JPH05347760A - Optical catv system - Google Patents

Abstract

PURPOSE:To prevent the degradation of image quality due to the multipath reflection caused on an optical transmission line in an optical CATV system which transmits the video signals by the optical signals. CONSTITUTION:An optical CATV system includes an optical image transmitting part 1 which converts the transmitting video signals to the optical signals by an optical transmission part 6, an optical transmission line 2 which transmits the optical signals, and a reception part 3 including an optical reception part 7 which converts the optical signals transmitted via the line 2 into the electric signals and a monitor part 8 which performs the video display. Then the part 1 is provided with a multiplexing part 4 which multiplexes the multi-carrier signal or the noise signal which is off the band width of the video signal received from an outside and width signal output part 5 on the video signals. The signal obtained by multiplexing the transmitting video signal to the outside band width signal is converted to an optical signal and sent to the line 2.

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 transmitting a video signal as an optical signal. CATV (C
The Able Television (Television) method is equivalent to cable broadcasting in which a cable is laid between a broadcasting station and each receiver to transmit a video signal, and an optical CATV system using the cable as an optical transmission line using an optical fiber is known. Has been. There is a demand for image quality improvement in such an optical CATV system.

[0002]

2. Description of the Related Art FIG. 5 is an explanatory view of a conventional example, 30 is a transmitting section, 31-1 to 31-n are video signal output sections, and 32-1.
˜32-n is a modulator, 33 is a frequency multiplexer, 34 is an optical transmitter, 35 is an amplifier, 36 is a laser diode, 3
7 is an optical transmission line, 38 is a receiving unit, 39 is an optical receiving unit, 40 is a photodiode, 41 is an amplifier, 42 is a receiving tuner, and 43 is a monitor.

The video signal output units 31-1 to 31-n include a television camera, a video device, a video signal output unit for television broadcasting, etc., which output video signals corresponding to channels, and the video signals have different frequencies. Modulator 32-1
.. 32-n, and is frequency-multiplexed by the frequency multiplexer 33. The frequency-multiplexed video signals of a plurality of channels are amplified by an amplifier 35 of an optical transmitter 34, converted into an intensity-modulated optical signal by a laser diode 36 such as a DFB (distributed feedback type) laser, and an optical transmission line 37. Sent to. The optical transmission line 37 is composed of, for example, a single mode optical fiber.

In the receiving section 38, the optical signal transmitted through the optical transmission line 37 is incident on the photodiode 40 of the optical receiving section 39, converted into an electric signal, and amplified by the amplifier 41. The output signal of the amplifier 41 is a frequency-multiplexed video signal of a plurality of channels. Therefore, by selecting a channel by the reception tuner 42, a video signal of an arbitrary channel is selected to monitor 43.
Can be received by. That is, after being converted into an electric signal by the optical receiver 39, it is possible to perform channel selection and display an image on the monitor 43 using a cathode ray tube as in a normal television broadcasting system. Further, a plurality of receiving units 38 can be connected to the optical transmission line 37 via any number of optical branching units.

[0005]

Multiple reflection occurs in the optical transmission line 37, and there is a problem that the image quality is deteriorated by this multiple reflection. An isolator is provided between the laser diode 36 and the optical fiber forming the optical transmission line 37.
Even if the return light to the laser can be reduced, the multiple reflection cannot be sufficiently suppressed, and there is a problem that the optical signal from the laser diode 36 contains a distortion component and the image quality is deteriorated. An object of the present invention is to reduce image quality deterioration in an optical CATV system with a simple structure.

[0006]

An optical CATV system of the present invention will be described with reference to FIG. 1. An optical video transmission unit 1 for converting a transmission video signal into an optical signal by an optical transmission unit 6, and an optical video transmission unit A receiver 3 having an optical transmission line 2 for transmission, an optical receiver 7 for converting an optical signal transmitted via the optical transmission line 2 into a video signal of an electric signal, and a monitor 8 for adding the electric signal. And a multiplexing unit 4 for multiplexing the out-of-band signal of the transmission video signal in the optical video transmission unit 1, and after multiplexing the out-of-band signal from the out-of-band signal output unit 5 to the transmission video signal, It is converted into an optical signal and transmitted.

