JPH10191298A - Optical catv system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical CATV system which can increase the number of light branchings and which can give satisfactory operability for a user. SOLUTION: AM modulators 12 which AM-modulate video signals and FM modulators which FM-modualte the AM video signals at different frequencies are provided for respective channels for a transmission station 10. FM signals are frequency-multiplexed and transmitted. A subscriber 30 uses an AM video signal receiver 33 receiving the AM video signal and fits an FM signal reception adapter 40 for selecting the FM signal in synchronism with the channel selection of the AM video signal receiver 33 from the frequency multiplexed FM signals and demodulating it to the AM video signal. Since FM transmission is executed, the number of light branchings is increased. Since the signal is demodulated into the AM video signal and an image is received, the user views it with the same operation as that at the time of AM transmission.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical CATV system for delivering a video signal through an optical fiber, and more particularly, to an optical CATV system.
The present invention relates to an optical CATV system that can increase the number of optical branches and is easy for a user to use.

[0002]

2. Description of the Related Art FIG. 2 shows a configuration example of a conventional optical CATV system. In this optical CATV system, a transmission station 10 having an optical transmitter and a subscriber 30 having an optical receiver are connected by an optical fiber 20. A branch by an optical coupler 21 is provided in the middle of the optical fiber 20, and a plurality of subscribers 30 are connected to one optical fiber 20 drawn from the transmitting station 10. In the transmitting station 10, the video signal is converted by a VSB-AM (hereinafter referred to as AM) modulator 12 into an AM video signal of the same format as that of a normal television terrestrial broadcast. That is, one channel ch1
The frequency of fch1 and the frequency of channel 2 are fch1
Multi-channel video signals such as ch2 are carried on carriers of different frequencies. This is called frequency multiplexing. In the optical transmitter 11, the laser light is intensity-modulated by these frequency multiplexed signals. This is called subcarrier multiplexing.

[0003] The optical signal is transmitted and branched through the optical fiber 20 and the optical coupler 21 and then delivered to the subscriber 30. The AM video signal converted into an electric signal by the optical receiver 32 in the set-top box 31 in the subscriber 30 is:
After being amplified (not shown), it is sent to an ordinary household television receiver (AM video signal receiver) 33 or a video recorder 34.

In this configuration of the optical CATV system, the distribution from the transmitting station 10 to the subscriber 30 is entirely performed using optical signals. High quality images can be obtained. The optical signal is obtained by subcarrier multiplexing the AM video signal, and the AM video signal output from the optical receiver 32 of the subscriber 30 has the same format as that of the terrestrial television broadcasting. If you connect to a TV receiver, you can watch immediately.

FIG. 2 shows a home television receiver A.
The configuration of the tuner inside the M video signal receiver 33 is shown. The AM video signal receiver 33 includes channel selection means such as a remote controller, a local oscillator 36 and a frequency converter for converting the frequency of the AM video signal to an intermediate frequency, and an AM demodulator 35 for demodulating the AM video signal. And a display for displaying an image based on the image signal.

[0006]

The optical CATV system shown in FIG. 2 has a problem that the number of optical branches in the optical coupler 21 cannot be made too large. Since the AM video signal can have a narrow signal band, the number of channels can be increased,
Although the use efficiency of the transmission path band is good, the noise of the transmission path has a great influence on the video itself. Therefore, the subscriber 30
In order to provide a good image, it is necessary to keep the CN ratio of the AM image signal high. If the level of the optical signal input to the optical receiver 32 of the subscriber 30 is increased, the CN ratio of the AM video signal can be increased. However, the level of the optical signal that can be output by the optical transmitter 11 of the transmitting station 10 has an upper limit. In the end, the number of optical branches cannot be increased so much. For these reasons, in the optical CATV system according to the prior art, it is necessary to increase the number of optical fibers 20 drawn from the transmitting station 10 and to use many expensive optical transmitters 11, thereby increasing the cost of the system.

FIG. 3 shows an example of an optical CATV system using an FM video signal instead of an AM video signal in video distribution. That is, in the transmitting station 10, the video signal of each channel is converted into an FM video signal of a different frequency by the FM modulator 13. In the subscriber 30,
After being converted into an electric signal by the optical receiver 32 in the set-top box 31 and demodulated by the FM demodulator 38, the television receiver 33
And the video recorder 34. With this method, the number of optical branches can be greatly increased. Since the FM video signal has a wider signal band than the AM video signal, there is a drawback that the utilization efficiency of the transmission path band is low. However, even if the CN ratio of the received signal is low, the quality of the demodulated video signal does not easily deteriorate. In the case of FM video signals, the CN ratio required to obtain the same video quality is much lower, such as 20 to 30 dB for AM video signals. Therefore, the level of the optical signal input to the optical receiver 32 is reduced, and the number of optical branches and the transmission distance can be increased.

