Stereo microphone unit

Abstract: A stereo microphone unit adapted to be installed in a video camera (200) or the like, comprising a cabinet (3), two nondirectional microphones disposed with a predetermined distance therebetween on the wall of the cabinet (3) for collecting outside sound signal, delay means for delaying each output of the microphones by a time period corresponding to the predetermined distance, and subtracter means for subtracting the output of the delay means relative to one microphone from the output of the other microphone. A stereo audio signal is provided only with regard to the output sound signal of any frequency lower than a predetermined frequency of 6 kHz or so.

Description: The present invention relates to a built-in type stereo microphone unit adapted to be installed in a video camera or the like where sound is recordable in a stereophonic mode. For processing input sound in a stereophonic mode, it is customary to use two unidirectional microphones 300 as illustrated in Fig. 6. The two microphones 300 are so disposed that the sound receiving directions thereof form an angle ϑ (ranging from 90° to 130°) and also that the sound receiving positions thereof are spaced apart from each other by a distance greater than 15 cm. The unidirectional microphone 300 generally has such directional characteristic as represented by a curve P in Fig. 7, and the curve P is obtained when the microphone 300 is disposed at a center O. As will be understood from Fig. 8 also, if the microphone 300 is used in a state incorporated in a microphone capsule 300A, the characteristic shown in Fig. 7 is achieved upon mutual combination of sound waves A and B emitted from a sound source and received at the front and the back of the microphone capsule 300A. It is therefore necessary to ensure, behind the microphone capsule 300A, a sufficient space without any impediment or reflective surface so as to prevent reflected sound waves from the sound source denoted by C being received by the microphone 300. There has been proposed a technique of performing a stereophonic recording operation by the use of two nondirectional microphones each having such directivity characteristic as represented by a curve Q in Fig. 9, but such technique is not employed in general. However, if unidirectional microphones 300 are installed in a casing of a small-sized video camera for example, a sufficient space is not existent around microphone capsules 300A so stereophonic recordings tend to fail. Therefore it is customary that one-point stereo microphones are provided in a posture projecting outward from the casing. Since restriction is unavoidable with respect to dimensional reduction of a video camera and the degree of freedom in design and so forth, it has been urgent heretofore to realize development of superior microphones which are adapted to be installed in a casing and are still capable of performing a stereophonic sound processing operation. It is an object of the present invention to provide an improved built-in type stereo microphone unit which executes a stereophonic sound processing mode in a state installed in a casing of a video camera or the like. For the purpose of achieving the object mentioned, the stereo microphone unit of the present invention comprises a cabinet, two nondirectional microphones disposed with a predetermined distance therebetween on the wall of the cabinet for collecting outside sound signal, delay means for delaying each output of the microphones by a time period corresponding to the predetermined distance, and subtracter means for subtracting the output of the delay means relative to one microphone from the output of the other microphone. In such stereo microphone unit, a stereophonic effect is attained with regard to relatively higher-pitched sound by the distance existent between the two microphones and, with regard to relatively higher-pitched sound, by electrically processing the audio signals inputted to and outputted from the two microphones, thereby enabling a stereophonic operation with the microphones installed in a casing of a video camera or the like. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Fig. 1 is a perspective view of a preferred embodiment of the built-in type stereo microphone unit according to the present invention; Figs. 2 and 3 illustrate the structure of a cabinet, in which Fig. 3 is a sectional view taken along the line III-III in Fig. 2; Fig. 4 is a block diagram of an electric constitution in the embodiment; Fig. 5 graphically shows the frequency characteristics of microphones; Fig. 6 illustrates a fundamental constitution in stereophonic recording; Fig. 7 shows the directivity characteristics of a unidirectional microphone; Fig. 8 illustrates an exemplary state where a unidirectional microphone is used; and Fig. 9 shows the directivity characteristics of a nondirectional microphone. Hereinafter a preferred embodiment of the built-in