GB2092411A - Telephone answering machines - Google Patents

Abstract

A telephone answering machine has a loudspeaker which enables a user to listen to a caller on a telephone line. The machine senses when a telephone receiver is brought into use, and disables the audible output in response thereto. In the preferred embodiment, the audible output is disabled by arranging for the machine to drop the telephone line. The sensing of the receiver being brought into use is preferably achieved by storing a voltage corresponding to that on the telephone line, and detecting when the voltage on the telephone line drops below the stored voltage. This arrangement is also used to detect when the caller has hung up.

Description

SPECIFICATION Telephone answering machines This invention relates to telephone answering machines.

Some such machines have means for providing an audible output so that, during operation of the machine, a user can listen to a caller on the telephone line. It is desirable, during operation of the answering machine, for the user to be able to pick up a telephone receiver, so that he can carry on a conversation with the caller.

In accordance with one aspect of the invention, a telephone answering machine has means for providing an audible output to enable a user to listen to a caller on a telephone line, and a sensor to detect a drop in voltage on the line due to a telephone receiver being brought into use, the machine being responsive to said detection for disabling said audible output.

The machine may be provided with its own loud-speaker, or may have an output socket by means of which the audible output can be delivered to an external speaker. The disabling of the output can be done by eiectronically or mechanically preventing an audio signal from reaching the speaker, in which case the machine can be arranged to continue a recording operation, or, as in the preferred embodiment described below, the entire machine can be arranged to "drop" the telephone line.

Both of these arrangements have the advantage that feedback, which may occur if the loudspeaker is operating at the same time as a telephone receiver is being used, is prevented. The latter arrangement has the additional advantage that it prevents the drop in line voltage which would otherwise occur when the telephone receiver is brought into use, and the telephone line current is thus shared between the receiver and the answering machine.

The use of a voltage sensor provides an automatic operation to prevent the disadvantages mentioned above without having the disadvantages of poor reliability and difficulty of installation which would occur if a mechanical switch were used.

Problems may be encountered in the detection of the voltage drop, due primarily to the fact that the voltage which appears on a telephone line during its use tends to vary from time to time by a substantial amount, for example from about 6 to 16 volts. Thus, although the voltage tends to remain fairly stable over short periods, for example over the period of a conversation, the fairly large variations over longer periods creates difficulties in the design of a sensor which is suitable for detecting the picking up of a receiver but is unaffected by the voltage variations.

In accordance with another aspect of the invention, a sensor for sensing when a receiver on a telephone line has been brought into use comprises means for storing a voltage having a level determined by the voltage on the telephone line, and means responsive to the appearance on the telephone line of a voltage which is less than the stored voltage for providing an output signal in response to the receiver having been brought into use. The sensor is preferably arranged to have a threshold so that the output signal is not provided unless the voltage on the line is at least a predetermined threshold amount less than the stored voltage.

The use of a stored voltage which is dependent upon the line voltage means that the sensor can operate independently of the absolute voltage on the line during its use.

In a preferred embodiment the sensor is operable not only to sense the voltage drop caused by a receiver being brought into use, but also what is known as a "K-break", which is a short period, occurring after a caller has hung up, during which there is no voltage on the telephone line. In this way, the sensor can have a dual function in that it can be used to indicate both when the receiver has been picked up and when a caller has hung up. If desired, the sensor could alternatively be used only to detect when a caller has hung up, and not for detection of a receiver being brought into use.

The sensor is preferably used in a telephone answering machine, and the output signal can be arranged to stop a recording operation of the machine.

The storage means in the sensor is preferably a capacitance connected to the telephone line so that the voltage across the capacitance tracks the voltage on the line. The output signal providing means is responsive to the difference between the voltage on the line and that stored by the capacitance.

A Zener diode may be used to provide the desired threshold level. The arrangement is preferably such that a voltage stored by the capacitance which exceeds the voltage on the telephone line reverse biases the Zener diode. An output signal is not, however, produced until the breakdown voltage of the Zener diode is reached.

It is desirable from the point of view of safety and to avoid interference with the telephone line for the sensor to be isolated from the rest of the circuitry in the answering machine. Therefore, there is preferably no d.c. coupling between the sensor and the rest of the machine. In a preferred embodiment there is in fact no electrical coupling at all, and instead an opto-coupler is used. It is also preferable for there to be no power supply to the sensor, which is preferably instead powered merely by the voltage on the telephone line.

The sensor may also be used for the detection of a ringing current on the line. In this case, the output signal would be repeatedly provided at a frequency determined by that of the ringing signal. The output signal can then be delivered to a discriminator which provides an output signal indicative of the presence of the ringing signal, as will be described more fully later.

The sensor may incorporate a rectifier, preferably a full wave rectifier, which may be a.c. coupled to the telephone line while awaiting calls. The a.c. coupling may be replaced by a d.c. connection when the line is taken up on receiving a call.

