Receiving system having pre-selected directional response

Claims

What is claimed is:

1. A receiving system comprising a plurality of spaced apart receiving means for producing a time varying output signal responsive to incident time-variable stimuli, and signal processing means for combining the output signals from the receiving means to produce a selected response pattern for the stimuli, the improvement wherein the signal processing means includes a difference channel for subtracting the output signals of a pair of the receiving means to obtain a difference signal, and an integrating channel for integrating the difference signal with respect to time to obtain an integrated difference signal thereby imposing a figure eight response on the response characterized by the inputs to the difference channel.

2. A receiving system having a plurality of spaced apart receiving members, each of which is responsive to incident time-variable stimuli for producing a corresponding time-variable output signal, and signal processing means for combining the output signals from the receiving members to produce a selected response pattern for the stimuli, the signal processing means including means for subtracting the output signals of a pair of members to obtain a difference signal, the improvement wherein said signal processing means includes:

an integrating channel for integrating the difference signal with respect to time to obtain an integrated difference signal;

a summing channel for adding the output signals of a pair of the members to obtain a sum signal; and

combining means for combining the outputs of the summing and integrating channels.

3. A receiving system according to claim 2 wherein the combining means includes means for adding the outputs of the summing and integrating channels.

4. A receiving system according to claim 2 wherein said combining means includes means for subtracting the outputs of the summing and integrating channels.

5. A receiving system according to claim 4 wherein said combining means further includes means for also adding the outputs of the summing and integrating channels.

6. A receiving system according to claim 2 wherein the combining means includes gain control means for setting the gain of at least one of said channels relative to the other.

7. A receiving system according to claim 2 wherein there are four aligned receiving members, the members whose outputs are added in the sum channel to form the sum signal being intermediate the members whose outputs are subtracted to form the difference signal.

8. A receiving system comprising:

a plurality of receiving members, each of which is responsive to incident time-variable stimuli for producing a corresponding time-variable output signal; and

signal processing means for combining the output signals from the receiving members to produce a selected response for stimuli at preselected values of frequency and angles of incidence, said signal processing means including:

a summing channel for adding the output signals of a pair of the members to obtain a sum signal;

a difference channel for subtracting the output signals of a pair of members to obtain a difference signal;

an integrating channel for integrating the difference signal with respect to time to obtain an integrated difference signal; and

combining means for combining the outputs of the summing and integrating channels.

9. A receiving system according to claim 8 wherein the combining means includes gain control means for setting the gain of one channel relative to the other to define a gain controlled integrated signal and a gain controlled sum signal, and means for adding the two gain controlled signals.

10. A receiving system according to claim 9 wherein the relative gain of the gain controlled signals is selected such that the output of the combining means approaches zero for incident stimuli at low frequencies making a predetermined angle with the axis of the array.

11. A receiving system according to claim 9 wherein the members are laterally spaced in an elongated array and the relative gain of the gain controlled signals is selected such that the output of the combining means approaches zero for incident stimuli at low frequencies and oriented in a given direction with the array.

12. A receiving system according to claim 8 wherein the plurality of members includes four receiving members, the members whose outputs are added in the sum channel to form the sum signal being intermediate the members whose outputs are subtracted to form the difference signal.

13. A receiving system according to claim 11 wherein the receiving members are microphones and the incident stimuli is sound having a component in the audio range.

14. A receiving system according to claim 13 in combination with the noise source and mounting means for mounting the microphone oriented with respect to the noise source such that the receiving system rejects noise from the source.

15. A receiving system according to claim 8 wherein the array includes two receiving members whose outputs are added to obtain the sum signal and whose outputs are subtracted to obtain the difference signal.

16. In a receiving system having a plurality of receiving members arranged in an array with each member being responsive to incident time-variable stimuli for producing a corresponding time-variable output signal, the improvement comprising signal processing means for combining the output signals from the receiving members to create a cardioidal-like response over a band of frequencies of the stimuli, said signal processing means including:

a summing channel for adding the output signals of a pair of members to obtain a sum signal;

means for subtracting the output signals of a pair of the members to obtain a difference signal;

an integrating channel for integrating the difference signals with respect to time to obtain an integrated different signal; and

combining means for combining the outputs of the summing and integrating channels.

