Talk-through unit with voice controlled switching with turn off delay variable from 250-500 milliseconds depending on voice amplitude

There is disclosed a compact battery-powered, self-contained talk-through unit for passing voice communications through a partition. Typically, the unit is installed in a window partition separating an attendant from the public. Facing the outside of the unit area a microphone and a speaker. Facing the inside, a speaker is mounted within the unit and a microphone is contained at the end of a gooseneck connector. In the quiescent condition, speech from the outside is level-compressed and heard on the inside; speech on the inside, e.g., between two attendants, is not amplified and heard on the outside. It is only when an attendant talks within approximately 2 inches of the gooseneck microphone that the outside-to-inside amplification is turned off and the inside-to-outside amplification is turned on. In this way, the attendant has complete, hands-free control of voice communications; there is no competition between talkers and there is no acoustic feedback.

This invention relates to talk-through units, and more particularly to such 
units which are completely self-contained, battery-powered, simple to 
install, and under the exclusive and hands-free control of an "inside" 
attendant. 
There are many situations in which voice communications must be 
accomplished through a partition (usually the partition is transparent, 
although not always). For example, a glass or plastic partition usually 
separates a ticket seller in a theater from members of the public who 
desire to purchase tickets. To facilitate voice communications, a metal 
grille is often provided in the partition. Especially in high-noise 
environments, the parties may have difficulty in hearing each other 
through a grille. Also, when it is cold on the outside, the draft through 
the grille may make it very uncomfortable for the ticket seller on the 
inside. 
For this reason, it is known in the prior art to provide talk-through units 
which amplify voice communications in both directions. But prior art units 
have suffered from several disadvantages. Because they have not been 
self-contained, installation has been a major problem. It is not only that 
a power line has been required (which, in addition to the cost of 
installation, generally mars the appearance). Very often the speakers have 
not been mounted on the units themselves, but have been placed separately, 
for example, on walls. The reason for doing this is that if on each face 
of the unit there is mounted a microphone and a speaker, voice 
communications in one direction can be fed back in the other, thus 
resulting in "singing". To prevent this type of feedback, a press-to-talk 
switch may be provided under attendant control, but this does not permit 
hands-free operation. Alternatively, if a foot switch is provided, 
installation costs increase and it is bothersome to use. Another serious 
shortcoming of prior art systems is the difficulty in maintaining a 
constant outside-to-inside voice level. Depending upon the standing 
position of the "customer", the level of the speech heard inside the 
attendant's booth may vary widely. 
It is an object of our invention to provide a self-contained, talk-through 
unit which overcomes the prior art shortcomings. 
In the illustrative embodiment of our invention, a battery-powered unit is 
easily mounted in a circular cutout in a window or other partition. A 
microphone and speaker are mounted within the unit facing the outside. A 
speaker is mounted within the unit facing the inside, but the inside 
microphone is contained at the end of a gooseneck or other elongated 
connector which comprises part of the overall housing. At any single 
instant of time, there is amplification of the voice signal in only one 
direction. In the quiescent condition, the amplification is from outside 
to inside. This permits the attendant to monitor what is going on outside 
his booth. The voice signal from outside to inside is level-compressed, 
that is, the speech which the attendant hears is at a more or less 
constant level no matter how loud the voice of the outside customer. A 
control knob permits the attendant to select this level. (Another control 
knob serves to vary the inside-to-outside amplifications.) 
When the attendant desires to talk, he moves his head so that this mouth is 
within approximately 2 inches of the gooseneck microphone. As soon as a 
signal is detected, the outside-to-inside amplification is shut off, and 
the inside-to-outside amplification is turned on. The attendant thus has 
complete conrol over the communications. The operation is entirely 
hands-free, and it is a normal reaction for a speaker to move his mouth in 
proximity to a microphone when he desires to talk; thus the unit is 
naturally adapted for use and requires no "training" or the learning of 
"new" manual procedures. 
The system is provided with a threshold adjustment which determines the 
signal level at the inside microphone which controls a switch from 
outside-to-inside amplification to inside-to-outside amplification. This 
initial adjustment allows the attendant to tailor the automatic switchover 
to his own operating procedures and voice level, and the reverberation 
characteristics of his booth. For example, an attendant who always works 
alone and who speaks in a soft voice may set a low threshold level. On the 
other hand, if the booth is often occupied by two attendants, it is wise 
to set a high threshold level so that their conversation (especially about 
customers) is not inadvertently amplified and heard on the outside. 
