Note: Descriptions are shown in the official language in which they were submitted.
558 31.0Y~lti~
BAC~CGROU~ID OF l'HE: INVENTION
This invention relate!s to a receiving system having
.I preselected directional re~ection characteristic, and more
particularly to a receiving system utilizing a microphone
array having a cardioidal-like rlesponse.
:-
Amateur photographers who have made sound accompanied
home movies with conventional equipment are familiar with the
problem of minimizing camera ~ound pick~up during filming
saquences. Failure to minimi~e camera sound pick-up is
10 evident during projection of film in that the camera noise
will freq`uently mask the sounds whose recording is de~ired
in connection with the film.
One approach to solving this problem is to physically
separate the microphone from the camera, but this requires
15 an assistant to coordinate recording with picture taking.
In many circumstances, this is inappropriats. 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
20 forwardly of the camera in the direction in which photography
takes place, but out of the field of view of the camera. An
.. : ~
inexpensive cardioid microphone so 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 frequen~y
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 100Hz to about 6000Hz
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with a peak occurring around 200Hz just in the region of
maximum hearing perception. While th~ usual inexpensive
cardioid microphone is often adequate for rejecting l~w
f-requency sounds originating from the camera, its spatial
pattern of response is not fixed with respect to frequency
over the relatively wide spectr~n 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, i-t has been found that the camera noise dominates.
While it may be possible to design a special
: ,:
microphone having the capability of rejecting noise from a
`-~ camera over a relatively wide frequency band, such a micro-
phone is likely to be very expensive and sensititve to
mechanical damage by reason of the fragile nature of the
~; elements of the microphone. It is therefore an object of
; the present invention to provide a new and improved recording
~ystem whose rejection characteristics in terms of anqularity
and frequency are determined by the type of signal proces~ing
utilized rather than by mechanical details of the elemRnts of
the microphone.
'''.~ .
SUMMARY OF THE INVENTION
~, .
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~ According to the present invention there is provided
,i
;~ 25 a receiving system having a preselected directional rejection
characteristic for incident stimuli comprising an array of
receiving members, each of which is responsive to incident
time-variable s~imuli for producing a corresponding time
variation output signal, and signal processing means to combine
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the output signals from the receiving members for cau~ing
the system to reject stimuli at ~alues of frequency and angles
of incidence which depend on the spacing of the receiving
members. The signal processing means includes a summing
channel for adding the output signals of a pair of the
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
signal to obtain an integrated 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, while the magnitudes of
the signals can be made equal for that frequency and fbr a
pre-selected angle of incidence by a proper selection of the -
gain applied to each of these signals before they are subtracted,
and by a proper selection of the spacing of tlle receiving
members of the array. Specifically, the relative gain of the
gain controlled signals is selected such that upon subtraction,
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 mer~ers 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.
:
- When the receiving system of the present invention
is incorporated lnto a sound motion picture system, the
signal processing means causes the array of receiving members
to act as a cardioidal microphone at low frequencies, and
- 4
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causes complete rcjection along the axis o~ t'ne cardioid for the frequency
that dominates camera noise.
~ he invention also consists in a sound motion picture system
comprising a motion picture camera and a sound recording system associated
with the camera. Such system includes a linear array of microphones fixed
to the camera and located out of the field of view thereof, the array prefer-
ably, 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 spa~ed that the array preferentially
rejects sound from the camera when it is operational. Specifically, low
frequency sounds originating from a source aligned with the array are
significantly rejected as are sounds originating at the camera at a frequency
in the range within which hearing is most perceptive.
A sound recording system according to the present invention is
thus capable of utilizing relatively simple and inexpensive microphones
because the preselected directional rejection characteristics are derived
entirely from the signal processing means employed and the spacing between
the microphones.
According to the broadest aspect of the present invention, there
is provided a receiving system comprising: a plurality of spaced apart
receiving members, each of which is responsive to incident time-variable
stimuli for producing corresponding time-variable output signals; signal
` processing means for combining the output signals from the rec0iving members
to produce a selected response pattern for the stimuli, the signal process-
ing 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 differ-
ence signal with respect to time to obtain an integrated signal.-
The invention will now be described in greater detail with refer-
ence to the accompanying drawing wherein:
FIG. 1 is a perspective view of a motion picture cc~mera into which
.
