Microphone actuation control system suitable for teleconference systems

A microphone and loudspeaker arrangement for use in a teleconference system, wherein a plurality of microphones are held in a fixed relationship to a loudspeaker. The microphones are independently gated ON in response to (1) speech picked up by the microphone, (2) a loudspeaker signal driving the loudspeaker and (3) an electrical signal related to the microphone signals of the other associated microphones. A noise adapting threshold circuit generates a voltage level representative of background noise which is compared with the microphone signal of a respective microphone for determining whether the microphone is receiving speech. A decisional circuitry monitors the microphone signal of the associated microphone with respect to a MAX bus which carries microphone signals representative of the level of microphone signals at the other microphones. The decisional circuitry generates a signal indicating that the associated microphone is the first loudest microphone signal.

RELATED PATENT APPLICATIONS 
The following application relates to U.S. patent application Ser. No. 
06/597,734, filed Apr. 6, 1984, entitled "Teleconference System" by 
Stephen D. Julstrom, which application is owned by the same entity as the 
present application. 
BACKGROUND OF THE INVENTION 
The present invention relates to an automatic microphone control suited for 
use in sound reinforcement, recording, broadcast, teleconference and other 
applications. 
Because of the number of participants involved or the number of locations 
needing sound pickup, multiple microphones are often used in applications 
such as churches, courtrooms, broadcasting studios, legislative chambers, 
and conference rooms, and in particular, in teleconferencing. The outputs 
of these microphones are usually combined in a mixer to feed a sound 
reinforcement system, a recording device, or a transmission link to a 
remote location. If a conventional mixer is used with multiple 
microphones, the room noise and reverberation pickup is increased as 
compared to a single microphone; also, the tendency for howlback is 
increased, even though typically only one or two microphones are receiving 
useful acoustic input (speech) at any given time. This is discussed 
extensively in "Direction-Sensitive Gating: A New Approach to Automatic 
Mixing" by Stephen Julstrom and Thomas Tichy, JAES, Vol. 32, Nos. 7/8, 
1984, July/August. 
Automatic mixers have more recently been employed to "gate" ON (pass to the 
mixer output) only signals from microphones receiving useful acoustic 
input. The relative effectiveness of these mixers is primarily a function 
of the means used to decide when a microphone should be gated ON. A 
microphone should gate ON quickly and independently in response to valid 
speech input over a wide dynamic range. Yet it should not respond to 
background room noise, nor to talkers who would be better picked up by 
another microphone. Additionally, for proper operation of many 
teleconference systems, including the system of the aforementioned related 
application, the room loudspeaker providing audio from the remote location 
should not trigger microphone gating. 
A gating method dependent on a representation of the microphone output 
level exceeding a fixed threshold level satisfies none of these criteria. 
Various prior art references suggest different gating methods. 
For example, in Dugan (U.S. Pat. No. 3,814,856), the threshold level for 
each microphone's gating tracks an estimate of the background room noise 
made from a distant sample taken from a separate noise-sampling microphone 
or from the average of all microphones in the system. This is done to 
improve the gating sensitivity while avoiding response to room noise. 
In Breeden (U.S. Pat. No. 3,751,602), a simple microphone is, in essence, 
gated for use in a speakerphone (a simple teleconference system). Here, 
the noise reference is taken from the single microphone with its level 
representation processed through a very slow attack, rapid decay circuit. 
Additional circuitry inhibits microphone gating for loudspeaker sound in 
most room acoustic environments. 
In Maston (U.S. Pat. No. 3,755,625), one and only one of a plurality of 
microphones is gated ON at any time. To gate ON (and thus gate OFF the 
already ON microphone), a microphone's level must exceed a fixed threshold 
and exceed the level of the already ON microphone by a preselected amount, 
such as 3 dB. 
In Kahn (U.S. Pat. No. 4,099,025), to prevent gating a plurality of 
microphones for a single source, during the time when a microphone's level 
exceeds a threshold, triggering of all other microphones is prevented for 
the duration plus a short additional time roughly corresponding to the 
transit time for the sound to travel to the farthest microphone in the 
system. 
In Schrader (U.S. Pat. No. 4,090,032), a preselectable, fixed threshold is 
overridden as soon as at least one microphone's level exceeds it and is 
gated ON. The threshold then varies between a high maximum level and 
approximately the level of the gated ON microphone with the highest level. 
Gating ON of more than a few microphones simultaneously, even for multiple 
sound sources, is strongly inhibited. 
In Anderson et al. (U.S. Pat. No. 4,489,442, owned by the same entity as 
the present application), each "microphone" actually consists of an array 
of typically two unidirectional microphones mounted back-to-back in a 
common housing whose output levels are compared. When the level of the 
"front" microphone exceeds the level of the "rear" microphone by a 
predetermined amount, typically 9.54 dB (indicating the sound source is 
within an "acceptance angle"), gating is triggered for the front 
microphone's signal. 
This Anderson arrangement (which also forms part of the preferred 
embodiment of the related Julstrom application previously referenced), has 
an effective gating threshold which inherently tracks at about 5 dB above 
the room noise level at the microphone's location. The Anderson 
arrangement results in direction-sensitive gating which limits the number 
of microphones which gate ON for individual sound sources while not 
causing the gating of any microphone to inhibit the desired gating of any 
other microphone for other sound sources. Also, the Anderson arrangement 
allows positioning a teleconference system loudspeaker in such a way that 
it will not trigger gating of any microphone and will not significantly 
inhibit desired gating for local talkers. 
However, the operating principle of Anderson requires some care in 
microphone and loudspeaker placement. Anderson also allows a single sound 
source to trigger gating in a plurality of microphones with overlapping 
acceptance angles. Anderson requires typically two high-quality matched 
transducer elements for each "microphone" even though only one of the pair 
is ever heard. Most significantly, Anderson can have proper gating 
inhibited by acoustically reflective objects close to the rear of the 
microphone or by placement of the microphone too far away from the sound 
source in relation to the reverberant field of the room, thus preventing 
the microphone from accurately assessing the direction of the sound 
source. 
The sound which a microphone "hears" in a room can be simply described as 
consisting of two parts: a direct sound which decreases in level 6 dB each 
time the distance from the source is doubled; and a reverberant field, 
coming from all directions, which stays substantially uniform in level 
throughout the room as it decays away. 
The direction-sensitive gating technique works well unless the microphone 
is so far from the sound source that the reverberant field dominates in 
the microphone's sound pickup. In larger rooms, this will not occur until 
the microphone is five feet or more away from a talker. However, at this 
distance, its pickup would be hollow, "barrelly" and perhaps 
unintelligible. In smaller rooms, such as offices and many conference 
rooms, the reverberant field may dominate at distances of two feet or 
less, preventing proper gating using the direction-sensitive microphone 
technique at convenient talker-to-microphone distances. However, in 
contrast to the larger rooms, in many of these smaller rooms, the sound 
pickup quality, even if predominantly reverberant, is still intelligible 
and subjectively acceptable due to the quick reverberant decay time. The 
microphones would be usable if they gated properly. 