During normal speaking, there are numerous short gaps, e.g., between 
sentences and when pausing to take a breath. To prevent "choppy" voice 
communications, it is desirable to wait a fraction of a second after the 
cessation of the attendant's talking before returning the system to its 
quiescent state, in the event the attendant resumes to speak. For this 
reason, a short delay is built into the system for preventing a return to 
the quiescent condition until a fraction of a second after the attendant 
stops talking; only if he does not resume talking does the system switch 
back to its quiescent state. Since people who talk in a loud voice usually 
pause a little longer between speech segments, the system waits longer 
before turning off the inside-to-outside amplifier when the attendant 
speaks loudly. 
One of the main problems with prior art talk-through units is acoustic 
feedback; it is possible for speech in one direction to be picked up and 
amplified in the other direction, giving rise to "singing" or 
oscillations. In the unit of the invention, for example, speech from the 
outside of the unit is ordinarily heard on the inside and, were the 
threshold to be exceeded, the outside-to-inside amplifier would be shut 
off before the customer finishes to talk. In the past, the microphones and 
speakers have been mounted separately and apart from each other to avoid 
this type of acoustic feedback. In the illustrative embodiment of our 
invention, the problem is avoided by mounting the inside microphone at the 
end of the gooseneck connector away from the speaker, and by adjusting the 
system so that sound inside the attendant's booth is not sufficient for 
controlling an automatic switch-over. The attendant need merely speak into 
his microphone in order for the threshold to be exceeded to switch the 
amplifiers.

The unit 10 of FIG. 1A consists of three parts which permit rapid mounting. 
The outer part 14 is simply a circular plate with a central conical 
cut-out 14a and a perforated grille 31 (FIG. 1C), and it is provided with 
four screw holes for insertion of screws 13. Part 16 is simply a circular 
plate with a central cut-out and four holes for allowing screws to pass 
through. The first step in mounting the unit is to place parts 14 and 16 
on opposite sides of a partition 12 which contains a cut-out made for the 
purpose of the unit's installation. Four screws 13 are then used to secure 
the two parts to each other on opposite sides of the partition. 
The third part 26 contains the working elements of the system. It is fitted 
against plate 16, with the outside microphone/speaker assembly 24 inserted 
through the cut-out in plate 16 and partition 12. In the drawing, the 
symbol MO represents the "outside microphone" and the symbol SO represents 
the "outside speaker". Part 26 includes a cylindrical sleeve, the end of 
which fits over plate 16. Four screws 22 are used to secure the sleeve to 
the plate, thus completing the installation. 
Within part 26 there are ten 1.25-volt batteries, shown by the numeral 28. 
These batteries are mounted in two planes, the five batteries in each 
plane being arranged around the periphery of the unit in the shape of a 
pentagon. The batteries are connected in series (not shown) in order to 
provide a 12.5-volt supply. 
A circuit board 30 is mounted within the unit, the circuit board containing 
the various electrical components included in the system. The detailed 
mounting of the components on the board is not important for an 
understanding of the invention, it being understood that FIG. 1 is 
designed to depict the overall arrangement, rather than structural 
details. However, attention should be paid to potentiometer 11 mounted on 
the circuit board. This potentiometer serves as the threshold adjustment. 
Access to the potentiometer is gained by removing four screws 32 which 
secure the circular sleeve to a front plate assembly in the system. By 
removing the front plate assembly, the potentiometer can be adjusted. 
On the inside of the unit, an "inside speaker" SI is mounted. The "inside 
microphone" MI is secured at the end of 12-inch gooseneck connector 23. In 
general, microphone MI is preferably mounted at least several inches 
(i.e., three inches) in front of speaker SI. 
On the inside face of the unit, as seen most clearly in FIG. 1B, there are 
provided a grille 29 for permitting sound from speaker SI to pass through, 
a main power switch 15 and a charging jack 21 for allowing the batteries 
to be recharged. A conventional charging circuit (not shown) can be used 
with the system, the batteries (preferably, of the nickel-cadmium type) 
being recharged, for example, overnight when the unit is not in use. Also 
mounted on the inside face of the unit are two potentiometer control knobs 
17 and 19 for respectively adjusting the voice level on the outside of the 
unit (talk) and the inside of the unit (listen). 
