This invention relates to telecommunications and, more specifically, to a telephone ring signal detector which detects a telephone ring signal on a telecommunication line during a first quarter cycle time period of a first telephone ring signal, by using a comparator to compare a magnitude of a telephone ring signal voltage with a threshold voltage which is adapted to a peak ring signal voltage.
In the telephone art, several different electrical voltages are transmitted by a Telephone Company Central Office (CO) on a telecommunication line. The line usually consists of two wires coupled from the CO to at least one premise telephone device at a subscriber premise. For example, the CO typically provides a nominal 48 volt direct current (DC) battery voltage between the wires for providing an electrical interface which allows the CO to appropriately communicate by electrical signals with a premise telephone. During an incoming telephone call to the premise, the CO also transmits a telephone alternating ring signal voltage between the wires for activating an audible alerting device typically associated with the premise telephones. The CO may further provide a telephone alternating pre-ring signal voltage between the wires prior to the ring signal. For example, ANSI document T1-401-1993 defines an Open Switch Interval (OSI) as a switching time prior to the ring signal when the battery voltage is disconnected from between the wires and when a ring signal generator is being switched to the wires. According to Bellcore documents TR-TSY-000821 and TR-NWT-000505, the CO may also activate a per-call subscriber line test prior to the ring signal. This line test is provided for detecting any excessive voltage on the line and for measuring line impedance between the wires and from each wire to ground. Bellcore document GR-506-CORE further indicates that during a Loop Current Feed Open (LCFO) Interval, when the battery voltage is disconnect for a duration not to exceed 0.3 seconds, each wire may be independently or simultaneously grounded, and a CO voltage may be independently or simultaneous applied to each wire with respect to ground. Multiple LCFOs can also occur, separated by at least a 0.1 second time interval when the battery voltage is momentarily connected to the line. When the CO generates OSIs and LCFOs for per-call line tests prior to the ring signal, the resulting pre-ring signal voltage on the line can exhibit a peak pre-ring signal voltage and an alternating pre-ring signal cycle that is similar to a peak ring signal voltage and an alternating ring signal cycle associated with the telephone ring signal. As a result, many known telephone ring signal detectors cannot distinguish between the pre-ring signal voltage and the ring signal voltage, causing these detectors to provide an invalid response to the pre-ring signal voltage.
Recently, enhanced telephone ring signal detectors have been developed to allow an automated telephone answering device (TAD) to silently answer incoming calls without activating any audible ringer mechanisms associated with premise telephones connected to the wires. These enhancements are described in the cross referenced patent application Ser. No. 08/876,446 by Davis, and in U.S. Pat. No. 5,544,241 to Dibner, on Aug. 6, 1996. According to Davis, a ring detect signal is provided when a first ring signal voltage exceeds a threshold voltage prior to a first quarter cycle time period of a first telephone ring signal cycle time period. According to one aspect of Dibner's ring signal detector, the ring detect signal is provided after the magnitude of the first ring signal voltage exceeds the threshold voltage for a predetermined time interval, which can be less then the first quarter cycle time period of the first telephone ring signal cycle time period. For both the Davis and Dibner ring signal detectors, a telephone device off-hook terminal impedance is coupled to the wires in response to the ring detect signal. As a result, the CO detects the off-hook impedance and terminates the first ring signal voltage before it can achieve sufficient magnitude or energy to activate any of the audible ringer mechanisms coupled to the wires. The off-hook impedance is a terminal impedance exhibited by the telephone when the telephone's receiver is removed from the telephone's cradle and hook switch.
Since the enhanced ring signal detectors described by Davis and Dibner both initiate an output response when the first ring signal voltage exceeds the threshold voltage, an invalid ring detect signal can result for both detectors when a first pre-ring signal voltage exceeds the threshold voltage. As a result, the off-hook impedance is coupled to the wires in response to the invalid ring detect signal prior to an activation of the ring signal voltage. Since the CO recognizes the off-hook impedance as a busy subscriber telephone line, the CO sends an audible busy signal to a caller without activating the ring signal voltage, and the off-hook impedance remains coupled to the wires at the premise in an erroneous off-hook state. Thus the enhanced ring signal detectors by Davis and Dibner may erroneously respond to the pre-ring signal voltage, producing an undesired telecommunications behavior just described.
Although a high frequency line noise voltage on the line can be filtered prior to being applied to the ring detector, many known telephone ring signal detectors utilize a delay time generator or a time averaging circuit to reject high frequency noise for discriminating between the line noise voltage and the ring signal voltage. Dibner's detector employs the delay time generator, and U.S. Pat. Nos. 4,939,775 to Houck et al, on Jul. 3, 1990 and 4,491,691 to Embree et al, on Jan. 1, 1985, employ the time averaging circuits to provide a valid ring detect signal after the magnitude of the ring signal voltage exceeds the threshold voltage for a predetermined time interval. However, these detectors can also provide the invalid ring detect signal, by detecting the pre-ring signal voltages when the pre-ring signal voltage exceeds the threshold voltage for a transition time interval greater than the predetermined time interval. In Dibner's example, any peak pre-ring signal voltage greater than 45 volts for a transition time interval between 0.005 and 0.05 seconds will produce the invalid ring detect signal. In Houck's example, any peak pre-ring signal voltage greater than 28 volts for a transition time interval between 0.05 and 0.1 seconds will produce the invalid ring detect signal.
Furthermore, variations in the peak pre-ring signal voltage and the peak ring signal voltage at the CO, variations in a line resistance between the CO and the premise due to variations in the line length, and variations in the number of ringer mechanisms coupled to the line at the premise all contribute to a difficulty for ring signal detectors to distinguish between the pre-ring signal voltage and the ring signal voltage and to provide the valid ring detect signal in the presence of the pre-ring signal voltage.
Hence there is a need for an improved telephone ring signal detector, which can distinguish between pre-ring signal voltages and ring signal voltages, being unresponsive to pre-ring signal voltages and be only responsive to ring signal voltages, and providing the ring detect signal during the first quarter cycle time period and being independent of variations in line voltage magnitudes, line length, and line loading conditions.