17. The improvement of claim 16 wherein the combining means includes gain control means for setting the gain of one channel relative to the other to define a gain controlled integrated signal and a gain controlled sum signal, and means for adding the two gain controlled signals.

18. A sound photographic system comprising:

a camera having a lens assembly with a predetermined field of view which defines the camera taking axis, said camera during its operation providing sound extending over a given range of frequencies; and

a sound recording apparatus associated with said camera, said sound apparatus comprising:

an array of microphones fixed to said camera outside of said predetermined field of view such that said camera sound is oriented at a predetermined angle of incidence to said array; and

means for spacing the microphones in said array and for combining the output signals thereof so that array preferentially rejects sound from said camera when the latter is made operative, said spacing and combining means includes means for subtracting the output signals of a pair of said microphones to obtain a difference signal, and an integrating channel for integrating said difference signal with respect to time.

19. The sound photographic system of claim 18 wherein said microphones are ominidirectional microphones and in combination with said spacing and combining means provide a cardioidal response having its area of maximum rejection of said range of camera sound frequencies located at said predetermined angle of incidence.

20. A sound receiving system for use with a camera having a lens assembly with a predetermined field of view which defines the camera taking axis, said camera during its operation providing camera sound extending over a given range of frequencies, said sound system comprising:

an array of spaced microphones;

means for mounting said array on said camera outside of said predetermined field of view such that said camera sound is oriented at a predetermined angle of incidence to said array;

means for combining the output signals of said microphones so that said array preferentially rejects sound from said camera when the latter is made operative, said combining means including:

means for subtracting the output signals of a pair of said microphones to obtain a difference signal; and

an integrating channel for integrating said difference signal with respect to time.

21. A sound receiving system according to claim 20 additionally including:

a summing channel for adding the output signals of a pair of microphones to obtain a sum signal; and

means for combining the outputs of said summing and said integrating channels.

22. A sound receiving system according to claim 21 wherein said combining means includes means for adding the outputs of said summing and integrating channels.

23. A sound receiving system according to claim 21 wherein said combining means includes means for subtracting the outputs of said summing and integrating channels.

24. A sound receiving system according to claim 21 wherein said combining means further includes means for also adding the outputs of said summing and integrating channels.

25. A sound receiving system according to claim 21 wherein said combining means includes gain control means for setting the gain of one channel relative to the other to define a gain controlled integrated signal and a gain controlled sum signal, and said combining means comprises means for adding the two gain controlled signals.

26. A sound receiving system according to claim 25 wherein the relative gain of said gain controlled signals is selected such that the output of said combining means approaches zero for low frequency sound incident on said array at said predetermined angle, and the lateral spacing between said microphones is selected to provide an equal amplitude of both gain controlled signals for a given frequency incident at said predetermined angle.

27. A sound receiving system according to claim 21 wherein said microphones are provided in two arrays at an angle to each other, the respective microphones of each array are subtracted and integrated with time to produce first and second integration signals, and the respective microphones of each are summed to produce first and second summed signals respectively, and said combining means includes means for combining the first integration signal with the first summed signal to produce a first stereo channel and the second integration signal with the second summed signal to provide a second stereo channel.

28. A sound receiving system according to claim 21 wherein said array includes four microphones, the microphones whose outputs are added in summing channel to form said sum signal being intermediate the microphones whose outputs are subtracted to form said difference signal.

29. A sound receiving system according to claim 28 wherein said four microphones are spaced generally side by side in a plane.

30. A sound receiving system according to claim 28 wherein the distance between said intermediate microphones is approximately one-half the distance between the outer microphones of said array.

31. A sound receiving system according to claim 28 wherein said combining means includes gain control means for setting the gain of one channel relative to the other, and said gain control means and the lateral spacing of said microphones in said array are selected to provide an equal amplitude of both gain controlled signals for a frequency of approximately 8,000 Hz incident on said array at said predetermined angle.

32. A sound receiving system according to claim 31 accomodating a range of frequencies extending from low frequencies up to approximately 6,000 Hz.

33. A receiving system having a preselected response pattern for incident sound comprising a plurality of microphones, each of which produces a time variable output signal in response to incident time variable sounds, at least a pair of said microphones being arranged in a first array defining a first array axis; and signal processing means for integrating with time the difference between the output signals of the microphones of the first array to provide a first integrated signal.