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the present invention is incorporated and showing orthogonal, low frequency
cardioidal response characteristics of the sound recording system;
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causes complete rejection along the axis of the cardioid for
the frequency that dominates camera noise.
The invention also consists in a sound motion picture
system comprising a motion picture camera and a sound recording
system associated with the camera. Such system includes a
linear 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 combiining
- l0 the output signals of the microphones which are so spaced
that the array preferentially rejects sound from the camera -
when it is operational. Specifically, low frequency sounds
~- originating from a source aligned with the array are!
- significantly rejected as are sounds originating at the camera
at a frequency in the range within which hearing is most
perceptive.
`~ A sound recording system according to the present
: .
invention is thus capable of utilizing rela~ively simple and
inexpensive microphones because the preselected directional
rejection characteristics are derived entirely from the
~ signal processlng means employed and the spa~ing between the
; microphones.
; BRIEF DESCRIPTION OF THE DRAWING
. - --. . . _
Embodiments of the invention are shown in the
accompanying drawing wherein:
FIG. l is a per~pective view of a motion picture
camera into which the present invention is incorporated and
showing orthogonal, low frequency cardioidal response
` characteristics of the sound recording system;
- 51-
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FI~. 2 is a qualitative showing of a typic~l noi.~e
spectrum associated with a movie camera;
FIG. 3 is a prespectilve view of a linear array of
receiving members showing ~le il~cidence thereon of a plane
wave of arbitrary frequency and making an ar~itrary angle
of incidence with the array;
FIG. 4 is a block diagram of a receiving system
according to the present invention showing details of the ~ :
signal processing means;
FIG. 5 is a polar-plot of the response chara~teristic
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;
FI~. 6 is a composite plot of the respective
lS 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 betwe~n
the pair of microphones whose output is subtracted i8 the same
: 20 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 amplitud~
` of the sum and integrated channels is made zero for ~timuli
: of a predetermined, ~on-zero frequency, incident on and aligned
. :
with the array;
. FIG. 8 is a plot similar to FIG. 7 but showing
~-` 30 the amplitudes of the sum and integra~ing signals for stimuli
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incident on the array at an angle of approximately 30;
FI~. 9 is a plot similar to ~IG. 8 but showing
the situation when the stimuli is incident on the ~rray at
an angle of cibout 60 ~; and
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
four~microphone version o the present invention.
DESCRIPTION OF THE PREF13R~ED EMBODI~ql3NTS
Referring now to FIG~ 1, reference numeral 10
designates a sound motion picture system according tv the
~-rcsent invention comprising a motion picture camera 11 and
a sound recording system 12 associated with the camera.
System 12 comprises a linear array of microphone elements
~not shown), collectively referred to as a microphone and
designated by reference numeral 13, secured in
fixed position to the camera 11 by boom 14, and signal
processing means 15 connected to the camera by cable 16.
Camera 11 includes a conventional housing 17 containing
film, film drive means (not ~hown), and lens assembly lB
through which light from a scene being photographed pa~se~
onto the film contained within the camera housing.
Aligned witll the optical axis Z of the lens
,
system of the camera is a viewfinder sys~em 19 through
which an operator views the scene being filmed~ In addition,
the camera is provided with grip 20 allowing the operator,
with one hand~ to hold the camera and actuate the same by
:- .
~ squeezing trigger switch 21 with one finger, and, with the
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other hand, to steady tlle camera.
The linear array oE microphone elements that
constitute micropllone 13 are aligned along the X-axis
w}~ich is do~nwardly inGlined at a small angle (e.g., 20)
~ 5 to ~eoptical axis z and lies in a plane co~non to the
`~ Z-axis and grip 20. Microphone 13 thus projects forwardly
and downwardly from the camera by reason of boom 14 and is
~ out of the field of view of lens assembly 18.