However, since the tuner in the television receiver 33 or the video recorder 34 is not usually adapted to receive the FM video signal, it is necessary to provide a separate tuner for the FM video signal. The BS tuner is a tuner for FM video signals, but has a different frequency band from that of the optical CATV system.

In the example of FIG. 3, the set-top box 31
A channel selection circuit 37 and an FM demodulator 38 are arranged therein, and a channel is selected and demodulated here. However, such a method prevents the subscriber 30 from receiving different channels on the plurality of television receivers 33, respectively. In addition, since the channel selection operation on the television receiver 33 cannot be performed, a remote control device for the set-top box 31 is required in addition to the remote control device for the television receiver 33. I cannot use the channel setting function for the timer recording function.

An object of the present invention is to solve the above problems and to provide an optical CATV system that can increase the number of optical branches and is easy for a user to use.

[0011]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a method of connecting an optical fiber between a transmitting station having an optical transmitter and a subscriber having an optical receiver, or connecting the transmitting station to an optical station. The receiver is connected with an optical fiber, the optical receiver is connected with a subscriber through an electric cable, and a transmitting station distributes video signals of a plurality of channels. Each subscriber is provided with one or more receivers. In an optical CATV system provided, a transmitting station has an AM modulator for AM-modulating a video signal for each channel and the AM modulator.
An FM modulator for FM-modulating video signals at different frequencies is provided, and the FM signals of these FM modulators are frequency-multiplexed and converted into optical signals.
Using an AM video signal receiver that receives M video signals,
For each video signal receiver, select an FM signal from the frequency multiplexed FM signal converted from the optical signal into an electric signal in synchronization with the channel selection of the AM video signal receiver and demodulate the FM signal into an AM video signal. An adapter is attached.

The inter-channel frequency interval of the FM modulator is set to an integer N times the inter-channel frequency interval of the AM video signal receiver, and the local oscillation in the AM video signal receiver is set in the FM signal receiving adapter. A frequency multiplying circuit for taking out a signal and multiplying it by an integer N, a frequency converting circuit for converting the frequency of the FM signal selected from the frequency multiplexed FM signal into a predetermined frequency by using the N-times signal, An FM demodulator for demodulating to an AM video signal may be provided.

The frequency of the AM modulation at the transmitting station is common to each channel, and the FM signal receiving adapter includes
A frequency conversion circuit for converting a common frequency of the AM video signal demodulated from the M signal into a frequency of the channel in the AM video signal receiver may be provided.

[0014]

An embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

In the optical CATV system of the present invention, the transmitting station modulates the video signal by AM, and further modulates the AM video signal by F-mode.
The M signal is multiplexed and the FM signal is multiplexed, and the subscriber attaches an FM signal receiving adapter to each of the television receiver and the video recorder. The FM signal receiving adapter extracts a local oscillation signal from the television receiver and the like, It is used for selecting and demodulating an FM signal linked to a receiver or the like, and the demodulated AM video signal is received by a television receiver or the like.

As shown in FIG. 1, a transmitting station 10 has an AM modulator 12 for AM modulating a video signal for each channel.
And an FM modulator 13 for FM-modulating the AM video signal at different frequencies. In addition, each FM
An optical transmitter 11 for frequency-multiplexing the FM signal of the modulator 13 and converting it into an optical signal is provided. In this optical CATV system, a transmission station 10 having an optical transmitter and a subscriber 30 having an optical receiver are connected by an optical fiber 20. A branch by an optical coupler 21 is provided in the middle of the optical fiber 20, and a plurality of subscribers 30 are connected to one optical fiber 20 drawn from the transmitting station 10.

The subscriber 30 is provided with a set-top box 31 in which an optical receiver 32 is accommodated, and a television receiver (AM video signal receiver) 3 is provided from the set-top box 31.
3 and the video recorder 34 are connected by a coaxial cable or the like. An FM signal receiving adapter 40 is attached to each AM video signal receiver 33.

