Two way telephone communication system

A two way telephone communication system includes a station having a signal transmitting channel for conveying a signal received at a microphone to another station, a signal receiving channel for conveying signals received from the other station to a speaker, and switches permitting the two signal channels to be selectively enabled or disabled. Two signal detection paths are associated with the transmitting and receiving channels, each containing circuitry which detects the presence of signals in one of the channels and responds by generating actuating signals. Control circuitry responds to the detection path which first generates an actuating signal by enabling its associated signal channel and simultaneously disabling the other signal channel, triggering the switches appropriately. Delay circuitry associated with each signal detection path can be selectively enabled to delay the generation of an actuating signal in the associated signal detection path. The delay circuitry of the signal detection path associated with the disabled signal channel is enabled so long as signals are present in the enabled signal channel. This arrangement tends to maintain the enabled signal channel until it is no longer required to convey signals.

BACKGROUND OF THE INVENTION 
This invention relates to two way telephone communications systems. More 
specifically, this invention relates, in its preferred embodiment, to what 
could be referred to as an automatic direction decision system for a two 
way telephone communications system, particularly, but not necessarily, of 
the hands-free type.

FIG. 1 shows a typical, prior art, hands-free telephone system having the 
same components at each of two stations 10a and 10b. Since the components 
at the two stations are identical, they will be assigned identical 
reference numerals in the following description but followed by "a" or "b" 
depending upon whether the components are at station 10a or at station 10b 
respectively. Thus, at stations 10a, 10b there are microphones 11a, 11b; 
loudspeakers 12a, 12b; four wire to two wire converters 13a, 13b; control 
circuits 14a, 14b; switches 15a, 15a', 15b, 15b' and amplifiers 16a, 16a', 
16b, 16b' all connected as shown in FIG. 1. 
Control circuits 14a, 14b control switches 15a, 15a' and switches 15b, 15b' 
respectively. 
Four wire to two wire converters 13a and 13b are connected by a two wire 
communications line 17. 
With the system shown in FIG. 1 operating ideally, a mode of operation 
which, from a practical point of view is not possible to attain, when a 
voice signal is received by microphone 11a, its presence is detected and 
causes control circuitry 14a to close switch 15a and open switch 15a' 
preventing this voice signal from being broadcast by speaker 12a, and the 
voice signal is transmitted via line 17 to station 10b where it is 
broadcast by loudspeaker 12b. At that station the presence of a voice 
signal from station 10a is detected and causes control circuitry 14b to 
close switch 15b and open switch 15b', preventing any voice signals 
received by microphone 11b from being transmitted to station 10a and 
enabling loudspeaker 12b to broadcast the voice signal received from 
station 10a. 
Unfortunately systems of the type shown in FIG. 1 exhibit a phenomenon 
known as "echo", whereby a voice signal received by microphone 11a, for 
example, will be "echoed" to loudspeaker 12a and then picked up by 
microphone 11a. It is possible in some systems for a signal indicating 
presence of the "echo" to reach control circuitry 14a before a signal 
indicating presence of the voice signal at microphone 11a creating a 
"race" condition. Under these circumstances control circuitry 14a will 
open switch 15a and close switch 15a', producing an entirely 
unsatisfactory result. 
One known technique for avoiding this problem is shown in FIG. 2, which 
shows only station 10a, station 10b being identical, and in which station 
10a is the same as in FIG. 1 except for the inclusion of a delay network 
18a in the signal path. This delay network always delays any signal on the 
line to loudspeaker 12a, including the "echo" signal, and prevents control 
circuitry 14a from being responsive to a signal indicating presence of the 
"echo", since it will be received by control circuitry 14a after reception 
of a signal indicating presence of the voice signal at microphone 11a. The 
problem with introduction of any form of delay into the signal path is 
that distortion is introduced thereby. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided an automatic 
direction decision system which functions like an ideally functioning 
system of the type shown in FIG. 1 and which obviates the distortion 
problem created by the addition of delay network 18a (FIG. 2) that 
compensates for the fact that the system of FIG. 1 cannot operate in the 
ideal mode previously discussed. 
In its broadest sense the instant invention resides in introduction of 
delay into the signal detection path rather than into the signal path 
itself, as in the case of the system shown in FIG. 2. 