The telephone answering machine preferably has means for detecting the presence of a "pay tone" (i.e. the tone which appears when a call is made from a coin-operated call-box before a coin is inserted).

The output of this detector can be used to delay the operation of the answering machine until the coin has been inserted.

In accordance with a further aspect of the invention, a telephone answering machine is provided with an amplifier which is operable in each of three modes, the amplifier in the first mode having its input coupled to a telephone line and its output to a pay-tone detector, in a second mode having its input connected to the telephone line and its output connected to a recording means for recording messages transmitted on the telephone line, and in the third mode having its input connected to a play-back means and its output connected to the telephone line for transmitting announcements from the play-back means to the telephone iine.The amplifier is preferably also usable in a fourth mode in which it amplifies signals from an audio input (for example connected to a microphone) and delivers them to the or another recording means for recording an announcement to be played back by the play-back means.

The selective connection of various devices to the input and output of the amplifier can be achieved using mechanical switching andlor electronic analogue switches.

Another problem encountered in the design of telephone answering machines concerns the detection of a ringing current on the telephone line. The machine must be able to distinguish between such a ringing current and dialling pulses to avoid erroneous operation. This can cause problems, because the voltage levels present when a ringing current is on the line can be within the range encountered when dialling pulses are on the line.

Furthermore the frequency of ringing is very similar to that of dialling pulses.

In accordance with another aspect of the invention there is provided means for determining when a ringing current is present on a telephone line, comprising means responsive to the ringing current for providing square-wave pulses at a frequency determined by that of the ringing current, the duration of the pulses being less than the duration between pulses, and means operable to detect whether said pulses have less than a predetermined duration and, if so, to provide an output signal indicative of the presence of the ringing current.

This provides a very effective way of discriminat ing between ringing current and dialling pulses.

Dialling pulses are at a slightly lower frequency than ringing current, and therefore the duration of the pulses produced when a ringing current is present is slightly less than when dialling pulses are present.

However in the arrangement of the invention this distinction is advantageously increased by convert ing the alternating sine-wave ringing current into a square-wave pulse train with a substantial difference in the durations of the pulses and the spaces therebetween, while the square-wave dialling pulses create a pulse train having a 63-70% duty cycle.

This can be achieved using a wave shaping means responsive to the ringing current for providing the train of square-wave pulses. The threshold of the wave shaping circuit is arranged to produce an appropriate mark-space ratio.

The waveform presented to the wave shaping circuit can in fact be derived using a full-wave rectifier, so that the pulses in fact have twice the frequency of the ringing current.

The wave shaping circuit can in fact take the form of the sensor described above.

In a preferred embodiment the detection of the duration of the pulses is done using a time constant circuit including a capacitor which is charged slowly and rapidly discharged (or vice versa). The time constant is chosen so that relatively long pulses occurring during dialling permit both charging and discharging of the capacitor, whereas the pulses produced when a ringing current is present are too short to allow the relatively slow charging (or discharging). In this manner, a steady level is produced at the output of the time constant circuit when a ringing current is present, and an alternating level is produced when the dialling pulses are present.

This output can be delivered to another time constant circuit, which is arranged so that the alternating level created during dialling is ineffective to permit charging (or discharging) of a further capacitor, but the steady level does allow this, so that the output of the second time constant circuit has a constant level which indicates whether or not the ringing current is present.

An arrangement embodying the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure l is a block diagram of a telephone answering machine in accordance with the invention, Figure 2 is a detailed circuit diagram of a sensing section of the machine of Figure 1, and Figure 3 is a circuit diagram showing a muting device used in the machine of Figure 1.

Referring to Figure 1, the telephone answering machine 2 is connected to telephone lines A and B.

The machine 2 comprises an interface 4 which receives audio signals from the telephone lines and delivers these to the rest of the machine, and also receives audio signals from the machine to deliver to the telephone lines. The machine incorporates two magnetic tape cassette decks, one of which, indicated at 6, is used to play back pre-recorded announcements which are transmitted on the telephone line after a caller has come onto the line. The other deck, 8, records any message the caller may wish to leave.

The machine also incorporates an amplifier 10, and this has an input and an output both of which are connectable via audio paths (indicated in broken lines) to the tape decks 6 and 8, and via analogue switches indicated at 12 to the interface 4. The amplifier's input is also connected to a microphone 14, by means of which a user can record announcements to be played back by deck 6, and its output is also connected to a speaker 16 which can be used when playing back messages recorded on deck 8, and when a user wishes to hear a caller on the line.

The machine can be controlled manually by manual controls indicated schematically at 18, and automatically in response to signals from a sensing section indicated at 20. The manual controls 18 incorporate mechanisms for changing the mode of operation of each of the tape decks 6 and 8, and also switches for selectively opening and closing the audio paths in accordance with the desired mode of operation. The machine also incorporates logic circuitry indicated at 22 which responds to signals from the manual controls 18 and the sensing section 20 to control the operation of the machine.