None of the prior art disclosed in the above cited patents fully addresses 
the goal of attaining maximum gating sensitivity in the presence of 
varying background room noise, preventing a plurality of microphones from 
gating ON for a single talker, allowing with little mutual inhibition a 
plurality of talkers to simultaneously gate ON a plurality of microphones; 
and doing all this even when operating in a near totally reverberant 
(i.e., small room) acoustic environment. Additionally, sound from a 
teleconferencing system loudspeaker must not gate microphones ON, yet 
desired gating for simultaneous local speech should not be significantly 
inhibited, again in a very reverberant environment. 
It is therefore an object of the present invention to provide an automatic 
microphone gating method which maximizes sensitivity to speech while 
avoiding sensitivity to varying background room noises. 
It is yet another object of the present invention to allow only the single 
most appropriate microphone in a system to gate ON for a single talker. 
It is another object of the present invention to allow a plurality of 
talkers to simultaneously gate ON a plurality of microphones, with minimal 
mutual inhibition of gating. 
It is another object of the present invention to prevent teleconference 
system loudspeaker sound from gating microphones ON, with minimal 
inhibition of desired microphone gating for locally originating speech. 
It is another object of the present invention to enable all the other 
objects to be met even in near totally reverberant, smaller room acoustic 
environments. 
It is another object of the present invention to allow the creation of 
"dead zones" in a room where sound sources do not trigger any microphone 
gating. 
It is another object of the present invention to provide a variation 
whereby microphone gating information can be used to control other 
functions, such as automatic video camera switching. 
It is another object of the present invention to link a gating method 
described herein with an automatic gain adjusting means to maintain 
constant reverberant field pickup as the number of widely spaced 
microphones gated ON varies above zero. 
It is another object of the present invention to provide a variation 
whereby an automatic gain adjusting means maintains constant reverberant 
field pickup as the number of very closely spaced directional microphones 
gated ON varies above zero. 
It is another object of the present invention to use a gating method 
described herein in the teleconference system of the previously referenced 
related Julstrom application yielding the benefits described therein. 
It is still another object of the present invention to employ such a 
teleconference system in a combined loudspeaker-microphone arrangement 
which optimally exploits the characteristics of the gating method, is easy 
and foolproof to set up, provides improved sound pickup and production 
through optimized acoustical and electrical design, works in almost any 
acoustical environment, and is easily expandable. 
SUMMARY OF THE INVENTION 
These and other objects are achieved in a preferred embodiment of the 
invention which uses processing circuitry which responds to accurate, 
rapidly varying DC level representations of the frequency equalized output 
of each microphone and, in a teleconference embodiment uses the electrical 
drive signal to the loudspeaker, similarly equalized. 
Each microphone circuit block of the preferred embodiment further processes 
its DC level representation through a very slow attach, immediate decay 
circuitry, yielding a noise adaptive threshold (NAT). This threshold 
adjusts to the background noise level present during brief pauses in 
speech, but does not particularly respond to speech waveforms. To satisfy 
the NAT criterion and potentially initiate gating, a microphone's DC level 
representation must exceed its NAT by an amount, established at 6 dB, just 
sufficient to guard against false triggering due to random background 
noise fluctuations. 
This enables a high gating sensitivity (low threshold) to speech, even in 
the presence of room noise, which is further improved by the use of 
directional microphones. In the teleconference system of the preferred 
embodiment, the microphones reject room noise pickup typically 5 to 7 dB 
as compared to direct speech pickup. Thus, direct speech will typically 
gate ON a microphone at about the measured room noise level (as measured 
by a standard omnidirectional noise measurement device). 
The NAT gating criterion also works well for microphones located in the 
predominantly reverberant field of a talker, at least over the range of 
reverberation buildup and decay rates found in acoustically poor, small to 
medium-sized conference rooms, albeit without the extra sensitivity boost 
afforded by directional microphones. (The microphones would not be used in 
the predominantly reverberant field of very large rooms with very long 
rverberation buildup and decay rates which might cause gating problems, 
since the sound pickup quality of even a directional microphone used at 
the implied distance from the source in this environment would be 
unusable). 
To prevent a plurality of microphones from gating ON for a single talker, a 
second criterion (MAX bus) is required to be satisfied in addition to the 
NAT criterion before gating of a microphone is triggered. The MAX bus is a 
single interconnection between the system's individual microphone circuit 
blocks which maintains a varying DC level equal to the maximum of the 
varying DC level representation of each of the gated ON microphones and 
the level representations reduced by a fixed amount, established at 6 dB, 
of each of the gated OFF microphones. To satisfy the MAX bus criterion, a 
microphone's circuit block must, at least momentarily, be "holding up" 
(providing the maximum voltage to) the MAX bus. 
Only when the MAX bus and NAT criteria are simutaneously satisfied for a 
microphone will the microphone be gated ON. This simultaneously results in 
a trigger signal which is extended by a retriggerable one-shot for a "hold 
time", established at 0.4 seconds, sufficient to bridge gaps in triggering 
and pauses between words. A downstream circuit controls the attack and 
decay rates of the audio signal switching to improve the subjective effect 
of the gating. These trigger extending and switching rate controlling 
circuits are substantially identical to those disclosed in the previously 
referenced patent of Anderson et al. 
While not necessary in many instances, in the preferred embodiment all 
microphones are identical and operated at the same relative gains, 
differing only in their orientation and positioning relative to the 
various sound sources. A talker's speech reaches the microphone closest to 
him (the one best suited to picking up his voice) before any other 
microphone and gates it ON. (If the microphones were not identical, were 
operated at differing relative gains, or were positioned either poorly or 
very close together, their relative levels would become more important in 
addition to the relative arrival times, but even then a microphone 
wall-suited to picking up the talker's voice would gate ON.) 
The gated ON microphone immediately gains a 6 dB advantage over the gated 
OFF microphones on the MAX bus, effectively preventing other microphones 
from gating ON for that talker. The 6 dB advantage, in combination with 
the filter time constants and characteristics of the DC level 
representation circuits, prevents secondary microphone gating (for that 
talker) even for impulsive sounds or sounds that terminate abruptly (later 
at more distant microphones) and for reverberant decays of sounds, which 
reach all microphones at approximately equal levels. The 6 dB advantage to 
the ON microphone insures that the reverberant sound will not trigger 
gating of an OFF microphone. The filter time constants, established at 11 
milliseconds, are barely slow enough to reliably prevent abruptly 
terminating sounds from triggering more distant microphones. 
