US2270247A - Sound recording system - Google Patents

Description

- Jan. 2o, 1942.

c. N.A BATSEL soUNo RECORDING SYSTEM Filed oct. 2o, i939 www Patented Jan. 20, 1942 SOUND RECORDING SYSTEM Cecil N. Batsel, North Hollywood, Calif., assignor to4 Radio Corporation of America, a. corporation of I Delaware Application october' 2o, 193,9, serial No. 300,401

9 Claims.

This invention relates to sound apparatus and particularlyv to a sound film recording system whereby greateri flexibility of recording and better' reproduction of the records made with the system are obtainable. i

vThe invention is particularly related to the variable area type of sound recording whereby a trace or traces longitudinally of the film sound track area separate a varying width opaque portion `from a varying width transparent portion, the transverse variations of the trace varying linearly with the amplitude of the signal being recorded. This linear relationship is` desirable to accurately record and reproduce the sound source without distortion. However, it is` well known that the sound track area of.' a vfilm is of limited Width andv that frequently the signal peaks are higher. than the upper limit of' the film. In such instances the peaks are cut off, producing square-topped areas between the point where the signal Went beyond the film width `and again returned thereto, and thus introducing a large amount of distortion. In recording practice, an operator attempts to control the signal level in the electrical transmission circuit between the microphone and light modulator. He attempts to bring the highv peaky levels within the iilm range by reducing the gain or increasing the attenuation in the electrical? circuit. This operation is, of course, beneiicial but it is frequently difficult for the operator to determine the exact point at which the-level will go beyond the film width. Furthermore, the change in gain of the circuit introduces varying amounts of distortion and sometimes eliminates or' at least reduces the dramatic effect'intended. It has also been found, in the reproduction of signals in theatres, that the average level of theatre reproduction is ,15 or 20 db. greaterthan the original level, particularly in the case of dialogue recording. At normal speech'loudness, that is, the level at which speech is naturallyl head without artificial aid, considerable volume latitudev is permissible without annoyance; When this average speech is reproducedscme 20 db. higher without compression of momentary peaks, the loudness at these-points causes extreme annoyance to the listener. This condition is found theatre reverberation serve to heighten this' effect. These energy peaks are not necessarily expressive dynamics used bythe actor, butr may befcaused by lack of breath control or othervocal CID defects and the less-trained' the actor the more noticeable the defect; However, the dynamics of even the well trained are uncomfortably exaggerated when reproduced at theatre loudness.

It is well known in the art that electronic. compressors have been used to control the sound level in the electrical transmission circuit between themicrophonefand sound recorder, the high energy peaks being' automatically brought Within the iilm range. This, of course, requires additional electrical circuits in which distortion may' occur, while the compression is generally effective over a considerable range of' the signal, necessitating in many cases the use of.` eX- panders to reconvert the signal to normal. Such f a system is disclosedin U; S. Patent 2,136,723 of November 15, 1938. Curved recording apertures to produce ygeneral compression, as shown in U. S. Patent 1,767,790 of June 24, 1930, have also been employed, this type of system also requiring expansion reconversion circuits to bring the signal to normal. The present invention, therefore, is directed to a simple arrangementof a recording aperture whereby the necessary expressive dynamics are brought within the film range with intelligibility and without destroying the linearity of the signal over the major portion of the lm range, while the record made with the invention is reproducibley with the normal reproducing system.

The principal object of the invention, therefore, is to improve the variable area type ofI sound recording.

Another object of the invention is to record sound in aY variable area type of sound record over a wide range of levels without loss of the expressive dynamics of the original signal.

A further object of the invention is to increase the iilm amplitude range of the soundtrack area of a lm without disturbing the normal recording or reproducing circuits.

A further object of the invention is to compress a range of 8 db. into a film width range of 2 db. without any substantial loss in intelligibility when normally reproduced.