-~ In operation, an operator grasps grip 20 with
one hand, steadies the camera with the other hand, and views
the scene to be filmed through the viewfinder means l9.
Squeezing the trigger causes the camera and the microphone
to be actuated whereby the scene withln the field of view of
the viewfinder means is photographed, and sound3 from the
scene are synchronously recorded. By reason of the
orientation of boom 14, microphone 13 is positioned to receive
:` :
sound from the scene being photographed. As explained
-
below, microphone 13 has a cardioidal-like response
(multiplied by the response of each element).- The spatial
- 20 characteristics of the response, as a function of ~reqyency,
are determined in accordance with signal processing means 15.
- Essentially, microphone 13 rejects sound incident on the
microphone within a predetermined rejection cone having a
- solid angle that comprehends camera ll as suggested by
chain-lines 22 of FIG. 1. The angularity of the X-axis
relative to th~e Z~axis, and the distance of the microphone
` from the camera are factors that depend upon the solid angle
- of the re~ection cone whose apex coincides with microphone 13,
such solid angle depending upon the operation of signal
processing mPans 15 and selectable within wide li~its to
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` accomodate a given camera.
.
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FIG. 1 shows orthogona] response patterns 23 and
~4, respectively, of the microphone showing the dependency
of the response on the direction of incidence of sound, the
patterns being symmetrical about the X-axis. The patterns
are shown qualitatively, but are typical of microphone 13
- o~er a band of frequencies of interest.
FIG. 2 shows, in a qualitative way, the frequency
distribution of noise associated with a typical movie
camera. It has been found that noise associated with operation
of the camera has very low frequency components, and a
significant peak around 2000Hz, which is within the range
- most perceptible by the human ear. The higher frequençy
components of naise associated with the operation of the
camera decrease significantly about 6000Hz. By reason of
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the operation of signal processing means lS, the response
characteristic of micropllone 13 can be ad~usted to
preferentially reject noise emanating from the c~mera
throughout a relatively wide band of frequencies with
particular emphasis on re~ection around 2000Hz~
In order to explain the manner in which the signal
processing means of the present invention preferentially
modifies the re~ection characteristics of microphone 13,
reference is made to FIG. 3 which sllows the interaction
between plane sound wave 30 and a linear array of four
microphone elements M1, M2, M3 and M4 which, collectively,
constitute microphone 13. The microphone elements are
shown uniformly spaced along the X-axis to facilitate the
analysis that follows, but the spacing need not be uniform.
The intermediate pair of elements M2 and M3 are spaced apart
a distance dl, and the outer pair of elements Ml and M4 are
spaced apart a distance d2. For purposes of simplifying
the analysis, it is assumed that the space between elements
Ml and M2 is the same as the space between elements M3 and M4.
The sinusoidal plane sound wave 30 has a frequency ~, and is
incident on the array of elements. The direction of
propagation of the wave is generally from the positive X-axis
and along the ~-axis which makes an angle ~ with the
X-axis, intersecting the same at the point 31, midway
~5 between elemen~s M2, M3. Because the plane wave varies with
` time, FIG. 3 shows the position of the wave at an instant
in time. The amplitude of the wave at this instant along
the X-axis is shown by the chain line 32 which is defined
~ b~ the intersection of a plane containing the Y-axi~ passing
- 30 through the X-axis and being perpendicular to the plane
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defined by the A - and X-axes. The Y-axis passes through point 31. The
amplitude of wave 32 at any instantl w.ith respect ~o a point on the X-axis,
is a measure of the instantaneous sound ~nplitude at that point.
The distance between corresponding points on the plane wave, as
measured along the ~A~-axis, is related to the distance between these points
as measured along the X-axis by the cosine of d , the angle of incidence
of the plane wave. Designating the wavelength of wave 30 along the ~h~-axis
is ~O , the wavelength xO along the wave in plane 33 defined by the X-Y
axes, is related to ~O as follows:
10 (lj ~ ~ CoS ~
where V is the speed of propagation of the plane wave, and f = ~lr. The
; speed of sound at 20C at sea-level is 344 m/s.