Thus, a single talker gates ON only one microphone, but a plurality of 
talkers speaking normally can reliably gate ON a plurality of microphones, 
if that is what is required to optimally pick up the talkers' voices. The 
use of precision full wave rectification proceeded by attenuation of low 
frequencies in obtaining the DC level representations of the microphone 
outputs allows the rapid 11 millisecond filter time constants. Normal 
speech patterns have frequent peaks, dips, dropouts, and pauses which are 
accurately represented in these rapidly varying DC levels. Since a 
plurality of talkers cannot synchronize these variations, there are 
frequent opportunities for each talker's closest microphone's circuit to 
momentarily "hold up" the MAX bus, resulting in a gating trigger which is 
extended as described above. The rapid DC level variations from each voice 
are normally much greater than the differences in average level between 
individual voices. This allows a softer talker microphone circuit to hold 
up the MAX bus to gate ON his microphone even though a louder talker is 
simultaneously talking ON another microphone. This is not significantly 
altered by the 6 dB advantage given already ON microphones on the MAX bus, 
which is small in proportion to the microphone level variations. The 6 dB 
advantage does mean, however, that a talker will tend to keep gated ON an 
already ON microphone if he is only slightly closer to a gated OFF 
microphone. 
To prevent sound from a teleconference loudspeaker from gating ON a 
microphone, a "speaker inhibit signal" is generated. The speaker inhibit 
signal is a rapidly varying DC level representation (substantially 
identical in character to those already discussed) of the electrical drive 
signal to the loudspeaker. This is scaled to represent approximately the 
worst case (highest) DC level representation expected from the microphones 
due to loudspeaker sound. 
A speaker inhibit signal such as this could be used in a direct microphone 
level comparison or applied to the MAX bus to prevent microphone gating 
from direct loudspeaker to microphone sound. This would have minimal 
effect on desired microphone gating for simultaneous local speech 
(necessary to interrupt the distant talker) due to the rapid time 
constants involved, as just discussed in connection with multiple 
microphone gating. However, in most practical rooms, sufficient energy 
would be available in the reverberant decay of the loudspeaker sound to 
trigger microphone gating after the speaker inhibit signal was gone, if 
applied in this manner. The decay time of the speaker inhibit signal would 
need to be lengthened to several tenths of a second or more (conceivably 
much more to allow for particularly poor rooms) to prevent false 
microphone gating. This would remove the rapid level variation from the 
signal and make interruption of an incoming talker by gating ON a 
microphone very difficult. 
Instead, the speaker inhibit signal, with its rapid level variations, is 
applied in a specific manner to the NAT of each microphone. Each 
microphone's NAT can be "held up" (with the same meaning as used in 
connection with the MAX bus) by either its own action or the speaker 
inhibit signal, whichever is greater. The speaker inhibit signal is scaled 
appropriately to prevent microphone gating due to the worst case 
loudspeaker to microphone coupling, and rapid time constants are 
maintained to minimize inhibition of desired microphone gating for 
excellent interruptability. 
The NAT level is not, however, just the maximum of the speaker inhibit and 
original NAT levels. When a speaker inhibit signal decays down, the 
modified NAT level is not what it would have been before the application 
of the speaker inhibit signal. The modified NAT follows the decaying 
speaker inhibit level until it reaches the level representation of the 
microphone output, and from there follows the latter level in normal NAT 
action. In the absence of local speech, the microphone acoustic input 
immediately following the decaying away of the speaker inhibit signal is 
the loudspeaker sound's reverberant decay. This does not trigger 
microphone gating since the 6 dB microphone level-to-NAT level criterion 
is not satisfied, even for uneven reverberant decays. By inherently 
tracking the actual reverberant decay of a room, the speaker inhibit 
function done in this manner precludes the need for designed-in-safety 
margins based on the worst and most unpredictable room acoustics to be 
encountered, and enables a sensitive local interruption capability to be 
maintained throughout the range of usable room acoustics. The speaker 
inhibit function may be beneficially used with a plurality of microphones 
in conjunction with the NAT and MAX bus, or with a single microphone in 
conjunction with the NAT without the MAX bus circuitry, as described 
hereinafter. 
While the microphone gating scheme disclosed herein may be used with a 
variety of additional circuitry to complete a teleconference system, its 
characteristics are most advantageously exploited when used as part of a 
system such as disclosed in the previously referenced related Julstrom 
application. This enables the benefits of that system, including total 
system gain and feedback control with sensitive, fast, 
conversationally-oriented send/receive direction switching, to be achieved 
in smaller, acoustically poorer rooms and with less restrictive setup 
requirements. 
In the preferred embodiment disclosed herein, a small upward-facing 
loudspeaker is combined in a common low-profile housing, or module, with 
three outward-facing surface-mounted directional microphones evenly spaced 
around the circumference. When used in this manner, the acoustical design 
can be optimized to yield smooth, wide-range speech pickup and 
reproduction without the frequency response aberrations commonly 
encountered when acoustic transducers are used in the vicinity of 
reflective surfaces. The loudspeaker distributes its sound uniformly to 
all conference participants around the module. In combination with the 
disclosed gating method, the microphones pick up speech substantially 
uniformly around the module with a significant reduction in room noise and 
reverberation pickup as compared to a single omnidirectional microphone. 
In addition, the directional microhpones are placed around the loudspeaker 
such as to maximally reject the direct loudspeaker sound. Even though 
allowance has to be made for strongly reflected loudspeaker sound from 
closely placed objects, this still allows some reduction in the speaker 
inhibit signal level and the feedback controlling suppression setting 
(described in the related Julstrom application) as compared to what would 
otherwise be required with such close transducer spacing. 
The loudspeaker-microphone modules can be used singly or "daisy-chained" to 
give better acoustic coverage of longer tables or a plurality of tables. 
Interconnection between modules is made through a single multiconductor 
cable which, among other things, links the gated microphone outputs, the 
MAX bus, and the loudspeaker power amplifier drive signals. The single 
microphone closest to the talker still gates ON for his speech. 
At the user's discretion, a module may be "muted", which prevents the audio 
signals of its microphones from gating ON, but allows its circuitry to act 
normally in MAX bus interactions, as if the microphones were gating. This 
creates a "dead zone" around the muted module where speech and other 
unwanted sounds (paper rustling, etc.) cannot triggeer microphone gating 
and, therefore, cannot interrupt a teleconference. This muting action can, 
of course, be applied to single microphones also. 
As a variation, applicable to teleconferencing or more general use, instead 
of spacing microphones apart, a plurality of directional microphones may 
be arrayed very close to each other such that, over a majority of the 
audio frequency range, sound waves from various directions arrive at the 
microphones essentially in the same phase. Under this condition, when 
various combinations of microphones in the array gate ON, new polar 
directional patterns are formed. In essence, the array forms a single 
microphone whose polar directional pattern and orientation automatically 
adjusts to optimize the pickup of the talkers around it.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, an embodiment of a teleconference system is shown. The 
system includes a microphone/loudspeaker until 11 approximately one foot 
in diameter which is positioned in the center of a conference table 13. 