Although the novel features which are believed' to be characteristic of this invention are pointed 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 Fig. I is a diagrammatic view of a variable area sound recording'system embodying the invention;

Fig. 2 is an elevational view of the aperture mask employed in Fig. 1;

Fig. 3 is a view showing the specic details of the mask of Fig. 2; and

Fig. 4 is a graph showing what occurs to a signal recorded with the system of Fig. 1.

Referring now to Fig. 1, light from a constant light source 5 is collected by a lens 6 and projected to an aperture mask 1 having apertures 8 and 9 therein. The light passing through the apertures 8 and 9 is controlled by a pair of shutters II operated by an electromagnetic coil I2 connected to a noise reduction unit 4 over conductors 3.

The light beams passing the apertures 8 and 8 and the shutters are projected by a lens I4 on a galvanometer mirror I5 actuated by an electro-magnet I 6 connected to an amplier 2 over conductors I3, which, in turn, is connected to a signal source I through amplifier The reilected light from the mirror I is projected to a slit mask I1 having a slit I8 therein, the light beams being shown on the mask I1 as shaded portions I9 at times of no signal. The light passing to slit I8 is projected by objective lenses 2| to the sound track portion 22 of a lm 23. This sound recording system just described is similar to the well known variable area sound recording system as disclosed in Patents 2,102,776, 2,102,777 and 2,102,778.

Referring now to Fig. 2, the general arrangement of the mask 1 is shown wherein the apertures 8 and 9 are shown in detail as having straight portions 25 and vcurved portions 26 and 21. portions of the apertures, reference is made to Fig. 3 wherein the slit I8 is shown with respect to the straight portion 25 and curved portions 26 and 21 of aperture 8, for example. Only one aperture is illustrated in Fig. 3 since the other aperture has identical dimensions, the two apertures producing the well known standard duplex track.

Assuming the slit I8 has a denite Ilength representing the full-track recording width, it is to be noted that the straight section 25 is at an angle of 26 and 34' with respect to the slit and crosses the slit at its center. It will also be noted that the curved ends are drawn on a radius of .093 inch. Thus the light beam may be vibrated or oscillated normal to the slit the distance from the center line of the slit to the extreme ends of the curves, which distance, shown in the practical example is .1094 inch. Thus linear modution occurs over a light beam movement'of .0428 inch, while the compression portion represented by the curves 26 and 21 permits a light beam movement of another .0666 inch, these dimensions being preferable for the usual optical arrangement of standard variable area recording systems.

Considering this particular aperture from the track width standpoint, the linear modulation exists over 80% of the track width which corresponds, from the signal amplitude standpoint, to l For the details of the curvedand linear of the normal lm range, However, with the particular curvature shown, the last 2 db. of film track range will now accommodate a 6 db. increase in signal level beyond normal linearity which would correspond to a track width extending to the points y, as shown by the extensions of the straight portion 25. To bring about this relationship, it was found necessary to make the distance b, which is the distance between the points x, substantially equal to the distance a-f-a', as shown in Fig. 3, this amount of compression being particularly suitable to prevent listener annoyance at theatre levels and obtain the flexibility of recording mentioned above. It also permits reproduction of the record without expansion as will now be explained.

To illustrate exactly what occurs to a signal beingrecorded with the system of Fig. 1, reference is made to Fig. 4, wherein two cycles of a ine wave are shown, the first cycle 38 having an amplitude which normally would come just within the upper limit of the sound track area as shown at 3|. Because of the curvatures 26 and 21 of the slit, the peak of cycle 30 or other waves of this amplitude will be slightly flattened, as shown by the dot-and-dash line 36, this amount of flattening being insuilicient to effect intelligibility. Furthermore, the major portion of any recorded signal lies below the line 35 so that very little of the signal will suffer any distortion whatsoever.