34. A receiving system according to claim 33 wherein the microphones are omnidirectional, and the signal processing means causes the response pattern of the first pair of microphones to be directional along the first array axis.

35. A receiving system according to claim 33 including a second array having a second pair of directional microphones, each of whose major axis is parallel to the major axes of the first pair of microphones, the second pair of microphones defining a second array axis, and wherein the signal processing means includes means for integrating the difference between the output signals of the second pair of microphones to obtain a second integrated signal whereby the response pattern of the second pair of microphones is directional along the second array axis.

36. A receiving system according to claim 35 wherein the signal processing means includes means for adding the output of the microphones of the first array to obtain a first sum signal and for adding the outputs of the microphones of the second array to obtain a second sum signal, and means for adding the first sum signal to the first integrated signal to obtain a first processed signal and for adding the second sum signal to the second integrated signal to obtain a second processed signal whereby the response pattern of the first array of microphones is a generally cardioidal pattern directed along the axis of the first array and the response pattern of the second array of microphones is a generally cardioidal pattern directed along the axis of the second array.

37. A receiving system according to claim 36 wherein the two microphone axes intersect and one microphone is common to both, the first and second array.

38. A sound receiving system comprising at least a pair of microphone arrangements laterally spaced in a first array and defining a first array axis, the microphone arrangements each including means for producing a direction response along parallel microphone axes which are oblique to the first array axis, and signal processing means for integrating with time the difference between the outputs of the microphone arrangements of the array to provide a first integration signal providing a response pattern rotated from the microphone axes.

39. A sound receiving system according to claim 38 wherein said processing means includes means for adding the outputs of the pair of microphone arrangements to produce a first sum signal and means for combining the first sum signal with the first integration signal.

40. A sound receiving system according to claim 39 wherein said combining means includes means for adding the first sum signal and the first integration signal to produce a first stereo channel and means for subtracting the first sum signal and the integration signal to produce a second stereo channel.

41. A sound receiving system according to claim 38 including a second array having at least a pair of second microphone arrangements laterally spaced in a second array and defining a second array axis at an oblique angle to the first array axis, the second microphone arrangements each including means for producing a directional response along parallel microphone axes which are oblique to the axis of the second array, said second array being arranged at an angle to said first array, and the processing means includes means for integrating with time the difference between the output signal of the microphone arrangements of the second array to provide a second integration signal providing a response pattern rotated from the microphone axis, said first integrated signal providing a first stereo channel directed generally along the first array axis, and said second integration signal providing a second stereo channel directed generally along the second array axis.

42. A sound receiving system according to claim 41 wherein the two array axes intersect and wherein one microphone is common to both the first and second array of microphones.

43. A sound receiving system according to claim 41 wherein the axes of the first and second array are located at an angle of about 90.degree. to each other, wherein the major axis of each microphone is about 45.degree. to the axis of its respective array.

44. A sound receiving system according to claim 41 wherein the signal processing means includes means for summing the outputs of the first pair of microphones to form a sum signal, and means for adding the sum signal to the first integrated signal to obtain a first processed signal, and for subtracting the sum signal from the first integrated signal to obtain a second processed signal, whereby the response pattern of the microphones based on the first processed signal is directional along a first axis about 45.degree. to the first microphone axis and the response pattern of the microphones based on the second processed signal is directed along a second axis 90.degree. to the first axis.

45. A receiving system comprising a first and second microphone arrangement laterally spaced apart to define an array axis, said microphone arrangements including means for producing a cardioid-like response directed at an oblique angle to the array axis, and means for subtracting the outputs of the microphone arrangements and for integrating the difference with time to obtain an integration signal thereby altering the response pattern of each microphone arrangement and rotating their major axes with respect to the array axis.

46. The system of claim 45 including means for adding the outputs of each microphone arrangement to produce a sum signal, means for adding the sum signal and the integration signal to produce a first stereo channel and means for subtracting the sum signal and the integration signal to produce a second stereo channel.

47. A sound receiving system comprising:

(a) an array of microphone elements for producing output signals in response to incident sound;

(b) signal processing means for providing the array with pre-selected directional acceptance and rejection characteristics including:

(1) means for integrating the difference between the output signals of a pair of elements to produce an integrated signal;

(2) a high pass filter for suppressing lower frequency components of the integrated signal to produce a filtered integration signal;

(3) a low pass filter for suppressing high frequency components in the output signal of at least one of the elements to produce a filtered output signal; and

(4) means for adding the filtered integrated signal to the filtered output signal.

