Sensing exchange signalling requirements

Abstract: According to the present invention there is provided a dual signalling standard subscriber's telephone equipment including means wherein the equipment is initially set to perform dialling signalling in a first mode, means responsive to initiation of the handset 'OFF-HOOK' condition to determine the presence or absence of significant line signal for a predetermined period of time (10-13), and means responsive to the output of the determining means to reset the equipment to the first or a second mode (15) for performing subsequent dialling signalling.

Description: This invention relates to dual standard signalling telephone instruments. In the United Kingdom at the present time there is a programme of telephone exchange replacement in which the previously traditional exchanges having "loop disconnect" (LD) signalling from subscribers equipments are being replaced by new electronic exchanges which require "multifrequency" (MF) signalling from subscriber equipment. Because there currently exists, and will continue to exist for some time to come, a dual signalling standard network various subscriber equipments are being produced that incorporate both types of signalling facilities which are switch selectable by the subscriber. Such equipments are popular for subscribers because, since the recent liberalisation of the telephone system, more and more subscribers are purchasing their own telephones and wish to be able to use them no matter to which type of exchange they are connected. This is particularly advantageous when the subscriber is connected to an older LD exchange which is later replaced by a MF exchange, since the subscriber does not have to then purchase a new telephone. Also if the subscriber moves to an area served by a different type of exchange he can continue to use his existing telephone since relocation and reconnection are simply a matter of plugging the telephone into a socket. GB-A-2016869 (Roche) provides a telephone station with automatic switch over between pulse code dialling and multifrequency code dialling. However, there are problems arising out of the development of such switched dual standard telephones. One main problem is that subscribers can alter the setting of the switch(es) accidentally or while "experimenting" with the telephone, often resulting in a call out of the public switched network (PSTN) engineer. According to the present invention there is provided a dual signalling standard subscriber's telephone equipment including means wherein the equipment is initially set to perform dialling signalling in a first mode, including means responsive to initiation of the handset 'OFF-HOOK' condition to determine the presence or absence of significant line signal for a predetermined period of time, means responsive to the output of the determining means to reset the equipment to the first or a second mode for performing subsequent dialling signalling, characterised in that the means to determine the presence or absence of line signal includes means responsive to the 'OFF-HOOK' condition to sample the line signal peak amplitude for a first predetermined period of time, means for storing the first signal level occurring during the sampling period, means responsive to release of the first dialling key to re-enable periodically the sampling means, means for comparing the periodic samples of the line signal with the stored sample to determine presence or absence of significant line signal for a predetermined period of time whereby if at least a "gap" in the line signal (dial tone) is detected during a second predetermined time period immediately following the release of the first dial key press, the telephone is set to the first mode, whereas if no such "gap" is found, the equipment is set to the alternative, second signalling mode. According to a preferred embodiment of the invention the equipment includes means for retaining the reset dialling mode so long as the equipment remains connected to the exchange line, the equipment being set to the first mode when disconnected from the line and reconnected to the same or another line. Embodiments of the invention will now be described with reference to the accompanying drawings in which:- Fig. 1 is a block diagram of an autosensing dual signalling standard telephone equipment, Fig. 2 is a flow diagram of the operations carried out in the autosensing telephone equipment of Fig. 1, and Fig. 3 is a timing diagram relating to the flow diagram of Fig. 2. Experiments have shown that even when an equipment is connected to a PABX and dialling in the correct mode, after dialling a first 'access' digit to gain access to an external PSTN line there is a significant, albeit brief, detectable virtual loss of signal on the subscriber's line before dial tone is replaced by the same or another dial tone. Therefore autosensing of the signalling requirements, i.e. loop disconnect (LD) or multifrequency (MF) signalling, is based on the fact that after dialling of the first digit there will be either:- a) a complete loss of dial tone on the subscriber's line when the equipment is connected direct to a public switched telephone network (PSTN) exchange, or b) a brief temporary loss of dial tone on the subscriber's line when the equipment is connected to a local private automatic branch exchange (PABX) if in both cases the equipment is dialling in the mode, LD or MF, required by the exchange. If the equipment is dialling in the wrong mode then the exchange will not respond by removing dial tone from the line. Therefore in one embodiment of the invention, to determine whether an exchange will respond to MF or LD dialling a first, single digit of the wanted number is transmitted in MF format and the equipment looks for a gap in the line signal immediately following the digit tones. Because of the variability of different exchanges and the existence of transients on the lines a 'gap' is defined as a given short period of time, e.g. 25ms, within a longer period of time, e.g. 600ms, following the transmission of the digital tones in which the amplitude of the line signal is significantly less than that of the first dial tone, i.e. the dial tone preceding transmission of the first digit. Referring to the drawings, the first time that the handset is lifted 'OFF-HOOK' after the equipment is installed, the timing and control circuitry 10 (Fig. 1) initiates a sequence of events shown in Figs. 2 and 3. To begin with the circuitry checks whether a 'configuration' flag is set. This flag will not be set if the equipment is newly installed and will be set if this is not the first time that the equipment has accessed the exchange since installation. If the configuration flag is not set the circuitry mutes the microphone for a short period of time e.g. 900ms, during which a peak detection circuit 11 is enabled. During this time the peak amplitude of the filtered line signal, i.e. the dial tone, is measured. At the end of this period the sample and hold circuit 12 is taken to the 'hold' mode and records the peak signal occurring during this period. This period must be long enough so that certain special dial tones, (e.g. 750ms tone; 750ms silence, used for indicating that certain PABX facilities have been activated), may be properly characterised. During this sequence the microphone is muted so that unwanted audio signals are not interpreted as dial tone. The microphone is then unmuted. The release of the first dial key press (which must not occur until the initial signal measurement sequence is completed) initiates the sequence shown in the second half of the timing diagram. The peak detector circuit 11 is re-enabled for a further short period, e.g. 25ms, and at the end of this period the maximum signal level found is compared in gap detector 13 with a signal which is half the amplitude of the original dial tone, which is obtained from the hold circuit via a 6dB attenuator 14. If the newly detected signal is less than half the original dial tone a 'gap' is deemed to have occurred. The peak detector is reset and another 25ms measurement period is started. This search for a gap continues for, say, 600ms. At the end of this time the output of the gap detection circuit is latched and, depending on the output of the gap detection circuit, the equipment is set, i.e. configured, to the correct signalling mode. If a gap is detected then this is indicative that the exchange has responded to MF signalling so the configuration flag is 'set' without altering the signalling mode of the equipment. If no gap is found then this is indicative that the exchange has not responded to MF signalling and so the equipment is set to the LD signalling mode and the configuration flag is set. The output of the gap detection circuit is held in latch 15 so long as the equipment remains connected to the exchange line and the line voltage is maintained. Therefore, once the equipment has been configured to the correct signalling mode it will remain in that mode until such time as it is unplugged from the line and re-installed, at which time it will default to the MF mode and repeat the autosensing sequence. Finally, if the equipment is re-configured to the LD mode the first digit is re-dialled, since the exchange will not have responded to the first dialling of the first digit. In the MF mode the exchange will have already responded to the first digit so re-dialling is not required. It should be noted that although the autosensing sequence requires a comparatively long period of time, e.g. 1-2 seconds, this is no serious inconvenience to the user since it occurs only on the first dialling attempt after plugging into the line socket. Since the dialling mode is thereafter 'frozen' (indicated in the flow diagram by the 'configuration flag' being set) the correct signalling requirement for the exchange is maintained for the duration of the equipment connection and normal dialling procedures prevail.

