IE904343A1 - A System for the distribution of very high frequency signals - Google Patents

Abstract

The invention relates to a distribution system for very high frequency signals. This system comprises the use of a dual frequency conversion for the purposes of making it possible in particular to eliminate parasitic signals. It applies most particularly to the distribution of audio and video signals coming from satellites.

Description

Α SYSTEM FOR THE DISTRIBUTION OF VERY HIGH FREQUENCY SIGNALS The present invention relates to a system for the distribution of very high frequency signals and, in particular, audio-visual signals coming from satellites.

Generally speaking, the signals transmitted by a satellite at a very high frequency, for example, between 10 and 15 GHz, are picked up by an antenna, for example, a parabolic antenna, and converted into signals with a lower frequency (of the order of 1 to 2 GHz) before being passed, usually by cable, to a distribution system (in the case of a collective or cable TV distribution).

The distribution system comprises, in essence, a group of receiver-modulator subassemblies corresponding to the various television channels, the various radio frequency signals transmitted by the modulators in accordance with the standards of the current communications regulations (for example, AM audio and AM video, or AM video/FM audio) being passed to the U.H.F. or V.H.F. television receivers by means of a coaxial cable, via a coupler.

Most frequently each subassembly is set in the factory to the frequency to be received. The active elements allowing the frequency of each subassembly to be selected may be frequency synthesizers receiving, from a central unit, commands which are converted into instructions for tuning the local oscillators. These synthesizers may, for their part, be constituted by a phase-locked loop (or P.L.L.).

According to a generally used technique, the audio and video signals received by the modulator modulate an intermediate frequency of the order of 30 MHz; the intermediate frequency signals are filtered with a view to reducing the band width of the amplitude-modulated transmitted signals, translated - 2 (by frequency multiplication), then amplified and are finally filtered to the frequency for the chosen channel, that is to say between 470 and 860 MHz with a band width not exceeding 8 MHz. In other words, in these systems frequency multiplication is effected on the basis of the intermediate frequency up to the frequency value of the chosen channel. One of the major problems of these systems lies in the filtering of the signals, resulting from this frequency multiplication, exactly to the chosen value, for example 500 MHz, both because the signals at 500 MHz are accompanied by interference signals subject to small frequency differences for example, between 470 and 530 MHz, and because it is necessary to provide one filter per channel (or frequency).

The invention therefore proposes a distribution system for signals transmitted at a very high frequency, comprising:- a group of receiver/modulator subassemblies; means for connection to the receiving antenna and to the output device, wherein the signals received by the modulator modulate the intermediate frequency carrier; wherein the thus obtained modulated signals are then translated to the desired channel, subsequently amplified and passed to the final receiver; wherein the signals modulated in an intermediate frequency are subjected to a first frequency conversion up to a fixed frequency above the frequency band of the output channel, and then after frequencies lower than said fixed frequency have been eliminated by filtering, they are subjected to a second frequency conversion by means of a local oscillator controlled by a phase locked loop synthesizer delivering signals at a second frequency equal to the sum of said first frequency and the desired output channel frequency; and wherein the new signals are finally filtered so as to retain only said output channel frequency.

The output device may be an audio/visual receiver such as a - 3 television receiver.

For the sake of simplification, the set of devices comprised between the modulator (included) and the final filter shall be termed the modulator means.

In contrast to the system described above, the first conversion to a frequency higher than the frequency desired for the output signals permits a maximum widening of the frequency bands of the signals with a view to facilitating their filtering. Indeed, as has been recalled, a translation from 30 MHz to 500 MHz leads to the presence of signals at 470 MHz and of signals at 530 MHz (in addition to the desired signals at 500 MHz) and the filtering of the correct freguency is difficult. Moreover, it is necessary to have a filter available for each desired frequency. The use of a frequency exceeding the desired frequency by preferably at least 100 MHz, for example, a frequency of 1 GHz for the same desired frequency of 500 MHz leads to signals at 500, 1500 and 2500 MHz, whose separation by filtering is considerably easier.

Moreover, after the filtering to eliminate the frequencies lower than said chosen fixed frequency (1 GHz in this example) the use of a synthesizer controlled by a central unit that can be connected to and controlled by a simple keyboard permits a direct selection of the output frequency, the said fixed frequency being itself eliminated by the band-pass filter.

In other words, this system not only makes it possible to arrive easily at the desired frequency, for example, by controlling the second converter in a simple manner by a numeric keyboard, but also enables control at will of the desired frequency without having to interpose into the circuit filters corresponding to each one of the desired - 4 frequencies. Because of this, and in contrast to the conventional systems, the adjustment of the distribution system, that is to say the choice of various output frequencies (corresponding for example to various television channels) need no longer be effected by the manufacturer, that is to say in the factory, but may instead be effected directly at the place of use.

