Automatic disconnect circuit

An automatic disconnect circuit is described for disconnecting a remote telephone in resonse to a hang-up click generated when the remote telephone goes on-hook. To distinguish a remote hang-up click from a local hang-up click, voice signals, and the like, all signals on the line are passed through a discriminator for attenuating high frequencies. Signals passing through the discriminator are sensed both as to their amplitude and their duration. Any such signal whose amplitude is too small results in no disconnect. Similarly, any such signal whose duration is atypical of a remote hang-up click results in no disconnect. Only a signal whose amplitude and duration are within predetermined limits, typical of a remote hang-up, causes the remote telephone to be disconnected.

BACKGROUND OF THE INVENTION 
This invention relates generally to automatic disconnect systems for 
telephone circuits, and is particularly directed to a system for 
disconnecting a telephone circuit in response to a "hang up" by one party 
on a telephone line. 
In the type of telephone system under consideration, a telephone line and 
intermediate switching circuitry couple a calling party's line to a called 
party's line. Generally, the system also includes means for detecting when 
one of the party's lines goes "on-hook" so that his telephone can be 
disconnected and made ready to receive or initiate further calls. 
Typically, the disconnect occurs approximately twenty seconds after the 
"on-hook" condition is detected. 
Recently, apparatus for automatically recording messages from a caller and 
for transmitting recorded messages to a customer or client have been 
introduced. When a party is in communication with such apparatus, it is 
highly desirable that the customer's telephone line be quickly 
disconnected when he hangs up, i.e., goes "on-hook". For example, if the 
customer does not desire to hear the recorded message, he must have the 
ability to hang up and have his telephone freed or disconnected in order 
to make and receive further calls. 
In the past, disconnect circuitry has typically sensed the hang-up click 
generated when a party hangs up, and based on that click, a disconnect was 
effected. Typical of such circuitry is that described in U.S. Pat. No. 
3,676,605. 
One of the assumptions that such prior art circuitry relied on was that the 
hang-up click was greater in amplitude than the voice signals. Thus, when 
a signal greater in amplitude than voice signals was sensed, that signal 
was used to trigger the disconnect. Of course, when a signal other than a 
hang-up click exceeded the expected amplitude of voice signals, the 
disconnect was improperly effected. 
In modern telephone systems using electronic switching systems, the hang-up 
click is not invariably greater in amplitude than the voice signals. In 
fact, it is usually lower. Hence, amplitude discrimination alone is no 
longer effective for sensing a hang-up click. 
Further, automatic message transmitting/receiving equipment is frequently 
used in connection with a local operator who initiates a call to a 
customer, places the apparatus on line with the customer, and then hangs 
up. Thus, the customer will be in communication with the automatic message 
transmitting equipment and the local operator is free to initiate another 
call. 
Obviously, any hang-up click generated when the local operator hangs up 
must be distinguished from the hangup click of the customer. Otherwise, 
the customer's line would be disconnected when the local operator hangs 
up. The type of prior disconnect circuitry discussed above is generally 
not capable of reliably making that distinction. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of the invention to provide an improved 
disconnect system for use in telephone communications. 
It is a more specific object of the invention to provide a disconnect 
system for use with automatic message transmitting/receiving apparatus.