The microphone/loudspeaker unit 11 is connected to a control unit 15 of 
the system by a cable 35 which may pass through holes drilled in the table 
or may rest on the table top. 
Control unit 15 is interconnected to a telephone line wall jack 19 by means 
of conventional telephone wire 31 for conventional "two-wire" connections. 
A telephone 17 is also connected to control unit 15 by telephone wire 33. 
The user is able to automatically connect either telephone 17 or 
microphone/loudspeaker unit 11 and associated circuitry in control unit 15 
to telephone line wall jack 19. 
Although shown connected to a two-wire link (combined send/receive path), 
the system may also be used with a four-wire link (separate send and 
receive paths) as described in the cited Julstrom application. The 
four-wire link may be hard-wired or may be a radio or satellite link, 
which may include time delay. The radio link may be that of a mobile 
telephone system, although a more appropriate physical arrangement of the 
microphone(s) and loudspeaker than that to be described hereinafter would 
be used. 
As shown in FIGS. 2 and 3, microphone/loudspeaker unit 11 includes three 
outwardly facing unidirectional (cardioid) microphones 21, 23 and 25 which 
are equally spaced about the circumference of a base 27 of unit 11. The 
base 27 is of a flattened, generally conical shape as shown. A loudspeaker 
29 is positioned at the acoustic null of the three microphones 21-25 and 
faces upwardly in direction being flushly mounted in base 27, as shown. 
Base 27 preferably rests on the tabletop of conference table 13. 
With such a mounting, microphones 21, 23 and 25 operate very near the 
surface with minimal acoustical interference from the base 27. Also, the 
loudspeaker 29 operates essentially as if flush-mounted in a large, flat 
surface. As is described in the literature, both microphones and 
loudspeaker may then have smooth, uniform frequency responses throughout 
the voice range without the usual deleterious effects of table surface 
reflections. In addition, the directivity indices of both microphones and 
loudspeaker are increased by an amount approaching 3.0 dB (as compared to 
free space), increasing clarity of sound pickup and production. Horizontal 
dispersion of the loudspeaker sound is achieved primarily through its 
small size (approximately 2 inch diameter cone), minimizing "beaming". 
Each microphone 21-25 reduces undesired sound pickup (room noise, 
reverberation and loudspeaker sound) by being maintained in a normally OFF 
condition until needed. A microphone is automatically gated ON only when 
it is the closest microphone to a speech-like sound source other than the 
loudspeaker. A microphone will not gate ON for background room noise or 
for loudspeaker sound. 
In the absence of local speech, the microphones are gated OFF and totally 
break the acoustic coupling path from loudspeaker to microphone. No echo 
of loudspeaker sound will be returned to the far end of the communication 
link. The microphones will turn ON quickly and reliably in response to 
speech, without chopping syllables or words. Through their directionality 
in conjunction with the acoustic arrangement of speaker and microphones as 
previously described, the microphones minimize acoustic coupling from the 
loudspeaker and pick up of room noise and reverberation while gated ON. 
Several units 11 may be used along the conference table 13. Two or more 
units 11 may be electrically connected in tandem, as discussed above and 
further described hereinafter. 
Referring to FIG. 4, microphone/loudspeaker unit 11 houses mike/speaker 
circuitry 41, while control unit 15 houses control circuitry 43. 
Mike/speaker circuitry 41 receives signals from microphones 21-25 and 
responsively generates gated and non-gated microphones signals along 
respective conductors 45, 47. The microphone signals are received by 
control circuitry 43 for generating a send signal for transmission to wall 
jack unit 19. The audio information received from wall jack 19 is utilized 
by circuitry 43 for generation of a speaker signal along a conductor 49. 
The speaker signal on conductor 49 is received by mike/speaker circuitry 
41 for responsively driving loudspeaker 29. 
Control circuitry 43 may be constructed utilizing the circuitry of the 
preferred embodiment described in the cited related application. 
Specifically, substitution may be made in FIG. 3A of the related 
application for its microphones 11, microphone interface/gating circuitry 
353, power amp 321, and loudspeaker 13 by the present application's 
microphone/loudspeaker unit 11. The gated microphone signal may be applied 
to the related application's conductor 358 and the non-gated signal to its 
conductor 1003. The speaker signal may be taken from the output of the 
related application's mixer/limiter stage 334. As will suggest itself, 
other types of control circuitry can be used as circuitry 43 in order to 
transmit microphone signals and receive loudspeaker signals along a 
telephone line or other telecommunication link. 
Control circuitry 43, as described in the preferred embodiment of the 
subject matter of the cited Julstrom application provides suppression 
(attenuation) of loudspeaker level when local speech interrupts received 
speech by gating ON a microphone, but only to the degree necessary to 
reliably maintain feedback stability. When local speech is interrupted by 
the far end, the outgoing microphone signal will be suppressed and the 
loudspeaker heard at normal level. Circuitry of the cited application also 
determines necessary send/receive direction switching for suppression to 
occur in an unobtrusive, conversationally oriented manner. When both ends 
talk simultaneously, priority is given to the interrupting party, 
maintaining natural interaction. Either end can always "get through" 
without yelling. 
The combined gated microphone signal from all gated ON microphones is fed 
along conductor 45 to the control circuitry 43 of unit 15. The gated 
microphone signal may be passed, for example, through a current control 
amplifier (as in the cited application) prior to entry into a conventional 
hybrid circuit for placing the microphone signal onto the telephone line 
via jack 19. As described in the cited Julstrom application, the control 
current input to the current controlled amplifier is governed by a 
send/receive direction decision. Potential feedback loop gain 
instabilities are controlled by inserting suppression in the send path 
during the receive mode and in the receive path during the send mode. 
A number of circuits presently on the market also may be utilized as 
control circuitry 43 in order to transmit the gated microphone signal to 
telephone jack 19 for transmission to the far end of the telecommunication 
link and to extract the receive signal from the telephone jack 19 for 
transmission to the loudspeaker 29. The mike/speaker circuitry 41 utilizes 
a different gating structure than that disclosed in the cited Julstrom 
application. This will become evident as circuitry 41 is described 
hereinafter. 
Referring to FIG. 5, mike/speaker circuitry 41 includes gating circuitry 50 
comprised of three like mike gating circuits 51, 53, 55, associated with a 
respective microphone 21, 23, 25. A conventional power amplifier 59 
responds to the speaker signal from control circuitry 43 appearing on 
conductor 49 and generates a similar signal on a conductor 58 of 
sufficient power level to drive loudspeaker 29. An inhibit signal 
generates 57 receives the loudspeaker drive signal along conductor 58 and 
responsively generates an inhibit signal to each gating circuit 51-55. 
Each gating circuit 51-55 is alike in structure and one will be described 
with reference to FIGS. 6 through 12. A MAX bus conductor 56 interconnects 
the mike gating circuits, but need not connect to control circuitry 43, as 
described hereinafter with reference to FIGS. 6 and 11. 