The machine as so far described is quite standard, and particular arrangements for carrying out desired functions in response to signals appearing on the telephone lines and the operation of the controls by a user will be apparent to those skilled in the art.

The sensing section 20 of the machine has a number of advantages over the sensing arrangements used in the prior art. The sensing section 20 is connected to receive signals from the telephone lines. The sensing section 20 includes a sensor 24 which is connected directly to the telephone line A, and indirectly, via a relay contact in the interface 4, to telephone line B, as will be described more fully later. The sensing section 20 is operable to provide, on a path 26, a signal which indicates when a ringing current is on the telephone lines. This signal is delivered to the logic circuitry 22, and is operable to cause the machine to take up the line and begin its operation, which commences with the playing back, by deck 6, of an announcement which is fed to the telephone line.

The sensing section 20 is also operable to deliver, on a path 28, a signal which indicates that the caller has hung up. This signal is fed to the logic circuitry 22, and is operable to terminate the operation of the answering machine.

As will be described more fully later, the sensing section 20 also receives signals on a path 30 from a switch in the recording deck 8. The sensing section 20 uses these signals to determine when the tape on the deck has run out, and in response to this provides a further signal on path 28 to terminate operation of the answering machine.

The sensing section 20, and in particular a paytone discriminator 32 thereof, receives audio signals on a path 34 from the amplifier 10to determine whether a pay-tone is present on the telephone line.

The pay-tone discriminator 32 is operable to deliver signals on a path 36 to control the analogue switches 12 in accordance with whether or not the pay-tone is present, and is also operable to provide a disabling signal on a path 38 which is delivered to the tape deck 6 to disable the latter and thus prevent the playing back of an announcement until the paytones on the line have ceased.

The basic operation of the sensing section 20 is as follows. The sensor 24 provides output signals indicative of signals appearing on the telephone line.

The output signals are delivered via a path 40 to a ringing discriminator 42. The discriminator 42 determines, in a manner to be described below, whether or not a ringing current is present on the line, and if so delivers a signal to an adjustment circuit 44. After a predetermined number of rings (determined by manually setting the adjustment circuit 44), an output signal is delivered on path 26 to the logic circuitry 22. This causes the answering machine to take up the telephone line, during the course of which a relay in the interface 4 is energised, as is standard.

As the answering machine takes up the telephone line, an output signal is provided by the sensor 24 on a path 46. This is delivered to a three second delay circuit 48. A signal on an output path 50 of the delay circuit 48 causes a detection circuit 52 to be armed.

The detection circuit 52 is also operable to receive output signals from the sensor 24 on path 46. A change in state of the signal on path 46 is caused when a caller hangs up, and this is used by the detection circuit 52 to provide an output signal on path 28. The detection circuit 52 is inoperable until it is armed by the signal on path 50, which occurs three seconds after the answering machine has come on line, and this ensures that a stable state is reached in the sensing circuitry before the detection circuit 52 is allowed to operate. The three second delay is also used for other purposes, as will be described.

The path 30 is connected to an on-off switch which is coupled to the shaft carrying the supply spool of the tape deck 8. The path 30 therefore carries pulses having a frequency which is reiated to the speed of rotation of the supply spool. These pulses are delivered to an end-of-tape detector 54 in the sensing section 20. When the tape runs out, and the supply spool stops rotating, this is sensed by the detector 54 which then delivers an output signal to path 28, and also delivers an inhibit signal on a path 56 to the ringing discriminator 42. The signal on the path 56 prevents the discriminator 42 from operating, and therefore stops the machine from answering any further calls.

When a caller rings, and the machine takes up the telephone line, the pay-tone discriminator 32 provides signals on its output paths 36 and 38 to delay the operation of the tape deck 6 and to keep the input of the amplifier connected to the interface 4 to the telephone line and the output of the amplifier disconnected therefrom. Thus any pay-tones on the lines will be amplified by the amplifier 10 and delivered via path 34 to the discriminator 32.

The pay-tone discriminator 32 receives the output signal from the three second delay circuit 48 on a path 58 (via the ringing discriminator 42, for the purposes described later). Three seconds after the machine has taken up the telephone line, the signal on path 58 causes the pay-tone discriminator 32 to change the states of the analogue switches 12, and to enable the tape deck 6 to operate, unless a pay-tone is present on the telephone line. If a pay-tone is present, the discriminator 32 continues to disable the announcement operation until either the pay-tone disappears, or a further twelve seconds have elapsed. A twelve second delay circuit 60 has an input connected to receive the output of the three second delay circuit 48, and an output which delivers a signal on a path 62 to the pay-tone discriminator 32 to cause the latter to permit the announcement operation to proceed, if it has not already done so, after fifteen seconds from the machine taking up the telephone line.

