 mounted between the active edges of the shutters. It is also to be understood that the point of minimum separation of the shutters may be controlled electrically. The narrow light beam passing the remaining opening in the aperture 8 appears upon the slit mask 20 as shown by the graduated beam 44, this beam being the penumbra caused by the stop l5. The position of the light beam .2

44 on the mask 20 is adjusted by the screw 39 to take the position shown in the drawing.

In describing the operation of the above-described system, reference is made to Fig. 2 in which a film 46 having sprocket holes 41 therein has shown thereon' a sound track in which the points of no signal are shown at 48 and 49, and the point of maximum signal is shown at 50. To condition the apparatus for recording, the system is adjusted so that when no current flows through conductors 35, 36 and 31, the shutters I0 are at their point of minimum separation, and the galvanometer is mechanically tilted 50 that the beam 44 is projected upon the mask 20 as shown in Fig. 1. In this adjustment the preferable shutter position represents a 12 db. variable area squeeze, and the galvanometer position represents a 6 db. noise reduction, the 6 db. noise reduction meaning that just one-half as much average light passes through the print at this point as at 100% modulation or full signal. The average light passing to the film at full signal should correspond approximately to an exposure point near the center of the straight-line portion of the characteristic curve of the emulsion.

The next adjustment is made by increasing the output of voltage amplifier 21 so that the shutters reach a position to provide a beam wide enough to just cover the sound track area, as

shown at point 50 in Fig. 2. By means of the shunt resistance 4|, the mirror I! may be adjusted to bring the slit at the midpoint or middensity of the light beam. With the shutters I0 in this position, the variable gain amplifier 28 is adjusted so that the variable density component the amplifier 29 is set for minimum gain.

For another check on the system, the signal output from the voltage amplifier 21 is reduced 6 db. At this point the shutters I should have moved in so that they cover one-half of the sound track area; and if they do not, they may be adjusted by manipulation of the shunt resistance l3. Also, at this point the variable gain which the signal oscillations occur is now halfway between the full-signal modulation point Thus, the shutters cover one-half the sound track when the signal of the track is near its maximum. At this point I is reduced 6 db., and one-half of the remaining sound track when the signal is reduced another 6 db. Although this is a preferred adjustment, it is to be understood that other settings may be made, the track in Fig. 2 representing a slightly greater squeeze than 12 db.

In the systems known in the art, when the variable area portion operates to decrease the track width, the variable density component is usually increased to maintain the output signal level linear with respect to the recorded input level. The present invention not only maintains this relationship, but provides an increase in noise reduction so that the signal to noise ratio will be maintained higher at all points immediately below full-track modulation. The shutters and galvanometer are so adjusted that when the shutters reach their minimum separation determined by the block 42, the noise reduction action becomes a maximum at 6 db. From this point, the variations in the variable density modulation component are linear, the variable area component remaining fixed at a constant width. This action is obtained by biasing the noise reduction unit 28 to an input level point below which there is no output from the amplifier 28. Without an output from unit 28, the variable gain amplifier 29 has a constant gain and a linear relationship between its input and output. The system thereby provides for the highest possible signal to noise ratio obtainable, since at all amplitudes the noise reduction action is functioning to maintain this ratio at a maximum, while all advantages of the combination area and density track are secured.

It is also to be noted that the bias of the galvanometer I8 is obtained mechanically and not by the introduction of a secondary electric current to the winding 34, so that the galvanometer may be adjusted easily and the light beam maintained in its proper position at all times. Furthermore, the output of the noise reduction amplifier 28 directly operates the shutters I0 and the galvanometer l8. By use of the shunt resistances l3 and 4|, the operation of the shutters l0 and galvanometer l8 may be given any desired operating characteristic over the operating range of the system. It is also to be understood that the system may be adjusted so that the noise reduction may continue to increase after the shutters have reached their minimum separation and the variable gain amplifier has become linear.

