Use of an echo canceller for detecting the signals CAS

0001A telephone equipment comprising:<ul><li>an input for receiving the useful signals to be transmitted over a go transmission path, and an output for outputting signals Useful received on a transmission channel feedback, said channels transmission to and being coupled to a telephone line by coupling means,</li><li>a service signal detector transmitted by a central telephone on the telephone line and likely to superimposed on the useful signals.</li></ul>

0002The invention also relates to a detection method service signals transmitted by a telephone exchange in such a telephone equipment, said service signals being may be superimposed on the useful signals.

0003The invention has important applications in the field telephone transmissions, for example for the implementation services such as call waiting to notify a user which is already online that someone else is trying to reach him.

0004The recommendation ETS 300 778-1 ETSI (English European Telecommunications Standard Institute) describes in particular the procedure that is implemented for this purpose, between a called and telephone exchange on which it depends. This procedure is summarized 1a and 1b. When the central office receives a 1 call to the called 2, it sends a signal service named CAS (English Call Alert Signal) on the telephone line 3. The called 2 detects this signal, cut audio tracks, and sends a DTMF digit (English Dual Tone Multi Frequency) "D" to the telephone exchange 1. Upon receipt of the DTMF digit "D", the telephone exchange 1 sends the coordinates of the caller (name, phone number ...) to the called, FSK modulation (English Frequency Shift Keying). Then the telephone call in progress is restored.

0005Japanese Patent No. 08214055 describes summary including a receiving circuit for telephone capable of operate in a CAS detection mode and in a mode of FSK detection to implement such a procedure.

0006The essential problem for the detection of service signals, including CAS signals, as follows. When the service signal may be superimposed on other signals, it is necessary to use criteria less stringent detection (including frequency criteria of duration, signal level ...) to ensure its detection some the level of these other signals. But using criteria less rigorous risk cause false detections, that is to say, service signals of detections in a useful signal (the a corresponding voice in the case of a CAS service signal).

0007The invention aims to propose equipment telephone and a service signal detection method provide an effective solution to this problem.

0008For this, a telephone according to the invention and as defined in the opening paragraph is characterized in that it includes an echo canceller to cancel the echo of the useful signals from said input returning to said exit, service signal detector being connected between the canceller echo and said output at least when the telephone equipment is online.

0009Similarly, a service signal detection method according to the invention and as described in the paragraph Introductory is characterized in that it consists in using a echo canceller to cancel the echo signals from useful said inlet returning on said output, and performing said detection on the signals output of the canceller echo.

0010Thus, the service signal detector receives no the signals emitted by the corresponding or the echo of the signal transmitted by Called returning on the way back when said means coupling and telephone line are not well adapted impedance. The detection criteria used may be more rigorous which reduces the risk of false detections and improves greatly system reliability.

0011The invention will be better understood and other details will be apparent from the following description with reference the accompanying drawings which are given by way of examples not limiting and in which:<ul><li>1a and 1b summarize the procedure is implemented between the central office and the called telephone equipment for the implementation of a call signaling service,</li><li>Figure 2 gives a schematic representation of a first embodiment embodiment of a telephone apparatus according to the invention,</li><li>Figure 3 gives a second embodiment of a device telephone of the invention.</li></ul>

0012The invention will now be described in the case of a phone with a CAS signal detector. It is of course not limited to this example. It applies to the detection of any service signal may be superimposed on the useful signals, and to any type of telephone equipment, including for phones with or without son, analog or digital.

0013Figures 1 to 3 are diagrams of which only the elements necessary for understanding the invention have been represented. From Figure 2, a telephone according to the invention comprises a converter 2 son / 4 son 10, which is coupled to a share a microphone 11 by a transmission path go A, and secondly to an earphone 12 through a return transmission path A. The microphone 11 is the input of the phone; he is destined to receiving the useful signals which are voice signals. The earpiece 12 constitutes the output of the phone; it is intended to deliver useful signals which are voice signals. The phone Also an echo canceller 13 shown schematically by a adaptive filter 14. The adaptive filter generates, from Data collected on the go channel A, a replica of the echo present on the return channel R. A subtraction unit 16 subtracts the echo replica of the signal provided by the converter 10. The adaptive filter 14 is set taking into account the sampled signal on the return path R, and taken from the output of the canceller signal echo 13, to cancel as much as possible the echo signal to the output of the subtraction unit 16.

0014A detector 20 is connected between CAS signals the echo canceller 13 and the earpiece 12. And a detector 21 signals FSK is connected to the telephone line.

0015This embodiment applies in all countries where CAS signals are not used when the phone is off line. FSK signal detection Note that does not pose the same problems as the CAS signal detection since the channels audio are cut at the FSK transmission (no voice signal therefore just be added to the useful signal). It is therefore possible connecting the signal detector FSK directly on the line phone which allows it to be operational when the phone is off-line (in this case, in fact, must be taken from the useful signal before the converter 2 son / son 4 which is not powered and does not work).

0016In Figure 3, there is shown another embodiment of embodiment of a phone according to the invention, applicable when the CAS offline detection signal is used.

0017In this embodiment, a switch 30 is placed between the telephone line 3 and the output of the echo canceller 13. With the phone line that connects the switch 30 CAS signal detector at the output of the echo canceller 13. When the phone is offline, it connects to the line phone 3, thus allowing the detection of signals that the CAS phone either online or offline.

0018In this embodiment, the FSK signal detector is advantageously integrated to the CAS signal detector.

0019The echo canceller 13 and CAS signal detectors and FSK are for example made from a PCD Circuit 5091 manufactured by Philips Semiconductors.

0020The echo cancellation techniques are well known of the art. In this respect we can for instance refer to the article entitled "An Adaptive Algorithm for Sign-Step Fast Convergence of a Data Echo Canceller "published in IEEE Transactions on Communications, Volume COM-35, No. 10, October 1987.