. The period To of the plane wave is as follows:
~ (2) rO = ~ X o Cl~S C~C
- From equation (2), it can be seen that time ~ for point 34 on
~ wave 30, whose projection on the X-axis is the point 31 midway between -~
- elements M2 and M3, to move to point 35, whose projection on the X-axis is
` a distance ~1/2)dl from point 31 where element M3 is located is as follows:
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(3) ~ = ~ cOsc~
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The time ~or the pOil~t 34 on the plane wave to reach
microphone M4 is;
~ (4) ~ = dZ G~cx
- r~ith equations (3) and (4) in mind, it can be
;- 5 seen that an assumption of the analytical form of the wave ?
at point 31 will yield the analytical expression for the
wave at ~he four locations of the microphone elements in
- terms of phase differences with respect to the assumed form
of the wave. If it is assumed that the wave at point 31
- 10 has the form Sin(~- ~d)~ then the wave at each of the four
elements will be given as follows:
~' : ~: .
(5A) At Ml: Stn C~
(5B) At M2: SJn ~-~o f ~ )
(SC) At M3; Srn ~-7~ - 2
- 15 (5D) At M4: S/n ~(~
where Z represents the time required for the wave to move i;: ~ :
.~ through a quarter-wavelength along the A axis (enab1ing ;' .
. t~e expressions that follow to be applicable to either a
- sine or cosine wave of unit amplitude). ~ecause any complex
.~ 20 wave can be synthesized by a Fourier sine or cosine seriesi .
` the analysis that follows is of general applicability even
. though the equations refer to a single sinusoidal wave
having an angular frequency of ~. -~.
~ Reference is now made to FIG. 4 which shows details
of the signal processing means 15. Each of the microphone
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elements ~1 and ~4 is responsive to an incident time-variable
stimuli, such as a sound wave, for producing a corresponding
time-variable output signal which is processed in accordance
with the block diagram shown in FIG. 4. Specifically, means
15 includes, in addition to the microphone elements, summing
channel 40 in the form of an adder for adding the output
signals of the inner pair of microphone elements M2 and M3,
~ and subtractor 41 for subtracting the output signals of
~ the outer pair of microphone elements Ml and M4. The
difference signal produced by subtractor 41 is integrated
in integrating channel 42 which is in the form of an integrator
whose output is termed an integrated signal. Finally, means
- 15 includes combining means 43 for combining the outputs
-~- of the summing channel andthe integrating channel.
` 15 Specifically, combining means 43 includes a gain control
means in each of the channels for setting the gain of one
channel relative to the other to define a gain controlled
-: integrated signal that appears at the output of amplifier 44
which has a gain B, and a gain controlled sum signal which
appears at the output of amplifier 45 which has a gain A.
Combining means 43 also includes means for adding the two
gain controlled signals and is in the form of adder 46.
The output of adder 46, which appears in line 47, constitutes
the output of microphone 13.
: 25 Where the inputs to microphones Ml and ~14 are as
indicated in equation (5) above, the sum signal S appearing
at the output of adder 40 is as follows:
(6) S = [2 c~ ] sin ~(Y ~0)
13 -
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while the difference signal D aE)pearing at the output of
subtractor 41 is as follows: '.
(7~ D = [- 2 S~n ~ ~ c~s~f~ z~)
where the minus sign indicatès a phase inversion with respect
:: 5 to the sum signal.
Integrating the difference siynal in integrator 42
yields an integrated signal I given as follows: :
; (8) I = [_ ZS~C~ S~n ~J~ 0)
`: After the sum and integrated signals are passed
` 10 through amplifiers 45 and 44 respectively, it can be seen
that each of the resultant gain controlled signals has the
~-` same phase allowing arithmetic addition of the amplitudes of
.,. ; . .
the signals to take place. The amplitude of the gain
controlled sum signal ~ ) is given by:
. ~ .
