US2062275A - Sound-transmission, sound-recording, and sound-reproducing system - Google Patents

Description

Nov. 24,1936. 4, D, B L N 2,062,275

SOUND TRANSMISSION, SOUND RECORDING, AND SOUND NEPRODUOING SYSTEM Filed Nov. 8, 1934 2 sheets-sheet 1 Nov. 24, 1936. A. D. BLUMLEIN 2,062,275

SOUND TRANSMISSION, SOUND RECORDING, AND SOUND REPRODUCING SYSTEM Filed Nov. 8, 1934 2 Sheets-Sheet 2 may assume, but it must be understood that.

Patented Nov. 24 1936 UNITED STATES PATENT orr cs 2,062,275 SOUND-TRANSMISSION,

ING, TEM

SOUND-RECORD- AND SOUND-BEPRODUCING SYS- Alan Dower Blumlein, West Ealing, London, England, assignor to Electric 82 Musical Industries Limited, Hayes, England Application November 8, 1934, Serial No. 152,147 In Great Britain February 10, 1934 6 Claims. (01. 179-1) This invention relates to systems for the electrical transmission of sound, and comprises an improvement in or modification of the invention disclosed in the specification of application No. 647,057 filed the 13th day of December,

1932. In that earlier specification are shown systems in which sounds are picked up by a plurality of microphones for transmission 'in such a manner that when reproduced the sounds convey a binaural impression to a listener. In the The arrangement may consist in the combination of a velocity microphone and a pressure mi-' Volume V, page 139 et seq., October, 1933, and

diiierent forms of the present invention may be variously regarded as a modified use of such a device in a system such as described in application No. 647,057 filed the 13th day of December,

1932, or as a system such as previously described in application No. 647,057 filed the 13th day of December, 1932, embodying one or more microphones of that character in place of the microphones originally indicated. Alternatively there may be employed in accordance'with the presentinvention other forms of microphones embodying combined velocity-responsive and pressure-sensitive elements, different from that described in the above-mentioned publication.

The invention will be readily understood from the following description of a few forms which it these forms are described merely by way of iilustration and not of limitation to the scope of the invention. Understanding of thedescription will be facilitated by reference to .the accompanying drawings representing arrangements in accordance with the invention.

In these drawingsz- Figures 1 and '2 represent two forms ofthe invention in symbolic fashion:

Figures 3 to 6 represent diagrammatically one form of an arrangement previously described in the specification of U. S. A. patent application No. 647,057 of 13th December, 1932, and

Figures 7 and 8 represent front and side elevations respectively of a form of known compound microphone which may be used in a novel manner in connection with the present invention;

In carrying the invention into eifect in one convenient manner a sound transmission system is arranged whereby sound impulses picked up by a plurality of microphones are transmitted over suitable channels and reproduced in a plurality of loud speakers, substantially .as described in application No. 647,057, filed the 13th day of December, 1932. The system may incorporate recording and reproducing stages, and thetransmission is not necessarily over telephone or like lines but may, for example, be eifected by duplex wireless telephony.

To facilitate understanding of the present specification a brief description of the invention described in the above mentioned prior specification will be given. One arrangement of that invention is shown in Figures 3 to 6 herein. .Sounds from a source s are picked up by two pressure microphones a1, a2, amplified if necessary by amplifiers 111,172, passed through interacting networks 01 (described more fully below), the outputs again amplified if necessary by amplifiers d1, d2, and 'fed to loud speakers e1, e2. The interacting networks 01 include initially an arrangement of transformer windings shown in Figure 4 so arranged that their outputs comprise respectively half the sum and half the difference of the inputs. be represented diagrammatically by the symbol shown in' Figure 5. Figure 6 represents the complete network arrangement 01 of the. circuit shown in Figure 3, and it will be seen that after passing through transformer windings .a: the impulses pass through networks comprising, in one channel the resistances 94, I14 and i4, and in the other channel resistances d4, erand I04, and condenser ii. The modified impulses are again passed through transformer windings 3 (whereby half their. sum and half their-difference: is again obtained), and fed to the amplifiers (11, (In and loud speakers e1, 62. 'It is shown in the prior specfication referred to that by suitable choice and arrangementof the elements in this circuit a Such an arrangement may directional sound effect is conveyed to a listener by the loud speakers an e: depending upon the d!- mitting microphones a1, a: may be replaced by a velocity microphone, such as a moving strip microphone, in close .iuxtaposition to a pressure microphone, such as a carbon or crystal microphone. While the invention is not limited to these two forms of microphone (i. e. moving strip and carbon orcrystal) they are mentioned in view of the fact that they can be made of very small dimensions which allow them to be brought close) together. These two microphones are connected together so that their outputs 'are fed to two channels leading to later stages of the system, and the arrangement is such that one of these channels receives and transmits the sum of the microphone outputs while the other re-.