SUMMARY OF THE INVENTION 
The automatic disconnect circuit described herein disconnects a remote 
telephone when the remote telephone goes on-hook. The disconnect circuit 
includes a signal discriminator coupled to the telephone line for 
attenuating voice signals, high frequency components of noise signals, and 
the like. Clicks generated by a telephone going on-hook, and low frequency 
components of the DC level shift resulting from the local telephone going 
on-hook, both pass through the discriminator. A threshold detector 
generates a control signal whenever the magnigude of a signal from the 
discriminator exceeds a predetermined level. Hence, low amplitude signals 
are ignored. The control signal is received by a signal generator which 
senses the duration of the control signal and generates a disconnect 
signal only when the duration of the control signal is longer than a first 
predetermined interval and shorter than a second predetermined interval. A 
control signal generated in response to the DC level shift caused by the 
local telephone going on-hook is too long, i.e., beyond the second 
predetermined interval, and hence results in no disconnect signal. A 
control signal developed by a short duration or transient signal is 
shorter than the first predetermined interval and, hence, results in no 
disconnect signal. Only a control signal generated by a remote hang-up 
click has a duration greater than the first interval and shorter than the 
second interval. Accordingly, a disconnect signal is generated only in 
response to a hang-up click generated by the remote telephone going 
on-hook. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The automatic disconnect circuit according to the invention may be used in 
a variety of applications. However, because it is particularly useful in a 
communication system wherein a remotely located telephone is placed in 
communication with a local automatic message transmitting/receiving 
apparatus, the invention is described below in terms of that environment. 
Referring now to FIG. 1, there is shown a detailed circuit diagram of a 
preferred embodiment of the automatic disconnect circuit of the invention. 
As illustrated, a telephone line having tip and ring lines is connected to 
a remote telephone (not shown) and, through a pair of relay-operated 
switches 10 and 12 to a local telephone 14 and an automatic message 
transmitting/receiving apparatus 16. Typically, an operator using the 
local telephone 14 may establish communication with another party at the 
remotely located telephone and inquire if that party desires to hear a 
message recorded on the apparatus 16. If the remote party assents to 
hearing the message, the local operator will activate the apparatus 16 for 
automatically transmitting the recorded message and then place the 
telephone 14 on-hook. Thereafter, the operator may again use the local 
telephone 14 to establish communication with yet another party while the 
apparatus 16 is transmitting its message to the first party. 
Should the party who is in communication with the apparatus 16 place his 
remote telephone on-hook to terminate communication with the apparatus 16, 
it is desirable that the remote telephone be quickly disconnected so that 
it is free to make and receive further telephone calls. In other 
applications, such disconnect of a telephone has been effected by 
apparatus which senses the click which is generated when a telephone goes 
on-hook. However, in many applications, such as the one illustrated in 
FIG. 1, merely sensing the telephone click is not sufficient. For example, 
when the local operator places the local telephone 14 on-hook, a click is 
generated on the telephone line. However, if that click were used to 
disconnect the telephone line, the apparatus 16 would be disconnected 
improperly from the remote telephone. Hence, in the illustrated 
application and in other applications, the disconnect circuitry must 
ignore the click generated by the on-hook condition of the local telephone 
14 but must quickly disconnect the telephone line, i.e., open the switches 
10 and 12, when the remote telephone goes on-hook. 
Before describing the operation of the automatic disconnect circuit herein, 
the effect of the local telephone 14 going on hook will first be 
described. Assuming that the switches 10 and 12 are open, a potential of 
approximately 50 volts will appear across the tip and ring lines. When the 
operator uses the local telephone 14 to establish communication with the 
remote telephone, the switches 10 and 12 close (by conventional apparatus 
not illustrated in FIG. 1) and the potential across the tip and ring lines 
drops to approximately 5 volts. When the called party answers the remote 
telephone, a click appears on the telephone line but no substantial DC 
level shift occurs across the tip and ring lines. When the local operator 
places the apparatus 16 on line, the potential across the tip and ring 
lines then drops to approximately 2.5 volts, and when the operator then 
places the local telephone 14 on-hook, the potential across the tip and 
ring lines rises to approximately 5 volts. The last mentioned change in 
potential must not be interpreted by the disconnect circuit as a hang-up 
click generated by the remote telephone. When the party then places his 
remotely located telephone on-hook, a click appears on the telephone line 
but the potential across the tip and ring lines stays at approximately 5 
volts. With the above described DC level shifts in mind, the automatic 
disconnect circuit of the invention will now be described. 
