Interactive video distribution systems – Video distribution system components – Transmission network
Reexamination Certificate
1999-12-02
2004-08-31
Srivastava, Vivek (Department: 2611)
Interactive video distribution systems
Video distribution system components
Transmission network
C725S150000, C725S127000, C725S130000
Reexamination Certificate
active
06785908
ABSTRACT:
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a tap device of a cable broadcasting system in which a part of transmission signal running through a transmission line leading to a terminal side from a center apparatus is branched and transmitted to a subscriber's terminal device, particularly to a tap device of a cable broadcasting system in which it can be switched on the side of the center apparatus whether or not the transmission signal is transmitted to the terminal device.
(ii) Description of the Related Art
In a conventional cable broadcasting system such as CATV system in which a broadcasting signal such as a television signal is transmitted to a subscriber's terminal device via one transmission line formed of a coaxial cable, and the like, the transmission line is provided with a tap device, so-called tap-off, for leading signals in order to branch the broadcasting signal from the transmission line and leading the signal into a subscriber's house.
Moreover, in a known tap device, it can easily be switched according to a subscriber's request or the like whether or not the broadcasting signal is distributed to the subscriber's house. Specifically, the tap device is provided with a relay disposed in a signal path between a directional coupler for branching a part of the broadcasting signal from the transmission line and a branched output terminal for outputting the branched broadcasting signal toward the subscriber's terminal device, in which a high-frequency broadcasting signal can be passed with a reduced loss. The relay is a so-called high-frequency relay, and a latching relay which can hold a switched state is usually used. By driving the relay in response to a command signal transmitted from the center apparatus via the transmission line, the connected/disconnected state of the signal path, in other words, the output or cut-off of the broadcasting signal to the terminal device can easily be switched on the side of the center apparatus.
When the conventional tap device is installed on the transmission line, an individual address is allocated to the device in order to distinguish the device from the other electronic apparatuses connected to the transmission line. Additionally, when the center apparatus switches the output of the broadcasting signal to the branched output terminal of the tap device, a command signal including command data and the individual address of the tap device is generated. This command signal is sent to the transmission line. In this case, on the side of the tap device, the command signal having the corresponding address is selected from various command signals transmitted via the transmission line, that is, the command signal from the center apparatus is extracted. By driving the relay based on a command included in the command signal, it is switched whether or not to transmit the broadcasting signal to the branched output terminal.
Therefore, according to the conventional tap device, it can easily be switched on the side of the center apparatus whether or not to transmit the broadcasting signal to the subscriber's terminal device from the branched output terminal. Therefore, an operator does not have to go to a place where the tap device is installed for the switching operation.
Additionally, in the conventional tap device, in order to receive the command signal from the center apparatus to drive the relay, electric power has to be supplied from the outside for the switching operation. Furthermore, it is troublesome to place a separate cable for the power supply, which enormously increases costs.
In the cable broadcasting systems such as CATV system, the power is usually supplied to the electronic apparatuses such as an amplifier connected to the transmission line via the transmission line. Basically, the broadcasting signals or high-frequency signals such as television signals are transmitted via the transmission line. However, by using a frequency band different from that of the broadcasting signal, the above-described command signal and the power signal can simultaneously be transmitted. Therefore, in the conventional system, by supplying an alternating-current power signal of several tens of Hz(hertz) to the transmission line with several tens of V(volts), the alternating-current power signal is used to supply electric power to various electronic apparatuses connected to the transmission line.
For this purpose, the tap device is usually provided with a power supply circuit in which the alternating-current power signal supplied to the transmission line is taken into the device, and rectified and smoothed to generate a direct-current voltage for operating an inner circuit. With the direct-current voltage generated by the power supply circuit, a drive circuit for driving the relay, a receiving circuit for receiving the command signal, a control circuit for turning on/off the relay via the drive circuit based on the received command signal, and the like are operated.
In the receiving circuit and the control circuit of the conventional tap device, semiconductor integrated circuits can be used, and the operation voltage can be set to a low voltage of 5V or 3.3V as in a general one-chip microcomputer. However, a high voltage is advantageous for the relay switch, because the relay can be driven with less current.
Therefore, the power supply circuit to be incorporated into the tap device is preferably constituted such that a direct-current high voltage of 10V or more (hereinafter referred to as the first power voltage) is supplied to the relay drive circuit, and that a direct-current low voltage lower than the first power voltage (hereinafter referred to as the second power voltage) can be supplied to the receiving circuit and the control circuit.
In the conventional tap device, for example, the power supply circuit is constituted so as to rectify/smooth the alternating-current power signal transmitted via the transmission line, then generate the first power voltage using a three-terminal regulator, a Zener diode, and the like, and further to generate the second power voltage for driving the control circuit from the first power voltage.
On the other hand, since each cable broadcasting system in each service area is independent, the alternating-current power signal supplied to the transmission line differs in each system, and the voltage of the alternating-current power signal differs by about several tens of V(volts) among systems. Therefore, when the tap device is used only in a specific system in which the alternating-current power is constant, the power supply circuit to be incorporated into the tap device may be designed in accordance with the alternating-current power to be used so that the power loss is minimized. However, when the tap device is provided with general-purpose properties so that the tap device of the same specification can be used in the systems different from one another in terms of the voltage of the alternating-current power, a problem arises that the power loss in the power supply circuit is increased in the system with a high voltage.
Specifically, for example, when the common tap device is used in the systems in which the voltages of the alternating-current power signals are 45V and 90V, the power supply circuit needs to generate the first and second power voltages in the range of the alternating-current power signal of 45V to 90V. In this case, the power supply circuit has to be designed such that even when the voltage of the alternating-current power signal is 45V, the first and second power voltages can be generated and that when the voltage of the alternating-current power signal is 90V, the power for the voltage difference is consumed by the three-terminal regulator, the Zener diode, and other components forming the power supply circuit. Therefore, when the conventional tap device can be utilized in the systems different from one another in terms of the voltage of the alternating-current power signal running through the transmission line, the
Davis & Bujold P.L.L.C.
Srivastava Vivek
LandOfFree
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