The operation of the answering machine 2 is thus as follows. When a ringing current appears on the telephone lines, the sensor 24 delivers output signals to the ringing discriminator 42, which, after a predetermined number of rings, causes the adjustment circuit 44 to deliver a signal on path 26 to the logic circuitry 22. This causes the machine to come on line, during the course of which the relay in the interface 4 is energised. At this stage, the tape deck 6, which is used for delivering announcements to the caller, is disabled by the pay-tone discriminator 32, which also causes the telephone line to be connected to the input of the amplifier 10 and disconnected from the output of the amplifier. For the next three seconds the discriminator 32 monitors the output of the amplifier 10 to determine whether pay-tones are present on the telephone line.If they are not, at the end of three seconds the announcement operation is commenced. If pay-tones are present, the announcement operation is further delayed until the pay-tones have disappeared, or until a further twelve seconds have elapsed. The announcement operation is commenced by the pay-tone discriminator 32 producing signals on path 36 to connect the output of the amplifier to the telephone interface 4, and to disconnect the input of the amplifier therefrom. The discriminator 32 also produces a signal on path 38 to permit the operation of the tape deck 6, which then delivers the announce menttothe input of amplifier 10.

The operation of the machine continues as is standard, and enters a recording stage during the course of which the input of the amplifier is connected to the interface 4, and the output disconnected therefrom, and the tape deck 8 is operable to record messages left by the caller. The recording operation is shut down after a predetermined period, or when the caller has hung up if this occurs prior to the expiry of the predetermined period. After the initial three second period, the detector 52 is armed so that it can detect when the caller hangs up and as a result produce an output signal on path 28.

Figure 2 is a detailed circuit diagram of the sensing section 20, and the same reference numbers as those used in Figure 1 indicate the various portions of the sensing section The sensor 24 has a first input lead 64 connected directly to the A telephone line. It also has two further input leads, 66 and 68, which are connected to the telephone interface 4 (part of which is indicated schematically in Figure 2). A relay contact 70 connects one or other of the leads 66 and 68 to the B telephone line in dependence upon whether or not the relay (not shown) in the interface 4 is energised.

The input lead 64 is connected to one terminal of a bridge rectifier 72 in the sensor 24. The opposite terminal is connected to input lead 66, and also via a series resistor-capacitor network 74 to the input lead 68.

When the relay is de-energised, which is the situation when the answering machine is awaiting calls, the contact 70 is connected to the input lead 68, so that the telephone lines A and B are a.c. coupled to the bridge rectifier 72 via the series capacitanceresistor network 74. After having detected a ringing current and taking up the line, the relay is energised so that the contact 70 is connected to the input lead 66 and the telephone lines A and B are directly coupled to the rectifier 72.

The sensor 24 also includes an opto-coupler 76 including a light emitting diode element 78 and a light sensitive element 80. The anode of the light emitting diode element is connected to the positive terminal of the bridge rectifier 72, and the cathode is connected to the negative terminal via a network 82 which incorporates a resistor 84 connected in parallel with a series connected capacitor 86 and Zener diode 88.

In use, when a ringing current appears on the telephone line, pulses at ringing frequency are delivered to the rectifier 72 via the series connected resistor and capacitance network 74. This causes pulses to appear at the output of the rectifier 72 (at twice the ringing frequency because of the full wave rectification). Each pulse causes current to be momentarily transmitted through the light emitting diode element 78 and the network 84, thus causing a momentary pulse of light to be emitted from the diode element 78. Each pulse of light causes an increase in the conductivity of the light sensitive element 80.

One terminal of the light sensitive element 80 is connected to earth, and the other is connected via resistance 90 to a positive supply potential. Thus, increases and decreases in conductivity of the light sensitive element 80 cause low and high voltage levels, respectively, to appear at the junction of the light sensitive element 80 and the resistor 90. The output of the sensor 24 (corresponding to path 46 in Figure 1) is taken from this junction.

The ringing pulses appearing atthe output of the sensor 24 are detected as will be described later, and the result of this is that the answering machine takes up the telephone line, and energises the relay so as to connect telephone line B directly to the rectifier 72 via contact 70 and input lead 66.

The output of the rectifier 72 thus has a d.c. level substantially equal to the d.c. level on the telephone lines A and B. This causes a steady current to flow through the light emitting diode element 78, and thus the output of the sensor 24 adopts a steady low level indicating that the answering machine has taken up the line.

At this stage, the capacitor 86 will charge up (via the light emitting diode element 78 and the Zener diode 88) to a voltage which corresponds to the d.c.

voltage on the telephone line. The voltage across the capacitance and that on the telephone lines will not actually be equal, due to voltage drops encountered in the rectifier 72, the light emitting diode element 78 and the Zener diode 88, but nevertheless there will be a relationship between them so that higher voltages on the telephone will produce higher charged voltages across the capacitor 86.