As shown, the tip and ring lines are coupled via a capacitor 18 and a 
transformer 20 into the first stage of the automatic disconnect circuit, 
that first stage being a signal discriminator comprising a buffer 
amplifier 22 and a pair of amplifiers 24 and 26 which are connected so as 
to form a low pass filter. The output of the low pass filter is fed to a 
threshold detector comprising a pair of comparators 28 and 30, an inverter 
32, and a NOR gate 34. As is described in more detail below, the threshold 
detector generates at its output lead 35 a control signal whenever a 
hang-up click is sensed. 
The output of the threshold detector is coupled into a disconnect signal 
generator which generates a signal for opening the switches 10 and 12 in 
response to a hang-up click generated by the remote telephone going 
on-hook. The threshold detector includes a transistor 36, a pair of 
comparators 38 and 40, and NAND gates 42, 44 and 46. The disconnect signal 
appears at the lead 48 coupled to the output of the NAND gate 46 and that 
signal is applied to a driver 50 for energizing a relay coil 52, the 
latter of which opens the switches 10 and 12 when the disconnect signal is 
generated. 
Referring more specifically to the first stage of the automatic disconnect 
circuitry, the secondary of the transformer 20 is coupled via a resistor 
54 to the input of the buffer amplifier 22. Diodes 56 and 58 are coupled 
as shown to the secondary of the transformer 20 to prevent large amplitude 
signals from overloading the buffer amplifier 22. A resistor 60 is coupled 
between the input and output of the amplifier 22 for setting the gain of 
the latter. 
The output of the amplifier 22 is coupled via resistors 62 and 64 to the 
input of the amplifier 24, the latter of which is interconnected with the 
resistors 66 and 68 and capacitors 70 and 72 as shown to form a part of a 
low pass filter of conventional design. 
The output of the amplifier 24 is coupled via resistors 74 and 76 to the 
input of the amplifier 26, the latter of which is connected as shown to 
resistors 78 and 80 and capacitors 82 and 84 to form the second half of 
the low pass filter. The illustrated resistors and capacitors which are 
connected to amplifiers 24 and 26 are selected to provide a 120 Hertz low 
pass filter. With this arrangement, the low pass filter substantially 
attenutates voice signals and high frequency components of noise, and yet 
passes the low frequency components attributable to the DC level shift 
which occurs when the local telephone 14 goes on-hook. 
The output of the low pass filter is coupled to the inputs of the 
comparators 28, 30 for generating a control signal when the receipt of any 
signal from the low pass filter which is in excess of a predetermined 
magnitude. Referring to the comparator 30, it is coupled by resistor 86 to 
a 5 volt supply, and by a resistor 88 to a 10 volt supply. A resistor 90 
and a diode 92 provides feed back from the output to the input of the 
comparator 30. With this arrangement, a quiescent bias of approximately 
51/2 volts is applied to the positive terminal of the amplifier 30. When a 
signal from the low pass filter is approximately equal to 51/2 volts, the 
output of the comparator 30 goes low, thereby turning off the diode 92, 
whereupon the bias at the positive terminal of the comparator 30 drops to 
about 5.1 volts. As the signal at the negative input terminal of the 
comparator 30 declines in amplitude, it will eventually reach a 5.1 volt 
level, whereupon the output of the comparator 30 will then go high. The 
output of the comparator 30 is essentially a rectangular pulse type signal 
which goes from a normally high level to a normally low level and is 
converted by the inverter 32 to a positive-going pulse on the lead 94. 
Because of the hysteresis associated with the comparator 30, low level 
noise which is superimposed on a click or other signal which causes the 
comparator 30 to change states will have no effect on the status of the 
comparator 30. As a result, the output of the comparator 30 and that of 
the inverter 32 is a clean rectangular pulse whose duration corresponds to 
the duration of the incoming signal which caused the comparator 30 to 
change states. 
Referring now to the comparator 28, its positive input terminal is coupled 
to ground via resistor 96 and to the 5 volt supply by resistors 98 and 86. 