In the system described above, use is made of synthesizers, filters, amplifiers, receivers, modulators, frequency con10 verters, and local oscillators which are individually known per se and do not, on their own, constitute any aspects of the invention.

In fact, the invention lies, in this combination of devices and more precisely in the use of converters in the conditions set out above.

In order that the present invention may more readily be understood the following description is given, merely by way of example, with reference to the accompanying drawings, in which the sole Figure shows a diagram of the installation of these various elements.

In this drawing the symbols and reference marks have the following meanings: 1. Video Frequency input 2. Audio Frequency input 3. Intermediate Frequency modulator 4. First Intermediate Frequency filter . First frequency converter 6. Oscillator associated with the first frequency converter 7. Second Intermediate Frequency filter 8. Second frequency converter - 5 9. Oscillator associated with the second frequency converter . Frequency synthesizer 11. Output filter 12. Amplifier 13. Final receiver (for example, a television set) 14. Central unit . Programming keyboard The indications given above relate to the modulator means as defined above. This means that on either side of the modulator means, the distribution system may comprise conventional equipment. In other words, ahead of the modulator means, there may be used a conventional receiver antenna, in particular a parabolic antenna, a converter that is itself conventional, and a conventional receiver.

Similarly, downstream of the modulator means, the signals may be passed to a conventional coupler before being passed to the final receiver, if required jointly with the signals picked up by conventional antennas.

However, and this constitutes a variant of the distribution system in accordance with the invention, it is possible to associate with the modulator means a receiver that is also active, that is to say it comprises frequency synthesizers constituted by a phase locked loop (P.L.L.) and receiving their commands from a central unit. In such a system, the signals arriving at the (parabolic) antenna at very high frequencies (for example, 10 to 15 GHz) pass into a frequency converter, then an amplifier and are then translated into an intermediate frequency set at the desired frequency by the synthesizer controlled by the central unit, and are then filtered and demodulated so as to extract the video signals and audio signals. As has been indicated, the modulator means embodying the invention offers a means for generating signals at a desired frequency (output frequency) - 6 both in a very efficient manner as regards the elimination of interference signals and in a simple manner, both because of the elimination of the filters corresponding to each of the output frequencies and because of the scope offered for adjusting the frequencies directly at the place of use, whereas the conventional systems require a frequency by frequency presetting in the factory and, in the case of a breakdown, necessitate keeping spare preset modulators corresponding to the defective modulator.

In other words, in a distribution system comprising several tens of channels, the new modulator means allows any one of the defective modulators to be replaced without the need for keeping as many replacement modulators as there are modulator types in use.

In the case where the active modulator means is associated with an active receiver as described above, it will be found that one and the same receiver/modulator pair permits the distribution of all the frequencies corresponding to the above mentioned tens of channels by a single command, for example by means of a numeric keyboard.

The modulator means (and the receiver) have been described above more precisely in the application to television signals transmitted by satellite. It goes without saying that the system in itself can receive and distribute other types of signals, such as for example digital data, and the invention does, of course, extend to all other applications of this system.

Claims (9)

1. A distribution system for signals transmitted at a very high frequency, comprising:- a group of receiver^/modulator subassemblies; means for connection to the 5 receiving antenna and to the output device, wherein the signals received by the modulator modulate the intermediate frequency carrier; wherein the thus obtained modulated signals are then translated to the desired channel, subsequently amplified and passed to the final receiver; 10 wherein the signals modulated in an intermediate frequency are subjected to a first frequency conversion up to a fixed frequency above the frequency band of the output channel, and then after frequencies lower than said fixed frequency have been eliminated by filtering, they are subjected to a 15 second frequency conversion by means of a local oscillator controlled by a phase locked loop synthesizer delivering signals at a second frequency equal to the sum of said first frequency and the desired output channel frequency; and wherein the new signals are finally filtered so as to retain 20 only said output channel frequency.

2. A system according to claim 1, wherein said fixed frequency exceeds the maximum desired output channel frequency by at least 100 MHz.

3. A system according to either of claims 1 and 25 2, wherein the receiver comprises frequency synthesizers constituted by a phase-locked loop controlled by a central unit.

4. A system according to any one of claims 1 to 3, wherein said output channel frequency of the modulator is 30 controlled by a keyboard.

5. A system according to any one of claims 1 to - 8 4, wherein said output channel frequency is controlled by said keyboard.

6. A system according to any one of claims 1 to 5, wherein the means effecting said final filtering are a band5 pass filter.

7. A system according to any one of claims 1 to 6, wherein said output device is an audio/visual receiver.

8. A distribution system for very high frequency signals constructed and adapted to operate substantially as 10 hereinbefore described with reference to, and as illustrated in, the accompanying drawing.

9. The use of a system according to any one of claims 1 to 8 for the distribution of audio and video signals from a satellite.