Two-way communication system with selective muting

A bidirectional communication channel provides communication across a sound proof wall used, for example in a ticket booth. Normally communication from outside into the booth is muted by inhibiting the gain of a listen channel to a preselected percentage of its maximum gain level. After an attendant talks into an internal microphone, the muting is temporarily inhibited and normal communication is allowed between an attendant and a customer. After the conversation is completed, the system reverts back to a muted operational mode.

BACKGROUND OF THE INVENTION 
A. Field of Invention 
This invention pertains to talk-through units, and more particularly to a 
talk through unit in which speech is automatically muted at least in one 
direction, and restored in response to voice signals in the opposite 
direction. 
B. Description of the Prior Art 
In many instances it is desirable to provide a two-way communication system 
for bridging a partition. For example, many ticket booths used for 
dispensing tickets at train stations, theaters, sporting arenas and so 
forth are provided with bullet proof windows for the protection of the 
attendant. Communication between a potential customer and the attendant is 
provided by a two way talk unit, such as as described in commonly assigned 
U.S. Pat. No. 4,049,911, incorporated herein by reference. The unit 
described in this reference, is preferably mounted on the window partition 
and includes two sets of speakers and microphones. An external microphone 
is used to pick up sounds from the customer and transmit them to an 
internal speaker. An internal microphone, preferably disposed on a 
flexible extension, referred to as the "goose-neck", transmits the sounds 
from the attendant to an external speaker. Control circuitry is provided 
to selectively amplify the sounds in both directions. More specifically, 
sounds from inside the booth are not picked up and transmitted unless the 
attendant speaks within a short distance, i.e., two inches, from the 
internal microphone. External sounds are compressed so that they are 
reproduced at a preselected constant maximum amplitude. 
A problem with this system is that sudden loud background external noises, 
such as generated for example by a large crowd at a sports arena, or by a 
train in a train station, are propagated indiscriminately into the booth, 
and can be discomforting to the attendant, especially over long time 
periods. 
Other two way communication systems are known in the art. For example, U.S. 
Pat. No. 3,291,911 discloses a system wherein one of the two way channels 
is partially muted when not in use. The channel automatically turns off 
its own muting when it receives an input signal. U.S. Pat. No. 4,588,859 a 
two channel system is disclosed wherein each channel mutes the other 
channel when receiving an input signal. 
Japanese Patent 63-48937 discloses a muting circuit in a telephone system 
with a predetermined time delay. 
The following references also disclose other communication schemes: 
3215780; 3999015; 43084278; 4319086; 4691348. 
None of these schemes solve the abovementioned problems satisfactorily. 
OBJECTIVES AND SUMMARY OF THE INVENTION 
In view of the above-mentioned disadvantages of the prior art, it is an 
objective of the present invention to provide a two channel communication 
system wherein external background noises are automatically reduced to 
eliminate discomfort to the attendant. 
A further objective is to provide a system which can be readily implemented 
by modifying an existing system thereby reducing design costs. 
Other objectives and advantages of the invention shall become apparent from 
the following description. 
Briefly, a communication system constructed in accordance with this 
invention provides two channels: a listen channel for transmitting 
external sounds into the booth, and a talk channel for transmitting sounds 
from the attendant in the booth to a potential customer outside the booth. 
Importantly, the listen channel is partially muted so that it transmit 
sounds at a fraction of its maximum amplitude, such as 30%. In this 
manner, the attendant is protected from sudden loud external background 
noises. When a customer approaches the booth, the attendant speaks into 
the internal mike, causing the muting to be disabled. This action defeats 
the muting of the listen channel for a preselected time duration, i.e., 30 
seconds. During this time, the sounds uttered by the customer are 
amplified at full amplitude and reproduced in the booth so that normal 
communication can take place between the customer and the attendant.