A resistor 100 serially connected with a diode 102 couples the positive 
input to the output of the comparator 28. With this arrangement, the 
positive terminal of the comparator 38 is at a potential of approximately 
4.5 volts and its output is low under that condition. When the negative 
input terminal of the comparator 28 receives a negative-going signal which 
is equal to or more negative than 4.5 volts, the output of the comparator 
28 goes high, the diode 102 turns off, and the potential at the positive 
terminal of the comparator 28 rises to approcimately 4.9 volts, at which 
point, the output of the comparator 28 again goes low. As with the 
comparator 30, the hysteresis built into the comparator 28 insures that 
low level noise superimposed on a click or other signal does not affect 
the status of the comparator 28. 
The output of the inverter 32 and the output of the comparator 28 are each 
coupled to one input of a NOR gate 34. Hence, when either a positive-going 
or a negative-going signal from the low pass filter causes either of the 
comparators 28, 30 to change states, the output of the NOR gate 34 goes 
low. This output is designated herein as the "control signal" which 
appears on the lead 35 and which takes the form of a rectangular, 
negative-going pulse whose duration corresponds to that of a signal from 
the low pass filter. For example, waveform A of FIG. 2 illustrates a 
negative-going signal 104 and a positive-going signal 106, each of which 
are received by the threshold detector from the low pass filter. In 
response to these signals, control signals 108 and 110 (waveform B of FIG. 
2) are developed. In the discussion below, it is assumed that the control 
signal 108 is developed in response to a click generated by the remote 
telephone going on-hook and the longer duration control signal 110 is 
developed in response to the click and DC level shift caused by the local 
telephone 14 going on-hook. Accordingly, only the control signal 108 will 
cause a disconnect signal to be generated. 
Referring again to FIG. 1, control signals on the lead 35 are coupled 
through a resistor 112 to the base of the transistor 36 which is normally 
on. The collector of the transistor 36 is coupled to a 10 volt supply via 
a resistor 114 and to one side of a grounded capacitor 116. Hence, the 
capacitor 116 remains discharged while the transistor 36 is on. 
The ungrounded side of the capacitor 116 is also coupled to the positive 
input terminal of the comparator 38 and to the negative input terminal of 
the comparator 40. The other input terminals 118, 120 of the comparators 
40, 38 are coupled to reference voltages generated by the voltage divider 
comprising resistors 122, 124 and 126. In this example, the potential at 
the terminal 118 is approximately 6 volts and the potential at the 
terminal 120 is approximately 1 volt. The outputs of the comparators 38 
and 40 are coupled to corresponding inputs of the NAND gate 42. 
When a control signal is received by the transistor 36, the latter is 
turned off, whereupon the capacitor 116 charges toward 10 volts. Waveform 
C of FIG. 2 depicts the voltage across the capacitor 116. When that 
voltage reaches 1 volt (the reference potential at input terminal 120), 
the comparator 38 (whose output is normally low) generates a 
positive-going pulse 128 which starts at time t.sub.2, as indicated by 
wafeform D of FIG. 2. The pulse 128 continues as long as the control 
signal 108 continues, i.e., until time t.sub.3, whereupon the capacitor 
116 discharges (waveform C) and the pulse 138 terminates. 
The comparator 40 normally has a high output. Hence, the combination of the 
pulse 128 and the high output of the comparator 40 cause the NAND gate 42 
to generate a negative-going pulse 130 (waveform F of FIG. 2) at time 
t.sub.2 and to continue the pulse 130 until shortly after time t.sub.3. 
Because of the delay inherent in the discharge of the capacitor 116, the 
comparators 38 and 40, and the NAND gate 42, the pulse 130 continues for a 
short interval past time t.sub.3. Because of the short duration of the 
control pulse 108, the voltage across the capacitor 116 does not reach 6 
volts. Hence, the comparator 40 does not change states. 
