US1794664A

US1794664A - Method of and system for recording and reproducing sound

Info

Publication number: US1794664A
Application number: US407925A
Authority: US
Inventors: James R Balsley
Original assignee: Fox Film Corp
Current assignee: Twentieth Century Fox Film Corp
Priority date: 1929-11-18
Filing date: 1929-11-18
Publication date: 1931-03-03
Anticipated expiration: 1948-03-03

Description

2 Sheets-Sheet l J. R. BALSLEY Filed Nov.

METHOD OF AND SYSTEM FOR RECORDING AND REPRODUCING SOUND mp.. m

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March 3, 1931.

J. R'. BALSLEY Filed NOV. 18,` 1929 2 SheeLs-Sheety 2 liar-neg.

Patented Mar. 3, 1931 UNITED STATES PATENT :OFFICE JAMES R. BALSLEY, -OF BEVERLY HILLS, CALIFORNIA, ASSIGNOR TO FOX V".liILM COR- AIPORAELION, OF HOLLYWOOD, CALIFORNIA, A CORPORATION OF NEW YORK METHOD OF AND SYSTEM-FOR RECORDING AND REPRODUCING SOUND Application led November 18, 1929. Serial No. 407,925.

1'5 illuminating the sound record on the moving film through av narrow slit by means of a beam of light of constant intensity, the portion of the light beam transmitted through the film being modulated ,by the sound record thereon, and then striking a photo-electric cell to generate a modulated electrical current therein, which is finally reproduced as sound waves by means of a speaker unit.

The width of the light-slit is determined as '25 a compromise between two opposing requirements. The'narrower the slit is made relative to the physical lengths of the opacity .variations of the sound record, the more accurate is/reproduction; but the lower is vthe light intensity on the film, and consequently the lower is the level of reproduction. Thus an effort to increase accuracy of reproduction by narrowing the width of the slit is defeated beyond a certain point by an accompanying decrease in light intensity, which causes distortion by reason o'fjcausing operation on the non-linear portion of the-film characteristic curve- A slit widthof between :0008 and .001

of an inch is now accepted as the best compromise between these opposing factors in certain present systems; although a greater width would give better light intensity on the film, while a narrower width would give betteraccuracy of reproduction.

Stated in one aspect, is an object of my invention to provide a sound recording and reproducing system in which increased accuracy in reproduction is gained with the use of a light-slit of a given width.

This object is accomplished, briefly, by

-thatit becomes a step-down frequency congained. In reproduction, the electrical cur'- rent produced from' the divided-frequency record 1s put through a frequency multiplier, properly tuned and balanced to match the -frequency divider, which reproduces the orig-Y f mal microphone current. The'ou-tput of the frequency multiplier is then amplified and reproduced as sound waves' in the usual manner.

The invention will best be understood from the followmg detailed description of systems for carrying the invention into effect, reference for this purpose being had to the accompanying drawi gs, in which:

F1 g. lis a diagram showing a sound recordlng system including step-down frequency converter or frequency divider;

Fig. 2 1 s a diagram showing a sound record reproducing system including a step-up frequncy converter or frequency multiplier; an

Fig 3 is a diagram showing a variational type of frequency multiplier.

The frequency divider utilized in the present invention may be of any' type suited to the purpose.- For instance, it may be the type of c1rcu1t included in the system yillustrated in Fi 1 of the drawings, which circuit is descri ed in an article in the Proceedings 'of the Institute of Radio Engineers for February 1929, on pages 255-259.

As described in the article referred to, this u I circuit is similar to the Abraham-Bloch multivibrator circuit, modified in such a' way verter. The multivibrator consistsessentially of an aperiodic circuit in which oscillations are sustained'by a ltriode excited by a second triode which provides the proper phase relation for maintenance.y -In its usual 100 form the multivibrator acts as source of high frequency oscillations controlled by a low frequency injected oscillation. `In its usual form it therefore constitutes a frequency multiplier or step-up frequency converter. The circuit in this usual form may be used in reproduction according to the present invention, as will hereinafter be explained.

As arranged in Fig. 1, however, the circuit is controlled at a fundamental frequency which is an integral submultiple of the injected control frequency, thus making it a step-down frequency converter.

Referring now to Fig. 1, the letter M indicates a microphone which is connected to an amplifier 10, conventionally illustrated as being of the vacuum tube type. The output leads 11 and'12 of amplifier 10 are connected v Ainto the plate circuit 13 of a frequency divider 14 through thee medium of a vacuum tube 15, which tube acts as a one way relay to isolate amplifier 10 from reactions due to the frequency divider. For certain separated ranges of values for condensers 16, the fundamental frequency of the frequency divider assumes values which are submultiples of the frequencies injected through leads 11 and 12. The orde;` of control of the frequency divider, or', in other words, the ratio of the injected frequency to the oscillation frequency of the frequency divider circuit, vis .dependent upon the various constants of the frequency divider circuit; and afrequency division by a relatively small number or b a number as high as 100 or more is readily o tainable by varying these constants. The output leads 20 and 21 of the frequency divider, which carry an electrical current of stepped-down frequency, may be connected to a vacuum tube amplifier 24, and the output of amplifier 24 is conducted by means of leads 25 and 26 to a recording lamp 27. A light-slit 28 is positioned before lamp 27 and a lens 29 casts an image of the illuminated slit 28 upon a moving film 30 to print a record-31 thereon of the divided-frequency current. It will be obvious that the individual opacity variations of record 31 will be increased in physical length overA Vwhat they would be in usual recording, .by a

factor-equal to thestep-down ratio of the frequency divider.

