Telephonic communications – Substation or terminal circuitry – Conversion of signal form
Reexamination Certificate
1998-02-23
2001-05-29
Chan, Wing F. (Department: 2643)
Telephonic communications
Substation or terminal circuitry
Conversion of signal form
C379S032010, C379S038000, C379S393000
Reexamination Certificate
active
06240177
ABSTRACT:
FIELD OF THE INVENTION
The subject of the invention is a communications device including detection of the unhooking of telephone instruments, modems or communications apparatus linked to the same telephone line. The invention applies particularly to automatic detection of the use of a telephone instrument by a data transmission modem.
BACKGROUND OF THE INVENTION
There is increasing use of apparatus of a type in which a switched telephone network (hereinafter “STN”) is connected in parallel with a conventional telephone instrument. Such items of apparatus commonly employ a modem connected to the STN line to modulate or demodulate the data to be sent or to be received. However, apparatus now exists, such as certain digital television decoders, which uses a modem to establish a connection back to the program provider. In such digital television decoders, it is common practice to establish the connection back to the service provider at night time so as to avoid any inconvenience to the user.
The viewer therefore risks finding the telephone line busy when he picks up the handset. This can be inconvenient and even dangerous when an urgent call has to be made.
SUMMARY OF THE INVENTION
The subject of the invention is a communications device connected in parallel with at least one item of communications apparatus on a transmission line, the said device and the said apparatus conforming to a given template in terms of DC components,
the said device being characterized in that it comprises:
means setting the operating point of the said device alone, in terms of DC component, outside regions of the said template where the DC component of the current may be zero,
means of determining the DC component of the current flowing in the said device,
means of detecting a variation in the said DC current component.
Given that the behaviour, in terms of DC components, of the other apparatus connected to the transmission line cannot be controlled, the operating point of the device in accordance with the invention is chosen appropriately in order to be able to obtain a detectable variation in the DC component of the current in the said device when one of the items of apparatus comes into operation.
By choosing the operating point of the device (when it is operating alone) outside of regions of the template where certain items of apparatus may (but do not necessarily) have a zero or low DC current component, a variation of the current in the said device is ensured.
According to one particular embodiment, the said means of setting the operating point comprise means of generating a characteristic, in terms of DC components, such that it possesses intersections with the possible load characteristics of the transmission line which are situated in regions of the said template where the DC current component cannot be zero.
The intersection between the load characteristic of the line and the characteristic of the device gives the operating point sought.
According to one particular embodiment, the said characteristic in terms of DC components comprises a point of intersection with the 1400 ohm load characteristic, the said point being situated beyond 8.6 V.
According to one particular embodiment, with the said template including a maximum current limit, the characteristic of the said device includes a maximum slope in the region bounded by the load characteristics and the said maximum limit, the said characteristic having a point which is common with each possible load characteristic.
The characteristic of the device extends throughout the possible operating region defined by the different load characteristics, the maximum slope maximizing the variation in current introduced, for example, upon unhooking a telephone instrument connected to the same line.
According to one particular embodiment, the said means of detecting a variation in the said DC component comprise a circuit forming the derivative of the signal corresponding to the said defined DC component.
According to one particular embodiment, the said means of detecting a variation in the said DC component comprise means of comparing the said defined DC component and the DC component at the operating point of the said device alone.
According to one particular embodiment, the said device comprises a bridge rectifying the voltage of the line interposed between the transmission line and an audio signal processing circuit.
According to one particular embodiment, with the positive output of the rectifier bridge being connected to the collectors of a first and second transistor mounted as an amplifier, the said positive output being linked to the base of the first transistor through a first resistor, the said amplifier feeds into a second resistor linked to the neutral output of the said rectifier bridge.
According to one particular embodiment, the said device further comprises a capacitor linking the base of the first transistor to the neutral output of the rectifier bridge.
According to one particular embodiment, the said device further comprises, in series between the input of the amplifier and the neutral output of the rectifier bridge, a third resistor, an on/off means and a first reference voltage generator, the said on/off means being closed when the line voltage exceeds a given value.
According to one particular embodiment, the said on/off means and the said voltage source comprise a Zener diode.
According to one particular embodiment, the said means of determining the DC component measure the voltage at the terminals of the second resistor.
According to one particular embodiment, the said means for determining the DC component tap off some of the current at the output of the amplifier towards a current-measuring circuit.
According to one particular embodiment, the current-tapping branch comprises, in series, a second reference voltage generator and a fourth resistor.
According to one particular embodiment, the said detection means carry out a comparison between the value of the DC component measured by the said current measuring circuit and the value of the DC component at the operating point of the said device.
According to one particular embodiment, the said comparison is carried out with respect to a range around the value of the DC component at the operating point of the said device.
According to one particular embodiment, the said determination circuit comprises a microprocessor connected to a digital-analog converter and to a memory in which is stored the value of the DC component at the operating point, the said digital-analog converter being linked to the terminals of a resistor connected to the tapping-off branch via an optocoupler.
The invention employs a detector circuit placed in parallel on the signal path. Thus a voltage drop possibly caused by a detector in series is avoided.
REFERENCES:
patent: 4686697 (1987-08-01), Shapiro et al.
patent: 4998271 (1991-03-01), Tortola et al.
patent: 5521974 (1996-05-01), Hayashi et al.
patent: 5592529 (1997-01-01), Linsker
patent: 0613286 (1994-08-01), None
*Patent Abstracts of Japan, vol. 19, No. 635, & JP 07-023129 Ricoh Co. Ltd.
Guntzburger Philippe
Moraillon Jean-Yves
Chan Wing F.
Thomson Multimedia S.A.
Tripoli J. S.
Wein F. A.
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