Referring to FIG. 1 again, the control signal (waveform B) is applied as an 
input to the NAND gate 46, and the output of the gate 42 (waveform F) is 
applied as one input to the NAND gate 44. The other input to the latter 
gate is coupled to the output of the gate 46 via a capacitor 132 and 
through a resistor 134 to a 10 volt supply. A diode 136 parallels the 
resistor 134. 
Normally, the output of the gate 44 is low, and the output of the gate 46 
is high. When the output of the gate 46 goes low, that change of status is 
treated as a disconnect signal, whereupon the driver 50 energizes the 
relay coil 52 for opening the switches 10 and 12. 
In operation, the leading edge of waveform B is received by the gate 46 at 
time t.sub.1, but this causes no change in the status of the gate 46, 
i.e., its output remains high. At time t.sub.2, the gate 44 receives the 
leading edge of the waveform F, whereupon the output of the gate 44 goes 
from low to high. However, since waveform B is still low, the status of 
the gate 46 still does not change. 
At time t.sub.3, the waveform B goes high, the output of gate 44 is high 
(due to the fact that the waveform F is still low), and the gate 46 
develops a low output, i.e., a disconnect signal. As described above, this 
causes the switches 10 and 12 to open for disconnecting the remote 
telephone. 
In order to retain the output of the gate 46 at a low level long enough to 
open the switches 10 and 12, the negative-going disconnect signal is 
transiently coupled back to one input of the gate 44. Hence, the latter's 
output remains temporarily high and the output of the gate 46 is held low 
for a predetermined interval, dependent on the time constant associated 
with the capacitor 132. 
To generate a disconnect signal as described above, the waveform F should 
continue at least as long, and preferably slightly longer, than the pulse 
108. This condition will occur whenever a click is generated in response 
to the remote telephone going on-hook, but it will not occur when the 
local telephone 14 goes on-hook first. 
Referring to FIG. 2, the control signal 110 is generated in response to the 
local telephone 14 going on-hook. This control signal is longer in 
duration than the control signal 108 because of the low frequency 
components of the DC level shift. Accordingly, the capacitor 116 charges 
for a longer interval and ultimately reaches (and exceeds) the 6 volt 
level (waveform C of FIG. 2) required to trigger the comparator 40. 
As shown, the control signal 110 begins at time t.sub.4 and the potential 
across the capacitor 116 reaches 1 volt at time t.sub.5. At this point, 
the comparator 38 develops a pulse signal 138 and the NAND gate 42 
generates a pulse signal 140. 
At time t.sub.6, the potential on the capacitor 116 reaches 6 volts and the 
comparator 40 generates a pulse signal 142. In response to the signal 142, 
the NAND gate 42 discontinues its pulse signal 140 (time t.sub.6). The 
signals 110, 138 and 142 all continue until time t.sub.7, but the signal 
140 is shorted by the length of the interval t.sub.6 -t.sub.7 during which 
the signal 142 is generated. Hence, the signal 140 is discontinued prior 
to the discontinuance of the control signal 110 and no disconnect signal 
is generated. This result can be appreciated by recalling that both inputs 
to the gate 46 must be high in order to generate the disconnect signal. 
That condition exists only when the control signal (waveform B) is 
discontinued (i.e., goes high), and waveform F is low. As indicated in 
FIG. 2, when the local telephone goes on-hook, the waveform F goes high 
before the control signal 110 discontinues. Accordingly, no disconnect 
signal is generated. 