An aspect of the invention is as follows: 
In a two way telephone communication system including a first station 
having a first signal receiving source, a second signal receiving source, 
signal broadcasting means, a first signal path connected to said first 
signal receiving source for transmitting a signal received by said first 
signal receiving source to a location for transmission of said signal to 
another station, a second signal path connected between said second signal 
receiving source and said signal broadcasting means for carrying a signal 
received by said second signal receiving source from another station to 
said signal broadcasting means, switching means in each of said signal 
paths for selectively enabling one of said signal paths and disabling the 
other of said signal paths, control means for controlling said switching 
means, signal detection paths connected between said first signal path and 
said control means and between said second signal path and said control 
means for detecting the presence of signals in said first and second 
signal paths and for activating said cohtrol means to control said 
switching means responsive to the one of said signals in said signal 
detecting paths first received by said control means, the improvement 
comprising signal delay means in at least one of said signal detection 
paths, and means responsive to the presence of a signal in one of said 
signal paths for selectively enabling or disabling said signal delay 
means. 
DETAILED DESCRIPTION OF THE INVENTION INCLUDING THE PREFERRED EMBODIMENT 
An embodiment of the invention is shown in FIG. 3. FIG. 3 shows only 
station 10a, and it will be understood that station 10b will be identical. 
The system of FIG. 3 differs from that of FIG. 2 in that there is no 
signal distorting delay network 18a in the signal path. Rather, delay 
networks 19a and 19a' are provided in the signal detection path, i.e., in 
the path wherein the presence of a signal in the signal path is detected 
and used to control the state of control circuit 14a and hence of switches 
15a, 15a'. 
A complete schematic of a system embodying the present invention (station 
10a only) is shown in FIG. 5. A somewhat simplified version of the system 
of FIG. 5 is shown in FIG. 4. 
In FIG. 4 each of amplifiers 16a and 20a are operational amplifiers (op 
amps). Amplifier 16a' is an audio amplifier. Each network 21a, 21a' is a 
background level sensing network. Components 22a, 22a' are comparators. 
Components 23a, 23a' are retriggerable monostable timers. Component 24a is 
a NAND flip flop, while components 25a and 26a are a NAND gate and an 
inverter, respectively. 
Attenuation is provided in the signal detection path from pin 7 of op amp 
20a to pin 3 of comparator 22a by a resistor R8 and a resistor R16. 
Additional attenuation (delay) is provided by a resistor R17 that can be 
placed in circuit or out of circuit by means of a switch that may be in 
the form of a transmission gate (TG) 50a (see also FIG. 5). 
Attenuation is provided in the signal detection path from terminal 27a to 
pin 5 of comparator 22a' by a resistor R24 and a resistor R25. Additional 
attenuation (delay) is provided by a resistor R33 that can be placed in 
circuit or out of circuit by means of a switch that may be in the form of 
a TG 50a'. 
For the purpose of explaining the operation of the system of FIG. 4, the 
function of background sensing level networks 21a and 21a' can be 
disregarded, and it can be assumed that manually adjustable or preset 
voltages are applied to pin 2 of comparator 22a and pin 6 of comparator 
22a'. These adjustable or preset voltages represent a background level 
(noise) that must be exceeded by a voice signal at pin 3 of comparator 22a 
or pin 5 of comparator 22a' to change the states of the comparators. 
In describing the operation of the system shown in FIG. 4 it will be 
assumed that a voice signal at microphone 11a has been received previously 
and has established the microphone path as having been selected. Under 
these circumstances Q of component 24a is high, the output at the SPKRENA 
terminal is low and the output at the MIKENA terminal is high. The output 
at the SPKRENA terminal being low sets TG 50a to the open position and 
does not increase the attenuation afforded by resistors R8 and R16 which, 
in the embodiment illustrated, is .div.3 attenuation. The output at the 
SPKRENA terminal being low also sets TG 15a' to the open position, meaning 
that no voice signals can be broadcast by loudspeaker 12a. The output at 
the MIKENA terminal being high sets TG 15a to the closed position, 
enabling voice signals received by microphone 11a to be transmitted to 
station 10b (not shown). The output at the MIKENA terminal being high also 
sets TG 50a' to the closed position, bringing resistor R33 into the 
attenuation circuitry and increasing by, say, a factor of two, the 
attenuation afforded by resistors R24 and R25, which may have been, say, 
.div.3. In other words, with resistor R33 out of circuit, there is a 
.div.3 attenuation factor, whereas with resistor R33 in circuit, there is 
a .div.6 attenuation factor. 
With the system so set as described hereinbefore, assume that a voice 
signal is received at microphone 11a. It also will be received at terminal 
27a because of the echo effect. It is essential to the proper functioning 
of the system that, under these circumstances, the system will maintain 
the microphone path as the correct path and will not choose the speaker 
path as the correct path. With .div.6 attenuation in the signal detection 
path between terminal 27a and pin 5 of comparator 22a' and only .div.3 
attenuation in the signal detection path between pin 7 of op amp 20a and 
pin 3 of comparator 22a, the system is forced to retain its selection of 
the microphone path (TG 15a closed, TG 15a' open) because comparator 22a 
will produce a trigger signal at its pin 1 before comparator 22a' will 
produce a trigger signal at its pin 7. The reason for this has to do with 
the different attenuations in the two signal detection paths and will 
become more apparent hereinafter. 