DETAILED DESCRIPTION OF THE INVENTION 
A communication system 10 constructed in accordance with this invention is 
shown in FIG. 1. This system 10 is a two way audio system that provides 
personal communication between people separated by a sound-proof barrier 
14. The system consists of two channels: a listen channel 20 and a talk 
channel 22. The listen channel 20 consists of an electret microphone 24, 
disposed outside the barrier 14. The signals from the microphone 24 are 
fed to a high pass filter 26 and then to a preamplifier 28. The output of 
the preamplifier 28 is fed to an averaging circuit 30. The averaging 
circuit 30 takes an average of the preamplifier output and feeds it to a 
gain control circuit 32 which controls the gain of preamplifier 28. In 
this manner the pre-amplifier 28 performs an amplification function and in 
addition, together with the averaging circuit 30 and the automatic gain 
control circuit 32 compresses and limits the signals from the microphone 
24 to a preselected range. 
The signals from amplifier 28 are also fed to a volume controller 34, which 
may be for example a variable resistor. The signals from the volume 
controller 34 are fed to a power amplifier 36. The gain of power amplifier 
36 is controlled by an automatic gain controller 38. The output of power 
amplifier 36 is fed to an internal speaker 40, as described below. 
The listen channel 20 can be disabled by an electronic switch 41 as 
discussed below, by grounding the output at filter 26. 
The talk circuit 22 includes an internal microphone 42, preferably mounted 
on a relatively stiff armored cable assembly, commonly called a 
`goose-neck` (not shown). The signals from the microphone 42 are filtered 
by high-pass filter 44 and then amplified by preamplifier 46. The gain of 
the preamplifier 46 is controlled through a volume controller 48. The 
output of the preamplifier 46 is fed through an electronic switch 50 to a 
power amplifier 52. The gain of amplifier 52 is controlled by an automatic 
gain controller 54. The output of the amplifier is fed through barriers 14 
to an external speaker 56. 
The system 10 further includes a VOX circuit 60 consisting of a voice 
detect circuit 62 and a 200 msec delay circuit 64. The sensitivity of 
voice detect circuit 62 is controlled through a sensitivity controller 66. 
The voice detect circuit 62 is used to detect signal T from preamplifier 
46 having an amplitude exceeding a preselected threshold set by 
sensitivity controller 66. The delay circuit generates a signal T/L which 
goes high substantially when signal T exceeds the threshold and stays high 
for a predetermined time (i.e., 200 msec) after the signal T falls below 
the threshold. 
The system 10 is further provided with a muting control circuit consisting 
of a one-shot multivibrator 72 triggered by the T/L signal. The 
multivibrator 72 generates a signal M of preselected duration (for 
example, 30 seconds) after the T/L signal goes low. This signal M is used 
to control AGC 38. 
A manual mute switch 74 is also provided to control the automatic gain 
controller 38, as discussed below. 
The signal T/L is used to control switches 50 and 71. 
In order to eliminate cross-talk, the system is further provided with an 
amplifier 76 and an average determining circuit 78. Signals from the 
preamplifier 28 are amplified by amplifier 76 and averaged by circuit 78. 
If this output of circuit 76 exceeds a predetermine threshold, the signal 
from circuit 76 is used to disable the voice detect circuit 62. 
The operation of the system 10 shall now be explained in conjunction with 
FIG. 2. Initially, the system 10 is in a quiescent mode. In this mode, 
assuming that the manual mute switch 74 is open the signal M from the 
multivibrator 72 is low, and the gain controller 38 is set to adjust the 
gain of amplifier 36 to about 30% of its maximum output. Thus, any loud 
ambient noises (indicated in FIG. 2 by dotted line 90) picked up by 
microphone 24 are automatically muted so that they will not interfere with 
the attendant inside the booth. Signal T is low and the voice detect 
circuit 62 keeps switch 50 open through signal T/L, unless the attendant 
talks directly into microphone 42. This feature permits, for example, an 
attendant to listen to a radio, talk on the telephone or talk to another 
person in the booth in privacy. 
At t=T1, a customer approaches the booth and the attendant inside the booth 
leans toward the microphone 42 and says something (for example `May I help 
you?`) to switch the system 10 to the talk mode. At the beginning of this 
mode, the sounds from the attendant are sensed by voice detector 62, which 
in response causes the output of the voice detector 62 to go high. This 
output remains high as long as voice signal T is detected. The output of 
detector 62 is fed to delay circuit 64 which in response generates signal 
T/L as shown in FIG. 2. Signal T/L is high while the output of detector 62 
is high and is used to close electronic switches 41 and 50 thereby 
enabling the talk channel 22 and disabling the listen channel 20. The 
voice signals from preamplifier 46 are amplified by amplifier 52 and 
reproduced by speaker 56. While the attendant is speaking, the listen 
channel 20 is therefore off. 
At t=T2 the attendant stops talking, however, the signal T/L remains high 
until t=T3 due to delay circuit 62. The delay circuit 62 is used to insure 
that the talk channel 22 is not disabled while the attendant is talking. 
Its delay is selected to extend over pauses between words uttered by the 
attendant. After the attendant stops speaking and the period set by the 
delay circuit 64, the signal T/L goes low at time t=T3, disabling the talk 
channel 22 and enabling the listen channel 20, thereby initiating a listen 
mode. 
The listen mode is initiated by signal M from the multivibrator 72, which 
is triggered by signal T/L going low. When signal M goes high, it causes 
gain controller 38 to switch the gain of amplifier from a low value of, 
for example 30%, to 100%. (During period T1&lt;L&lt;T3, the gain of AGC 38 is 
immaterial since the LISTEN channel is off). The signal M remains high for 
a predetermined time, for example, 30 seconds. During this time the gain 
of amplifier 38 remains high thus allowing the attendant to hear the 
customer loud and clear. The duration DL of the listen mode is preselected 
to be long enough to allow a customer to complete his sentence. For 
example, as shown in FIG. 2, signal 92 indicating voice signals from the 
customer is expected to terminate prior to duration DL. Alteratively, the 
circuitry could be made so period DL is not fixed, but instead it 
terminates a preselected time after the customer stops talking. At the end 
of the listen period, i.e. at t=T4, the signal M goes low, causing the 
gain of controller 38 to fall back to 30%. In this manner, the system 10 
reverts to the quiescent mode. 
If desired, the muting of the listen channel can be disabled by turning 
manual switch 74 on. This switch then sets the gain of amplifier 36 to 
100% independently of the operation of the multivibrator 72. 
Although the invention has been described with reference to several 
particular embodiments, it is to be understood that these embodiments are 
merely illustrative of the application of the principles of the invention. 
Accordingly, the embodiments described in particular should be considered 
exemplary, not limiting, with respect to the following claims.