I claim as my invention:

1. A sound recording system comprising a source of light, means for forming light from said source into a beam, means for vibrating said beam in accordance with the instantaneous values of sound Waves to be recorded, and means for simultaneously varying the width of said beam and shifting the average position of said beam in accordance with the average value of said sound waves to be recorded.

2. A sound recording system in accordance with claim 1 in which said light beam is graduated in intensity and a slit mask is provided for passing a selected portion of said beam in accordance with both the instantaneous and average values of said sound waves.

3. A sound recording system in accordance with claim 1 in which said means for varying the Width of said beam is a shutter and said means for shifting and vibrating said beam is a galvanometer.

4. A sound recording system comprising means for producing a light beam having a definite shape, means for vibrating said beam in accordance with the instantaneous values of sound waves to be recorded, a film, means for maintaining one dimension of said beam constant as transmitted to said film, said vibrating means varying the intensity of said portion of said beam reaching said film, means for varying one dimension of said beam in accordance with the average value of said sound waves, and means for inversely relating the variation of said beam vibrating means with said beam dimension varying means.

5. A sound recording system in accordance with claim 4 in which the point of vibration of said vibrating means is varied in accordance with the average value of said sound Waves.

6. A sound recording system in accordance with claim 4 in which said vibrating means includes a galvanometer mechanically biased in position at times of no signal and electrically varied by both the instantaneous and average values of said sound waves.

'7. A sound recording system in accordance with claim 4 in which said relating means includes a variable gain amplifier for transmitting electrical currents corresponding to said sound waves and a rectifier having its output connected to said amplifier for controlling the gain of said amplifier, the output of said rectifier simultaneously controlling said beam dimension varying means.

8. A photographic sound record comprising a variable density component having a frequency Variation in accordance with the instantaneous values of certain sound waves, and a variable area component having a frequency variation in accordance with the average value of said sound waves, said variable density component having an average density variation in accordance with the average value of said sound waves.

9. A photographic sound record in accordance with claim 8 in which said variable area component has a fixed value over a predetermined range of amplitudes of said sound waves.

10. A sound recording system comprising a source of light, a light-sensitive medium, means for exposing said light-sensitive medium to light from said source, means for varying said exposing light in accordance with the instantaneous values of sound waves to be recorded, and means for simultaneously varying the area and average intensity of said exposing light in accordance with the average value of said sound waves to be recorded.

11. A sound recording system comprising a source of light, a light-sensitive medium, means for exposing said light-sensitive medium to light from said source, means for varying the intensity of the exposing light in accordance with sound Waves to be recorded, means for varying the area of said exposing light in accordance with said sound Waves to be recorded, and means for inversely relating the variations in intensity of said exposing light with the variations in area of said exposing light.

12. A sound recording system comprising a source of light, a light-sensitive medium, means for exposing said light-sensitive medium, means for varying said exposing light in accordance with the instantaneous values of sound waves to be recorded, said last-mentioned means being adapted to expose said medium to a lesser amount of light during small instantaneous values or said sound waves than during large instantaneous values of said sound waves, and means for mechanically adjusting said last-mentioned means Whereby said light-sensitive medium is exposed to the minimum amount of light at times of no signal.

13. A sound recording system in accordance with claim 12 in which said exposing light varying means includes a galvanometer mirror adapted to be mechanically translated to reduce the exposing light reaching said medium to a minimum and to be electrically vibrated to vary the exposing light reaching said medium.

14. A sound recording system in accordance with claim 12 in which said exposing light varying means includes a galvanometer mirror adapted to be mechanically translated to reduce the exposing light reaching said medium to a mini-- mum and to be electrically vibrated to vary the exposing light reaching said medium in accordance with the instantaneous and average values of said sound waves to be recorded.

15. A sound recording system in accordance with claim 12 in which said exposing light varying means includes a light deflector and means for mechanically biasing said deflector to a point whereby said medium is exposed to a minimum of light at times of no signal with additional means for electrically actuating said deflector with a current varying in accordance with the instantaneous values of said sound waves to be recorded, and a current varying in accordance with the average value of said sound waves, said currents simultaneously increasing and decreasing.

CECIL N. BATSEL.

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