DESCRIPTION

The present invention relates to a method for filtering data signals according to the preamble of claim 1.

The invention also relates to a filtering unit and to a signal distribution method.

The invention is preferably and advantageously applicable to systems for distributing signals within a building, such as, for example, MATV (Master Antenna TV) systems. In these systems, the signals are received by antennas and satellite dishes connected to a head-end station, where the signals are collected and distributed to the different housing units in the building.

Within each housing unit, television receivers are used (such as television sets, video recorders and set-top-boxes) which pick up the signal from a signal receptacle connected to the distribution system.

Since along their aerial path the television signals undergo different attenuations, they are also received by the antennas of the MATV system with different attenuations. Before being distributed to the housing units, the received signals must therefore be amplified and possibly equalized.

To do this, it is necessary to select the different signals from the received data streams. In particular, when signals at adjacent frequencies undergo similar attenuations and distorsions, in order to cut costs down it is advantageous to select groups of signals on adjacent carriers; for example, in the case of digital television signals it would be advantageous, in terms of cost and ease of installation of the MATV system, to be able to filter groups of transponders (signals modulated on one carrier, each carrying information of multiple TV channels modulated on subcarriers) which are adjacent and undergo similar attenuations, so as to reduce the number of filters required at the system's head-end station.

It is known that filters which can be frequency-tuned with a variable passband can be obtained by using variable inductive and/or capacitive elements; in particular, variable capacities (varicap diodes) can be used.

These elements however have low quality factors (Q), and therefore such filters are not very selective; as a consequence, the filtering of a group of transponders will output not only the desired signals, but also lateral residues, the importance of which will increase the broader the band to be filtered.

As a result, these solutions are not suited to selecting transponders or, in general, signals modulated on different carriers. In particular, they are not suited to selecting a large number (e.g. 4 to 6) of transponders.

It is therefore an object of the present invention to provide a filtering unit and a filtering method which can overcome the drawbacks of the prior art.

It is another object of the present invention to provide a filtering unit and a filtering method which allow to accurately select a variable number of signals on different carriers within a signal stream.

In particular, it is an object of the present invention to provide a filtering unit and a filtering method which are particularly suitable for use in a system for distributing television signals.

It is a further object of the present invention to provide a system for distributing television signals within a building which is easier to set up and less costly than the prior art.

These objects are achieved through a filtering unit and a filtering method incorporating the features set out in the appended claims, which are intended as an integral part of the present description.

The present invention is based on the general idea of filtering a data stream (in particular a group of transponders) by using one cascade-connected pair of selective filters having a predefined passband, whose input signals are frequency-translated by using corresponding oscillators whose oscillation frequency is controlled on the basis of the number of signals to be extracted from the received stream.

In this way it is possible to use filters having a predetermined passband and select the signals to be filtered by varying the oscillators' frequencies, so as to select a more or less large number of channels or transponders.

Further objects and advantages of the present invention will become apparent from the following description and from the annexed drawings, which are supplied by way of non-limiting example, wherein:

Fig. 1 shows a building equipped with a television signal distribution system according to the present invention;

Fig. 2 is a block diagram of a portion of the distribution system of Fig. 1;

Fig. 3 is a block diagram of a filtering unit according to the present invention; Figs. 3 a, 3b and 3 c schematically show the results of the filtering occurring in different points of the filtering unit of Fig. 3.

Fig. 1 shows a building 1 equipped with reception means for receiving data signals, in particular television signals.

In the following description, the term "data signals" will broadly refer to any form of signal carrying information other than pure noise.

In the example of Fig. 1 said reception means comprise antennas 200, 201 and 202 and satellite dishes 3; of course, other reception means may be used, such as a gateway for receiving cable-transmitted signals, e.g. IPTV signals or more in general signals intended for a web browser used by a user for connecting to the Internet.

For simplicity, but with no limitations whatsoever to the present invention, in the following examples reference will be made to receiving, filtering and distributing television signals within a building.

The antennas 200, 201 and 202 and the satellite dishes 3 are connected to a head-end station 4 that receives the television signals and distributes them to the different housing units 10 in the building.

Each television signal received is characterised by a respective modulation and a respective carrier.

The distribution of the signals to the housing units 10 takes place thanks to a distribution network 5 made out of coaxial cable or optical fibre.

In the example of Fig. 1 the network 5 includes a backbone cable 50 attached to floor shunts 51; cables 52 connect said floor shunts 51 to the signal receptacles 6 of the housing units 10.

The distribution network 5 may alternatively employ a star connection, wherein each signal receptacle 6 of the different housing units is connected to the head-end station 4 through a dedicated line.

Referring back to the example of Fig. 1, a receiver 7, e.g. a television set or a set-top box, is connected to each receptacle 6.

Each housing unit may be fitted with multiple receptacles 6 and receivers 7; in this regard, the example of Fig. 1 must not be considered to be limiting either the number of receptacles per housing unit or the number of receivers that can be connected to a single signal receptacle.

In one embodiment, two or more cables can be connected to a single receptacle, so that multiple receivers can be connected to one signal receptacle. Fig. 2 shows a block diagram of a portion of the distribution system according to the present invention.