In order to ignore short noise pulses and the like, the system is designed 
to generate a disconnect signal only in response to a control signal which 
has at least a minimum duration. For example, FIG. 2 shows that the 
comparator 38 generates the control pulse 128 only at time t.sub.2 in 
response to the voltage across the capacitor 116 reaching 1 volt. If the 
control signal terminates before the voltage on the capacitor 116 reaches 
1 volt, the comparator 38 will not generate a pulse signal; nor will the 
gate 42. Hence, no disconnect signal will be generated. Thus, any output 
from the low pass filter must exceed a predetermined amplitude and 
continue for a predetermined interval (t.sub.1 -t.sub.2) in order to 
generate a disconnect signal. The reference potential at the negative 
input terminal 120 of the comparator 38 is selected so that the latter 
changes state only after the minimum duration (t.sub.1 -t.sub.2) of a 
control signal generated in response to the remote telephone going 
on-hook, typically about one millisecond. 
In addition, and as indicated above, the control signal must discontinue 
prior to the time t.sub.6 at which the comparator 40 changes state. Thus, 
the interval t.sub.4 -t.sub.6 is selected to be shorter than the duration 
of a control signal generated in response to the DC level shift which 
occurs when the local telephone goes on-hook. That time is selected by the 
reference potential applied to the input 118 of the comparator 40. Thus, 
any control signal must have a duration of at least t.sub.1 -t.sub.2 or 
t.sub.4 -t.sub.5, and less than t.sub.4 -t.sub.6 in order to generate a 
disconnect signal. Typically, the interval t.sub.4 -t.sub.6 is about 10 
milliseconds. Thus, a time "window" is established for discriminating 
between pulses responsive to remote hang-up on the one hand, and control 
pulses responsive to either local hang-up or short term transient signals 
on the other hand. 
From another point of view, the signal generated by the NAND gate 42 may be 
thought of as a "window" signal which establishes a time "window" only 
during which a disconnect signal can be generated. If that window signal 
is too short (non-existant) or too long (longer than t.sub.6), no 
disconnect signal will be generated. That is, the window signal is 
generated only when the control signal continues for the first, minimal 
interval (t.sub.1 -t.sub.2 or t.sub.4 -t.sub.5), and the window signal is 
discontinued on the first to occur of: (a) the control signal 
discontinuing (signal 108 at t.sub.3) and (b) the control signal 
continuing for the second, maximum interval (signal 110 at t.sub.6). Using 
this terminology, the disconnect signal is generated when the control 
signal discontinues, and then the window signal discontinues. Accordingly, 
control signals which are shorter than the minimum interval do not result 
in a "window" signal or a disconnect signal; control signals which are 
developed in response to the hang-up of a local telephone and which have a 
duration longer than the maximum interval result in the discontinuance of 
the window signal prior to the discontinuance of the control signal and do 
not, therefore, generate a disconnect signal. 
The automatic disconnect circuit described above may be used in a variety 
of applications, but is particularly useful in connection with automatic 
message transmitting/receiving apparatus. It enables a local operator to 
initiate communication with a remote telephone, to transfer communication 
to the automatic message transmitting/receiving apparatus, and then to 
hang up without disconnecting the remote telephone. However, if the party 
using the remote telephone hangs up, his telephone is promptly 
disconnected, principally because the automatic disconnect circuit 
discerns the difference between a local hang-up and a remote hang-up. That 
same disconnect is not achieved by substituting for the disclosed window 
generator a bandpass filter to pass substantially only those frequencies 
generated by a hang-up click. Such a bandpass filter would attenuate the 
characteristics relied on by the automatic disconnect circuit to 
distinguish a local hang-up from a remote hang-up. 
To enhance the effectiveness of the automatic disconnect circuit in certain 
environments, it may be desirable to "roll-off" the voice frequencies 
generated by the local telephone and/or those transmitted by a local 
automatic message transmitting/receiving apparatus to prevent low 
frequency components of voice signals from developing a disconnect signal. 
No such "roll off" is required for the remote telephone because the 
telephone line acts as a filter to remove such low frequency components. 
Although the invention has been described in terms of a specific 
application and a preferred structure, it will be obvious to those skilled 
in the art that many alterations and modifications may be made without 
departing from the invention. Accordingly, all such modifications and 
alterations aredeemed to be within the spirit and scope of the invention 
as defined by the appended claims.