Referring to FIG. 6, microphone 21 is electrically connected to a 
preamplifier/interface circuit 61. After preamplification, the microphone 
signal is output as a nongated microphone signal along conductor 47 and 
gated through an opto coupler switch 69 for output as a gated microphone 
signal along conductor 45. 
The remaining circuitry of FIG. 6 controls the gating of opto coupler 
switch 69 by generation of an opto coupler control signal along a 
conductor 71 to the opto coupler switch. The signal on conductor 71 is 
developed in accordance with (1) the preamplified microphone signal of its 
associated microphone (appearing on a conductor 73), (2) a signal related 
to the other microphones (appearing on MAX bus 56) (3) the loudspeaker 
drive signal (appearing on conductor 58) and (4) a mute logic signal 
(appearing on conductor 75). 
The microphone signal on conductor 73 enters a trim calibration circuit 81. 
Circuit 81 is utilized to calibrate its associated mike gating circuit 51 
(FIG. 5) so that each of the three mike/gating circuits 51-55 functions 
identically in its comparison analysis, as will be understood. 
The microphone signal is then fed to a frequency equalization/rectification 
circuit 83 which frequency equalizes the audio signal. Low frequencies, 
and to a lesser extent high frequencies, are reduced in level relative to 
mid frequencies. Circuitry 83 also serves to precision full wave rectify 
the audio signal and filter the resultant. The output of circuitry 83 is a 
varying DC voltage level signal which carries information of the amplitude 
and time of occurrence of speech as well as noise in the room as picked up 
by the associated microphone 21. 
The output of circuitry 83 is fed to a noise adapting threshold circuit 85 
which generates a threshold voltage level representative of room noise in 
the vicinity of the microphone 21. Circuitry 85 generates the threshold 
voltage level by effectively following the DC microphone signal using a 
very slow attack and immediate decay following. As the DC microphone level 
signal increases, a capacitor is slowly charged over a long RC time 
constant, and then as the DC microphone level signal is removed, the 
capacitor is quickly discharged at the same rate as the DC microphone 
level signal decrease. Because of the patterns of ordinary speech, the 
resulting voltage appearing on the capacitor is representative of noise in 
the room The noise adaptive threshold voltage will adapt to steady 
background room noise which does not go below a certain level. Normal 
speech will not significantly charge the capacitor, which will continually 
discharge to the background noise level during even very brief pauses in 
speech. 
The DC microphone output of circuitry 83 is also fed to an attenuator 
circuit 87 where the DC microphone signal is attenuated by 6 dB (a factor 
of 2). The output of attenuator circuit 87 and the output of noise 
threshold circuit 85 are fed to a voltage comparator 89. Comparator 89 
generates an output signal indicating when the rapidly fluctuating speech 
exceeds by 6 dB the threshold level representative of continuous noise in 
the room. Thus, the output of comparator 89 represents an independent 
decision with respect to one microphone that speech is occurring. 
As understood, each microphone gating circuit 51, 53, 55 (FIG. 5) will make 
a similar decision at its respective comparator 89 as speech from a single 
talker reaches each of the microphones 21, 23, 25. Since it is desired to 
limit the number of microphones gated ON for a single sound source, the 
output of comparator 89 is ANDed at 91 with a second decision signal for 
determining whether the associated microphone should be gated ON via opto 
coupler 69. 
The second decisional process determines which of the microphones has 
received the loudest speech first. The MAX bus 56 receives inputs 
representative of the other microphone signals for use in the second 
decisional process. The MAX bus is connected to a decisional circuit 97 
where the other microphone signals will be compared to the signal of the 
associated microphone 21. 
The DC microphone signal from circuitry 83 is first attenuated by a 6 dB 
attenuator 93 prior to input to decisional circuit 97. Attenuator 93, 
however, is electrically actuable along a conductor 95 for removing the 6 
dB attenuation when the microphone is gated ON. The output of the 
defeatable 6 dB attenuator 93 is connected to decisional circuitry 97 
which compares it to the comparable signals in the other microphone 
circuits by way of the MAX bus interconnection for determining whether it 
is momentarily the maximum of all such comparable signals. The signal 
level on MAX bus 56 is controlled by decisional circuitry 97 and the 
comparable circuits associated with the other microphones to be equal to 
such maximum. 
When both the noise adapting threshold criteria is satisfied (i.e., speech 
is occurring) and the MAX bus criteria is satisfied (i.e., the associated 
microphone 21 is momentarily receiving the loudest speech as slightly 
modified by the defeatable 6 dB attenuation of each microphone circuit), 
an output trigger signal is generated along a conductor 98 for actuating a 
retriggerable one shot 99. The output of one shot 99 actuates an opto 
coupler driver 101 which in turn drives opto coupler switch 69, gating the 
microphone signal of the associated microphone onto conductor 45. One shot 
99 provides a 0.4 second hold time after each trigger on its input 
conductor 98. The output of one shot 99 is fed back to the control input 
of defeatable 6 dB attenuator 93 along conductor 95. Attenuator 93 
responds to the HIGH signal from one shot 99, and removes the 6 dB 
attenuation. 
The net result of the MAX bus interaction, as described above, is that a 
single talker gates on only one microphone, but a plurality of talkers 
speaking normally can reliably gate ON a plurality of microphones. 
The loudspeaker 29, of course, will present speech signals to each of the 
microphones 21-25. In order to prevent gating of the microphone channels 
by speech from the loudspeaker, a speaker inhibit signal generator 57 is 
utilized. Generator 57 receives the loudspeaker drive signal 58 for 
responsively generating a speaker inhibit signal along a conductor 105. 
Generator 57 frequency equalizes, rectifies, and filters the loudspeaker 
driving signal for generating a DC output onto conductor 105. The speaker 
inhibit signal appearing on conductor 105 feeds each of the mike/gating 
circuits 51-55. 
The speaker inhibit signal is fed to the noise adapting threshold circuit 
85 for affecting the noise threshold level in the manner described above 
and hereinafter with reference to FIG. 10. Microphone gating for 
loudspeaker sound and its reverberant decay is prevented with minimal 
inhibition of desired microphone gating for local speech. 
As will suggest itself, when two or more microphone/speaker units 11 are 
connected in tandem along the top of a conference table, all MAX buses are 
connected together, all loudspeaker power amplifier inputs are connected 
together for being driven by the speaker signal from control unit 15, and 
all gated microphone outputs 45 are connected together as are all nongated 
outputs 47. 
Mute input 75 can be actuated by appropriate logic circuitry to defeat 
individual microphone gating without altering the MAX bus interactions, 
yielding the results described above. 
It will be understood with reference to FIGS. 7-13 that all op amps and 
comparators are connected to well regulated and filtered, balanced voltage 
supplies of .+-.15 volts, as is well known in the art. Referring to FIG. 