FIG. 2 depicts the block diagram schematic of the system. Preamplifier 41 
and automatic gain control circuit 42, arranged in a feedback 
configuration, comprise a level compressor; the output of outside 
microphone MO is amplified, the amplification depending on the signal 
level, and a relatively constant signal level is applied to the signal 
input of gate 43. The control input of the gate is connected to the Q 
output of monostable multivibrator 48. In the quiescent condition, the Q 
output is high so that gate 43 is enabled and speech from outside the 
booth is passed through the gate. Power amplifier 44 amplifies the signal 
and applies it to inside speaker SI. Potentiometer 19 controls the level 
of the speech heard by the attendant. 
When the attendant speaks into the inside microphone, the signal is 
preamplified by amplifier 45 and applied to the signal input of gate 49. 
The control terminal of the gate is connected to the Q output of 
multivibrator 48. Until the multivibrator is switched to the 1 state, gate 
49 remains off. It is only when the multivibrator is switched that gate 49 
is enabled, at which time power amplifier 50 applies an amplified signal 
to the outside speaker SO. Potentiometer 17 controls the outside speech 
level. 
Elements 46 and 47 determine when multivibrator 48 is switched to the 1 
state, thereby enabling gate 49 and disabling gate 43. The signal at the 
output of preamplifier 45 is amplified by driver 46, and the amplified 
signal is rectified by rectifier 47. Although not shown in FIG. 2, a 
threshold control (potentiometer 11 in FIG. 1) is provided for controlling 
the level at the output of the rectifier which triggers the multivibrator. 
When the threshold level is exceeded, the multivibrator is switched to the 
1 state. The inside-to-outside amplification persists to the exclusion of 
the outside-to-inside amplification until after a fraction of a second has 
elapsed following the cessation of speech by the attendant. 
The detailed circuit is shown in FIG. 3. The system includes three 
integrated circuits IC1, IC2, and IC3, each of which is a dual 741C type 
operational amplifier. Each of these eight-pin packages contains two 
difference amplifiers shown symbolically in the drawing. The difference 
amplifiers are shown only to facilitate an understanding of the signal 
flow. The pins of each integrated circuit are numbered 1 through 8 in the 
counter-clockwise direction, starting with the upper right-hand corner. 
There are two power supplies for the integrated circuits, the numerals 18 
and 20 being used to designate them. As shown at the bottom of FIG. 3, the 
ten batteries are arranged in series to provide a 12.5-volt potential at 
terminal 18. Terminal 20 is connected to the center of the battery string 
to provide a 6.25-volt potential. The batteries function to power the 
system only when the main power switch 15 is closed. In order to charge 
the batteries, a low DC current is applied from a conventional charging 
circuit at charging jack 21. Capacitors 105 and 106 are standard filter 
capacitors known in the art. 
Amplifier 62 in integrated circuit IC1 provides preamplification for the 
signal picked up by outside microphone MO. Resistor 56 is a feedback 
resistor coupled from the output of the amplifier to the input. The 
amplification is approximately equal to the magnitude of resistor 56 
divided by the input resistance between the microphone and the input 
terminal of amplifier 62 (pin 2). This latter resistance is the parallel 
combination of resistor 55 and the channel resistance of FET transistor 
Q1. When the transistor is fully conducting, its channel resistance is low 
and thus the overall amplification is high. When the transistor is not 
conducting, the channel resistance is high and the overall amplification 
is low. The overall amplification is approximately 50 when the transistor 
conducts, and approximately 2 when the transistor does not conduct. 
Depending on the DC potential at the transistor gate, the overall 
amplification varies between 2 and 50. 
