Modem loop current detect system to detect an off-hook condition

Details Modem loop current detect system to detect an off-hook condition Modem loop current detect system to detect an off-hook condition

1998-06-16

2000-10-17

Dorvil, Richmond

Telephonic communications

Subscriber line or transmission line interface

37910608, 379443, 379902, H04M 1106

Patent

active

061343218

DESCRIPTION:

BRIEF SUMMARY
The subject invention generally concerns the field of telephone line monitoring circuits.
Modern satellite television systems such as the RCA DSS.RTM. system, manufactured by Thomson Consumer Electronics, Inc., Indianapolis, Ind., include circuitry for automatically calling a predetermined telephone number to report the billable specifics regarding the use of the customer's integrated receiver decoder (IRD). Such billing is necessary, of course, because the user receives programming by subscribing to one or more of several program providers. Unfortunately, there is no time of day at which the IRD is guaranteed access to the telephone line, because the user may be placing a call, literally anytime (one cannot schedule emergencies, for example). It is desirable, therefore to be able to detect a condition in which the user picks up an extension phone, in order that the IRD may release the line to the user. However, heretofore the operation of telephone line monitoring circuitry has been somewhat inadequate, due to the drift of the state of the voltage provided by the telephone company. Many factors affect the telephone voltage level delivered to the user's home, such as, the length of the line from the central office, fluctuations in the signals in the line cards used at the central office, whether the connections to the user's home have become corroded, etc. The problem at hand is how to accurately determine if an extension telephone is picked up (causing a perturbation in the line voltage level) given the real world situation wherein changes in line level may be the norm.
Moreover, the measurement is even more complicated by the fact that one cannot reliably measure telephone line voltage at the user's location with respect to the user's ground reference without risking the introduction of ground loop voltages. It also must be noted that because devices coupled to telephone networks are required to have a relatively high input impedance, they tend to be susceptible to picking up noise transients, which may interfere with the measurement of the telephone line level.
European Patent Application No. 0 338 654 discloses a method and apparatus for use in an automatic tele-communication system for controlling communication by a modem over a telephone line. Means are provided for determining prior to initiating communication by the modem over the telephone line whether a telephone connected to the telephone line is off hook and for inhibiting communication by the modem over the telephone line if a telephone is off hook. The same means or further means are arranged for determining during communication by the modem over the telephone line whether a telephone connected to the telephone line goes off hook and for terminating such communication if a telephone goes off hook so as to release the telephone line for use by the telephone.
International Patent Application No. WO 95/14344 discloses a telephone line monitoring circuit for a MODEM monitors the TIP and RING lines of the telephone line. A first connecting circuit detects when a local phone goes off-hook while the MODEM is on-hook by sensing a decrease in a differential voltage between the TIP and RING lines. A second circuit detects the pick up of a local phone while the MODEM is off-hook by detecting a change in the impedance between the TIP and RING lines. A third circuit detects the hang up of a remote phone connected with the MODEM while the MODEM is off-hook by detecting a temporary break in the DC loop between the central office and MODEM. A snoop circuit monitors the telephone line for DTMF signals while the MODEM is on-hook.


SUMMARY OF THE INVENTION

A telephone line monitoring circuit uses a converting device such as an optoisolator to convert the telephone line current to a voltage which is applied to the input of an A/D converter. The A/D values are coarse values representing telephone line current. The A/D is sampled at a predetermined interval to identify changes introduced by the telephone company central office, and to take those changes into acc

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patent: 4958371 (1990-09-01), Damoci et al.
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patent: 5422939 (1995-06-01), Kramet et al.
patent: 5444772 (1995-08-01), Coker
patent: 5809109 (1998-09-01), Moyal et al.

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