TELEPHONE SYSTEM WITH ANSWERING MACHINE

Abstract: The present invention relates to a telephone system (1) comprising at least one telephone subset (3) and an answering machine (5) which are connected, in parallel, to an outside telephone line (9). The system (1) has a first line terminal (11) and a second line terminal (13). The answering machine (5) comprises tone-generating means (37) for generating a probe tone from the answering machine (5) to the first and second line terminals (11, 13). The system (1) further comprises detection means (41, 43, 45) for detecting a change in reflected residue of said tone in response to an impedance change between the telephone system (1) and the outside telephone line (9).

Description: Telephone system with answering machine. The invention relates to a telephone system comprising at least one telephone subset and an answering machine connectible, in parallel, to an outside telephone line. The invention also relates to a method of parallel set detection in such a telephone system. The invention further relates to an answering machine for use in a telephone system in which the answering machine and at least one telephone subset are connectible in parallel. When a customer buys a stand-alone answering machine or an answering machine incorporated in a telephone subset, said machine is usually operated in parallel to the conventional telephone. When the telephone answering machine is on-line, it will monitor the line for incoming calls. This monitoring can also taken place when there are still other persons in the house to pick-up the telephone when a call comes in. When the (telephone) answering machine picks up the telephone to respond to the incoming call, it will start playing the prerecorded message and will subsequently record the message which the calling party wants to leave behind. During this period of time, one of the persons in the house can pick-up the parallel-connected telephone. Then, one of the requirements is that the answering machine has to stop its operation and finish the call, leaving the parallel set, i.e. the telephone, in charge of the call. To this end, the answering machine should be able to detect that the parallel set has been picked up. Existing answering machines require external components such as transistors and passive components. A disadvantage thereof is that it is not possible to change, the detect threshold levels and/or the detect algorithm without changing component values or even the PCB design. Consequently, changes in design to comply with country-specific parameters is rather difficult and very costly. Furthermore, setmakers have found that non- adaptability in the parallel set detection greatly increases the risk of false detects. A reason therefor is that, nowadays, customers often use the Public Service Telephone Network in parallel for more than two telephone peripherals such as, for example, a modem and a facsimile apparatus, and that the threshold levels are different for different peripherals. It is an object of the present invention to provide a telephone system in which parallel detection is possible without external component requirements and which allows gained software programmability of the threshold levels, the detect algorithm .and the country-specific parameters. To this end, the telephone system according to the present invention comprises a first line terminal and a second line terminal, the answering machine comprising tone-generating means for generating a probe tone from the answering machine to the first and second line terminals, and detection means for detecting a change in reflected residue of said probe tone in response to an impedance change between the outside telephone line and the telephone system. The present invention relies on the fact that the total impedance seen by the outside telephone line changes when a parallel set is picked up in the telephone system. Under ideal conditions, the impedance of the telephone subset corresponds to the impedance of the telephone line, when the telephone subset is in the off-hook position while the answering machine is inactive. When the telephone is in the on-hook position and the answering machine is active, the impedance of the answering machine corresponds to the impedance of the outside telephone line. When the telephone subset is picked up while the answering machine is sending or recording a message, the impedance of the telephone system seen by the telephone line will change. In the telephone system according to the present invention, a probe tone is generated in the answering machine and directed towards the outside telephone line. Due to the change in the telephone system impedance, a changed residue of said generated probe tone will be reflected, for example, at the line terminals. In a preferred embodiment of the telephone system according to the present invention, the probe tone is a signal of the kind X(t) = A(t) sin (ω(t)t + φ t ), in which A is the envelope, ω(t) is the angular frequency and φ t is the phase of the probe tone.. A probe tone of this kind allows a number of possible signals to be generated and provides a more accurate detection so that the reliability of the measurement is . increased. In a further embodiment of the telephone system according to the present invention, the probe tone frequency f(t)=ω(t)/27r is out-of-band. An advantage thereof is that the tone, when reflected at the line terminals, will not be heard by the user, both at the far and near end, of the telephone because it falls outside the frequency range that covers telephone calls. Thus, out-of-band is in fact outside the speech band. In a further embodiment of the telephone system according to the present invention, the probe tone has a frequency within a type approval frequency mask. In order to obey the conditions