~' 15 (9) A( ~ ) = Z~ ~os~Jdl c~sc~
while the magnitude of the ga.in controlled integrated signal :~
Bt ~,~ ) is given by:
, (10)~ ) = ZB~cos~)~4~)]
~i The output of adder 46 is ~(~,~) and is ~ .
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given by:
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(12) ~ J= B [Z~os Z~c~s~ - ~cos~ ]
.~ where the minus sign takes into account the inversion that
occurs due to subtractor 4l.
From inspection of equation tl2), it can be seen
that the output of adder 46 can be reduced to zero,
: indicating the complete rejection of the incident wave, at
~ any desired frequency or angle oE incidence of the incidént
:~ wave on the array of microphone elements, by suitable
`-; selection of the relative gain A/B of the gain controlled
. lO signals and the spàcing dl, d2 between the microphone
elements. For low frequency incident waves where ~ tends
. to zero, equation (12) reduces to:
':
(12A) ~(O~X) = Z ~ L~ - Z~ Cr9S~]
. . .
From inspection of èquation (l2A), it can be
seen that the output of adder 46 will be zero when the
.: expression contained wi~hin the square brackets in this
equation is equal to zero. For a given angle of incidence
CCo, the relative gain A/B of the gain controlled signals
to achieve complete rejection at low frequencies is given
as follows:
. .
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; tl2B) ~ = z ~ CoS ~0 ~ '
', '~
. Inserting the relative gain from equation ~12B)
- into equation (12) provides the general expression for the
output of adder 46 that insures rejection of low frequency
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waves incident on tlle array of microphone elements at an
angle ~O ;
~ ) = ZB(z v) [c~so(; ~o~(z~ c~ o50~ Sin(~cOsP~)]
To re~ect low frequency sounds incident on the
array at ~O= o , equation (13) reduces to:
`:
; (13A) ~(o,c~J- z ~ d~ cos~)
Inspection of equation (13A) reveals the low
frequency response of the composite microphone is cardioidal,
with the axis of symmetry lying along the axis of the array
(i.e., along the X-axis), and arises solely as a consequence
` of the processing of the signals from the elements of the
array. The higher frequency response of the composite `
-- microphone for stimuli aligned with the array is obtained
from equation (13) with cyc o
- 15 (13B) a(c),o) C ZB(z~)[~os(~)-- ~i) ]
The spacing of ~le microphone elements from which
- the sum signal is derived, dl can also be selected to insure
rejection of a wave at any frequency ~ having an angle of
`71 incidence C~l . The value of dl is obtained by setting the
expression in the square brackets of eqUation (13) equal to
zero and solving for dl. This process yields the following:
;~i (14) d~= Z~- afi~ cos r ~7)]
~" L (Z,,)
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If C~ = ~ = O, which is to say that
rejection is achieved for waves of frequency approaching
the linear array along the X axi,s from the positive
direction, equation (14) reduces to the following;
~ 5 (14~) d, = Zv o~c ~05 [~ ]
FIG. 5, which is a plot of equation (13A) r is the
response of an array of microphone elements to low frequency
waves as a function o theirangles of incidence on the array
wllen the outputs of the ~lements are processed in accordance
` 10 with ~G. 4. Thus, it is seen that, a linear array of
omni-directional receiving members in the form of micxophone
elements is converted into a composite microphone having
a cardioidal re~ponse by rea~on of the signal processing
that is carried out by means 15. If the elements themselves
~, 15 have cardioidal responses, the signal processing creates
a higher order cardioidal response of the composite microphone.
The response of an array of microphone elements
' to higher frequency waves incident on the array in alignment
~` tnerewith ~i.e., C~ =o) is sh~n in FIG. 6 which is based
~; 20 on equation (13B). Curve 50 represents the variation of
co~ ( Z~d~ ) with the parameter ( ~Zd~ ~ for dl=d2 and
-`~i is the gain controlled sum signal at the output of amplifier
~ 45; and curve 51 represents the variation of 5; -(- ~
)d~ ~
-~ with the parameter ( ~~ ) and is the gain controlled
integrated signal at the output of amplifier 43. It should
be noted that when dl-d2, elements Ml and M2 merge and
elements M3 and ~4 merge creating a composite microphone of
~ - 17 -
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two, rather than four elements. Such a microphone has good
low frequency re~ection characteristics, but the ability
of the microphone to re~ect higher frequency waves decreases
markedly with frequency as indicated by curve 52 which is
the difference between curves 50 and 51 and represents
equation (13B) whicll is the output of adder 47.