7, microphone 21 is of the electret condenser type. The transducer is 
interconnected to preamp/interface circuitry 61, including a field effect 
transistor impedance converter 203 (Sanyo 25K156L), transducer calibration 
resistor R1 and bias resistors R2, R3, preamplifier components R4, R5, C2, 
C4 and operational amplifier 207, connected as shown. Resistor R1 is 
selected to assure uniform sensitivity of each transducer, impedance 
converter, R1 assembly. Resistor R5 sets the gain of the preamp and is a 
precision resistor so that identical acoustical sensitivities from each 
microphone will be presented to the audio mixing buses and to calibration 
circuit 81. Resistors R7 and R8 are also precision resistors. 
The microphone signal passes through capacitor C5, photoresistor R6 (part 
of opto coupler 69), resistor R7, and onto gated microphone conductor 45. 
The microphone signal is also passed through capacitor C5 and resistor R8 
onto nongated microphone conductor 47. The microphone signal is also 
transmitted along a conductor 73 to the trim calibration circuit 81, 
described in detail with reference to FIG. 8. The gated and non-gated 
buses 45, 47 are terminated with resistors to ground, 5.6 k, 1.0 k 
respectively, preferentially located in control circuitry 43 so that the 
values will not change as a plurality of units 11 are linked. These 
terminations assure that background noise and reverberation pickup remain 
substantially constant as the number of units 11 used varies and as the 
number of gated ON Microphones varies above 0. This method was described 
in Anderson et al. and discussed in the referenced article. 
Referring to FIG. 8, the microphone signal enters trim calibration circuit 
81 along conductor 73. Calibration circuit 81 includes an operational 
amplifier 211, trim resistor R10, and resistors R11, R12 connected as 
shown. Calibration circuit 81 is used to trim out component tolerance 
errors to give identical microphone input to MAX bus gains. In combination 
with the trimming of resistor R1 and the use of high quality electret 
transducers, the gating coverage areas of each microphone can be precisely 
matched and controlled. 
Referring to FIG. 9, frequency equalizer/rectifier circuit 83 receives the 
microphone signal from circuit 81 along conductor 213. Circuit 83 
emphasizes the speech portion of the frequency spectrum and attenuates 
very high frequencies somewhat and low frequencies which lie outside of 
the speech band considerably. Also, since there is less energy in the high 
frequency parts of the speech band, for example, "s" sounds, as compared 
to the energy in the low frequency parts of speech, equalizer/rectifier 
circuit 83 serves to emphasize the high frequency portions within the 
frequency band of speech. Overall, this greatly reduces the interfering 
effects of room noise on gating and enables rapid filter time constants to 
be used. 
Circuitry 83 includes an operational amplifier 215, resistors R13-R18 and 
capacitors C7-C10, connected as shown. Circuitry 83 further includes a 
pair of operational amplifiers 217, 219 interconnected with diodes D1-D4, 
resistors R19-R22 and capacitors C11, C12, connected as shown, for 
precision full wave rectifying and filtering of the microphone signal. 
Attack and decay filter time constants are equal at 11 msec. Circuitry 83 
provides accurate level sensing over a wide dynamic range, particularly 
with speech sounds. An operational amplifier 221 buffers the output of 
equalizer/rectifier circuit 83, appearing on conductor 223. 
Referring to FIG. 10, noise adapting threshold circuit 85 is illustrated in 
more detail. The signal appearing on conductor 223 is a linear amplitude 
representation of the frequency equalized microphone signal. This 
amplitude representation is compared with a voltage appearing on a 
capacitor C13 which represents room noise. The signal appearing on 
conductor 223 is applied to the noninverting input of a field effect 
transistor input operational amplifier 225 through resistor R68. As the 
signal at the noninverting input changes, capacitor C13 is charged and 
discharged accordingly. Resistor R27 is of a low value which is used to 
aid the stability of operational amplifier 225 and may be neglected in the 
circuit analysis. Transistor Q102 is interconnected between the output and 
the inverting input of operational amplifier 225 and is used as a low 
leakage current diode. 
When the noninverting input of op amp 225 is higher in voltage than the 
capacitor voltage, then op amp 225 holds its inverting input at the same 
voltage as its noninverting input through transistor Q102. Resistor R23 
charges capacitor C13 slowly with a 10 second time constant. When the 
signal on the noninverting input of the operational amplifier attempts to 
go lower than the voltage on the capacitor, then op amp 225 discharges 
capacitor C13 through diode D5. The operational amplifier pulls down at an 
appropriate rate in accordance with the feedback voltage appearing at the 
inverting input through resistor R23, which has a negligible voltage drop 
across it due to the low input current of op amp 225 and the low leakage 
current of Q102. This makes discharge diode D5 appear as a precision 
diode. 
Thus, the noise adaptive threshold voltage on capacitor C13 tracks the 
voltage on conductor 223 with a slow attack, immediate decay 
characteristic, seeking the lowest continuous background level. 
The threshold voltage appearing on capacitor C13 is buffered by a unity 
gain field effect transistor input operational amplifier 227. The output 
of operational amplifier 227 provides a signal representative of room 
noise. 
As shown in FIG. 10 6 dB attenuator 87 is constructed by resistors R24, R25 
which provide along conductor 229 a 6 dB modification of the input signal 
appearing on conductor 223. Comparator 89, which is formed from an open 
collector output LM339, receives the buffered output of the threshold 
voltage at its inverting input and receives the 6 dB attenuated signal at 
its noninverting input. A resistor R26 provides a small amount of 
hysteresis around comparator 89 for switching stability. Thus, the output 
of comparator 89 potentially provides a logic high signal along a 
conductor 98 indicating that speech is occurring at its associated 
microphone 21. The voltage level on conductor 98 is also dependent on 
decisional circuit 97 (FIG. 6) to be described with reference to FIG. 11. 
Referring to FIG. 11, 6 dB attenuator 93 is constructed by resistors R28, 
R29 which provide along a conductor 233 a 6 dB attenuation of the input 
signal appearing on conductor 223. A field effect transistor switch 235 
(p-channel, Vp&lt;3 volts) is connected between resistor R29 and ground for 
effectively removing the 6 dB attenuation when the FET switch 235 is 
turned off. Turning off of FET 235 effectively removes resistor R29 from 
the circuit. 
A signal appearing on conductor 95 passes through diode D6 for turning FET 
switch 235 OFF. A resistor R30 is connected between ground and the cathode 
of diode D6 as shown, and a capacitor C14 connects conductor 95 to ground. 
Resistor R30 and diode D6 help to provide proper control voltages to the 
gate of FET 235. Capacitor C14 slightly slows the voltage transition on 
conductor 95 to minimize capacitively coupled noise spikes to the 
defeatable 6 dB attenuator 93 and other parts of the circuitry. 
