US2094737A - Recording and reproduction of electrical impulses - Google Patents

Description

0ct 5,'1937. 6.1.. DIMMICK 2,094,737

- RECORDING ANDREPRODUCTION OF ELECTRICAL IMPULSES Filed June 8, i953 2 Sheets-sheaf; 1

INVENTOR GLEN/V LESL/f D/MM/C/f ATTORNEY Oct. 5, 1937. G, L, 1 M

RECRDING AND REPRODUCTION OF ELECTRICAL IMPULSES Filed June 8, 1933 2 Sheets-Sheet 2 INVENTOR 6LAW 1554/: BY Lg ATTORNEY Patented Oct. 5, 1937 PATENT OFFICE RECORDING AND REPRODUCTION OF ELECTRICAL IMPULSES Glenn Leslie Dimmick, Westmont, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 8, 1933, Serial No. 674,805

6 Claims.

This invention relates to the photographic recording and reproduction of electrical impulses, such as those of audio frequency, and has for its principal object the provision of an improved apparatus and method of operation whereby the reproduced impulse distortion which tends to result from the imperfect resolving power of the photographic record is minimized or avoided.

As pointed out in a copending application, Serial No. 610,302, filed May 9, 1932 and assigned to the same assignee as the present application, the limited resolving power of the photographic film available for recording audio or other impulses causes the valleys of the high frequency waves to be filled in or fogged, thus partially rectifying the higher frequency components of the recorded impulses. Also the highfrequency peaks are likely to be underexposed and soft. The effect on the reproduced impulses is the introduction of a second harmonic distortion and a distortion which results from. a change in the average transmission and follows the envelope of the higher frequencies. In accordance with this invention, both these types of distortion are avoided or minimized by the provision of two similar records displaced in phase by 180 degrees and arranged to be simultaneously reproduced and combined for operating a sound reproducer or loud speaker.

The fogging of the valleys of the high frequencies in a film recording results in the following types of. distortion. Due to the loss of transmission in the valleys the amplitude of the light modulation is decreased and an attenuation'of the high frequencies results. The change in the shape of the wave would result in the introduction of both even and odd harmonics. The reduction of the average transmission when high frequencies are recorded results in extraneous sounds, the frequencies of which depend upon the envelope of the high frequencies recorded. The latter distortion is by far the most serious since it produces disagreeable extraneous sounds accompanying the sibilants. The method of recording and reproducing outlined, however, eliminates this zero shift distortion resulting from the change in the average transmission and at the same time completely eliminates all even harmonic distortion. Odd harmonics and attenuation of the high frequencies are still present. The first odd harmonic, the third, is not of such great importance because the filling in of the valleys of the recorded waves does not usually become appreciable for frequencies below half of the frequency range recorded. For instance,

It will be apparent that the zero shift distortion and second harmonic distortion, is eliminated by the push-pull recording and reproducing system if it is considered that the resulting electrical wave would be symmetrical about the time axis. It is obvious that the total transmission at the instant W--W shown on Figure 2 is the same as the transmission at instant V-V which is displaced a half length along the time axis. The electrical wave will therefore have the same numerical value at W-W and at V-V, but will have opposite signs. It can be proven mathematically that a wave which is symmetrical about the time axis is free from even harmonic distortion and that the average value of the wave does not vary with its amplitude.

The invention will be better understood from the following description when considered in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring to the drawings:

Fig. 1 is a diagrammatic illustration of a recording system to which the invention has been applied,

Fig. 2 illustrates the type of record produced by the system of Fig. 1,

Fig. 3' is a diagrammatic showing of a system adapted to reproduce the record of Fig. 2,

Fig. 4 illustrates a modified form of recording system which facilitates the elimination of ground or background noise,

Fig. 5 illustrates a light mask control element which forms a part of the system of Fig. 4, and

Fig. 6 illustrates the type of record which is produced by the system of Fig. 4 and is adapted to be reproduced by the system of Fig. 3.

The recording system of Fig. 1 includes a source 50 from which a rectangularly-shaped light beam is applied through a lens II, a mask 52 and a lens 13 to a galvanometer mirror M which is vibrated in accordance with the impulses to be recorded. From the mirror M, the light beam is reflected through a lens IS, a light slit member l6 and lenses H and I8 to aphotographic or light sensitive record which is moved lengthwise by means of a drum 20 and the cooperating rollers 2i and 22.

