JP2008092500A - Optical subscriber terminal device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To change a reference signal for AGC control over an optical subscriber terminal device, by selecting the reference signal corresponding to the degree of optical modulation from among a plurality of previously set reference signals. <P>SOLUTION: A photodiode 14 photodetects an optical television signal supplied from a center device 2 to an optical fiber based on the television signal, and generates a received television signal. An amplifying unit 26 amplifies the received television signal from the photodiode 14. A CPU 34 controls the output level of the amplifying unit 26, based on the received television signal and reference signal. The CPU 34 has the plurality of reference signals that differ in value and uses a reference signal selected which is designated from the outside. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical subscriber terminal installed in a terminal of an optical transmission line in a system for transmitting a television signal using the optical transmission line.

  2. Description of the Related Art Conventionally, an optical receiver circuit of a transmission system using an optical transmission path is disclosed in, for example, Patent Document 1. According to the technique of Patent Document 1, an optical signal is received by a photodiode, a received light signal is generated, and the received light signal is amplified by an AGC amplifier. The output level of the AGC amplifier is detected, compared with a reference signal having a predetermined value in the comparison circuit, and the AGC amplifier is controlled based on the comparison result.

JP-A-5-145360

  When the optical receiver circuit disclosed in Patent Document 1 is implemented in an optical subscriber terminal device in a television signal optical transmission system, the optical signal is generated based on a plurality of television signals in the center device. When the wave number of the television signal is increased or decreased in the center device, the optical modulation degree of the optical signal changes. If there is a change in the optical modulation degree, and if automatic gain control is performed using a predetermined reference signal in the optical subscriber unit, the reference signal does not correspond to the change in the optical modulation degree and distortion occurs. Or the C / N ratio deteriorates. Therefore, it is desirable that the AGC control reference value can be changed in the optical subscriber terminal apparatus.

  An object of the present invention is to provide an optical subscriber terminal device capable of changing a reference signal for AGC control.

  The optical subscriber terminal device according to one aspect of the present invention includes light receiving means. The light receiving means receives the optical television signal based on the television signal supplied from the center device to the optical transmission path, and generates a reception television signal. As the light receiving means, a photodiode or a phototransistor can be used. The amplification means amplifies the received television signal from the light receiving means. The automatic gain control means controls the output level of the amplification means based on the received television signal and the reference signal. The automatic gain control means has a plurality of reference signals having different values in advance and uses a reference signal selected by an instruction from the outside. This external instruction can also be supplied from the center apparatus to the automatic gain control means via the optical transmission line. Alternatively, the user can give an instruction from the outside by operating the operation means provided in the automatic gain control means. The automatic gain control means can also be configured to construct a database of different instructions from the outside and reference signals of different values corresponding to these instructions, and read out the reference signals corresponding to the instructions from the outside. . Each reference signal may be set according to a different light modulation degree of the optical television signal.

  In this way, since a reference signal selected according to an external instruction is used among preset reference signals, for example, even when the light modulation degree is changed, a reference signal corresponding to the changed light modulation degree is used. It can be used, and the deterioration of the C / N ratio of the received television signal and the occurrence of distortion can be prevented.

  As described above, according to the present invention, the reference signal used for the automatic gain control of the optical subscriber terminal device can be changed to a desired one, for example, the optical modulation degree of the optical signal from the center device has been changed. Even in this case, the reference signal corresponding to the change can be used.

  An optical television signal transmission system in which an optical subscriber terminal device according to an embodiment of the present invention is implemented has a center device 2 as shown in FIG. The center device 2 is provided in a head end device or the like, for example, and has a television signal source 4. The television signal source 4 is composed of, for example, a plurality of terrestrial television broadcast signals in the UHF band and the VHF band, an independent broadcast signal of the joint viewing device, an intermediate frequency signal in the 1 GHz band of satellite broadcast and satellite communication signals, and the like. Outputs a television signal. This television signal is supplied to the optical transmission means, for example, the optical transmission unit 8 via the combiner 6. The optical transmitter 8 converts a television signal into an optical television signal and supplies it to an optical transmission line, for example, an optical fiber 10.

  This optical television signal is supplied to the optical subscriber terminal device 12 installed at home. In FIG. 1, only one optical fiber 10 is shown, and the center device 2 and the optical subscriber terminal device 12 are shown connected to both ends of the optical fiber 10. An optical subscriber terminal apparatus 12 is connected to each of the branched optical fiber terminals. Alternatively, the optical fiber branched at each access point is further branched via a coupler or a closure, and the optical subscriber terminal device 12 is connected to the terminal of each branched optical fiber.