In any event, under the previously described circumstances, a trigger 
signal at CL of retriggerable monostable 23a before there is a trigger 
signal at CL of retriggerable monostable 23a' triggers on monostable 23a 
which, in turn, locks off monostable 23a, so that it can't be triggered on 
by the output it subsequently receives from pin 7 of comparator 22a'. When 
monostable 23a is triggered on, its Q NOT goes low setting Q of NAND flip 
flop 24a high. The result is maintenance of the original condition, i.e. 
SPKRENA being low and MIKENA being high. 
On the other hand, if a valid signal is received at the SPKR IN terminal, 
its validity will be recognized by comparator 22a', and comparator 22a' 
will produce a trigger signal at its pin 7. Since no voice signal is being 
supplied from microphone 11a to pin 3 of comparator 22a at this time, 
comparator 22a will not produce any trigger signal. As a result, 
monostable 23a' is triggered on locking off monostable 23a and resetting 
the output of component 24a so that its Q is low. This, in turn, sets 
SPKRENA high and MIKENA low and changes the states of all of the 
transmission gates. More specifically, TG 15a opens, TG 15a' closes, TG 
50a closes and TG 50a' opens. As a result, .div.6 attenuation is 
introduced into the signal detection path between pin 7 of op amp 20a and 
pin 3 of comparator 22a; the attenuation in the signal detection path 
between terminal 27a and pin 5 of comparator 22a' reverts from .div.6 to 
.div.3; the path for signals from microphone 11a is open; and the path for 
signals from the terminal SPKR IN to speaker 12a is closed. 
An explanation now will be given, with reference to the preferred 
embodiment of the invention employing background level sensing systems 21a 
and 21a', how variation of the attenuation in the signal detection paths 
maintains the previously selected signal path, i.e., the signal path from 
microphone 11a or the signal path to loudspeaker 12a until an actual 
change in the signal path is required. 
Referring to FIG. 5, op amp 30a, diode CR2, resistor R13 and capacitor C6 
constitute a peak detector, the latter two components being the low pass 
and storage device of the peak detector. The output of op amp 30a and 
diode CR2 is, within very small tolerances, the output voltage expected 
from an ideal diode placed at pin 3 of op amp 30a, but the output signal 
of op amp 30a is buffered and is of low impedance to drive diode CR2, 
resistor R13 and capacitor C6. 
The output of capacitor C6 (at terminal A) with respect to the +4 volt 
reference voltage thus is the approximate peak voltage of the voice 
frequency characteristic signals (when present), and when the voice input 
is between syllables and words, the peak voltage decays down to the peak 
value of the characteristic background noise. 
Referring to FIG. 6a, the noise and voice signals which are applied to pin 
3 of op amp 30a are shown by reference numeral 2, the largest amplitude 
portion 3 of these signals being voice signals and the other portion 4 of 
the signals being noise signals. The peak detected voltage is shown at 5 
in FIG. 6b and appears at terminal A in FIG. 5. 
An op amp 31a and diode CR3 functioning as an ideal diode together with a 
resistor R18 and a capacitor C7 constitute what could be referred to as a 
valley detector or an inverted peak detector. Capacitor C6 (terminal A) is 
connected to one input terminal (pin 5) of op amp 31a, and op amp 31a with 
diode CR3 thus tracks the lowest voltage that appears of capacitor C6 
which, of course, is the background noise voltage. The buffered output of 
op amp 31a rapidly discharges capacitor C7 through diode CR3 to the 
minimum voltage on capacitor C6, i.e., the background noise voltage. The 
voltage on capacitor C7 at terminal B (FIG. 5) is shown at 6 in FIG. 6b. 
Op amps 30a and 31a and their associated components constitute background 
level sensing network 21a. 
Capacitor C7 can be charged in several ways, e.g., by a constant current; 
by a resistor to a constant voltage; by a resistor to a voltage 
proportional to the voltage (at terminal A) on capacitor C6; or by a 
current proportional to the voltage at terminal A. In the embodiment 
illustrated, which is the preferred embodiment, the technique of charging 
capacitor C7 by a resistor (R14) to a voltage proportional (1:1) to that 
at terminal A has been chosen. 