[0007]

When a noise signal, a multi-carrier signal or the like outside the used band of the video signal is superimposed from the out-of-band signal output unit 5 on the single-channel video signal or the multiplexed multi-channel video signal, the video signal In the same manner as when the spectrum of No. 1 is spread, if the video signal obtained by superposing this out-of-band signal in the multiplexing unit 4 is converted into an optical signal by the laser diode of the optical transmission unit 6 and sent to the optical transmission line 2. It is possible to reduce multiple reflections in the optical transmission line 2, reduce distortion of the laser diode, and prevent deterioration of image quality received by the monitor unit 8 of the receiving unit 3.

[0008]

EXAMPLE FIG. 2 is an explanatory view of an example of the present invention.
Is a transmission unit, 11-1 to 11-n are video signal output units, 12
-1 to 12-n are modulators, 13 is a frequency multiplexer, 14
Is an optical transmitter, 15 is an amplifier, 16 is a laser diode, 17 is an optical transmission line, 18 is a receiver, 19 is an optical receiver,
Reference numeral 20 is a photodiode, 21 is an amplifier, 22 is a receiving tuner, 23 is a monitor, and 24 is an out-of-band signal output unit.

The out-of-band signal output unit 24 is configured, for example, to output a multi-carrier signal in a band outside the video frequency band by the modulators 12-1 to 12-n, and the multi-carrier signal is sent to the frequency multiplexing unit 13. input. The frequency multiplexer 13 frequency multiplexes the modulated video signals from the modulators 12-1 to 12-n. Therefore, the spectrum of the video signal is spread, the frequency multiplexed signal is input to the optical transmitter 14 by the frequency multiplexer 13, and the optical signal converted by the laser diode 16 of the optical transmitter 14 is converted into the optical transmission line 17. To send to.

FIG. 3 is a band explanatory diagram of an embodiment of the present invention. FIG. 3A shows a configuration in which the out-of-band signal output unit 24 outputs a multi-carrier signal, and two main signals (video signals).
4 shows a case where four multicarrier signals from the out-of-band signal output unit 24 are superimposed on both sides of the use band. The amplitude of this multi-carrier signal can be at the same level as the main signal. The frequency interval of the multi-carrier signal can be the same as or different from the frequency interval of the main signal. Also, the multi-carrier signal is not only a plurality of carrier signals of different frequencies,
It includes a carrier signal of a single frequency outside the band of the main signal.

As described above, the frequency-multiplexed signal in which the out-of-band multi-carrier signal is superimposed is converted into an optical signal in the optical transmission section 14, and this optical signal is transmitted through the optical transmission line 17, thereby performing optical transmission. Image quality deterioration due to multiple reflections on the path 17 can be reduced. Also, for a single-channel main signal (video signal), by superimposing a multi-carrier signal outside the used band, it is possible to reduce the influence of multiple reflections by the optical transmission line 17 on the image quality.

Further, the out-of-band signal output unit 24 is configured to output a noise signal outside the band of the frequency signal, the noise signal is input to the frequency multiplexing unit 13, and is input to the frequency multiplexing unit 13 together with the modulated video signal. The frequency is multiplexed in the optical transmitter 1
At 4, it can be converted into an optical signal and sent to the optical transmission line 17.

FIG. 3B shows an out-of-band signal output section 24.
And a case where a noise signal from the out-of-band signal output unit 24 is superimposed on both sides of the use band of two main signals (video signals). The signal level is generally lower than the main signal level, but is selected according to the characteristics of the optical transmission line 17. The out-of-band signal output unit 24 in this case is
A white noise signal may be generated and added to the frequency multiplexing unit 13 via a band elimination filter excluding the used band of the main signal.