The output of amplifier 62 is extended through resistor 57 to an input of 
amplifier 63. Resistor 58 is a standard feedback resistor connected 
between the output of the amplifier and its input. The output of the 
amplifier is extended through diode 60 to the junction of resistor 59 and 
the gate of transistor Q1. Source 20 controls a positive potential at the 
gate of transistor Q1, the output of diode 60 decreasing the magnitude of 
the potential as the AC signal at the outputs of amplifiers 62 and 63 
increases. As the outputs of amplifiers 62 and 63 tend to increase, the 
gate potential of transistor Q1 tends to decrease, thus reducing the 
overall amplification. It is this feedback arrangement which provides the 
level compression, i.e., a relatively constant signal level at the output 
of amplifier 62 which is independent of the speech level at the input of 
microphone MO. Capacitor 61 shorts out high frequencies so that the 
overall amplification remains a function of the average speech level 
rather than varying with instantaneous changes in level. Amplifier 63 and 
transistor Q1 thus provide automatic gain control and eliminate the need 
for the attendant to constantly vary the "listen" volume control depending 
upon the voice levels of different customers. The circuitry associated 
with integrated circuit IC1 corresponds to elements 41 and 42 on FIG. 2. 
Diodes 64 and 65 correspond to gate 43 on FIG. 2. The left end of resistor 
71 varies in potential between approximately +5.5 and -5.5 volts. In the 
quiescent condition, the potential is +5.5 volts so that each of diodes 64 
and 65 is forward biased. The signal at the output of amplifier 62 (a few 
hundred millivolts) is thus extended through the gate to the primary 
winding of transformer T1. The transformer has a turns ratio of 1:100 and 
thus boosts the signal voltage, as well as isolating the power amplifier 
from the preamplifier. Capacitor 66 functions to flatten the response of 
microphone MO. A typical microphone has a greater response to higher 
frequencies and the capacitor attentuates them. Potentiometer 19 is the 
"listen" control shown on FIG. 1B. It is the signal at the potentiometer 
tap which controls the voice level inside the attendant's booth. 
Integrated circuit IC4 is an LM380P type, 2.5-watt power amplifier. It is 
arranged in a standard configuration as is known in the art, capacitors 
67, 68 and 69 being standard stabilizing capacitors which are recommended 
for audio applications. The output of the integrated circuit (at pin 
number 8 when counting starting at the upper right-hand corner and going 
in the counter-clockwise direction) is coupled to one input of inside 
speaker SI through decoupling capacitor 70, the other input to the speaker 
being grounded. 
The inside microphone MI is coupled through resistor 80 to an input of 
difference amplifier 82. Feedback resistor 81 and source resistor 80 are 
selected to provide modest amplification (approximately 15) to overcome 
attentuation introduced by the gate comprising diodes 97 and 98, the 
signal at the output of amplifier 82 being extended through the gate 
(comparable to gate 49 on FIG. 2) to the power amplifier which amplifies 
the attendant's speech. The outpuf of amplifier 82 is also extended 
through resistor 86 to an input of amplifier 83. This amplifier 
corresponds to driver 46 in FIG. 2 and provides the additional 
amplification necessary for triggering the multivibrator. The output of 
the amplifier is fed back to the input through resistor 87 and 
potentiometer 11, this potentiometer being set to control the threshold 
level. The output of the amplifier is extended through diode 88, this 
diode and capacitor 91 serving to provide a positive rectified signal 
indicative of the attendant's average speech level. It is the positive 
potential which is extended through resistor 90 to the negative input of 
amplifier 84 which functions to trigger the multivibrator, amplifiers 84 
and 85 corresponding to multivibrator 48 on FIG. 2. 
It is a sufficiently high positive potential at the cathode of diode 88 
which maintains the multivibrator in the 1 state so that it is the 
attendant's speech which is amplified to the exclusion of the customer's 
speech. The setting of potentiometer 11 controls the overall amplification 
of amplifier 83 since the potentiometer is in the feedback path of the 
amplifier. It is by initially setting potentiometer 11 that the attendant 
can adjust the unit to his particular requirements. 
Capacitor 91 charges to the average potential at the cathode of diode 88. 
When the attendant ceases to talk, the capacitor discharges through 
resistor 89 to source 20, through resistor 94 to ground, and through 
resistor 90. Not until the capacitor discharges below the threshold level 
does the multivibrator switch back to the 0 state. Consequently, the 
greater the potential across capacitor 91, the longer it takes for the 
multivibrator to switch back to the 0 state after the attendant has 
stopped talking. It will be noted that the louder the speech level of the 
attendant, the higher the potential across capacitor 91. Consequently, the 
delay between the cessation of the attendant's talking and the automatic 
switch-over of the multivibrator varies in direct proportion to the 
attendant's speech level. This is a desirable characteristic because in 
most cases it has been found that the louder the speaking voice of the 
attendant, the longer the pauses taken. In general, depending on the 
setting of potentiometer 11, it requires between 250 and 500 milliseconds 
after the cessation of speech of the attendant before the 
outside-to-inside amplifier is turned back on. 