. Now, referring to the second cycle, it will be seen that this cycle has an amplitude which would normally extend considerably beyond the lm width, as shown by the dotted line 32 and which might represent pistol shots or dialogue dramatics. By the invention, this peak is brought within the lm range by attening, the peak as shown by the broken line 33 between the lines 3| and 35. If the cycle were recorded normally, the top of the peak would appear as shown by the at top 34, which destroys intelligibility to a much greater extent than the shape 33. Furthermore, the wave form 33 is reproducible at a l5 or 20 db. greater level without appre-` ciable annoyance whatsoever as compared to the square-topped wave 34. Thus, by the application of applicants particularly curved aperture, as illustrated, he has retained the desirable linear relationship between signal variation and light variation over the major portion of the film track range and signal variations, and has brought substantially the entire range of energy levels of the signal within the lm width in a manner to reduce annoyance and improve the general overall efficiency and quality of the system. He has permitted the sound recording operator a greater latitude, produced a more flexible system, minimized the defects of a signal source, and permits reproduction of the record with the normal reproducing system.

Although one specific aperture has been described above, it is to be understood that for other types of signals, other aperture forms may be preferable.

I claim as my invention:

1. A sound recording system comprising a light source, means for forming light from said source into a beam of a predetermined shape, means for passing a portion of said beam to a film, and means for varying the amount of said portion of said beam passed to said film in accordance with sound waves to be recorded, said light beam forming means providing a linear relationship between the amplitude of said sound waves and the amount of said beam passing to said lm over a, range of signal amplitudes of said sound waves corresponding to 80% of the nlm range and a non-linear relationship between the amplitude of said sound Waves and the amount of said beam passing to said film over a higher range of amplitudes of said sound waves. said j higher range of amplitudes being recorded in the remaining 20% of lm range.

2. A sound recording system comprising alight source, means for projecting light froml said source to a film, said film having a predetermined width of sound track area, an aperture for forming said beam into a substantially triangular shape, and means for vibrating said beam in accordance with sound waves to be recorded, the position of said beam determining the amount of light passing to said iilm, said beam having one side thereof with a straight portion to pass light to said film in direct proportion to the amplitude variations of said sound waves and with a curved portion to increasingly decrease the light to said iilm per unit increase in amplitude of saidl sound waves, said straight portion of said triangular beam corresponding to substantially 80% of said predetermined Width lof the sound track area of Said 3. A sound recording system comprising a light source, a lm, means for forming a slit, and means for projecting certain amounts of said light to the sound track area of said lm through said slit in accordance with the amplitude of sound waves to be recorded, said means including a mask having a substantially triangular aperture, one side of said aperture having a straight portion corresponding to substantially 80% of said sound track area Width, and a curved portion corresponding to the remaining 20% of said sound track area width.

4r A sound recording system in accordance with claim 3 in which the straight portion of said aperture lies at an angle of substantially 2634 with respect to said slit.

5. A sound recording system Ain accordance with claim 3 in which the curved portion of said aperture is tangent tosaid straight portion and has' a radius of substantially .093 inch.

6. A light beam defining aperture mask for a sound recording system comprising a substantially triangular aperture having two straight sides and one side partially straight and partially curved, the curved portion being tangent to said partially straight portion and having a radius of substantially .093 inch.

7. The method of sound recording comprising linearly varying the amount of light reaching substantially 80% of the sound track recording area of a lrn in accorda-nce with the amplitude of sound waves to be recorded and continuously decreasing the variations in light reaching the lm with increasing sound wave amplitudesr over the remaining 20% of said sound track area.

8. The method of sound lm recording which comprises compressing into substantially the upper 2` db. range of the sound track, 8 db. of the signal range, 6 db. of said 8 db. normally lying beyond the upper sound track limit, and linearly recording over the remaining range of said sound track extending to the lower limit of the film range.

9. The method of sound film recording within a width of sound track area approximating a 30 db. range which comprises compressing into substantially the upper 2 db. range of said sound track width, 8 db. of the signal range, and linearly recording over the remaining range of said sound track width.

CECIL N. BATSEL.

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