US2124150A - Method and device for recording sound - Google Patents

Description

July 19, 1938. MRmHI/IRDI 2,124,150

METHOD AND DEVICE FOR RECORDING SOUND Filed March 27, 1957 2 Sheets-Sheet 1 Fig. 1-

IIII I I I I 1 II IIIII I I l I I I l l 7 2 3 4 5 6 7 8 9 70 7/ I2 I3I l151677787.9202177237425262726 July 19, 1938- P. c. RICCHIARDI 2,124,150

METHOD AND DEVICE FOR RECORDING SOUND v Filed March 27, 1937 2 Sheets-Sheet 2 Fir g. 4

@Wi/W Patented July 19, 1938 iJNlTED STATES B A-TENT OFFEQE METHOD AND DEVICE FOR RECORDING SOUND 4 Claims.

My invention. relates to sound recording and reproducing devices.

It is known that in existing sound recording and reproducing systems, although the recorded 5 .sound may be pure as regards the fidelity of the frequencies that are recorded, it nevertheless appears to be fiat and colorless in comparison with the sound source. This is due to the fact that the range of volume variations as electromagnetically recorded on a film, disc or band is limited to about 40 decibels whereas the power or volume range of an. average orchestra is around '10 or 75 decibels. The limiting of this power range corresponds to bringing down the volume of a large i orchestra to the sound level of a human voice.

It is therefore obvious that, under these conditions sound recording and reproduction can never be an accurate image of the original sound because the various proportions of volume or 0 power are not maintained.

The sound being limited in the recording to maximum variations in power of 40 decibels, when a sound strikes the microphone with higher power or volume the practice is to effect a compression by means of either a manually operated or an automatic attenuating device, in such manner as to reduce the original volume of the sound source to the maximum of 40 decibels which is allowed for recording.

An examination of the diagram of Fig. 1 shows how this compression is effected during recording. Curve A represents the power variations of the sound source, the variations in decibels of which can be observed in ordinates. It is found that these variations oscillate between 5 decibels and '72 decibels. When the sound level of the source exceeds 30 decibels, a compression is effected by acting upon an attenuating device associated with the amplifier. The action of this 40 attenuating device is illustrated by curve B, the

attentuation values of which, reckoned in decibels, are plotted on the right hand side abscissa.

It is found that when the sound source exceeds the 30 decibels above mentioned, the action of the attenuating device is called into play, in order to keep the record inside the range of these 30 decibels. Thus, at point 8 of the horizontal axis, curve A has reached 40 decibels and attenuating device B has produced a corresponding attenuation of 10 decibels. At l l,the curve A of the sound, source has reached '70 decibels and the attenuating device produces a reduction of 40 decibels. At l4, curve A drops 20 decibels and reaches 40 decibels, at the same time the attennation is reduced by 20 decibels, and so on.

From this compression, there results a final curve C, which shows the variations of the volume that is actually recorded. By comparing curve A, which corresponds to the original sound, with curve C, which corresponds to the recorded 5 sound, it is visible not only that the recording fails to maintain the levels of the sound source, which destroys colour and power, but also that there is introduced a sound level distortion which may be as high as 100%, since curve C has noth- 10-:

ing whatever in common with the original curve A.

In order to avoid this distortion, electronic devices are employed which apply to recording on discs as well as on films. are commonly called volume-expanders are intended to increase the power range by bringing it up to a level of and even decibels. There is thus obtained a volume variation the maximum level of which approximates the original, but the 20 very principle of these devices merely further increases the distortion. above mentioned.

Thus double recording devices have been utilized,'one of these recordings corresponding to the modulation and the other to the variations of the 25 level of this modulation. This second recording acting upon the factor of amplification of the amplifier, adds to the 30 decibels recorded with the modulation a supplement of 40 decibels or even more, thus increasing the range so that it 30 becomes equal to the necessary minimum, 70 decibels. But this supplement of amplification merely proportionally increases the rate of distortion, because the second recording, which controls the volume variations, being taken at the 35 output of the amplifier, can but correspond to the shape of the volume variations of the modulation that is recorded. And, as compression. has been effected during the recording, the variations of thesound level have been modified by this 4 operation, and no longer correspond to the volume variations of the sound source.