- For the condition that dl~l/2 d2, elements Ml-M4
are equally spaced with the result that the microphone has
four elements. Curve 53 represents cos( ~kJ~ ) for the
condition indicated, and it is apparent that curve 53
approximates curve 51 rather closely. The difference between
curves 51 and 53 is curve 54; and it is obvious that the
use of four elements with dl=1/2 d2 provides significantly
improved rejection characteristics as compared with a two
-
, element microphone.
- The frequency scale in FIG. 6 has been selected
by choosing d2=1" so that both curves 51 and 53 are zero
at ~d~ = 13~548EIz.~ A further improvement in rejection
of waves aligned with the array and in the band up to
: 20 6000Hz, which i5 the noise band of a camera, is pos~ible by
suitable choice of the ratio of dl to d2. Presently, it is
preferred to select dl such that the amplitude of the gain
controlled sum signal (i.e., the frequency of the cosine
curve) equals the gain controlled integrated signal (i.e.,
~ 25 the sinc curve) at a frequency of about 59~ of the gain
-1 controlled integrated signal. For d2~1"~, the e~uality occur-
at 8000Hz; and from equation (14A), dl_0.55".
Curve 60 in FIG. 7 represents the gain controlled
sum signal under these conditions, and curve 61 represents
the g~in controiled integrated signal which equal each other
:
- 18
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at 800~Hz. At frequencies less than this, the rejection
is extremely good as shown by curve 62 which is the difference
between curves 60 and 61 and represents the output 47 of adder 46.
The curves shown in FIG. 7 are applicable to input
waves aligned with the ar~ay. FIGS. 8 and 9 show the rej~ction
at angles of incidence of 30 and 60 respectively. In FIG. 8,
curve 63 represents the gain controlled sum signal for
= 30, and curve 64 represents the gain controlled
integrated signal. Curve 65 repr~sents the difference between
curves 63 and 64. In FIG. 9, curve 66 represents the gain
controlled sum signal for ~ =60, and curve 67 represents
the gain controlled integrated signal. Curve 68 represents
the difference betwecn curves 66 and 67. It is evident,
that the selcction of dl=0.55 inches and d2=1 inch provides
15 ~ extremely good rejection at C~ = 0 and through CX = 30
(which corresponds to a rejection conc whos~ apical angle
is 60).
Other rejection patterns can be created by suitable
selection of relative gain between the sum and integrated
channels, and the relative distances between the sum and
difference pairs of elements. Furthermore, other analytical
solutions are available when the spacing of the elements is
not uniform.
While the above description refers to sound waves
and microphones, it is clear that the present invention is
applicable to other stimuli to which elements respond by
producing output signals. For example, the invention is
applicable to radio waves and receiving antennae.
.
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81~
FIG. 10 is a circuit diayram for siynal
processing means according to the present invention for
~tfectinc3 a desired signal rejection characteri~tic.
Me~ns 15~ includes four microphonc ~lements Ml-M4 and a
pre-amplifier 70 associated with each element. The pre-
amplified output signals from elements M2 and M3 are added
in analog adder 71 to develop the sum signal. The pre-
amplified signals from~elements Ml and M4 are subtracted
and integrated in ~orton difference integrator 72, the
output of which is the integrated signal. The sum and
integrated signal are added in analog adder 73 to provide
the output signal.
It is believed that the advantages and improved
i results furnished by the method and apparatus of the present
- 15 invention are apparent from the foregoing description of
the several embodiments of the invention. Various changes
; and modifications may be made without departlng from the
spirit and scope of the invention as sougllt to be defined
in the claims that follow.
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