48. A sound receiving system according to claim 47 wherein the cutoff frequency of the filters is in the range 400-2,000 Hz.

49. A sound receiving system comprising:

(a) an array of microphone elements for producing output signals in response to incident sound;

(b) signal processing means having a left and right output channel for processing the output signal;

(c) means in each channel for integrating the difference between the output signals of an adjacent pair of elements to produce an integrated signal;

(d) means responsive to the integrated signal in each channel for producing left and right output signals representing sound originating in front of and respectively to the left and right of the array; and

(e) frequency responsive means for suppressing low frequency components of the integrated signal in each channel and for passing substantially equal amounts of low frequency components of sound picked up by the elements to each of the left and right output signals.

50. A sound receiving system according to claim 49 wherein the array comprises a triangular array of at least three elements, the apex of the triangular array being directed toward the sound source, and each output channel including means to subtract the outputs of a pair of elements which consists of a lead element and a trailing element for forming a difference signal, means for integrating the difference signal to produce an integrated signal, and a high pass filter for filtering the integrated signal to form a filtered integrated signal.

51. A sound receiving system according to claim 50 including a low pass filter for filtering the output of at least one of the elements to produce a filtered output signal and means in each channel for adding the filtered integrated signal in the channel to the filtered output signal.

52. A sound receiving system according to claim 51 wherein the array consists of three elements arranged in a triangle.

53. A sound receiving system according to claim 52 wherein the array consists of four elements with two of the elements at substantially at the apex of a triangle.

54. A sound receiving system according to claim 49 wherein the low frequency cutoff is in the range 400-2,000 Hz.

55. A sound receiving system according to claim 54 wherein the low frequency cutoff is less than 1,000 Hz.

56. A sound receiving system according to claim 55 wherein the low frequency cutoff is less than 500 Hz.

57. A sound receiving system according to claim 49 wherein the array comprises two elements arranged in a line perpendicular to the sound source, and the signal processing means includes:

(a) means to add the outputs of the elements to produce a sum signal;

(b) a low pass filter for filtering the sum signal to produce a filtered sum signal;

(c) means to subtract the outputs of the elements to produce a difference signal;

(d) means to integrate the difference signal to produce an integrated signal;

(e) means to add the integrated signal to the sum signal to provide one combined signal;

(f) means to subtract the integrated signal from the sum signal to provide another combined signal;

(g) a high pass filter for filtering the one combined signal;

(h) a high pass filter for filtering the other combined signal;

(i) an adder for adding the filtered sum signal to the filtered one combined signal for producing a first processed signal; and

(j) an adder for adding the filtered sum signal to the filtered other combined signal for producing a second processed signal.

58. A sound photographic system comprising:

(a) a camera having a lens assembly with a predetermined field of view which defines the camera-taking axis;

(b) sound recording apparatus associated with said camera and comprising:

(1) three microphone elements arranged in a triangle whose apex is directed toward the scene being photographed and lies along the taking axis, each element producing an output signal in response to incident sound;

(2) a low pass filter for passing low frequency components of the output signal of one of the elements for providing a filtered output signal;

(3) left and right output channels, each having means to subtract the output signal of the lead element from a different trailing element to produce two difference signals;

(4) an integrator in each channel for integrating the difference signals therein to provide an integrated signal;

(5) a high pass filter in each channel for filtering low frequency components from the integrated signal in the channel to provide a filtered integrated signal; and

(6) an adder in each channel for adding the filtered output signal therein to the filtered integrated signal in the channel to provide left and right output signals.

59. A sound photographic system according to 58 wherein the low pass filter suppresses signals greater than 2,000 Hz and the high pass filter suppresses signals less than 400 Hz.

60. A sound photographic system according to 58 wherein the cutoff frequency of the filters is substantially the same, and is in the range 400-2,000 Hz.

This invention relates to a receiving system having a preselected response pattern, and more particularly, to a receiving system utilizing a microphone array that permits monaural or stereophonic recordings to be made.

Amateur photographers who have made sound accompanied home movies with conventional equipment are familiar with the problem of minimizing camera sound pick-up during filming sequences. Failure to minimize camera sound pick-up is evident during projection of film in that the camera noise will frequently mask the sounds whose recording is desired in connection with the film.