ceives their difference. Any convenient arrangement of electrical elements may be used to provide this sum-and-dlflerence step, and several arrangements will be found described in the abovementioned specification and one such arrangement is shown in Figs. 4 and 5 and described above.

The outputs so obtained after this summing and differencing process consist of currents which fora particular sound source are in phase but whose relative magnitudes depend on the direction of the sound wave arriving at the microphone, and these can be made suitable for obtaining the desired binaural effect by arranging the velocity microphone to be insensitive to sound arriving from the central" direction, so that outputs, are obtained from microphones which are equivalent to the outputs of the first sum" and difference channels (Fig. 6 herewith).

For example, with such an arrangement the output of the pressure microphone (after amplification orattenuation) is comparable with the output obtained from the resistance In shown in Fig. 6 herewith (corresponding to Figure 5 of application No. 647,057, filed the 13th day of December, 1932) while the output from the velocity microphone is comparable with the output obtained from the resistance e4 of that figure, and by providing theabove-mentioned sum-and-diference process once only to these microphone im-- pulses, therefore, outputs are provided suitable for feeding to the loud speakers.

The transformer arrangement a: and the modii'ying networks comprising elements (14, 64, f4. 94,

71.4, 1'4 and k4 shown infFigure 6 can'thereiore be eliminated and-a much simpler arrangement is available giving the same effect.

This will be seen clearly by referring to Figure 1 wherein 4 represents a directionally sensitive velocity microphone, e. g. a strip microphone, while b represents a pressure microphone, which is not directionally sensitive. These are connected to a network represented at c by the symbol introduced in Fig. 5 herewith (corresponding to Figure 4 of application No. 647,057 filed the 13th day of December, 1932), and the outputs pass through. leads d and e to two loud spreakers.

The strip of microphone a is arranged along the axis of the sound field and is clearly insensitive to sounds from a sound source 81 on that axis, but has maximum sensitivity tosounds from a lateral source 8:, whereas the sensitivity of microphone b is substantially independent or the position of the source. Leads (1 and e transmit respectively half the sum and half the diflerence oi the microphone outputs, and clearly with the rection oi the sound source s. relatively to the misound source in position 81 the two outputs in leads d and e are the same (so that loud speakers connected thereto would be equally loud and indicate correctly a central position of the sound source) whereas with the sound source in position S: the microphone outputs are the same, and the impulses in leads d and e are therefore respectively a maximum value and zero (which provides full sound in one loud speaker and sube stantially none in the other, thus giving the correct impression of a laterally displaced sound source).

An alternative arrangement of the present in-- vention which may be used (especially where th microphones are of similar impedance, as in th case of microphones having two strip elements described more fully below) is shown in Figure and consists in connecting one terminal of on microphone to one of the other microphones and connecting the other microphone terminals respectively to opposite ends of the primary winding 1 of a transformer. The primary 9' of a second transformer is connected between the common microphone terminals and the mid-point of the first-mentioned transformer primary fmand it will be clear that while the output from the secondary of one transformer will consist in the sum of the microphone impulses that from the other will consist in their difference. By suitably arranging the orientation of the microphones (one of which is, of course, directionally sensitive), and by suitably choosing the transformer ratios in order to obtain the desired ratio of outputs in the two channels connected to the secondary windings of the transformers, the impulses can be controlled satisfactorily to provide the binaural eiIect aimed at by the system, and the relative amplitudes of impulses in the two channels determined in accordance with the.dl rection from which the sounds reach the microphones.