As above stated, this distortion is observed by examining the form of curve A, as compared to that of curve C. The second curve intended for 45 the volume expander in question can therefore but have a form proportional to that of curve C. Finallyywhen reproducing sounds, the result of the action of said volume expander is illustrated .by curve D, the characteristics of which 50 are worse than those of curve C, while corresponding to the desired range of 70 decibels. In other words, the action of the attenuating device (curve B, Fig. 1) is. exerted as well upon the modulation as upon the volume expander.

These devices, which 15 My invention has been developed with the above considerations in view and has for its objects to provide sound recording and reproducing means which will accurately record and reproduce the volume variations of the recorded sound; and to produce a sound record which may be reproduced or read by means of standard existing apparatus Without necessitating any important modification thereof.

For this purpose, the sound recording is double. In the case taken by way of example, I will refer io photo-electric records on a film, but it is ob- "vious that the invention applies as well to records aced on discs and to all forms of electro-me- .ical recording on bands or other means.

clone, in what follows the term sound band be employed for the sake of simplicity, but

clear that the invention is not limited to *ording on bands but applies to any other form of record or means.

According to the invention, which includes a double recording, one of the elementary records carries the modulation, with its variations of normal volume after compression, whereas the other one records level variations which have nothing in common with the variations of volume of the sound source or of the modulation but which are proportional to the compression action the function of which has been above explained.

Therefore, the difference between the system according to the invention and the volume expanders lies essentially in the fact that the supplement of amplification of the volume expanders is controlled by the variations of volume of the modulation or of the sound source, whereas, in the system according to my invention, the supplement of amplification is controlled by the compression action exerted upon the modulation for effecting the recording.

The advantages of the system according to the present invention, and also the manner in which it is working, are illustrated by the diagram of Fig. 2. In this diagram, the curve of the variations of the level of the sound source is 4 A; C shows the variations of volume such as they are normally recorded in a normal recording after the action of the compressing device, the action of which is illustrated by curve B. It will be seen that these three curves are identical to those of the diagram of Fig. 1, above described.

In the diagram of Fig. 2, there is shown a fourth curve, E, which illustrates the recording that pilots the second volume control, the action of which is analogous to that of the ordinary volume expanders.

By examining curve B, of the attenuating device acting as compressing means, it is found that it is proportional to the curve E of the expander control. This record, which pilots the power of amplification of the amplifier, acts simultaneously with the modulation either on the same reading means or on separate reading means. Therefore, to the variations of level of the modulation (curve C, Fig. 3), I add the variations of level of the expander control (curve E). The recording amplifier and the reproducing device are arranged in such manner that the variations of the expander control act in opposition and proportionally with the attenuating action of the compressing means.

The sum of the volume variations of the modulation and of those of the expander control is then illustrated by curve AB. When comparing this curve with curve A of Figs. 1 and 2, it is found that they are wholly identical, which shows that any distortion introduced by the level compression when recording is automatically eliminated by the second recording of the expander control.

When comparing the results obtained with a normal volume expander, that is to say the curve D of Fig. 1, and those obtained with the system according to the present invention, that is to say the curve AB of Fig. 3, it is found that the difference between the two systems involves considerable advantages for the system according to the invention.

The difference between the two systems lies in the fact that the records of the ordinary volume expanders are proportional to the sound level variations of the source, or of the modulation, Whereas the variations of the expander control according to the invention have nothing in common either with the variations of the sound source or with those of the modulation, but are merely proportional to the compression action exerted by the attenuating device upon the modulation, when recording.

A preferred embodiment of the present invention will be hereinafter described, with reference to the accompanying drawings, given merely by way of example, and in which:

Figs. 1 to 3 are diagrams showing explanatory curves above referred to;

Fig. 4 is a diagrammatical view of a device according to the invention;

Fig. 5 shows a sound band.

The embodiment of the invention which is shown by way of example, corresponds to the case of photo-electric records on films, but it is clear that the same principle applies to the case of recording on a band or disc through electromechanical means, since it sufiices to change the recording system to obtain the same results. Besides, even in the case of photo-electric recording means, the circuits, the type of recording devices and the like can be changed or modified without departing from the principle of the invention.