type stereo microphone unit of the present invention will be described in detail with reference to the accompanying drawings. In Fig. 1, a stereo microphone unit 1 according to the present invention is installed integrally with a front portion of a casing,of a video camera 200 for example. As will be understood from Figs. 2 and 3 also, the stereo microphone unit 1 comprises a substantially -shaped cabinet 3 in cross section and two microphones 10, 20 housed in the cabinet 3 for left (L) and right (R) channels respectively. In the cabinet 3, there is formed a space 3a where sound receiving surfaces c, d of the two microphones 10, 20 are positioned apart from each other by a distance ℓA of, e.g., 30 mm. The two microphones 10, 20 are nondirectional with the directivity characteristic represented by a curve Q in Fig. 9 and are disposed in such postures that the sound receiving surfaces c, d thereof are directed toward the outside of the cabinet 3. And protective net members 5, 5 are attached to the cabinet 3 at the fronts of the sound receiving surfaces c, d. As will be understood from Fig. 4, sound waves received by the microphones 10, 20 are converted into audio signals, which are then supplied to delay circuits 50, 60 respectively via buffer amplifiers 30, 40 and are further supplied to adders 100, 90 respectively via attenuators 70, 80. In this case the output signals of the attenuators 70, 80 are polarity-inverted, and the output signals of the buffer amplifiers 40, 30 are supplied also to the adders 100, 90. The output signals of the adders 90, 100 are supplied, as L-channel and R-channel audio signals respectively, to output terminals 130, 140 via equalizers (EQ) 110, 120 respectively. Each of the delay circuits 50, 60 may be composed of a general low-pass filter which principally comprises a resistor and a capacitor. In the stereo microphone unit 1 of the constitution mentioned, sound waves of any wavelengths smaller than the distance ℓA (in this embodiment, frequencies higher than 6 kHz) are so processed that, as will be understood from Fig. 5, level differences occur correspondingly to the distance ℓA among audio signals α, β, γ received and outputted from the front, back and face of the microphone 10 (or 20), so that a stereophonic effect can be achieved by such frequency characteristics. More specifically, with regard to any sound waves of frequencies higher than 6 kHz, a stereophonic sound processing operation is performed by the structure of the cabinet 3. This level difference is caused for what is called an acoustic shadow. That is, in Fig. 3, sound from left side of the figure, sound source A, reaches directly to the microphone 20(R). But only sound reflected from surrounding surface of the cabinets reaches to the microphone 10(L). Since the reflected sound travels a long way in the reflected path and loses energy at each reflection, the intensity of sound picked up by the microphone 10(L) is reduced. This acoustic shadow appears high frequency sound, more than 6 kHz, in this model. If the wave length of a sound is much longer than the distance between microphone 20(R) and 10(L), ℓA, in Fig. 3, acoustic shadow does not appear; but if the wave length of a sound is shorter than or as the same as the distance between two microphones the acoustic shadow appears. Meanwhile with regard to any sound waves of frequencies lower than 6 kHz, a stereophonic effect can be realized by the circuit constitution shown in Fig. 4. When the R-channel microphone 20 is positioned in greater proximity to sound sources A, B as shown in Fig. 3, the time required for sound waves to reach the L-channel microphone 10 is longer, as compared with the arrival at the other microphone 20, for a time period corresponding to the travel of the wound waves through a distance ℓA ( ad - ac ) relative to the sound source A or through a distance ℓB ( Bd - bc = Bd - be ) relative to the sound source B. Therefore the output signal of the buffer amplifier 40 is delayed by the delay circuit 60 for such time period, and then the delayed signal is subtracted from the output signal of the buffer amplifier 30, whereby the output of the adder 90 is substantially canceled to consequently attain a stereophonic effect. In case the sound sources are more proximate to the microphone 10, the R-channel output is cancelled correspondingly. Now the principle of the above operation will be described in further detail. As given below, Eq. (1) is obtained in relation to sound waves sin ωt arriving at the microphone 20 (where ω is angular velocity, and t is time); sound waves L (ω) received by the microphone 10; delayed phase angle  (function of ℓA and ω) at the microphone 10; variation a (≦ 1) in the attenuator 80; and phase angle ψ corresponding to the delay caused by the delay circuit. If the delay is set to be equal to the delayed phase angle (i.e.,  = ψ), the following is obtained from Eq. (1). L (ω) = (1 - a) sin (ωt - ψ) Setting the condition as a = 1, the above equation is L (ω) = 0 In this case the sound wave R (ω) received by the microphone 20 is expressed as R (ω) = sin ωt - a sin (ωt -  - ψ) Therefore, the above Eq. (2) may be rewritten as R (ω) = sin ωt {1 - cos ( + ψ)}+ cos ωt sin ( + ψ) + (1 - a) sin (ωt -  - ψ) Accordingly, when  = ψ, R (ω) = 2 sin  cos (ωt - ) + (1 - a) sin (ωt - 2) If a = 1, the above becomes R (ω) = 2 cos (ωt - ) sin  From Eqs. (1) and (3), the following can be defined. i) When the sound source is at a position where  = ψ, R (ω) = 2 sin  cos (ωt - ) + (1 - a) sin (ωt - 2) L (ω) = (1 - a) sin (ωt - ψ) (ii) When the sound source is at a position where  = ψ and a = 1, R (ω) = 2 sin  cos (ωt - ) L (ω) = 0 (iii) When the sound source is at a position where  ≠ ψ and a = 1, R (ω) = sin ωt {1 - cos ( + ψ)} + cos ωt sin ( + ψ) L (ω) = 2 sin  - ψ 2 cos (ωt +  + ψ 2 ) In each of the above three cases, a stereophonic effect (difference between left and right levels) is attainable as will be understood from the frequency characteristics shown in Fig. 5. Thus, according to the present invention, a stereophonic effect regarding sound waves of frequencies higher than 6 kHz can be achieved by the structure of the cabinet 3, and a stereophonic effect regarding sound waves of frequencies lower than 6 kHz can be achieved by the electric processing. Since the cabinet 3 is installed integrally with the casing of the video camera 200, a stereophonic sound receiving and recording operation can be performed in a state where the stereo microphone unit 1 is built in the casing of the video camera 200. Consequently it becomes possible to attain a dimensional reduction with incorporation of the stereo microphone unit 1, hence forming the video camera 200 into a further compact structure while enhancing the degree of freedom in entire design and so forth. As described hereinabove, according to the built-in type stereo microphone unit of the present invention, a stereophonic effect with regard to relatively higher-pitched sound is attained by the structure of the cabinet, and a stereophonic effect with regard to relatively lower-pitched sound is attained by the electric constitution inclusive principally of delay circuits and adders. Therefore, sound waves can be received and processed in a stereophonic mode even in a state where the cabinet is installed integrally with the casing of a video camera for example, i.e., the microphone unit is built in the casing. Consequently it becomes possible to realize a dimensional reduction of the microphone unit while enhancing the degree of freedom in design of the video camera or the like, hence further diminishing the entire dimensions thereof.

Claims: A stereo microphone unit for providing a stereo audio signal, comprising: a cabinet (3); two nondirectional microphones (10,20) disposed with a predetermined distance (ℓA) therebetween on the wall of said cabinet (3) for collecting outside sound signal; delay means (50,60) for delaying each output of said microphones by a time period corresponding to said predetermined distance; and subtracter means (90,100) for subtracting the output of said delay means relative to one microphone from the output of the other microphone. A stereo microphone unit according to claim 1, wherein a stereo audio signal is provided only with regard to said outside sound signal of any frequency lower than a predetermined frequency. A stereo microphone unit according to claim 1 or 2, wherein the sound receiving surfaces of said two microphones are directed outward, and the facing directions of said two microphones are opposite to each other. A stereo microphone unit according to claim 2 or 3, wherein said predetermined distance is approximately 30 mm, and said predetermined frequency is approximately 6 kHz. A stereo microphone unit according to claim 1, 2, 3 or 4 further comprising: attenuator means for controlling the output level of said delay means; and equalizer means for compensating the frequency characteristic of the output of said subtractor means. A stereo microphone unit according to any one of the preceding claims wherein each microphone is respectively positioned in the acoustic shadow of the other for sound approaching from the left or right direction. A stereo microphone unit according to any one of the preceding claims wherein said delay circuit comprises a low pass filter. A portable television camera comprising: a housing; a television camera supported in said housing and having an input for the light derived from image fields to be scanned by said camera; a stereo microphone unit according to any one of claims 1 to 7; and recorder means for recording the outputs of said television camera and said microphone means.

Inventors: HAMADA ICHIRO, OHTA KIYOSHI, ONO YOSHIHIRO, HAMADA, ICHIRO, OHTA, KIYOSHI, ONO, YOSHIHIRO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: H04R5
IPC codes: H04S1, H04R5, H04N5