The voltage decay of capacitor C6 to the minimum levels allowed by the 
noise peaks is via resistors R12 and R14, semiconductor leakage and bias 
currents. The voltage charging of capacitor C7 is via resistor R14, 
semiconductor leakage and bias currents. In order to prevent very small 
voice and noise peaks, as well as bias voltages, from activating the 
circuit, there may be provided a bias circuit (resistors R9 and R11) that 
provides a small bias which prevents comparator 22a, to be discussed 
hereinafter, from detecting below a minimum threshold. 
The result of the foregoing is that through the manipulation of the time 
constants in the circuit, different voice frequencies and noise 
frequencies can be detected with the valleys of the peak detected noise 
voltage, i.e., signal 6 in FIG. 6b, being placed on one input terminal 
(pin 2) of a comparator 22a. 
The other input terminal (pin 3) of comparator 22a is connected via an 
attenuator and, optionally, a filter, constituted by resistors R8, R15 and 
R16 and capacitors C3, C4 and C5 to the output terminal (pin 7) of op amp 
20a and thus receives attenuated and, optionally, filtered voice and noise 
signals. 
The voice and noise signals applied to pin 3 of comparator 22a are shown at 
7 in FIG. 6c, while the background noise level (shown at 6 in FIG. 6b) 
also is shown at 6 in FIG. 6c. These two signals 6 and 7 are referenced to 
one voltage, in the present circuit +4 volts, making ratiometric 
comparison possible, this being achieved by comparator 22a. As a result, 
whenever signal 7 exceeds signal 6, as shown at 8 and 9 in FIG. 6c, there 
is an output signal, which is shown at 8a and 9a in FIG. 6d, at the output 
terminal (pin 1) of comparator 22a that signals this condition and thus 
the presence of, in this case, voice signals. 
In the illustrated embodiment of this invention the voice and noise signals 
(signal 2) at the output terminal of op amp 20a are attenuated by a ratio 
of 1:3 before being applied to pin 3 of comparator 22a, as a result of 
which signal 7 is one third the amplitude of signal 2. If this were not 
done, the relative levels of signals 6 and 7 would be such that comparator 
22a could not differentiate between noise signals, on the one hand, and 
voice signals alone or with noise signals, on the other hand, because 
signal 6 always would be below signal 7 in level. It should be 
appreciated, however, that the same desired result may be achieved by 
amplifying signal 6 to, say, enhance its amplitude by 100%, as shown at 6a 
in FIG. 6b, while not attenuating signal 7, or attenuating it to a lesser 
degree than otherwise. The sensitivity of the circuit is directly related 
to the degree of amplification and/or attenuation. Care must be taken, of 
course, not to attenuate signal 7 or amplify signal 6 to the point where 
the voice signals do not exceed the level of signal 6 and hence never are 
detected. 
It will be apparent from the foregoing that background level sensing 
systems 21a and 21a' can be used to differentiate between low level noise 
signals and higher level voice signals, such that comparators 22a and 22a' 
will respond to voice signals, i.e., will produce an output in response to 
voice signals but not in response to lower level noise signals. 
As previously pointed out, when one of comparators 22a and 22a', say, 
comparator 22a, provides an output before comparator 22a', its monostable 
23a locks out monostable 23a', thus setting flip-flop 24a, TG 50a opens or 
remains open and TG 50a' closes or remains closed. Thus resistor R17 is 
out of circuit, while resistor R33 is in circuit. Consequently there is 
.div.3 attenuation in the path between terminal 65a (FIG. 5) and pin 3 of 
comparator 22a and .div.6 attenuation between terminal 27a and pin 5 of 
comparator 22a'. The result of this may be seen by comparing FIGS. 6c and 
6e, the former showing signal 2 attenuated by a factor of three and the 
latter showing signal 2 attenuated by a factor of six. Obviously the point 
where signal 7 exceeds signal 6 occurs considerably early in time in FIG. 
6c than in FIG. 6e, and this is what creates the required delay in the 
signal detection path (see FIGS. 6d and 6f). In other words, with the 
example chosen, a voice signal (having a non-step response) at microphone 
11a always will cause comparator 22a to provide an output before 
comparator 22a' because of the .div.3 attenuation in the signal detection 
path between terminal 65a and pin 3 of comparator 22a creating less of a 
delay than that created by the .div.6 attenuation in the signal detection 
path between terminal 27a and pin 5 of comparator 22a'. The slope of the 
rising voice signal thus causes the difference in delay in the detection 
circuit. 
It will be understood, of course, that while the invention has been 
described herein in connection with a telephone communication system 
wherein the two communicating stations are wired together, the invention 
is equally applicable to any mobile system. 