One approach to solving this problem is to physically separate the microphone from the camera, but this requires an assistant to coordinate recording with picture taking. In many circumstances, this is inappropriate. Therefore, to permit simultaneous recording and picture taking by a single person, it is conventional to attach a microphone to the camera by way of an extension that positions the microphone forwardly of the camera in the direction in which photography takes place, but out of the field of view of the camera. For stereophonic recordings, two widely spaced microphones are conventionally required. For monaural sound recordings, an inexpensive cardioid microphone positioned on the camera and oriented so that the null of the cardioid faces the camera will normally be adequate for recording sounds associated with the scene being photographed. Unfortunately, the frequency spectrum of the noise associated with an operating camera is so wide, that a considerable amount of noise is also recorded. Experience shows that the spectrum of many cameras extend from a relatively low frequency of around 100 Hz to about 6,000 Hz with a peak occurring around 2,000 Hz just in the region of maximum hearing perception. While the usual inexpensive cardioid microphone is often adequate for rejecting low frequency sounds originating from the camera, its spatial pattern of response is frequency dependent over the relatively wide frequency range of sound usually associated with the mechanical drive of the camera. As a result, the sound of the camera in operation is superimposed on the recording of the sound associated with the scene being filmed. Being much closer to the microphone than the subject, it has been found that the camera noise dominates.

While it may be possible to design a special microphone whose mechanical arrangement increases the capability of rejecting noise from a camera over a relatively wide frequency band, such a microphone is likely to be highly complicated and hence expensive. It is therefore an object of the present invention to provide a new and improved recording system whose frequency response patterns are determined essentially by the type of signal processing utilized rather than by mechanical details of the elements of the microphone, thereby permitting a cluster of microphones to act as a highly directional monaural or stereo composite microphone.

SUMMARY OF THE INVENTION

The present invention provides a receiving system comprising an array of receiving members, each of which is responsive to incident time-variable stimuli for producing a corresponding time-variable output signal, and signal processing means to combine the output signals from the receiving members for causing the system to have a response pattern that depends on the type of signal processing and the spacing and orientation of the receiving members. The signal processing means includes an integration channel for integrating with time the difference between the output signals of a pair of the members to obtain an integrated signal. Optionally, the signal processing means includes a summing channel for adding the output signals of the same or different pair of the members to obtain a sum signal, and combining means for combining the outputs of the two channels.

For a stimulus of a given frequency incident on the array, the sum signal and the integrated signal will be in phase and will vary with time in accordance with the time variation of the incident stimulus. Consequently, the magnitudes of these signals can be made equal for that frequency and for a pre-selected angle of incidence by a proper selection of the gain applied to each of these signals before they are combined, and by a proper selection of the spacing of the receiving members of the array. Specifically, the relative gain of the gain controlled signals is selected such that upon addition, the result approaches zero for low frequency incident stimuli (i.e., stimuli at frequencies approaching zero) which make a predetermined angle with the axis of the array. Furthermore, the spacing between the pairs of receiving members may be selected so that the amplitudes of the gain controlled signals are also made equal for any given frequency for stimulus at the predetermined angle with respect to the array.

A pair of members which individually have omni-directional receiving characteristics will exhibit a directional response in the form of a figure-eight when the output of the integrated channel associated with such pair is considered. When the output of the combining means is considered, however, the pair members will exhibit a more highly directional response of a cardioid. In each case, the direction of the major axis of the response characteristic will be aligned with a line interconnecting the pair of members.

Alternately, if each of the pair of members in the preceding arrangement has a directional and similar response, and the members are oriented in an array so that the major axes of the responses are perpendicular to the array axis, then the pair may be processed to exhibit a directional response whose major axis is rotated, for example, 45 degrees to the major axis of the individual members when the output of the integration channel is considered. The direction in which the major axis of the composite response is rotated will depend on the direction in which the subtraction process takes place.

The spatial position of pairs of microphones and the orientation of their individual directional responses relative to their spatial orientation permits closely spaced pairs of microphones to exhibit a single highly directional characteristic, or two highly directional angularly displaced characteristics. This versatility makes the recording system, according to the present invention, particularly well suited for incorporation into a sound motion picture system. In such case, the signal processing means may be arranged to cause the array of receiving members to act as a monaural cardioidal microphone at low frequencies with substantial rejection along the axis of the cardioid for the frequencies that dominate camera noise. Alternatively, the signal processing means can be designed to cause the array of members to act as a stereophonic microphone with adjustable separation between the two channels.