The multivibrator itself consists of amplifiers 84 and 85 arranged in a 
standard configuration. The output of amplifier 84 is coupled via resistor 
92 to an input of amplifier 85 and the output of amplifier 85 is coupled 
through resistor 93 to an input of amplifier 84. With a positive potential 
being extended through resistor 90 when the attendant is talking, the 
output of amplifier 84 is negative. The negative potential is extended 
through resistor 71 to the junction of diodes 64 and 65, thus back-biasing 
these diodes so that outside-to-inside amplification is cut off. 
Conversely, when the output of amplifier 84 is positive, diodes 64 and 65 
are turned on. While it is the output of amplifier 84 that controls the 
gate comprising diodes 64 and 65, it is the output of amplifier 85 which 
is extended through resistor 99 to the junction of diodes 97 and 98 to 
control the inside-to-outside amplification. It is only when the output of 
amplifier 85 is positive that the attendant's speech is amplified and 
heard on the outside. 
The power amplifier for driving outside speaker SO operates similarly to 
the power amplifier for driving inside speaker SI. Just as potentiometer 
19 controls the amplified voice level on the inside, potentiometer 17 
controls the amplified voice level on the outside. It should be noted that 
potentiometers 17 and 11 are independent of each other, the former 
controlling the attendant's amplified speech level and the latter 
determining whether there is any amplification in the first place. It is 
important to provide not only an adjustable speech level, but also an 
adjustable threshold. For example, many attendants keep a radio on while 
sitting in their booth and the threshold level must be adjusted to make 
certain that the level is not exceeded by the playing of a radio. An 
increased threshold level simply requires the attendant to talk closer 
into the inside microphone in order to switch the amplifiers. 
It should be noted that in the illustrative embodiment of the invention 
capacitor 66 is placed across the secondary winding of transformer T1 
while no such capacitor is placed across the secondary winding of 
transformer T2. Outside microphone MO is in front of speaker SO; the 
microphone is therefore relatively small (Dynamagnetic Devices model 
D457). This microphone is also quite sensitive as it should be to pick up 
sound even if the customer does not speak directly into it. But because 
the microphone has a peak response to high frequencies, capacitor 66 is 
utilized to attentuate them. Inside microphone MI, on the other hand, is 
not limited in size and is far less sensitive (Shure model 561). Because 
it does not exhibit a peak response to high frequencies, there is not need 
for a capacitor comparable to capacitor 66. Because one transformer 
circuit includes a capacitor and the other does not, the best performance 
is achieved by using different-valued potentiometers across the secondary 
windings. Potentiometer 19 has an impedance of 50K, while potentiometer 17 
has an impedance of 5K. In the illustrative embodiment of the invention, 
each speaker is a 2-inch Oxford model 2PX12-S. 
It is desirable to provide acoustic isolation of the two speakers from the 
other elements in the housing. Techniques for doing this are well known in 
the art; for example, each speaker may be surrounded by acoustic absorbent 
material contained, in a hard enclosure. 
In the preferred embodiment of the invention, transistor Q1 is of type 
2N5949. All diodes are of type HEP170. The resistor and capacitor 
component values are as follows: 
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Resistor Value Capacitor Value 
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11 1M 61 5 mfd 
55 56K 66 .1 mfd 
56 100K 67 120 pf 
57 20K 68 120 pf 
58 100K 69 5 mfd 
59 100K 70 220 mfd 
71 1K 91 10 mfd 
80 1K 101 120 pf 
81 15K 102 120 pf 
86 1k 103 5 mfd 
87 1K 104 220 mfd 
89 10K 105 220 mfd 
90 1K 106 220 mfd 
92 470K 
93 470 K 
94 100K 
99 1K 
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Although the invention has been described with reference to a particular 
embodiment, it is to be understood that this embodiment is merely 
illustrative of the applicaton of the principles of the invention. For 
example, instead of using batteries to power the unit a line cord could be 
extended to the unit, although this is not as pleasing aesthetically. Thus 
it is to be understood that numerous modifications may be made in the 
illustrative embodiment of the invention and other arrangements may be 
devised without departing from the spirit and scope of the invention.