If the voltage on the telephone lines, and thus the voltage at the output terminals of the rectifier 72, were to drop, then a situation may be reached whereby the stored voltage across the capacitance 86 exceeds that on the telephone line. This would reverse bias the Zener diode 88, but nevertheless current would still flow from the positive terminal of the rectifier 72 through the light emitting diode element 78 and the resistor 84 to the negative terminal. The output of the sensor 24 would therefore not change. If, however, the voltage on the telephone line were to decrease to such an extent that the break-down voltage of the Zener diode 88 is reached, an avalanche current will flow from the capacitor 86 through the Zener diode 88 and the resistor 84.This will maintain the voltage across the resistor 84, and thus the voltage drop at the output of the rectifier 72 reverse-biases the light emitting diode element 78 and prevents current from flowing through the latter. Thus light from the element 78 ceases, and the level of the output of the sensor 24 goes high.

The sensor is so arranged that it responds both to voltage drops on the line caused when a caller hangs up, and to voltage drops caused by a telephone receiver connected to the line being picked up. It has been found that the use of a Zener diode 88 wih a Zener voltage of 2.7 volts gives satisfactory results in that it permits the sensing of voltage drops due to the aforementioned causes, but does not cause false triggering due to relatively small voltage changes produced by voice signals or noise on the telephone line.

The sensor 24 has a number of advantages, including the fact that it operates independently of the absolute d.c. voltage level on the telephone lines, which means that it tolerates the substantial changes in that voltage level which are found to occur.

The sensor 24 also has the advantage that the parts thereof which are connected to the telephone lines are electrically isolated from the rest of the machine by the opto-isolator 76. This isolation could be achieved in other ways, for example by the use of an isolating relay.

The isolated part of the sensor 24 and its power supply is derived entirely from the voltage on the telephone lines.

The output of the sensor 24 is delivered on path 40 to the ringing discriminator 42.

One problem encountered in designing a telephone answering machine is ensuring that the machine is capable of distinguishing between ringing current on the telephone line and dialling pulses on the line. This is made difficult by the fact that voltages on the line during ringing lie in a range which overlap that within which the voltages caused by dialling pulses lie. Furthermore, the frequency of ringing is close to that of the dialing pulses.

These problems are solved in the present embodiment by the discriminator 42, which operates in the following manner.

When a ringing current appears on the line, the a.c. waveform is rectified by the rectifier 72, and the resultant waveform is effectively shaped by the opto-coupler 76 to produce square-wave pulses at the output 46 of the sensor 24. The shaping is achieved because the opto-coupler 76 effectively has a threshold, and currents through the diode element 78 which are below this threshold result in a high level at the output of the sensor 24, whereas currents above the threshold result in a low output. The threshold is such that the square-wave output has low periods which are substantially longer than the periods when the output is high. The threshold may for example be such that the output of the sensor 24 is low whenever more than about 4 volts is present across the bridge rectifier 72.

Dialling pulses appearing on the telephone line, however, have a square-wave shape, and the result of this is that the pulses appearing at the output of the sensor 24 are in the form of a square-wave wherein the low periods are substantially equal in duration to the high periods.

The result of this is that when a ringing current is present on the line, the output of the sensor 24 consists of pulses which have a high level which lasts for substantially less time than is the case when dialling pulses are present on the line, partly because of the discrepency between the durations of the high and low levels created during ringing by the threshold characteristics of the opto-coupler76, and partly because of the slightly lower frequency of the dialling pulses.

The ringing discriminator 42 uses this situation to discriminate between ringing current and dialling pulses on the basis of the duration of the high levels in the pulses from the output of the sensor 24.

The discriminator 42 comprises a NAND gate 92, which has its inputs both connected to the output of the sensor 24, and which acts as an inverter. The output of the gate 92 is connected to an R-C time constant circuit 94 comprising a resistor 96 connected in parallel with a diode 98, the resistor and diode being connected between the output of the gate 92 and one side of a capacitor 100, the other side of which is connected to earth.

When awaiting a call, the output of the sensor 24 is high, and therefore the output of the gate 92 is low, as is the junction 102 between the resistor 96 and the capacitor 100. When pulses appear at the output of the sensor 24, the low levels in the output are inverted by the gate 92 and thereby cause the capacitor 100 to be charged quickly through the diode 98. Any high levels in the output of the sensor 24 are inverted by the gate 92, and cause discharging of the capacitor 100 through the resistor 96.

The time constant produced by the resistor 96 and the capacitor 100 is selected such that, when dialling pulses are present, the capacitor can fully discharge through the resistor 96, but when ringing pulses are present the capacitor does not have sufficient time to discharge. Accordingly, during ringing, the capacitor 100 remains charged, so that a high level appears at the junction 102, whereas during dialling the level at junction 102 successively changes from high to low.

The voltage at the junction 102 is inverted by another NAND gate 104, and the output of this gate 104 is connected to a further time constant circuit 106 which is similar to circuit 94 except that the capacitor is connected to a positive supply potential rather than to earth, and the time constant is substantially longer.