imposed by the telephone exchange, the frequency of the tone generated within the answering machine and sent to the telephone line lies within the type approval frequency mask. When an impedance change of the telephone system occurs, there are several ways of detecting the change in reflected residue. In a first embodiment of the telephone system according to the present invention, the detection means comprise a bandpass filter and an energy detector. A bandpass filter with a center frequency around the frequency of the generated tone is used, so that the energy content of the reflected residue can be measured and compared with the energy content of the reflected residue before the impedance change. The detector is provided with a detection window around the energy level of the reflected residue before the impedance change. In a second embodiment of the telephone system according to the present invention, the detection means comprise a correlator. If the reflected residue is below the noise level of the system, a correlator is preferred to measure the reflected residue and thus to ascertain a parallel set pick-up. The invention further relates to a method of parallel set detection in a telephone system as described above. The invention also relates to an answering machine for use in a telephone system in which the answering machine and at least one telephone subset are connectible in parallel. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings: Figures 1(a) and 1(b) are simplified basic diagrams of a telephone system according to the invention, comprising a telephone subset and an answering machine connected, in parallel, to an outside telephone line; Figures 2(a) and 2(b) show a possibility for the received reflected frequency spectrum of a transmitted outgoing message plus probe tone due to an impedance change as the result of a parallel set pick-up, before and after parallel set pick-up; Figure 3 is a block diagram of a digital signal processor-based parallel set detection; and Figure 4 is a block diagram of an analog IC which can be used in the telephone system and answering machine according to the present invention to perform a parallel set detection. Figures 1(a) and 1(b) are simplified basic diagrams of a telephone system 1 comprising a telephone handset, or subset 3, and an answering machine 5. Figure 1(a) shows a system in which the telephone subset 3 is in the on-hook position while the answering machine is active, and Figure 1(b) shows a system in which the telephone subset 3 is in the off-hook position and the answering machine 5 is inactive. Reference numeral 7 denotes a telephone exchange to which the telephone system 1 with a first line terminal 11 and a second line terminal 13 is connected via an outside telephone line 9 in order to receive messages from, and send messages to, the telephone exchange 7. The telephone subset 3 has a first terminal 15 and a second terminal 17, and the answering machine 5 has a first terminal 19 and a second terminal 21. The answering machine 5 and the telephone subset 3 are connected, in parallel, to the first line terminal 11 and the second line terminal 13 of the telephone system 1. The telephone subset 3, the answering machine 5 and the telephone exchange 7 have an impedance denoted by 23, 25 and 27, respectively. When the answering machine is on-line, it will monitor incoming calls. When the incoming message is being recorded, the telephone subset may be picked up. A requirement is now that the answering machine stops operating and leaves the telephone subset in charge of the call. To detect whether the telephone is in the on-hook or off-hook position, i.e. whether a parallel pick-up occurs in the telephone system, the answering machine is provided with probe tone-generating means. The generated probe tone is sent towards the line terminals of the telephone system. If the telephone subset or the answering machine changes from the active to the inactive state, or vice versa, the impedance of the telephone system will change. Figure 1(a) shows the situation in which the answering machine 5 is active (switch 28 closed) and the telephone subset 3 is in the on-hook position (switch 29 open). When the telephone is. in the on-hook position, an impedance mismatch may or may not be present, depending inter alia on the quality of the telephone system and line length variations. When there is a parallel set pick-up, in the case of absence of a mismatch, a change in impedance between the telephone system and the telephone exchange will cause a mismatch. An already existing mismatch in impedance between the telephone system and the telephone exchange will increase or decrease due to the parallel set pick-up. Consequently, in all cases mentioned here, the reflection of the probe tone generated in the answering machine will be different before and after the parallel set pick-up which causes a change in impedance (mis)match. A difference in telephone exchange impedance due to line length variations does not affect the principle of detection of the changed reflected residue. When the telephone subset 3 is picked up while the answering machine 5 is, for example, still recording a message, the outside telephone line 9 will see an impedance change at the line terminals 11 and 13, as both devices are now connected in parallel. The impedance seen by the outside telephone line 9 will now be spread over the telephone subset 3 and the answering machine 5. Now, switch 28 is open and switch 29 is closed. This situation is illustrated in figure 1(b). In terms of the present invention, this means that the answering machine will be switched off. The change in reflected residue between the situation before parallel set pick-up and the situation after parallel set pick-up is measured. The result of this measurement can be