  The optical subscriber terminal device 12 includes a light receiving element, for example, a photodiode 14, a cathode of which is supplied with a voltage of + V via a high frequency blocking coil 16, and an anode which is supplied with a reference potential such as a ground via a load resistor 18. Connected to potential. By connecting in this way, a reverse bias is supplied to the photodiode 14. When the optical television signal enters the photodiode, a current corresponding to the optical television signal flows through the photodiode 14, and this is amplified by the amplifying means, for example, the amplifier 22 through the DC blocking capacitor 20. The output signal of the amplifier 22 corresponds to the television signal of the television signal source 4 and is hereinafter referred to as a reception television signal. The received television signal is supplied to amplifying means, for example, a two-stage amplifying unit 26 with AGC, via a branching means, for example, one branching device 24.

  The amplifying unit 26 includes an attenuator 28 including an attenuator, for example, a PIN diode, on the input side, and an output signal of the attenuator 28 is amplified and output by two-stage amplifiers 30 and 32. In order to adjust the attenuation amount of the attenuator 28, automatic gain control means, for example, a CPU 34 is provided. The CPU 34 is supplied with the voltage across the load resistor 18, that is, the voltage corresponding to the received television signal, digitized by the A / D converter 36. The CPU 34 compares this digital signal with a predetermined reference value, and generates a control signal for controlling the attenuator 28 so that the difference between the two is reduced. This control signal is converted into an analog signal by the D / A converter 38 and supplied to the attenuator 28 via the on / off circuit 39 and the buffer 40. The on / off circuit 39 is controlled by the CPU 34 and is turned on when the attenuator 28 is controlled, and is turned off when the control is not performed.

  By the way, the number of television signals from the television signal source 4 of the center device 2 may increase or decrease, for example. In this case, the optical modulation degree of the optical television signal output from the optical transmitter 8 changes. When the reference value used for AGC control by the CPU 34 of the optical subscriber terminal device 12 is fixed to a constant value despite the change in the optical modulation degree, the amplified reception television set output from the amplifier 32 is used. The John signal is deteriorated in C / N ratio or includes distortion. In order to prevent the deterioration of the C / N ratio and the occurrence of distortion, a reference value change instruction accompanying a change in the optical modulation degree is supplied from the center device 2 side to the optical subscriber terminal device 12.

  Therefore, optical modulation degree data representing the optical modulation degree is supplied from the optical transmission unit 8 to the control unit 42 in the center apparatus 2. The control unit 42 supplies the light modulation degree data to a control data generation unit, for example, the data transmission unit 44. The data transmission unit 44 modulates a carrier wave having a predetermined frequency with the optical modulation degree data, and performs, for example, FSK modulation to generate a modulation signal. This modulated signal is combined with the television signal by the combiner 6 and supplied to the optical transmitter 8. Accordingly, the optical transmitter 8 generates light control data based on the modulation signal, for example, the light modulation signal, in addition to the optical television signal. An optical signal composed of the optical television signal and the optical modulation signal is supplied to each subscriber terminal device 12 through the optical fiber 10.

  In each subscriber terminal 12, the output signal from the amplifier 22 based on the current flowing through the photodiode 14 is a reception signal including a reception modulation signal corresponding to the optical modulation signal in addition to the reception television signal. This received signal is supplied to the data demodulator 46 via the one branching device 24, where the received modulated signal is extracted from the received signal. From the extracted reception modulation signal, reception control data, for example, reception modulation degree data corresponding to the modulation degree data is demodulated and supplied to the CPU 34.

  In the CPU 34, a database in which various received modulation degree data and reference signals corresponding to these are stored in a storage unit (not shown) is constructed, and the received modulation degree data is received from the data demodulator 46. Is supplied, the corresponding reference signal is selected from the database and used as the AGC reference signal. Therefore, even if the optical modulation degree changes on the center device 2 side, the reference signal corresponding to the change is automatically used in each optical subscriber terminal device. Accordingly, it is possible to prevent the deterioration of the C / N ratio and the occurrence of distortion based on the change in the degree of light modulation.

  In addition, the optical subscriber terminal device 12 is installed in each home, but there are homes that cancel the reception contract even though the reception contract using the optical subscriber terminal device 12 has been made so far. Alternatively, some families have newly signed a reception contract. It is necessary to disable reception using the optical subscriber terminal device 12 after the expiration date of the contract at the home where the reception contract is canceled. Moreover, it is necessary to make it possible to receive from the date of issuance of the reception contract to a household that has newly signed a reception contract.