FIG. 4 is an S / N characteristic curve diagram.
25MHz (channel number 3) and 217.25MHz
Measurement of S / N when video signals of the respective television channels (channel number 12) (video signals of a single channel) are converted from the optical transmitter 14 into optical signals and transmitted by the optical transmission line 17. Curve A connecting points and 55.25MHz to 313.25MHz excluding those frequency bands
And a curve B connecting the measurement points of S / N when the multi-carrier signal of 6 MHz interval is superimposed on the above-mentioned video signal and converted into an optical signal. The amplitude of the multi-carrier signal in this case was set to be approximately the same as the amplitude of the video signal.

As can be seen by comparing the curves A and B,
Approximately 10 dB due to superposition of multi-carrier signals
It was possible to improve the S / N to some extent. For example, the S / N when the video signal of channel number 3 (103.25 MHz) is converted into an optical signal and transmitted through the optical transmission line 17.
Was about 41.5 dB, but when the multi-carrier signal as described above was superimposed and converted into an optical signal, 51.
It became 5 dB, and the S / N could be improved by 10 dB by superimposing the multi-carrier signals.

It has been confirmed that the S / N can be improved also when the video signals of a plurality of channels are multiplexed and the out-of-band multi-carrier signals (including a single carrier signal) are superimposed and converted into an optical signal. Similarly, it was confirmed that the S / N can be improved also when an out-of-band noise signal is superimposed on a video signal and converted into an optical signal. Further, it is considered that the number of multi-carrier signals and the like slightly differ in the S / N improving effect depending on the characteristics of the optical transmission line 17.

When the video signals of the television channels of channel numbers 1 to 12 are frequency-multiplexed (91.25
MHz to 217.25 MHz), 55.2 out of the used band
5MHz to 85.25MHz and 223.225MHz to
In the band of 312.25 MHz, it is possible to superimpose a multi-carrier signal at a 6 MHz interval which is the same as the frequency interval of the television channel.

In each of the above-mentioned embodiments, it is necessary to make the band of the amplifier 15 of the optical transmitter 14 wider than in the conventional example.
This amplifier 15 does not need to amplify multi-carrier signals and noise signals outside the used band without distortion, and it is sufficient if it has a configuration that does not increase harmonic components in the used band due to nonlinear distortion or the like. The transmitter 14 does not have a particularly expensive configuration. Further, the amplifier 21 of the optical receiver 19 does not need to amplify the multi-carrier signal or the noise signal outside the used band, and the configuration similar to the conventional example is sufficient, and the multi-carrier signal or the noise signal outside the used band is sufficient. A filter for removing the component can be provided after the photodiode 20.

[0019]

As described above, according to the present invention, an out-of-band signal such as a multi-carrier signal (including a single-carrier signal) out of the band, a noise signal or the like is added to a transmission video signal of single or multiple channels. The optical signal is multiplexed by the multiplexing unit 4, converted into an optical signal by the optical transmitting unit 6, and the optical signal is transmitted by the optical transmission line 2. Frequency multiplexing equivalent when the spectrum of the video signal to be transmitted is spread. By converting this into a signal and converting it into an optical signal and transmitting it, there is an advantage that the influence of multiple reflection by the optical transmission line 2 can be reduced and the image quality deterioration can be economically prevented.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory diagram of an example of the present invention.

FIG. 3 is a band explanatory diagram of an example of the present invention.

FIG. 4 is an S / N characteristic curve diagram of an example of the present invention.

FIG. 5 is an explanatory diagram of a conventional example.

[Explanation of Codes] 1 optical video transmission unit 2 optical transmission line 3 reception unit 4 multiplexing unit 5 out-of-band signal output unit

Claims (1)

[Claims] 1. An optical video transmission unit (1) for converting a transmission video signal into an optical signal, and an optical transmission line (2) for transmitting the optical signal,
An optical CATV system comprising: a receiving unit (3) for converting an optical signal transmitted through the optical transmission line (2) into an electric image signal and receiving the image. ), A multiplexing unit (4) for multiplexing an out-of-band signal of the transmission video signal is provided, and the out-of-band signal is multiplexed with the transmission video signal, and then converted into an optical signal for transmission. An optical CATV system characterized in that