transmitted to a controller in the answering machine, allowing the answering machine to be inactivated. In a preferred embodiment, the probe tone frequency is out-of-band, i.e. said frequency has a value which is different from the frequency band for outgoing messages. Consequently, the probe tone cannot interfere with the outgoing message, i.e. speech messages. The probe tone frequency preferably has a value in the range between 8 and 15 kHz. Figure 2(a) shows a possibility for the transmitted frequency spectrum and the received reflected frequency spectrum of an outgoing message and the probe signal before a parallel set pick-up. Figure 2(b) shows the transmitted frequency spectrum and the received reflected frequency spectrum of an outgoing speech message and the probe signal after a parallel set pick-up. The transmitted spectrum is indicated by a solid line and the received spectrum is indicated by a broken line. Both Figures illustrate a situation in which already a mismatch between the telephone line and the telephone system before a parallel set pick-up. When there is no outgoing message, the spectrum of the outgoing message will disappear. The envelope denoted by e in Figures 2(a) and 2(b) is the type approval frequency mask. It indicates the range of frequencies in combination with the signal level which a transmitted signal has to obey. Said ranges are imposed by the telephone exchange. The signal of the probe tone is of the kind X(t) = A(t) sin (ω(t)t + t ), in which A is the envelope, ω(t) is the angular frequency and φ t is the phase of the probe tone. ω(t) and φ t are both time-dependent. Such a signal represents a large number of different signals and yields reliable measurement results. The probe tone may be a continuous signal or a pulsed signal. Both the tone generation and the detection can be performed in a digital way or in an analog way. All answering machines currently available on the market comprise a digital signal processor with AD/DA converters. The tone-generating means for generating the probe tone may be part of this digital signal processor. Figure 3 shows a block diagram of parallel set detection based on a digital signal processor. The answering machine 5 comprises a block 33 besides the normal answering machine functionality 35. Block 33 comprises tone-generating means 37. The probe tone is generated independently of the presence of an outgoing message and is sent towards the line terminals. When there is an outgoing message, the probe tone and the message are jointly sent towards the terminals 19, 21, and the line terminals 11, 13, respectively. The changed reflected residue is detected in detection means 39. A change in reflection occurs due to the pick-up of a parallel set. Reflection of the residue takes place, for example, at the line terminus 11, 13. Detection of the reflected residue can be performed by using a bandpass filter 41 at the frequency of the generated tone, and an energy detector 43. The energy detector is provided with a detection window on an energy level corresponding to the energy level of the reflected residue before parallel set pick-up. This is illustrated in Figures 2(a) and 2(b) . The detection window extends through an energy range γ and is concentrated around the energy level of the reflected residue before parallel set pick-up. When the energy level of the reflected residue after parallel set pick-up is outside this range, a change in reflected residue is ascertained and the parallel set pick-up will thus be detected. If the reflected residue is below the system noise level, the detection based on a correlation technique by using a correlator 45 is preferred. In Figure 3, blocks 43 and 45 are thus exchangeable. The described parallel set detection employing a digital signal processor can be used in all answering machine applications in a customer environment, together with a PSTN (Public Service Telephone Network) line. Examples are stand-alone answering machines, analog cordless telephones with an answering machine in the base station, digital cordless telephones with an answering machine in the base station and add-on cards for PC- based answering machines. An analog set up may make use of an analog integrated circuit. Figure 4 is a possible diagram of an integrated circuit-based parallel set detection. Only the integrated circuit part 50 of the answering machine is shown. A probe tone X(t) is generated in an oscillator 51, coupled to the telephone subset line and sent towards the line terminals 15, 17, .and also the line terminals 11, 13 of the telephone system. The change in reflected residue is measured by means of a detector 53, the output 55 of which can be supplied to a microcontroller so as to deactivate the answering machine. In conclusion, the invention here presented allows detection of a parallel set pick-up by using remote sensing techniques. A probe tone is generated in the answering machine and is sent towards the outside telephone line. There is a change in reflection of said tone with a change in impedance (mis)match. This effect can be detected, for example, at the line terminals of the telephone system. The reflection of the probe tone is measured without the use of external components or expensive components needed for other applica- tions within the system. Here use is made of a digital signal processor which is present in answering machines, that are currently available on the market. The digital signal processor allows software programmability of both the threshold levels and country specific parameters. The telephone system described hereinbefore is not restricted to a system comprising an answering machine and one telephone subset, but the present invention is also applicable in telephone systems comprising an answering machine and a plurality of telephone subsets.