In the example illustrated by Fig. 4, reference character E designates a microphone and F an amplifier. The sound caught by the microphone E is first amplified by the pre-amplifier. The sound level is then controlled by the gain control A. B. and then amplified by amplifier G. Through the coupling transformer L, it is fed to the recording device M. The light rays that come from lamp N through the optical system 0 are reflected by device M through the usual optical system P Q S, so as to be impressed upon the film Z. By acting upon attenuating device AB, I therefore exert the necessary compression when the sound level from F exceeds the limit, so as to keep it within the range of 30 decibels that is allowed for recording. The action of this attenuating device is illustrated by the curve B of Fig. 1.

A rod K connects the handle of attenuating device B with the handle C of a rheostat D. Any action exerted upon attenuating device AB is therefore reproduced on rheostat DC.

The current flowing through this circuit therefore varies as a function of the resistance value of rheostat C. D. Reference character I designates an apparatus for measuring this current, whereas A acting as an ammeter, will vary the position of plate U proportionally to the intensity of the current flowing through the circuit. Plate U is capable of stopping the light rays emitted by lamp B and condensed by optical means X. The light rays which act upon the band of the expander control at V are more or less stopped by plate U, according to the value of the current flowing through the circuit of rheostat DC, and, finally, the action of said plate will be exactly proportional to that of the attentuating device AB. With this arrangement, I therefore produce upon the band of the expander control, a sound track the form and appearance of which are analogous to the disclosure of Fig. and in which the variations of amplitude will be piloted by the action of the attenuating device AB, which corresponds to carrying out into practice the diagram illustrated by Fig. 2 and above described.

By varying the recording systems, it is possible to obtain other arrangements which are within the scope of the invention.

The band of Fig. 5 includes a variable area record B for modulation, and a variable density record C for the expander control. This figure also shows, at D, a variable area record of the triangular type, for modulation, and at E a fixed density record for the expander control. The

' same principle is applicable for instance to a variable density record for modulation and a fixed or variable density record for the expander control. Thus, at F is shown the modulation and at T the expander control.

However, another characteristic feature of the present invention lies in the fact that the width of the two bands is equal to the width of a normal band, in order to permit reading by means of the standard optical means of the existing apparatus, without necessitating any important modification.

In a general manner, while I have in the above description, disclosed what I deem to be practical and efiicient embodiments of the present invention it should be well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the appended claims.

What I claim is:

1. A sound recording device which comprises, in combination, modulation recording means for tracing a sound track on a film, an attenuating device associated with said means for exerting a compression action on the sound level so as to keep it inside the permissible range, and a sound level recording means operatively connected with said attenuating device for tracing on said film a sound level track showing variations proportional to the compression actions exerted on the modulation when recording, said means being adapted to form parallel and adjacent sound tracks the total width of which is equal to the width of an. ordinary sound track, whereby both of the first mentioned sound tracks can be read by means of a single reading device, without necessitating any modification in existing apparatus.

2. A sound recording device which comprises, in combination, modulation recording means for tracing a variable area sound track on a film, an attenuating device associated with said means for exerting a compression action on the sound level so as to keep it inside the permissible range, and a sound level recording means operatively connected with said attenuating device for tracing on said film a variable density sound level track showing variations proportional to the compression actions exerted on the modulation when recording, said means being adapted to form parallel and adjacent sound tracks the total width of which is equal to the width of an ordinary sound track, whereby both of the first mentioned sound tracks can be read by means of a single reading device, without necessitating any modification in existing apparatus.

3. A sound recording device which comprises, in combination, modulation recording means for tracing a variable density sound track on a film, an attenuating device associated with said means for exerting a compression action on the sound level so as to keep it inside the permissible range, and a sound level recording means operatively connected with said attenuating device for tracing on said film a variable area sound level track showing variations proportional to the compression actions exerted on the modulation when recording, said means being adapted to form parallel and adjacent sound tracks the total width of which is equal to the width of an ordinary sound track, whereby both of the first mentioned sound tracks can be read by means of a single reading device, without necessitating any modification in existing apparatus.

4. A sound recording device which comprises, in combination, modulation recording means for tracing a variable area sound track on a film, an attenuating device associated with said means for exerting a compression action on the sound level so as to keep it inside the permissible range,

and a sound level recording means operatively

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