FIG. 1 shows the internal configuration of the FM signal receiving adapter 40 and the configuration of the tuner inside the AM video signal receiver 33. The AM video signal receiver 33 includes channel selection means such as a remote controller, a local oscillator 36 and a frequency converter for converting the frequency of the AM video signal to an intermediate frequency, an AM demodulator 35, and a display for displaying video. And a container. The FM signal receiving adapter 40 extracts a local oscillation signal from the AM video signal receiver and multiplies it by an integer N.
A frequency conversion circuit for converting the frequency of the FM signal selected from the M signals into a predetermined frequency, and an FM demodulator 43 for demodulating the FM signal of the predetermined frequency into an AM video signal are provided. Further, a local oscillator 42 and two frequency conversion circuits are provided.

The operation will be described. In the transmitting station 10, the video signals 1, 2,.
Are ch1, ch of a normal AM video signal as described later.
2, so that they can be viewed on the subscriber's television receiver. In this embodiment, a common carrier frequency by the video signals 1, respectively ... the AM modulator 12 f _A
Into an AM video signal. The AM video signal is converted into different carrier frequencies f ₁ and f by the FM modulator 13.
₂ , ... are converted to FM signals. Each FM signal is frequency multiplexed. The optical signal output from the transmitting station 10 is amplitude-modulated (intensity-modulated) by the frequency-multiplexed FM signal in the optical transmitter 11, and becomes a subcarrier multiplexed signal of the FM signal. Note that the common carrier frequency f _A is FM
In order to narrow the occupied band of the signal and increase the use efficiency of the frequency band, the frequency is set to be sufficiently lower than f _ch1 within a range allowed by the circuit characteristics of the AM modulation circuit 12.

In the subscriber 30, the subcarrier multiplexed signal is converted into an electric signal by the optical receiver 32 in the set-top box 31 and distributed to each AM video signal receiver 33. F attached to each AM video signal receiver 33
In the M signal receiving adapter 40, the selected FM signal is A
The video signal is converted into an M video signal, and the AM video signal is sent to the AM video signal receiver 33.

Hereinafter, the AM video signal receiver 33 is set to the normal A
Channel x (chx; x is 1, 2,...) Of the M video signal
The signal of each unit when the channel is selected will be described. The frequency of channel x is fchx, and A
When the intermediate frequency in the M video signal receiver 33 and f _IF1, the local oscillator 36 oscillates at a frequency of fLx = fchx-f _IF1, the frequency of the AM video signal from the FM signal receiving adapter 40 is converted to an intermediate frequency . The FM signal receiving adapter 40 extracts the local oscillation signal (frequency fLx) of the AM video signal receiver 33 and multiplies it by an integer N by the frequency multiplying circuit 41. Using this N-times signal, frequency multiplexing F
The frequency of the FM signal having the frequency fx in the M signal is converted to the intermediate frequency _fIF2 . This is demodulated by the FM demodulator 43.

Here, the carrier frequency fx of the FM signal corresponding to the channel x of the AM video signal is f _IF2 + N ×
fLx may be used. That is, the frequency interval between channels of the FM modulator in the transmitting station 10 is set to an integer N times the frequency interval between channels of the AM video signal receiver.

The AM video signal demodulated by the FM demodulator 43 in the FM signal receiving adapter 40 is frequency-converted using the signal of the frequency f _IF1 -f _A output from the local oscillator 42, and further converted to an AM video signal. The frequency is converted using the local oscillation signal (frequency fLx) of the receiver 33 and the frequency f
It becomes an AM video signal of chx.

For example, if the frequency of ch1, ch2, and ch3 of the AM video signal that can be viewed on the subscriber's television receiver is 9
1.25 MHz, 97.25 MHz, 103.25 MH
z, the intermediate frequency f _IF1 is 41.25 MHz, the intermediate frequency f _IF2 is 50 MHz, N is 5, and the carrier frequency f _A of the AM modulator is 2 MHz. The carrier frequencies f 1, f 2 and f 3 of the corresponding FM signals are 300 MHz, 3
It suffices to set 30 MHz and 360 MHz.

As described above, according to the present invention, F
An optical CATV system using M signals can be easily configured. When the AM signal is FM-modulated and transmitted as in the present invention, the CN ratio of the AM signal is improved by 20 to 30 dB over the FM signal when the FM signal is demodulated. Therefore, if the modulation degree of the optical signal is the same, the optical signal level may be a value smaller by 10 to 15 dB (a value of 1/10 to 1/30). For this reason, the number of optical branches by the optical coupler 21 can be increased 10 to 30 times. In practice, it is necessary to consider the excess loss of the optical coupler, but the number of optical branches can be greatly increased as compared with the case where the signal is transmitted as it is as an AM signal. as a result,
Generally expensive optical transmitter 1 used in transmitting station 10
1 can be greatly reduced, and the cost of the system can be reduced.