  Therefore, in each optical subscriber terminal device 12, on / off circuits 48 and 50 are provided in a supply path of a power supply voltage to the amplifiers 30 and 32, for example, + V. When the on / off circuits 48 and 50 are in a first state, for example, an on state, a voltage of + V is supplied to the amplifiers 30 and 32, and the amplifiers 30 and 32 start operating. Therefore, the received television signal can be output to the outside. When the on / off circuits 48, 50 are in the second state, eg, the off state, the + V voltage is not supplied to the amplifiers 30, 32, and the amplifiers 30, 32 stop operating and receive television. Output of the signal to the outside is stopped. The CPU 34 switches the on / off circuits 48 and 50 to an on state or an off state.

  In order for the CPU 34 to perform this switching control, amplifier control data is supplied as control data from the outside to the control unit 42 of the center apparatus 2. The amplifier control data includes an instruction to turn on or off the amplifiers 30 and 32 and data to instruct the optical subscriber terminal apparatus 12 including the amplifiers 30 and 32 to be turned on or off. The amplifier control data is supplied to the data transmission unit 44, and an optical modulation signal obtained by FSK-modulating the carrier wave is transmitted to each optical subscriber terminal apparatus 12 in the same manner as described above.

  In each optical subscriber terminal device 12, the receiving amplifier control data corresponding to the amplifier control data is demodulated from the optical modulation signal by the data demodulator 46 and output to the CPU 34 in the same manner as described above. The CPU 34 determines whether the data indicating the optical subscriber terminal device included in the reception amplification control data matches the data indicating the optical subscriber terminal device allocated to itself. The on / off circuits 48 and 50 are controlled in accordance with an on / off instruction included in the reception amplifier control data. On the other hand, when the data indicating the optical subscriber terminal apparatus does not match the data indicating the optical subscriber terminal apparatus allocated to itself, the on / off circuits 48 and 50 are not controlled.

  In the control unit 42, when the modulation degree data and the amplifier control data are simultaneously supplied to the data transmission unit 44, the optical transmission unit 8 transmits an optical modulation signal based on the modulation degree data and the amplifier control data.

  In the above embodiment, the modulation degree data is transmitted from the center device 2 to each optical subscriber terminal device 12, and based on this, the CPU 34 automatically selects the corresponding reference signal and uses it for AGC control. For example, an operation unit such as a numeric keypad may be provided in the CPU 34, modulation degree data may be supplied from the operation unit to the CPU 34, and a reference signal corresponding to the modulation degree data may be selected and used by the CPU 34. In this case, transmission of the modulation degree data from the center device 2 becomes unnecessary.

  In the above-described embodiment, the on / off circuits 48 and 50 are provided for the amplifiers 30 and 32. However, a common on / off circuit is provided for the amplifiers 30 and 32. The start and stop of power supply can also be controlled. In addition, the start / stop of the signal supply from the attenuator 28 of the amplifier 30 can be controlled by the on / off circuit instead of the start / stop of the power supply. Similarly, the output of the output signal of the amplifier 32 can be started. The stop can also be performed by an on / off circuit. Further, the on / off circuits 48 and 50 can be removed. Further, although the on / off circuit 39 is provided on the input side of the buffer 40, it is not necessary in some cases.

  In the above embodiment, the current flowing through the photodiode 14 amplified by the amplifier 22 is extracted from the cathode of the photodiode 14, but can also be extracted from the anode side of the photodiode 14. Similarly, the voltage across the load resistor 18 is A / D converted for AGC control, but the voltage between the cathode of the photodiode 14 and the ground potential can also be A / D converted. Further, although the photodiode 14 is used, a phototransistor can be used instead.

It is a block diagram of the remote control apparatus of the optical subscriber terminal device of one Embodiment of this invention.

Explanation of symbols

2 Center device 4 Television signal source 8 Optical transmitter (optical transmitter)
10 Optical fiber (optical transmission line)
12 Optical subscriber terminal device 14 Photodiode (light receiving means)
26 Two-stage amplifier with AGC (amplifier)
34 CPU (control means)
46 Data demodulator (control means)

Claims (3)

A light receiving means for receiving an optical television signal based on the television signal supplied to the optical transmission line from the center device and generating a received television signal;
Amplifying means for amplifying the received television signal from the light receiving means;
Automatic gain control means for controlling the output level of the amplification means based on the received television signal and a reference signal;
The automatic gain control means includes an optical subscriber terminal device that has a plurality of reference signals having different values in advance and uses a reference signal selected by an instruction from the outside.   2. The optical subscriber terminal apparatus according to claim 1, wherein each reference signal is set in accordance with a different degree of optical modulation of the optical television signal.   3. The optical subscriber terminal apparatus according to claim 2, wherein an instruction for selecting the reference signal is supplied from the center apparatus to the automatic gain control means via the optical transmission line.

Images