It also should be appreciated that the attenuating networks used herein as 
delay networks 19a, 19a' (FIG. 3) merely constitute one form of delay 
network that could be employed. Delay lines might be used, for example, 
these being capable of being by-passed by suitable switching devices, and 
the delay may be introduced anywhere between terminal 60a (FIG. 5) and pin 
3 of comparator 22a and anywhere between terminal 27a and pin 5 of 
comparator 22a'. It also may be possible to employ a single delay system 
or component, rather than the two shown in FIG. 3, and employ suitable 
switching devices to switch it either into the location of delay network 
19a in FIG. 3 or the location of delay network 19a' in FIG. 3. 
While the use of a retriggerable monostable timer as a time-out device for 
voice characteristic timing is common, a feature of the preferred 
embodiment of the invention shown in FIGS. 4 and 5 resides in the 
employment of a dual monostable timer 23a, 23a', or two separate timers 
interlocked so that the operation of one is dependent upon the other 
timing out. In this respect, pulses applied to pin 4 of monostable 23a 
will be allowed to discharge timing capacitor C9 (FIG. 5) if the interlock 
line (NOT SPKRT) (pin 3 of monostable 23a) is not held low. For the period 
that capacitor C9 is below the internal timing threshold, the output at 
pin 7 (NOT MIKET) of monostable 23a will be held low, thus preventing the 
pulses applied to pin 12 of monostable 23a' from discharging capacitor C17 
(FIG. 5). Thus the activation indication from one retriggerable 
monostable, is connected to the disable of the other monostable, as may be 
seen by the connection between pin 7 of monostable 23a and pin 13 of 
monostable 23a' and the connection of pin 9 of monostable 23a' to pin 3 of 
monostable 23a. 
The time out of each monostable is chosen for the best effect on normal 
speech patterns. It is set for monostable 23a by the values of capacitor 
C9 and resistor R22 and for monostable 23a' by the values of capacitor C17 
and resistor R35. Desirably it is about 150 milliseconds. 
It should be noted that the output of comparator 22a is timed by a timing 
network consisting of a resistor R19 and a capacitor C8, so that a set 
period of comparator output is needed to activate the following circuitry, 
i.e. monostable 23a. A similar timing network consisting of a resistor R34 
and a capacitor C16 is provided for comparator 22a'. The delay now put in 
the circuit is part of the filtering action, not allowing frequencies 
above, say, 5 KHz to activate the following circuitry. As voice 
characteristics have a slow rising amplitude, the effect of the detection 
and lower ratios to the noise background will allow the direction having 
the signal source decision and, hence, the lower ratio of voice to noise, 
to activate first. 
The holding of the decision for the last direction to be in use is 
necessary for the proper operation of the communications system. The 
direction that last had the control will need a lower signal level than 
the other direction, for reasons previously explained, allowing the 
circuit to differentiate between two signals that otherwise would be 
nearly the same or of the same characteristics. 
As previously disclosed, component 24a is a NAND flip flop. The activation 
of either of the direction monostables 23a, 23a' causes the output at pin 
11 of NAND flip flop 24a to reflect the last direction in control. The 
added signal labelled HOOK is used to lock out one direction without 
qualification. 
The time out of the monostables and the interlocking of the monostables 
selects the period of time that a decision must be held for. After the 
time out of the monostables, the direction decision may be changed if the 
opposite monostable than before is activated. 
As aforementioned, direction can be controlled by the externally generated 
HOOK voltage. The disabling of the further retriggering of a monostable is 
one way of preventing further signals from activating the path decision 
circuitry without necessarily changing the decision latching and output. 
The circuitry used for this consists of a logic level input driving diode 
CR4 and a capacitor C8 connected to pin 4 of monostable 23a. The holding 
of the logic level to a low prevents the charging of capacitor C8 via 
resistor R19. 
Strictly by way of example, the following table lists various components 
and parameters that may be employed in the system of FIG. 5. 
______________________________________ 
Component Type +8 v Ground 
______________________________________ 
16a, 20a MC 3458P1 8 4 
30a, 31a MC 3403P 4 11 
30a', 31a' 
15a, 15a' HCC/HCF 4066 14 7 
50a, 50a' 
22a, 22a' LM 2903N 8 4 
23a, 23a' HCC/HCF 4538 16 8 
16a' LM 380N-8 SHOWN 
U7 CD 4011 14 7 
U8 uA 78L08 -- -- 
______________________________________ 
While preferred embodiments of this invention have been disclosed herein, 
those skilled in the art will appreciate that changes and modifications 
may be made therein without departing from the spirit and scope of this 
invention as defined in the appended claims.