Instead of employing two separate microphones, the microphone elements may be arranged together to form a single instrument. An instrument of such a character has been described in the Journal of the Acoustical Society of America, Volume V, page 139 et seq., October, 1933, and also the specification of British Patent No. 405,497, and as shown in Figs. 7 and 8, comprises a strip a: supported from its ends between poles of a field magnet in and having midway along its length a supporting bridge ca which efiec-' tively divides the strip into two separately opertive portions, one of which is allowed to function as an ordinary velocity sensitive element, while the other is loaded on one side by an acoustic impedance (consisting of an adjacent tube (is packed with felt absorption pads) whereby it is rendered sensitive to pressure variations and substantially non-directional. Such a microphone may readily be employed in the present invention, the elements being arranged and electrically connected in the manner described above. Alternatively the microphone may be slightly modified to meet the requirements of this invention. For example instead of being formed of one strip two separate elements may be located between the magnetic pole pieces-side by side and substantially parallel to one another, one being acoustically damped as indicated above, this arrangement allowing relative angular displacement of the planes of the elements, should that be necessary or desirable to achieve the desired employed in respect of only part of the frequency range being transmitted. For example in copending application No. 750,898, filed October 31, 1934, it is shown that in some circumstances it isdesirable initially to transmit low frequencies over one pair of channels and high frequencies over another pair of channels, and the systems described in this specification may if desired be employed for one part of the range, in combination with a different system or systems for another part or parts of the range.

Finally it must be understood that the invention is not limited solely to the details of construction or arrangements described herein since modifications may be introduced as they become desirable or necessary in order to carry the invention into effect under different conditions and requirements which have to be fulfilled, without departing in any way from the scope of the invention.

Having now described my invention, what I claim as new and desire to secure by Letters Pattent is:- v

1. A system of sound transmission comprising a plurality of loud speakers, a velocity microphone element and a pressure microphone element, a plurality of channels connecting said microphone elements to said loud speakers, and means by which the impulse transmitted in one channel is composed of half the sum of the microphone outputs while the impulse transmitted in another channel is composed of half the diiference of the microphone outputs.

2. A system of sound transmission according to claim 1 comprising transformers in one of which the microphone outputs assist, and in another of which the microphone outputs oppose, one another.

3. A system of sound transmission comprising a plurality of loudspeakers, a velocity microphone element, and a pressure microphone element, and a plurality of channels, connecting said microphone elements to said loudspeakers,

so arranged that the output intensities of the loudspeakers differ from one another by an amount dependent upon the output of the velocity element and their relative output intensities vary in accordance with the direction from which the sounds are incident upon the microphone elements.

4. A system of sound transmission comprising a velocity-sensitive microphone element and a pressure-sensitive microphone element arranged in close juxtaposition, means for causing the output channels of said microphone elements to interact with one another, and a plurality of loudspeakers connected to receive impulses resulting from said interaction of the microphone outputs, the arrangement being such that the loudspeaker outputs diifer by an amount proportional to the output of the velocity microphone, and the relative intensities of the loudspeakers vary in ac- 1 cordai ce with the direction from which the sounds approach the microphones.

5. A system of sound transmission comprising a velocity-sensitive microphone element and a pressure-sensitive microphone element arranged together in a single instrument, each element being connected to the transmission circuit as a separate microphone, a. plurality of loudspeakers by which impulses from said microphones are reproduced, and means adapted automatically to control the relative intensities of said loudspeaker outputs in accordance with the direction from which the sounds to be transmitted reach said microphone elements.

6. In a system of sound transmission according to claim 5 a microphone comprising, in a single instrument, a strip element sensitive to the velocityof the air due to the sounds received, and another strip element acoustically loaded to render it sensitive to. pressure variations of the air due to the sounds received, each element being connected to the transmission circuit as a separate microphone.

ALAN DOWER Brim/mam.

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