The level at the output 108 of the time constant circuit 106 is normally high, but during ringing the high level at the junction 102 is inverted by the gate 104 to cause progressive charging of the capacitor of the time constant circuit 106 so that the output of this circuit adopts a low level. This is inverted by a further gate 110 to provide an indication that ringing current is present on the telephone line.

If dialling pulses are present, as has been described the voltage at the junction 102 of time constant circuit 94 successively changes from high to low, and this means that the capacitor in the time constant circuit 106, which can discharge quickly through the diode of that circuit, does not have sufficient time to be charged through the resistor, so that the output 108 ofthe circuit remains at a high level, and the output of the gate 110 remains low.

The time constant of circuit 94 is preferably less than about 55 ms, and that of circuit 106 may be aboutfourtimes longer.

The output of the discriminator 42 is delivered to the adjustment circuit 44, which basically comprises a further time constant circuit including a variable resistor 112 which can be manually adjusted to set the circuit to respond only after a desired number of rings.

The output of the sensor 24 is delivered to the three second delay circuit 48, which comprises a NAND gate 114 acting as an inverter, and having an output connected to a time constant circuit comprising a resistor 116 and a capacitor 118 connected in series between the output of the gate 114 and a positive supply potential. When the machine is awaiting calls, the level at the output of the sensor 24 is high, and hence the level at the output 120 of the delay circuit 48 is low. When pulses are present at the output of the sensor 24, due to ringing current or dialling pulses on the telephone line, the output 120 of the delay circuit 48 remains low because the pulses do not have sufficient duration to discharge the capacitor 118. After the machine has started taking a call there is a three second delay before the output 120 goes high.

The output 120 of the delay circuit 48 is connected to one input 122 of the detector 52 which is used to detect when a caller has hung up or when a telephone receiver has been picked up. The input 22 is connected to one input of a NAND gate 124, the other input of which is connected to the output of the sensor 24. The output of the gate 124 is connected to one input of a standard flip-flop 126, which has another input connected to the output 120 of the delay circuit 48.

In operation, the input 122 of the detector 52 is held low by the output of the delay circuit 48. The output of the gate 124 is therefore held high. The output 128 of the flip-flop 126 is low. When the answering machine takes up the telephone line after a caller has rung, the output of the sensor 24 goes low. Three seconds later, the output 120 of the delay circuit 48 goes high. Thus, the input 122 of the detector 52 goes high, and this "arms" the detector 52 because it permits the gate 124 to respond to changes in the output of the sensor 24.

At a later stage, when a caller hangs up or a telephone receiver is picked up the output of the sensor 24 goes high. This causes the output of the gate 124 to go low, and this low transition changes the state of the flip-flop 126 so that its output 128 goes high, thus providing a signal on path 28 to terminate the operation of the answering machine.

Three seconds later, the output 120 of the delay circuit 48 will go low, thus resetting the flip-flop 126 so that it is ready for its next operation.

The output 120 of the three second delay circuit 48 is delivered to the twelve second delay circuit 60, which comprises a NAND gate 130 acting as an inverter and another R-C time constant circuit (in this case adjustable). The inverted three-second delay signal from the output of the gate 130 is delivered, via a diode 134, to one input of the NAND gate 104 in the discriminator 42. The purpose of this is to ensure that the output of the discriminator 42 is rendered low after the initial three-second period initiated by the taking up of the telephone line by the answering machine. This is desirable because a continued high output from the discriminator 42 may otherwise cause undesired oscillations in the rest of the answering machine.

The three-second delay signal is again inverted by the gate 104, and it is then delivered on path 58 to the pay-tone discriminator 32.

The discriminator 32 receives audio signals on path 34 from the amplifier 10, and these audio signals are delivered to a frequency discriminator 136, which comprises two NE567 integrated circuits connected in a standard configuration. The level at the output 138 of the frequency discriminator 136 goes low when a pay-tone is present on the telephone line.

A NAND gate 140 in the pay-tone discriminator 32 receives the output 138 of the frequency discriminator 136 at one of its inputs, and the three-second delay signal on the path 58 at its other input. The output of the gate 140 is connected to one input of a flip-flop 142 which has its two opposite-polarity outputs connected to the two analogue switches shown at 12 in Figure 1 via paths which collectively correspond to path 36 in Figure 1.

In operation, after the telephone answering machine has taken up the line, the three-second delay signal on path 58 initially is at a low level. The output of the gate 140 is thus high. At the end of the three-second period, the signal on path 58 goes high, and if there is no pay-tone present on the telephone line the output 138 of the frequency discriminator 136 will also be high, so that the output of gate 140 goes low, thus causing the flip-flop 142 to change state. The discriminator 32 also incorporates a NAND gate 144 having one input connected to the output of gate 140 and the other input connected via path 62 to the output of the twelve-second delay circuit 60 (which output stays at a high level until fifteen seconds after the telephone answering machine has taken up the telephone line).Thus, the change in state of the output of gate 140 from a high level to a low level will cause the output of gate 144 to change from a low level to a high level. This output is connected path 38 to enable the tape deck 6 to start operating, and thus initiate the announcement operation.