Claims: CLAIMS: 1. A telephone system comprising at least one telephone subset and an answering machine connectible, in parallel, to an outside telephone line, the system comprising a first line teπninal and a second line terminal, the answering machine comprising means for detecting a change in impedance between the first line terminal and the second line terminal. 2. A telephone system comprises tone generating means for generating a probe tone from the answering machine to the first and second line terminals, and detection means for detecting a change in reflected residue of said probe tone in response to an impedance change between the outside telephone line and the telephone system. 3. A telephone system as claimed in Claim 2, wherein the probe tone is a signal of the kind X(t) = A(t) sin (ω(t)t + φ t ), in which A is the envelope, ω(t) is the angular frequency and φ t is the phase of the probe tone. 4. A telephone system as claimed in Claim 2 or 3, wherein the probe tone frequency f(t)=ω(t)/27r is out-of-band. 5. A telephone system as claimed in Claim 2, 3 or 4, wherein the probe tone has a frequency within a type approval frequency mask. 6. A telephone system as claimed in any one of Claims 2 o 5, wherein the detection means comprise a bandpass filter and an energy detector. 7. A telephone system as claimed in any one of Claims 2 to 5, wherein the detection means comprise a correlator. 8. A telephone system as claimed in any one of the preceding Claims, wherein the telephone subset is an analog cordless telephone, and the answering machine is accommodated in a base station. 9. A telephone system as claimed in any one of Claims 1 to 7, wherein the telephone subset is a digital cordless telephone, and the answering machine is accommodated in a base station. 10. A telephone system as claimed in any one of Claims 1 to 7, wherein the answering machine is an add-on card for use in a personal computer. 11. A method of parallel set detection in a telephone system having a first line terminal and a second line terminal and comprising at least one telephone subset and an answering machine, which are connectible, in parallel, to an outside telephone line wherein a change in impedance is measured between the telephone system and the outside telephone line. 12. A method as claimed in Claim 11, wherein a probe tone is generated by tone-generating means which are provided in the answering machine and a change in reflection of said probe tone at the line terminals is measured when the telephone subset is picked up, said measurement being in response to the impedance change between the telephone system and the outside telephone line. 13. A method as claimed in Claim 12, wherein the tone-generating means generate a signal of the kind X(t) = A(t) sin (ω(t)t + φ t ), in which A is the envelope, ω(t) is the angular frequency and φ t is the phase of the probe tone. 14. A method as claimed in Claim 10 or 13, wherein a probe tone is generated, whose frequency f(t)=ω(t)/2 is out-of-band. 15. A method as claimed in Claim 12, 13 or 14, wherein a probe tone is generated which has a frequency within a type approval frequency mask. 16. A method as claimed in any one of Claims 12 to 15, wherein a reflection of the probe tone at the first and the second line terminals is filtered with a bandpass filter which is active around the frequency of the probe tone, and the energy content of said changed reflected residue is measured with an energy detector. 17. A method as claimed in any one of Claims 12 to 16, wherein the detection of the reflection residue is measured by means of a correlation technique. 18. An answering machine for use in a telephone system having a first line terminal and a second line terminal, the answering machine comprising means for detecting a change in impedance between the first line terminal and the second line terminal. 19. An answering machine as claimed in Claim 18, comprising tone-generating means for generating a probe tone having a changed reflected residue in response to an impedance change between the telephone system and an outside line. 20. An answering machine as claimed in Claim 19, wherein the probe tone is a signal of the kind X(t) = A(t) sin (ω(t)t + φ t ), in which A is the envelope, ω(t) is the angular frequency and φ t is the phase of the probe tone. 21. An answering machine as claimed in Claim 19 or 20, wherein the probe tone frequency f(t)=ω(t)/27r is out-of-band. 22. An answering machine as claimed in Claim 19, 20 or 21, wherein the probe tone has a frequency within a type approval frequency mask. 23. An answering machine as claimed in any one of Claims 19 to 22, wherein the detection means comprise a bandpass filter and an energy detector. 24. An answering machine as claimed in any one of Claims 19 to 22, wherein the detection means comprise a correlator.

Inventors: MENNEN MARTINUS ERNASTINUS JEA, STIPHOUT EDUARD HENRI, MENNEN, MARTINUS, ERNASTINUS, JEAN, HUBERTUS, STIPHOUT, EDUARD, HENRI
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: H04M1
IPC codes: H04M1, H04Q7