JPS60177781A - Frequency synchronizing system - Google Patents

Abstract

PURPOSE:To prevent beat interference with a receiving radio wave when constituting a main line system of a CATV system by a coding system, by sending a TV signal synchronizing with clock extracted from broadcast television signals, and forming carrying to broadcast wave band in receiving side basing on the clock. CONSTITUTION:Clock synchronized with a TV signal inputted as digital coding clock is prepared using PLL circuits 4 provided in each channel. On the other hand, the receiving side converts signals from light transmitting paths OP1-OPn into electric signals. A PLL circuit 11 extracts clock from the output of a pho toelectric converter and converts to base band signal in synchronizing the clock, and remodulates to signals of VHF band by an up converter 10, and transmits to a distribution system. At this time, by making carrier wave for up converting from extracted clock, beat interference with a broadcast wave transmitted in the air can be eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency synchronization method for transmitting mainline transmission using a digital encoding method in an A'rV system.

In conventional 0ATV systems, the main line and distribution systems also normally transmit data using analog methods. In that case, the repeater for the main line should be of a cable quality different from that of the distribution system, and the coaxial cable should also be of large diameter (for example,
C! -2W) to maintain transmission quality and ensure transmission distance. However, as the scale of the ATV system increases, and when one large-scale ATV system is installed in an entire metropolis (1! It is difficult to construct a system from

The purpose of the present invention is to solve such a large-scale C! When configuring the main line of an ATV system using a digital (encoding) method, it is necessary to avoid beat interference with waves that are directly propagated in the air when the signal is converted back to an analog signal (Vi-[F band or UHF band). The purpose of this invention is to provide a frequency synchronization method.

The present invention includes a PLL (7 Azuron loop) circuit for receiving broadcast waves in the VHF band or UHF band and generating a clock synchronized with the broadcast carrier wave (corresponding to the TV double signal O frequency) of the TV channel to be transmitted. Transmission line clock based on oscillation frequency.

Constructs a frame and sends it to the digital transmission path.

- On the receiving side, the transmission lock frequency is extracted, a PLL circuit is used to configure the broadcasting carrier frequency based on it, the desired synchronization is achieved, and the TV signal is converted back into an analog signal and modulated. be.

FIG. 1 shows an example of the configuration of the present invention. The nine VHF band signals received by the antenna ANT are received and amplified by the HE (Head End) device 1 . Next, the down converter 2 corresponding to each TV channel (OH) demodulates the TV signal into a baseband signal, and the TV signal is sent to the encoding circuit 3.
Then, the band is compressed by digital encoding (in the example of FIG. 1, intraframe encoding (commonly known as DPOM) L32M1)/S. ) and a POM signal with an appropriate bit rate (
In FIG. 1, the optical transmission line OFI to O
Transmit to Pn.

The first point of this invention is that nine works can be produced by using a PLL (needs lock loop circuit) circuit 4 in which each channel is provided with a clock synchronized with the input TV signal as a clock for digital encoding. Yes On the receiving side, optical digital signals from the optical transmission lines OFI to OPn are converted into electrical signals by opto-electrical converters. P.L.
The L circuit 11 extracts the clock from the photoacid-air converter Deba,
In synchronization with this clock, the DPOM decoding circuit 9
It converts the output of the D/D converter 8 into a baseband signal and also outputs the VHFi signal to the up converter 10.
remodulates the signal. And no hybrid coupler 1
2 to the distribution system.

The second point of this invention is to create a carrier wave for upconversion from the clock extracted on the receiving side.
This eliminates beat interference with air-propagated broadcast waves.

While FIG. 1 shows an example in which an optical digital transmission path is provided for each channel of a TV transmitted in the trunk system, FIG. 2 shows an example in which they are multiplexed in the electrical domain.

A signal encoded with a clock synchronized with the carrier (broadcasting) frequency for each channel is sent to a digital multiplexer M.
Multiplexed in UX. In this case, since the clock frequency differs slightly for each channel, so-called stuff multiplexing is required. In this case, only one optical digital line is sufficient as the transmission path. In addition, the synchronized frequency configuration on the receiving side is determined by the receiving side demultiplexer DMUX.
The PLL circuit 11 may perform the processing from the separated signal for each OH.

As described above, according to the present invention, a large city can be covered by one 0AT.
Even for a large-scale 0ATV system such as a V system, it is possible to configure a main line system without a beat failure by using a digital system with no limit on transmission distance.

[Brief explanation of the drawing]

Claims (1)

[Claims] In a transmission system that receives a broadcast television signal, converts it to a baseband band, digitally encodes it, transmits it over a digital transmission path, and reconverts it to the broadcast wave band on the receiving side, the clock extracted from the broadcast television signal. A frequency synchronization method that is characterized by transmitting signals in synchronization with , extracting the clock on the receiving side, and generating a carrier signal for the broadcast wave band based on the clock.