In reproduction, the film 30 within its divided-frequency record 31 is operatedA in the usual manner in conjunction with a light-slit, to produce an electrical current wave at the reduced frequency, and this electrical current wave is thereafter stepped-up or multiplied in frequency by the factor originally utilized in frequency division, thereby reproducing the original sound wave current which may be reproduced as audible sound by meansof a usual speaker unit.

. For this purpose, the system of Fig. 2 may be utilized. The film 30 with the divided-fre.- quency record 31 is moved before a light-slit- 32, which slit is illuminatedk by means-of an quency multiplier 41, which, in the system illustrated in Fig. 2, embodies' an electrical circuit of the multivibrator type, similar to thatutilized inthe frequency divider of the recording system, By properly balancing the constants of the frequency multiplier circuit 42, osciillations may be set up therein at a stepped-up frequency; and by carefully oadjusting the frequenc divider and frequency multiplier with re erence to each other, the output frequency of the frequency multiplier may be perfectly matched with" the frequency of the original input current injected into the frequency divider."

The output leads 45 and 46 ofthe frequency multiplier may be connected to a vacuum tube amplifier 4 7, and the output leads 48 and 49 thereof are connected to a speaker unit 50.

-The general operation of the system may briefly be summed up as follows: The sound wave current produced by the 'microphone is divided in frequency by a selected number, and the divided-frequency electrical current thereby produced is recorded upon a photographic film. The physical lengths of the individual opacity variations ofthe photographic record are then obviously increased by the factor of frequency division over what they would have been had the original sound v wave current been recorded directly, and they are also increased by this same factor relative to the light-slit used in reproduction. And since, as has-previously been 'explained, the accuracy of reproduction of a photographic record depends upon the physical lengths of the individual opacity variation relative to the width of the light-slit, the divided-frequency record will be reproduced with an accuracy increased by the same factor over the highest accuracy possible in reproduction of usual full-Wave sound records. A divided-frequency electrical current is then produced from the record with the use of a light-slit in the manner explained, and this current is then multiplied in frequency to match the original sound Awave current injected into the frequency divider, the multiplied-frequency 4current being linally reproduced as audible sound in the forth merely for the purpose of simplicity of explanation.

Another form of frequency multiplier which may be mentioned is illustrated in Fig. 3. The frequency multiplier of this illustration, which will be understood to be adapted to replace the frequency multiplier in the reproducing system of Fig. 2, is a well known arrangement for doubling frequency, and will therefore be herein-but briefly described. The input circuit 60 leads to the series connected windings of a pair of transformers\61 and 62, each equipped with a tertiary circuit 63 applied with current from a common battery B. The steady current through these windings is adjusted to bring the transformer fluxes just to the point where saturation occurs. The two primary windings in series being connected to a pulsating current, during the'positive half cycles the flux in transformer 6 1 (wherein th m. f. of the primary winding assists the direct current winding), will change very little, while the flux in winding 62 will decrease considerably, since in this case the primarym. m. f. opposes the m. in. f. of the direct current winding. On the negative half cycle, the, reverse is true. The iux of one transformerthus has a large variation during one alteration while kthe flux of the other transformer changes only slightly; and on the next alteration these conditions are reversed. The

l electromotive forces induced in the respec- It will be recognized that the illustrative u system specifically described herein is capable of considerable modification and rear-V rangement without departing from the spirit and scope of my invention; and it is therefore j to beunderstood that the following claims embrace all such modifications and equivalent .arrangements as may fairly be construed to fall within the scope of\my invention.

1. The method of recording and reproducing sound, that includes transforming sound waves tinto electrical current Waves corresponding thereto, producing therefrom' an electrical current'wave of stepped down fref quency, makingl a record of said current Wave 'of stepped down frequency, reproducing from said record its recorded electrical current wave, producing therefrom an electrical current wave stepped up in frequency to match the Original sound Wave frequency and reproducing said last mentioned current wave as an audible sound wave.

2. The method of recording and reproducing sound, that includes transforming sound Waves into electrical current waves corresaid current wave of stepped down frequency, reproducing from said record its recorded electrical current wave, producing therefrom an electrical current wave stepped up in frequency to match theoriginal sound wave frequency, and reproducin said last mentioned current wave as an au ible sound wave. 3. The method of recording and reproducing sound,` that includes transforming sound waves into electrical current Waves corresponding thereto, dividing the frequency of said electrical current wave by a given number, making a record of .the di-vided-frequency current wave, reproducing from said record its recorded divided-frequency electrical current wave, multiplying the fre-- quency of said divided-frequency current Wave by the number originally Vused in fre-y ing the frequency of said divided-frequency* current wave by the number originally used 1n frequency dlvision, and reproducing said multiplied-frequency current wave as audible sound.

5. In a sound recording and reproducing system, the combination of means for transforming sound waves into electrical current waves, means for dividing the frequency of said current waves` by a given number, recording means for making a record of the electrical current wave soproduced, means for reproducing the current waves recorded on said record, means formultiplying the frequency of 'said current waves by the number used in original frequency" division, and means for reproducing sound waves from the multiplied-frequency current wave so produced. 6. In a souiid recording and reproducing system, the combination of means for transforming sound waves into electrical current waves, means for dividing the frequency of said current waves by a given number, photographic recordin means for making photographic record o Vthe electrical current wave so produced, meansfor reproducing the current waves recorded on said photographic record, means for multiplying the frequency of said current waves by the number used in original frequency division, and means for reproducing sound waves from the multi-A plied-frequency current wave so produced.

In witness that I claim the oregoin I have hereunto subscribed my name this 1 th day of September, 1929.

JAMES R. BALSLEY.