The invention also consists of a sound motion picture system comprising a motion picture camera and a sound recording system associated with the camera. Such system includes an array of microphones fixed to the camera and located out of the field of view thereof, the array preferably, but not necessarily, projecting forwardly and downwardly from the camera. Signal processing means are provided for combining the output signals of the microphones which are so spaced that the array preferentially accepts sound from a scene being photographed and rejects sound from the camera when it is operational. Specifically, the array can act as a monophonic or stereophonic microphone, depending on the spatial arrangement of the members of the array and on the type of signal processing employed. Low frequency sounds originating from the camera are significantly rejected, as are sounds originating at the camera at a frequency in the range within which hearing is most perceptive.

A monaural or stereophonic sound recording system according to the present invention is thus capable of utilizing relatively simple and inexpensive closely clustered microphones because the preselected directional characteristics, in terms of pickup and rejection, are essentially controlled by the signal processing means employed and the spatial location of the microphones.

To minimize wind noise pickup in a monaural or sterophonic system according to the present invention, low frequency components of an integrated signal produced by the system are suppressed to produce a filtered integrated signal, and high frequency components in the output signal of at least one of the elements are suppressed to produce a filtered output. When the filtered integrated signal is added to the filtered output signal, the low frequency components of the resultant signal essentially will be unprocessed, while the higher frequency components will be processed achieving the desired directional characteristics for these components. As a consequence, low frequency wind noises will be present in the resultant signal to an extent no greater than they would be present in a conventional recording system while higher frequency information signals will be directionally picked up. Lower frequency information signals will also be picked up from directions other than the preferred direction, but the reduction in wind noise more than balances this loss.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are shown in the accompanying drawings wherein:

FIG. 1 is a perspective view of a motion picture camera into which a monaural version of the present invention is incorporated and showing orthogonal, low frequency cardioidal response characteristics of the sound recording system;

FIG. 2 is a qualitative showing of a typical noise spectrum associated with a movie camera;

FIG. 3 is a perspective view of a linear array of receiving members showing the incidence thereon of a plane wave of arbitrary frequency and making an arbitrary angle of incidence with the array;

FIG. 4 is a block diagram of a receiving system according to the present invention showing details of one form of the signal processing means;

FIG. 5 is a polar-slot of the response characteristic of a receiving system according to the present invention for a particular value of relative gain as between the sum channel and the integrated channel for low frequency stimuli;

FIG. 6 is a composite plot of the respective amplitudes of the sum signal and the integrating signal for the system shown in FIG. 4, and showing the effect on the difference in magnitudes of the amplitudes of the sum and integrated channels for two situations, when the spacing between the pair of microphones whose output is subtracted is the same as, and is twice the spacing between the pair of microphones whose output is added for stimuli aligned with the incident on the array;

FIG. 7 is a plot similar to that shown in FIG. 6 except the spacing between the pairs of microphones has been selected so that the difference in magnitudes of the amplitudes of the sum and integrated channels is made zero for stimuli of a predetermined, non-zero frequency, incident on and aligned with the array;

FIG. 8 is a plot similar to FIG. 7 but showing the amplitudes of the sum and integrating signals for stimuli incident on the array at an angle of approximately 30.degree.;

FIG. 9 is a plot similar to FIG. 8 but showing the situation when the stimuli is incident on the array at an angle of about 60.degree.;

FIG. 10 is a block diagram of signal processing equipment suitable for carrying out the signal processing technique of the present invention as it is applied to a two-microphone version of the present invention;

FIGS. 11A-F are concerned with a stereophonic recording system utilizing a triangular array of omnidirectional microphones;

FIG. 12 is an elevational view of a sound camera system showing the relationship of an array of cardioid elements with each directed away from the camera;

FIGS. 13A-B illustrate an embodiment of a microphone which effectively provides a pair of microphones which achieve subtraction by means of the mechanical construction of the microphone.

FIGS. 14A-D are concerned with cardioidal microphones and circuitry therefore, which provides highly directional stereophonic receivers;

FIGS. 15A-B are concerned with a two-member array of omnidirectional microphones whose output can be processed to provide a stereophonic receiver;

FIG