Decisional circuitry 97 includes an op amp 237 having its noninverting 
input connected to receive the output of attenuator 93 along conductor 
233. The inverting input of op amp 237 is connected to the MAX bus via 
resistors R31 and R32. Resistor R32 is of a low value and aids circuit 
stability. If the noninverting input of op amp 237 attempts to go higher 
than the amplitude of the MAX bus, diode D7 is forward biased and will 
"hold up" the MAX bus to a level equal to the non-inverting input level. 
If the noninverting input of op amp 237 is lower than the voltage on the 
MAX bus, diode D7 is reverse biased. Diode D8 prevents excessive negative 
voltage excursions at the output of op amp 237. The bias condition of 
diode D7 is monitored by a comparator 239 (LM339), potentially providing a 
logic high indication at its output along conductor 98 only when diode D7 
is forware biased. Resistor R33 provides hysteresis for stabilizing 
comparator 239. Resistors R34, R35 are connected as shown. At least one 
resistor R36 should be included to guarantee a load under all conditions 
for the diode D7 which is holding up the MAX bus, and thus enable reliable 
forward bias sensing by comparator 239. 
The output of comparator 89 (FIG. 10) is connected to conductor 98 (FIG. 
11). Comparator 89 and comparator 239 thus serve to provide a hard wired 
AND (shown symbolically as 91 in FIG. 6) along conductor 98 because of 
their open collector configuration. Thus, only when both the noise 
adapting threshold criteria and the MAX bus criteria are satisfied, then 
retriggerable one shot 99 will be actuated. 
Retriggerable one shot 99 includes an open collector output comparator 241 
(LM339) interconnected to resistors R37-R41, as shown. Comparator 241 
responds to a logic HIGH input on its inverting input which serves to 
discharge a capacitor C15. A comparator 243, (LM339) interconnected to 
resistors R42-R47, monitors the voltage across capacitor C15 for 
generating a logic output onto conductor 95. Capacitor C15, comparator 
243, and associated components provide a 0.4 second hold time for the 
output signal appearing on conductor 95. This 0.4 second time bridges gaps 
in triggering, as previously described. When triggering stops, capacitor 
C15 begins to charge back up, taking 0.4 second before capacitor C15 
reaches a sufficient voltage to change the output of comparator 243. If 
triggering recurs before the 0.4 seconds is finished, the capacitor C15 is 
again discharged and the signal on conductor 95 does not change. 
The output of retriggerable one shot 99, which appears on conductor 95 is 
fed to opto coupler driver 101, as shown in FIG. 12. Driver 101 is used to 
provide a controlled attack and decay time to the opto coupler LED 248 and 
thus to the resistance change of photoresistor R6 (FIG. 7). The driver is 
comprised of op amps 247, 249, diodes D9, D10, capacitors C16-C18 and 
resistors R48-R55, connected as shown. The result is that the audio signal 
is gated with a 4 msec. attack time and a 0.3 sec. decay time, yielding a 
click-free, unobtrusive switching action. 
A mute circuitry comprised of resistor R69 and transistor Q113 serves to 
prevent the microphone gating signal without effecting the gating control 
circuitries 85, 87, 89, 93, 97, 99 (FIG. 6). Transistor Q113 has its 
collector and emitter connected across the input to LED driver 101 and 
ground for shorting the input to ground during muting. Resistor R48 
isolates the shorting signal from the other gate control circuitry. 
Appropriate logic circuitry (not shown) may be used to apply a positive 
voltage to conductor 75, thus turning on Q113 and activating muting. This 
particular type of muting permits construction of dead zones as previously 
described. 
Referring to FIG. 13, speaker inhibit signal generator 57 (FIG. 6) samples 
the loudspeaker drive signal along conductor 58. The inhibit generator 
generates a speaker inhibit signal along its output conductor 105. 
Generator 57 serves to frequency equalize, rectify and filter the 
loudspeaker driving signal for generating a DC output signal onto 
conductor 105. The DC output signal is representative of the amplitude 
level of speech from the loudspeaker. The frequency equalization and 
filter parameters are substantially identical to those of the similar 
circuit 83, but the gain is scaled appropriately for the described 
purpose. Inhibit signal generator 103 is formed from op amps 251, 253, 
255, 257, diodes D11-D14, capacitors C19-C24 and resistors R56-R65, 
interconnected as shown. 
Referring again to FIG. 10, the speaker inhibit signal appearing along 
conductor 105 (from FIG. 13) is fed to the non-inverting input of an op 
amp 259 for modifying the threshold voltage level appearing on capacitor 
C13. When the voltage level on conductor 105 is at ground (0 volts), or is 
less than the voltage on capacitor C13, op amp 259 and associated 
components do not effect NAT circuit 85 operation. Diode D14 prevents 
excessive negative voltage excursions on the output of op amp 259 which 
would otherwise occur due to positive voltage on capacitor C13 feeding 
back to the inverting input of op amp 259 through buffer op amp 227 and 
resistor R67. When the voltage level on conductor 105 attempts to exceed 
the voltage on capacitor C13, then op amp 259 charges capacitor C13 via 
diode D15 and resistor R66 to maintain the two voltages equal. 
The signal at the output of op amp 259 is also fed to the noninverting 
input of op amp 255 via diode D16 in order to override the otherwise 
present input voltage during the time when the speaker inhibit signal 
controls the voltage on capacitor C13 to prevent contention for the 
voltage level appearing across capacitor C13. The immediate positive 
change on the voltage of capacitor C13, as caused by the speaker inhibit 
signal, would normally cause op amp 255 to pull in the opposite direction 
through diode D5. By providing the inhibit signal to the noninverting 
input of op amp 255 at a slightly higher voltage level than on capacitor 
C13, this problem is avoided. This slight voltage difference is assured by 
resistor R66 and the higher current level in diode D15 relative to diode 
D16. As the speaker inhibit signal on conductor 105 decreases from its 
peak level, capacitor C13 is discharged exactly in step by op amp 225 
through diode D5, controlled by omp amp 259 through diode D16. 
The voltage on capacitor C13 and the output of buffer op amp 227 track 
exactly the voltage level and rapid attack and decay times of the speaker 
inhibit signal on conductor 105 until it drops below the microphone level 
representation on conductor 223, at which point normal NAT action resumes. 
At this time, the microphone signal will probably represent the 
reverberant decay of the loudspeaker sound, which the voltage on capacitor 
C13 will follow, as desired. 