It will be observed that the light beam is biased to a position where it illuminates a rectangularlyshaped area midway between the ends of the light slit in the member I6. When this beam is vibrated longitudinally of the light slit, the two records A and B of. Fig. 2 are produced. These records are degrees out of phase with one another and, when reproduced together, rectify the distortional elfects previously mentioned.

The records A and B are reproduced by the apparatus of Fig. 3. This apparatus includes a source 23 from which light is projected through an optical system 24, the records A and B on the film i9, a lens 25 and lenses 26 and 21 to a pushpull type of photo-electric or other light sensitive device 28. The device 28 includes an anode 29 which is connected through a battery 30 to an intermediate terminal of a transformer winding 3| and with cathodes 32 and 33 which are connected respectively to the opposite end terminals of the winding 3|. Inductively associated with the winding 3| is a secondary winding 34 which is connected through suitable amplifying apparatus to a sound reproducer in a manner well known to those skilled in the art.

Since the records A and B are not only identical but are180 degrees out of phase, it necessarily follows that their simultaneous reproduction results in the cancellation of the distortional effects previously resulting from fogging of the high frequency valleys and softening of the high frequency peaks. V V

The transparent section between the records A and B is of constant width and of course leaves something to be desired where ground noise due to film grain, scratches, dust etc. is an important consideration. The recording system of Fig. 4 is designed to eliminatesuch ground or background noise.

This apparatus differs from that of Fig. 1 in that the rectangularly-apertured mask i2 is replaced by a triangularly-apertured mask 35 which is arranged to be moved by an electric motor device 35 energized in accordance with the envelope of the impulses to be recorded in any suitable manner such as that disclosed by United States Patent No. 1,855,197.

As indicated more clearly in Fig. 5, the rectified or envelope current referred to in the aforesaid patent is supplied through a pair of terminals to the cells 38 to 4! which are so wound on the pole pieces of a magnet 42 as to cause the triangular light beam to move transversely across the light slit as indicated by the arrow C. This beam is also moved longitudinally of the light slit by the mirror i l which is vibrated in accordance with the impulses to be recorded. Under these conditions, the records E and F (see Fig. 6) are not evenly spaced from one another but the distance between them varies with the amplitude of the impulses, this result being effected by the movable mask 35 which moves the triangular light beam further across the light slit when the amplitude is high and vice versa. At all times, however, the spacing" between the records E and F is kept at the minimum permitting satisfactory reproduction, thus maintaining a low ratio of ground to signal noise. The records E and F may of 7 light source, means provided with course be reproduced by the apparatus of Fig. 3.

Having thus described my invention, what I claim is:

1. An impulse recording system including a light source, means provided with a triangular aperture for defining the shape of the beam of light from said source, a member provided with a narrow slit, means for imaging the said aperture on the said slit, means for vibrating the said beam in accordance with impulses to be recorded and means for moving the said aperture in accordance with the envelope of said impulses.

2. An impulse recording system including a light source, means provided with a triangular aperture for defining the shape of the beam of light from said source, a member provided with a narrow slit, means for forming an image of said slit on a film, means for imaging the said aperture on the said slit, means for vibrating the said beam in accordance with impulses to be recorded and means for moving the said'aperture in accordance with the envelope of said impulses.

3. An impulse recording system including a light source, means provided with an aperture of varying dimensions for defining the shape of the beam oflight from said source, a member provided with a narrow slit, means for imaging the said aperture on the said slit, means for vibrating the said beam in accordance with impulses to be recorded and means for moving the said aperture in accordance with the envelope of said impulses, the variation in dimensions of said aperture being transverse of the said slit.

4. An' impulse recording system including a light source, means provided with a triangular aperture for defining the shape of the beam of light from saidsource, a'member provided with a narrow slit, means for imaging the said aperture on the said slit, means for vibrating the said beam longitudinally of the said slit in accordance with impulses to be recorded and means for mov ing the said aperture in accordance with the envelope of said impulses.

5. An impulse recording system including a a triangular aperture for defining the shape of the beam of light from said source, a member provided with a narrow slit, means for imaging the said aperture on the said slit, means for vibrating the said beam longitudinally of said slit in accordance with impulses to be recorded and means for moving the said aperture transversely of the slit in accordance with the envelope of said impulses.

6. An impulse recording system including a light source, means provided with a triangular aperture for defining the shape of the beam of light from said source, a member provided with a narrow slit, means for forming an image of said slit on a film, means for imaging the said aperture on the said slit, means for vibrating the said beam longitudinally of said slit in accordance with impulses to be recorded and means for moving the said aperture transversely of the slit in accordance with the envelope of said impulses.

GLENN LESLIE DIMMICK.

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