Claims: A dual signalling standard subscriber's telephone equipment comprising means wherein the equipment is initially set to perform dialling signalling in a first mode; means (10-13) responsive to initiation of the handset 'OFF-HOOK' condition to determine the presence or absence of significant line signal for a predetermined period of time ; means (15) responsive to the output of the determining means to reset the equipment to the first or a second mode for performing subsequent dialling signalling, characterised in that the means to determine the presence or absence of line signal includes means responsive to the 'OFF-HOOK' condition to sample the line signal peak amplitude for a first predetermined period of time, means for storing the first signal level occurring during the sampling period, means responsive to release of the first dialling key to re-enable periodically the sampling means, means for comparing the periodic samples of the line signal with the stored sample to determine presence or absence of significant line signal for a predetermined period of time whereby if at least a "gap" in the line signal (dial tone) is detected during a second predetermined time period immediately following the release of the first dial key press, the telephone is set to the first mode, whereas if no such "gap" is found, the equipment is set to the alternative, second signalling mode. An equipment according to claim 1 further including means for retaining the reset dialling mode so long as the equipment remains connected to the exchange line, the equipment being set to the first mode when disconnected from the line and reconnected to the same or another line. An equipment according to claim 1 or 2 characterised in that said first signalling mode is multifrequency (MF) and said second signalling mode is loop disconnected (LD). An equipment according to claim 3 when dependent on claim 2 including means for muting the equipment microphone during at least said first predetermined period of time.

Inventors: BARNES GEORGE JOHN PRESTON, VERRELLS MICHAEL VINCENT, BARNES, GEORGE JOHN PRESTON, VERRELLS, MICHAEL VINCENT
Applicants: NORTHERN TELECOM LTD, NORTHERN TELECOM LIMITED
Application year: 1990
Main classification: H04M1
IPC codes: H04M1