JPH0481394B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a system for frequency multiplexing and transmitting a plurality of television signals, and in particular to a channel for converting multiplexed signals into optical signals and transmitting the signals. The present invention relates to a multiplex television signal transmission system that reduces inter-interference.

[Conventional technology]

A method that transmits multiple TV signals using a single optical cable by frequency multiplexing the frequencies in the VHF band to UHF band used for TV broadcasting not only reduces the cost of optical cables and their installation, but also reduces the cost of optical transmitters and optical fibers. Since only one pair of receivers is required, it is more economical than optical wavelength division multiplexing systems, etc., and is used in community reception systems in buildings and short-distance hard-to-view systems.

[Problem that the invention seeks to solve]

In television broadcasting, for example, suppressed carrier modulation is used as a modulation method, but when transmitting multiple multiplexed television signals over a single optical cable, cross-modulation and cross-modulation due to non-linearity of the optical transmission system occur. Inter-channel interference caused by intermodulation has become a problem.

One type of interference that is particularly noticeable is the so-called "wind wiper," a phenomenon in which horizontal stripes slowly move upward or downward on the TV screen, and this is caused by cross-modulation from the synchronization peaks of other channels. Contamination is the cause.

Inter-channel interference occurs because the modulation degree of the synchronization peak is the highest, or the lowest depending on the signal component, among the signal components, and deviates greatly from the average modulation degree. FM modulation other than suppressed carrier modulation used for
Even with pulse modulation, inter-channel interference similarly occurs, making it difficult to multiplex television transmission using optical communications.

SUMMARY OF THE INVENTION An object of the present invention is to provide a multiplex television signal transmission system for reducing the above-mentioned cross-modulation from the synchronization peak and transmitting high-quality television signals using optical fibers.

[Means for solving problems]

In a multiplex television signal transmission system in which a plurality of television signals are frequency-multiplexed and transmitted, the present invention synchronizes the vertical and horizontal synchronization signals of all baseband television signal sources to the same phase, and inverting the polarity of about half of the baseband television signal, modulating each of the inverted baseband television signal and the non-inverted baseband television signal and converting them to a high frequency domain, and then combining them; It is characterized by converting a composite signal into an optical signal and transmitting it.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 and FIG. 2 are diagrams for explaining one embodiment of the present invention, and show the configurations of a transmitting side and a receiving side, respectively. In this embodiment, a case will be explained in which the television signals to be transmitted are two channels.

Horizontal synchronization and vertical synchronization of the two television cameras 1 and 2 are inserted at completely the same timing by a synchronization generator 10. That is, the horizontal synchronization signals and vertical synchronization signals of the television cameras 1 and 2 are synchronized to have the same phase. The baseband television signal from the television camera 1 is input to the inversion circuit 3, and the baseband television signal from the television camera 2 is input to the suppressed carrier wave modulator 5. The inverting circuit 3 inverts the polarity of the voltage of the baseband television signal from the television camera 1 and inputs it to the suppressed carrier wave modulator 4 . The baseband television signal whose polarity has been inverted is modulated by a suppressed carrier wave modulator 4 and outputted as an intermediate frequency suppressed carrier wave signal. On the other hand, TV camera 2
The output is modulated with the same polarity by a suppressed carrier modulator 5, and output as a suppressed carrier wave signal of an intermediate frequency. The suppressed carrier wave signals, which are the output signals of the suppressed carrier wave modulators 4 and 5, are shown as a and b in FIG. Signal a is a suppressed carrier signal whose polarity has been inverted. The synchronization peaks of these suppressed carrier wave signals a and b are at the same timing and in opposite directions.

Frequency converters 6 and 7 convert these signals a and b into different frequencies, a combiner 8 combines the signals, and an optical transmitter 9 converts them into optical signals and transmits them to an optical fiber 11. .

On the receiving side in FIG. 2, in order to perform the opposite conversion to that on the transmitting side, the optical input from the optical fiber 11 is converted into an electrical signal by the optical receiver 12, and the frequency converter 13, 1
4 converts it to an intermediate frequency, and demodulators 15 and 16 reproduce it into a baseband television signal. Demodulator 1 corresponding to a channel in which an inversion circuit 3 is inserted on the transmitting side
The output of 5 is corrected in the inverting circuit 17 and has the polarity.
It is output from the terminal 18. On the other hand, the output of the demodulator 16 is directly output from the terminal 19.

According to the above multiplex television signal transmission system, as described above, the synchronization peaks of the suppressed carrier wave signals a and b, which are the outputs of the suppressed carrier wave modulators 4 and 5 on the transmitting side, occur at the same timing and in opposite directions. Therefore, the modulation degree of the synchronization peak is averaged, and in the optical transmission system such as the optical transmitter 9, the optical fiber 11, and the optical receiver 12, the modulation degree of the synchronization peak is particularly likely to increase. do not have. Therefore, the wind wiper phenomenon described above does not occur on the receiving side.

In the above embodiment, the case where the number of channels is 2 has been described, but it is clear that the same idea can be applied to a case where the number of channels is more than 2. In this case, if the number of channels is even, the polarities of the baseband television signals of half of the channels are inverted. Also, if the number of channels is an odd number, about half the channels (for example, 3 or 4 channels if there are 7 channels)
Clearly, reversing the polarity of the baseband television signal, even on odd channels, can be very effective.

In addition, although the suppressed carrier modulation method has been described in the above embodiments, the cross-modulation characteristics of the modulation-demodulation system can be improved even with other modulation methods by averaging the modulation depth of the synchronization peak, which is a feature of the present invention. It is also clear that improvements can be made.

〔Effect of the invention〕

As explained above, if the polarity of about half of all channels is inverted at the baseband stage before the modulation circuit, and all channels are synchronized in the part that generates the baseband signal, synchronization can be achieved. Since cross-modulation from the peak to other channels is reduced and the cross-modulation components are in a synchronous relationship with each other, there is an advantage that no wind wiper phenomenon occurs.

[Brief explanation of the drawing]

1 and 2 are for explaining one embodiment of the present invention, FIG. 1 is a block diagram showing the configuration of the transmitting side, FIG. 2 is a block diagram showing the configuration of the receiving side, and FIG. FIG. 3 is a diagram showing the relationship of waveforms between channels in the embodiment of FIG. 1. 1, 2... Television camera, 3, 17... Inverting circuit 4, 5... Suppression carrier modulation circuit, 6, 7, 1
3, 14...Frequency converter, 8...Synthesizer, 9...
... Optical transmitter, 10 ... Synchronization generator, 11 ... Optical fiber, 12 ... Optical receiver, 15, 16 ... Demodulator, 18, 19 ... Output terminal.

Claims (1)

[Claims] 1 The baseband television signals of the plurality of channels are multiplexed on the frequency axis by individually modulating the baseband television signals of the plurality of channels, converting them into high-frequency signals of mutually different frequencies, and then combining them. In a multiplex television signal transmission system in which the baseband television signals of the plurality of channels are synchronized in phase with each other, and Approximately half of the channels' baseband television signals modulate each channel after inverting their polarity, and the other channels' baseband television signals modulate each channel without inverting their polarity. A multiplex television signal transmission system characterized by modulating the signals, combining all these modulated signals to create one frequency multiplexed signal, converting this frequency multiplexed signal into an optical signal, and transmitting the optical signal.