If, after the initial three-second period, the output 138 of the frequency discrimintor 136 is at a low level, indicating the presence of a pay-tone, the output of gate 140 will remain high, thus preventing the announcement operation from being started.

This state continues until the pay-tone ceases, in which case the output 138 goes high and the output of the gate 140 goes low.

If however the pay-tone continues for fifteen seconds after the telephone answering machine has taken up the line, the output of the delay circuit 60 will change to a low level, which causes the output of the gate 144 to change from a low to a high level.

This output is connected via a diode 146 to one of the inputs of the gate 140 so that it can override the signal appearing at the output 138 of the frequency discriminator 136. The effect of this is that the output of the gate 140 changes to a low level, and the announcement operation commences.

Thus, the discriminator 32 monitors the telephone lines, and after the initial three-second period if a pay-tone is present it continues to prevent the announcement operation from starting until the pay-tone disappears or until a further twelve seconds have elapsed.

The end-of-tape sensor 54 is connected to a motion switch 150 which, as mentioned above, is switched on and off at a rate dependent upon the speed of rotation of the supply spool of the tape deck 8. This applies pulses via a capacitor 152 to a NAND gate 154 of the detector 54, the NAND gate 154 acting as an inverter.

The detector 54 has a further NAND gate 156 which has one input connected to the output of gate 154 and another input 158 connected to receive a signal indicating whether or not the drive motor of the deck 8 is operating. Assuming that the motor is operating, the gate 156 passes pulses from the output of gate 154 to a time constant circuit 160 which is similar in configuration to circuit 106 described above. The circuit 160 permits rapid discharging of a capacitor 162 and slow charging thereof. Thus, when the pulse rate is fairly high, the capacitor does not become charged, so that the output of the time constant circuit 160 remains high.

When the pulse rate is very low, the output of gate 156 remains low sufficiently long to permit charging of the capacitor 162, so that the output of the time constant circuit 160 goes low. This causes a flip-flop 164 of the detector 54 to change state, which thereby provides an output signal on path 28.

The time constant of the circuit 160 is chosen so that an output signal is provided only for very low pulse rates so that the output sigal occurs when the supply spool stops due to the tape in the cassette on the deck 8 having run out.

The change in state of the flip-flop 164 also causes a disabling signal to be delivered on path 56 to the gate 104 in the ringing discriminator 42. The result of this is that the discriminator 42 is disabled and no further calls will be taken by the machine until the cassette on the deck 8 has been replaced and the flip-flop 164 has been reset. Resetting occurs when the supply spool next rotates, which causes pulses to be applied to the gate 154. The first change in level of the output of gate 154 from high to low resets the flip-flop 164. The flip-flop is also reset whenever the machine is re-started by means other than ringing.

The end-of-tape detector is also operable, in the same manner, to detect when the tape has broken or a cassette has not been placed in the deck 8.

At the time a signal is provided at the output of the detector 54, a continuous tone is generated to advise the caller that the machine has ceased recording.

Instead of disabling the discriminator 42, the signal on path 56 could be arranged to alter the mode of the machine so that it only gives an announcement to a caller, and does not record messages. The announcement may differ from that given when the machine is in the normal mode in which it can record messages.

The machine 2 of Figure 1 incorporates a further improvement over the prior art. The amplifier 10, which may be formed by an integrated circuit, includes a preamplifier section, a main amplifier section, and an automatic gain control circuit, as is well-known. An output from the preamplifier section is delivered on an audio path 170 to a mute circuit 172. When the output of the preamplifier drops to a low level, the mute circuit 172 detects this and provides a signal on path 174 which overrides the automatic gain control and effectively cuts off the output of the amplifier 10.

The mute circuit 172 is shown in detail in Figure 3.

The circuit basically comprises a transistor connected in common-emitter mode and having its base connected to a variable resistor 176 which can be adjusted to adjust the bias applied to the transistor.

The base is also connected by a capacitor 178 to the path 170. The biasing of the transistor is set so that below predetermined levels of the signal applied to the capacitor 178, the transistor is switched off and the output on path 174 goes high to override the automatic gain control and thus disable the amplifier. With higher signal levels, the transistor is switched on.

The collector of the transistor is connected to ground via a resistor 180 and a capacitor 182. When the transistor is switched off, the capacitor 182 charges through the collector resistor 184 and the resistor 180. The collector is also connected via a Zener diode 186 and a diode 188 to the path 174.