Since the FM signal receiving adapter 40 is attached to each AM video signal receiver 33 and the FM signal receiving adapter 40 is operated in conjunction with the channel selection function in the AM video signal receiver 33, The user can use it in the same manner as receiving a conventional AM system CATV or terrestrial broadcast without being aware of the type of signal.

Since the FM signal receiving adapter 40 can be easily constituted by low-priced semiconductor parts, even if one FM video signal receiver 33 is connected to each, no significant increase in cost is required. Although it is necessary to extract a local oscillation signal from each AM video signal receiver 33, since only one lead wire is required,
Installation is very simple. Also, since the local oscillation signal generally does not differ depending on the model of the AM video signal receiver 33, it is not necessary to prepare a different FM signal reception adapter 40 for each model of the AM video signal receiver 33.

In the above embodiment, the all-optical type optical CA that connects the transmitting station 10 and the subscriber 30 with the optical fiber 20 is used.
Although the present invention is a TV system, the present invention can be applied to an optical CATV system in which an optical fiber and an electric wire are combined. A similar effect can be obtained in a system in which an optical amplifier is inserted into the output of the optical transmitter to increase the output level.

[0029]

The present invention exhibits the following excellent effects.

(1) Since FM transmission is performed, the number of optical branches can be increased, and the cost of the system can be reduced.

(2) Since the channel is selected by the FM signal receiving adapter, the subscriber can receive a different channel with each of the plurality of receivers, and the FM selection is linked to the AM selection of the receiver. Therefore, the user can view and listen as usual, and is convenient.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of an optical CATV system according to an embodiment of the present invention and an internal configuration of a transmitting station and a subscriber.

FIG. 2 is a diagram showing the overall configuration of a conventional optical CATV system using AM transmission and the internal configuration of a transmitting station and a subscriber.

FIG. 3 is a diagram showing the overall configuration of a conventional optical CATV system using FM transmission and the internal configurations of a transmitting station and a subscriber.

[Explanation of symbols]

 Reference Signs List 10 transmitting station 11 optical transmitter 12 AM modulator 13 FM modulator 20 optical fiber 30 subscriber 32 optical receiver 33 television receiver (AM video signal receiver) 40 FM signal reception adapter 41 frequency multiplier 43 FM demodulator

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI H04N 7/22

Claims (3)

[Claims] An optical fiber is connected between a transmitting station having an optical transmitter and a subscriber having an optical receiver, or an optical fiber is connected between the transmitting station and the optical receiver. In an optical CATV system in which a subscriber is connected with an electric cable and a transmitting station distributes video signals of a plurality of channels and each subscriber is provided with one or a plurality of receivers, the transmitting station transmits video signals to each channel. AM modulator for AM-modulating a signal and its A
FM that FM-modulates M video signals at different frequencies
A modulator is provided, and the FM signals of these FM modulators are frequency-multiplexed and converted into optical signals. On the subscriber side, an AM video signal receiver for receiving an AM video signal is used.
FM signal for synchronizing with the channel selection of the AM video signal receiver from the frequency multiplexed FM signal converted from the optical signal to the electric signal for each M video signal receiver and demodulating it into an AM video signal An optical CATV system comprising a receiving adapter. 2. An inter-channel frequency interval of the FM modulator is set to an integer N times an inter-channel frequency interval of the AM video signal receiver, and an FM signal receiving adapter is provided in the FM signal receiving adapter. A frequency multiplying circuit for extracting a local oscillation signal and multiplying it by an integer N; a frequency converting circuit for converting the frequency of an FM signal selected from frequency-multiplexed FM signals into a predetermined frequency by using the N-times signal; 2. The optical CA according to claim 1, further comprising an FM demodulator for demodulating the signal into an AM video signal.
TV system. 3. The frequency of AM modulation at the transmitting station is common to each channel, and the FM signal receiving adapter includes:
3. An optical CATV system according to claim 1, further comprising a frequency conversion circuit for converting a common frequency of the AM video signal demodulated from the FM signal into a frequency of the channel in the AM video signal receiver. .