The following circuit values are given: 
______________________________________ 
Resistors Resistance 
______________________________________ 
R2, R46 8.2K 
R3 9.1K 
R4, R27 200 
R5 470K 
R7 5.1K 
R8, R11 11K 
R31, R67, R50, R51, R52 R48 
100K 
R12, R55, R40 30K 
R13, R56 2.7K 
R14, R57 1K 
R15, R16, R20, R22, R47, R58, R59 
51K 
R17, R18, R60, R61 510 
R19, R21, R32, R66, R62, R64 
100 
R24, R25, R45, R69 10K 
R23, R44 2.2 M 
R26, R49, R42 1.5 M 
R28, R29, R34, R35, R68 
20K 
R30, R33, R36 1 M 
R37, R38 15K 
R39 1.5K 
R41 750 
R43 27K 
R53 680 
R54 390 
R63, R65 24K 
R1 Nominally 
1.2K 
R10 100K 
Trimpot 
R6 Photocell ON 
resistance 500 
Photocell OFF 
resistance 10 M 
______________________________________ 
Capacitors Capacitance 
______________________________________ 
C2 .33 
C4 10 P 
C5, C13 4.7 
C7, C9, C10, C19, C21, C22 
.15 
C8, C20 .068 
C11, C12, C15, C17 .22 
C14 .01 
C16 .1 
C18 .047 
C23, C24 .47 
______________________________________ 
Microphones may be used in separte enclosures, and directional patterns 
other than unidirectional may be used. A speakerphone (simple 
teleconference system) may be contructed with the control circuitry of the 
related application or other types of control circuitry using just one 
microphone when this is adequate. The advantage of excellent gating 
performance, even in the presence of incoming speech, would be maintained. 
Referring again to FIG. 6, defeatable attenuator 93, decisional circuit 
97, AND function 91, and MAX bus 56 would not be needed for a single 
microphone teleconference system. 
Likewise, multiple microphone pickup may be used alone without the use of a 
teleconference loudspeaker. Referring again to FIGS. 5 and 10, power 
amplifier 59, inhibit signal generator 57, and loudspeaker 29 would, of 
course, not be needed. Neither would op amp 259, diodes D14, 15, and 16 
and resistors R66, R67, R68, with R68 being replaced by a connection. 
It will also be evident that, referring again to FIG. 11, gating logic 
signal on conductor 95, for example, may be used to control other related 
functions, such as overhead loudspeaker muting, automatic video camera 
switching, or talker indicative lights. An output port 94 is provided to 
retrieve the logic signal. Accurate microphone gating coverage areas can 
be established with no overlap. 
As a further variation, directional microphones may be used in an array 
spaced very close to each other. This would typically be an array of 2, 3, 
or 4 unidirectional (cardioid) microphones operated at identical gains 
facing outward in the same plane at equally spaced angles. In this case, 
MAX bus decisions for an individual sound source are based primarily on 
relative microphone amplitudes rather than relative time-of-arrival of the 
speech. 
The directional pattern in the horizontal plane for an array of three 
unidirectional microphones is shown in polar coordinate form in FIG. 14. 
Graphs 401, 403, and 405 represent, by the distance of the line from the 
center point 411, the relative sensitivities of the three microphones (on 
a linear basis, not dB) to sound sources coming from any angle around the 
array. 
Graph 407 represents the directional pattern of the combination, or sum, of 
graphs 401 and 403. Similar graphs may be obtained for the sums of the 
other two pairs. The relative sensitivity of graph 407 is scaled to 
maintain the same total sensitivity as one of the single cardioid 
microphones to background room noise and reverberation coming equally from 
all directions. The directional pattern may be described as a wide-angle 
cardioid. 
Graph 409 represents the sum of graphs 401, 403 and 405, again scaled to 
maintain the same sensitivity to room noise and reverberation. This 
directional pattern is omnidirectional. In symmetrical, two or four 
microphone arrays, opposite pairs of cardioids combining, or all 
microphones combining will also form an omnidirectional pattern. In the 
four microphone array, two or three adjacent microphones combining will 
form different wide angle cardioid patterns. 
The variety of directional patterns and orientations just described will be 
obtained using the gating method of the present invention as varying 
numbers of talkers at various locations around the arrays speak in varying 
combinations, as understood. One or more of the microphones in an array 
may be muted to yield direction-sensitive gating similar to that disclosed 
in Anderson et al. (U.S. Pat. No. 4,489,442), but relatively less 
disturbed by close reflections and reverberation. 
It is generally desirable in an automatic microphone gating system to 
maintain constant pickup of room noise and reverberation as the number of 
gated ON microphones varies above zero. This eliminates audible background 
noise "pumping" and "breathing" effects and maintains constant feedback 
loop gain, assuming the microphones are in the reverberant field of the 
loudspeaker. Ordinarily, including in the preferred embodiment 
teleconference system of the present invention, the microphones are spaced 
sufficiently far apart to assume random phase relationships in their 
response to room noise and reverberation. To maintain constant pickup of 
these sounds, the gain should be attenuated according to the following 
formula, which has become standard practice: 
EQU Attenuation=10 log.sub.10 NOM 
where attenuation is in dB and NOM is the number of gated ON microphones. 
This is equal to 3 dB additional attenuation for each doubling of the NOM. 
The required attenuation law is different for the closely spaced arrays. To 
obtain the scaling characteristic shown in FIG. 14 for the three 
microphone array, requires the relative attenuations: 
______________________________________ 
1 mike ON 0.00 dB 
2 mikes ON -5.12 dB 
3 mikes ON -8.29 dB 
______________________________________ 
To maintain constant reverberant sound pickup in the four microphone array, 
of which the two microphone array is a subset, requires the relative 
attenuations: 
______________________________________ 
1 mike ON 0.00 dB 
2 opposite mikes ON -4.77 dB 
2 adjacent mikes ON -5.44 dB 
3 mikes ON -8.45 dB 
4 mikes ON -10.79 dB 
______________________________________ 
Referring again to FIG. 7, these attenuation characteristics may be closely 
approximated using a similar mixing bus arrangement as already employed on 
gated microphone mixing bus 45. If the bus is terminated with a resistor 
equal to 4.0 times the sum of the photoresistor R6 ON resistance and 
resistor R7, then the result is the relative attenuation characteristic: 
______________________________________ 
1 mike ON -0.00 dB 
2 mikes ON -5.11 dB 
3 mikes ON -8.30 dB 
4 mikes ON -10.63 dB 
______________________________________ 
In this case, the bus terminating resistor (not shown) would be 22K ohms 
rather than 5.6K ohms. 
An array may be used singly or in combination with other microphones or 
arrays spaced away. In these instances, the bus terminated with the 22K 
ohm resistor is isolated locally within an array by a buffer which feeds 
the overall system gated microphone mix bus through another opto coupler. 
Associated resistor values are as originally specified for this bus. This 
additional opto coupler is driven on whenever any microphone in its 
associated array is gated on. 
The above described option allows what are essentially automatically 
variable directional characteristic, automatically variable orientation 
microphones to be created and included in automatic microphone gating 
system. The option is obviously not limited to undirectional (cardioid) 
microphones nor to microphones oriented in the same plane. 
It should be understood, of course, that the foregoing description refers 
to a preferred embodiment of the invention and that modifications or 
alterations may be made therein without departing from the spirit or scope 
of the invention as set forth in the appended claims.