After the capacitor 182 has been charging for about two seconds, the charged voltage is sufficient to cause an avalanche current to flow through the Zener diode 186, and this delivers a signal to path 174 to mute the amplifier 10. When voice signals are applied to the amplifier 10, the transistor switches on and the capacitor 182 discharges through the resistor 180 and the transistor. The value of the resistor 180 is small, so that this takes place almost instantly and no voice signals are lost. The charging of the capacitor 182 through the resistors 180 and 184 takes place relatively slowly, to avoid muting the amplifier 10 in the pauses between words.

The advantage of this mute circuit is that it is ensured that no signals below, for example, -45 dB are delivered to the output of the amplifier 10.

Without such a circuit, very low level signals which could be picked up from other telephone lines might be undesirably delivered to the output of the ampli fier 10.

CLAIMS (Filed 9 June 1981) 1. Atelephoneanswering machine having means for providing an audible output to enable a user to listen to a caller on a telephone line, and a sensor to detect a drop in voltage on the line due to a telephone receiver being brought into use, the machine being responsive to said detection for disabling said audible output.

2. A machine as claimed in claim 1, having a control means operable on detection of a ringing current on the line to establish an operational connection with the line to allow the machine to record calls thereon, said control means being responsive to said detection for dis-establishing said connection and thereby disabling said audible output.

3. A machine as claimed in claim 1, responsive to said detection for disabling an audible signal path to a loudspeaker in order to disable said audible output.

4. A machine as claimed in any preceding claim, wherein said sensor is also operable to detect when a caller on the line has hung up.

5. A machine as claimed in any preceding claim, wherein the sensor comprises means for storing a voltage having a level determined by the voltage on the telephone line, and means responsive to the appearance on the telephone line of a voltage which is less than the stored voltage for providing a detection signal in response to the receiver having been brought into use.

6. A machine as claimed in claim 5, wherein the sensor is operable to provide a detection signal only if the stored voltage exceeds the voltage on the line by more than a predetermined threshold amount.

7. A machine as claimed in any preceding claim, wherein the sensor has a portion which is electrically connected to the telephone line and electrically isolated from the rest of the machine, means being provided to transmit signals from said portion to the rest of the machine.

8. A machine as claimed in any preceding claim, having means for detecting a ringing current on the telephone line, said detecting means being operable to provide square wave pulses at a frequency determined by that of the ringing current, said square wave pulses having a duration less than the duration between pulses, and being operable to detect whether the pulses have less than a predetermined duration, and if so to provide an output signal indicative of the presence of the ringing current.

9. A machine as claimed in any preceding claim, having an amplifier operable to: (a) amplify pay-tone signals on the telephone line and deliverthe amplified pay-tone signals to a pay-tone detector (b) amplify voice signals on the telephone line and deliver the amplified voice signals to a recording means, and (c) amplify voice signals from a play-back means of the machine and deliver the amplified voice signals to the telephone line.

10. A telephone answering machine substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. fier 10. CLAIMS (Filed 9 June 1981)

1. Atelephoneanswering machine having means for providing an audible output to enable a user to listen to a caller on a telephone line, and a sensor to detect a drop in voltage on the line due to a telephone receiver being brought into use, the machine being responsive to said detection for disabling said audible output.

2. A machine as claimed in claim 1, having a control means operable on detection of a ringing current on the line to establish an operational connection with the line to allow the machine to record calls thereon, said control means being responsive to said detection for dis-establishing said connection and thereby disabling said audible output.

3. A machine as claimed in claim 1, responsive to said detection for disabling an audible signal path to a loudspeaker in order to disable said audible output.

4. A machine as claimed in any preceding claim, wherein said sensor is also operable to detect when a caller on the line has hung up.

5. A machine as claimed in any preceding claim, wherein the sensor comprises means for storing a voltage having a level determined by the voltage on the telephone line, and means responsive to the appearance on the telephone line of a voltage which is less than the stored voltage for providing a detection signal in response to the receiver having been brought into use.

6. A machine as claimed in claim 5, wherein the sensor is operable to provide a detection signal only if the stored voltage exceeds the voltage on the line by more than a predetermined threshold amount.

7. A machine as claimed in any preceding claim, wherein the sensor has a portion which is electrically connected to the telephone line and electrically isolated from the rest of the machine, means being provided to transmit signals from said portion to the rest of the machine.

8. A machine as claimed in any preceding claim, having means for detecting a ringing current on the telephone line, said detecting means being operable to provide square wave pulses at a frequency determined by that of the ringing current, said square wave pulses having a duration less than the duration between pulses, and being operable to detect whether the pulses have less than a predetermined duration, and if so to provide an output signal indicative of the presence of the ringing current.

9. A machine as claimed in any preceding claim, having an amplifier operable to: (a) amplify pay-tone signals on the telephone line and deliverthe amplified pay-tone signals to a pay-tone detector (b) amplify voice signals on the telephone line and deliver the amplified voice signals to a recording means, and (c) amplify voice signals from a play-back means of the machine and deliver the amplified voice signals to the telephone line.

10. A telephone answering machine substantially as herein described with reference to the accompanying drawings.