The dotted line indicates the head-end station 4, which in the example of Fig. 2 is connected to three antennas 200, 201 and 202.

The antennas 200 and 201 are UHF antennas, whereas the antenna 202 is a VHF antenna.

The outputs of the antennas 200 and 201 are first filtered by the filtering units K1-K5 and then amplified, whereas the output of the antenna 202 is amplified by the amplifier 216 and is then added to the filtered and amplified signals coming from the antennas 200 and 201.

Fig. 3 schematically shows the operation of one of the filtering units K1-K5 of Fig. 2. The input signal 300 comprises a plurality of signals 301 modulated on different carriers. By way of example, reference will be made below to a signal 300 of the DVB type (Digital Video Broadcasting), which comprises a plurality of television transponders 301, each of which, as known, contains information of different television channels (e.g. Rai 1, Rai 2 and Rai 3) modulated on respective subcarriers. The filtering unit receives at its input the signal 300 from an antenna, and frequency- converts it by multiplying it by the output of the oscillator OL1.

The latter oscillates at a first oscillation frequency Fol, which can be set by a user, e.g. an installer.

Each transponder 301 is thus frequency-translated through its convolution with the output signal of the oscillator OL1.

By acting upon the oscillator OL1, the user thus frequency -translates the different transponders 301 in a manner such as to bring the transponders of interest within the passband of the filter 320 placed immediately downstream.

The frequency-translated signal 310 is then filtered through a band-pass filter 320 having very selective characteristics, in particular a SAW filter.

The band-pass filter 320 is centred on the frequency Fcl and has a predefined bandwidth that only lets through a maximum number of transponders 301.

By changing the oscillation frequency Fol of OL1, it is thus possible to select a first group of transponders which are filtered by the filter 320. For example, by changing Fol it is possible to select the passage of the transponders 20 to 25 (these numbers being those conventionally defined by the European regulation ITU-R-BT.470.6) in the UHF band. In the example of Fig. 3a, the filter 320 lets through six television transponders m, m+l,..m+5 (where 'm' is a positive integer) of 8 MHz each, and therefore it has a bandwidth of 48 MHz.

The signal 330 outputted by the filter 320, comprising the six transponders m-m+5, undergoes a further frequency conversion set by the oscillator OL2, which oscillates at an oscillation frequency Fo2.

The translated signal 340 thus obtained is filtered through a second band-pass filter 350 having preferably the same bandwidth as the filter 320.

The filter 350 is centred on a preferably fixed central frequency Fc2.

By acting upon the oscillator OL2 and changing its oscillation frequency, it is possible to bring a different number of transponders among those selected by the filter 320 into the passband of the filter 350, as shown in the examples of Figs. 3b and 3c.

In Fig. 3b all six transponders m..m+5 selected by the first filter 320 pass through the second filter 350.

In Fig. 3c, only a portion of the six transponders selected by the filter 320 pass through the filter 350.

In the example of Fig. 3 c, only the transponders m, m+1, m+2 and m+3 pass through the filter 350, whereas the other transponders are cut.

It is therefore apparent that the combined use of the cascade-connected filters 320 and 350, along with the possibility of controlling the frequencies of the oscillators OL1 and OL2, allows the input signal 300 to be selectively filtered without needing any variable filters like, for example, those employing varicaps. Unlike the latter, in particular, the solution of the present invention, described herein with reference to Figs. 3, 3a, 3b and 3 c, provides a very selective filtering due to the use of SAW filters.

With reference to the example of Fig. 3, before arriving at the output 370 of the filtering unit the signal 360 outputted by the band-pass filter 350 is frequency- converted again, so as to bring the filtered signals back to the frequencies originally received from the antennas.

This conversion, obtained by means of the oscillator OL3, makes the filtering process transparent to the television signal receiver, which will not notice the frequency conversions carried out on the signals received from the antennas.

Of course, this latter frequency conversion process is not strictly necessary, and the receivers 7 may receive the frequency-translated signals while knowing that they have been set to intermediate frequencies obtainable by knowing the oscillation frequencies Fol and Fo2 of the oscillators OL1 and OL2; such information may be sent to the receivers by the head-end station through known methods, e.g. as an additional signal placed in a frequency band known to the receivers.

Referring back to Fig. 2, it can be observed that downstream of the filtering units Kl- K5 there are amplifiers 21 1-215 which, according to the present invention, are of the variable-gain type.

Such a configuration implements an equalizer system that equalizes the input signals, i.e. it applies different gains to each group of transponders for the purpose of making their widths homogeneous.

This makes it possible to compensate for the distorsions caused by the passage of the signals through the transmission medium, in this case represented by the aerial path. The equalizer system allows the installer, or any maintenance technician, to control the gain of the amplifiers 21 1-215 in order to individually adjust the amplitude of the outputs of the filtering units K1-K5, so as to obtain signals having sufficient amplitude for their transmission on the building' s distribution network 5.

The filtering units K1-K5 are adjusted by the installer so as to select groups of adjacent transponders having undergone similar attenuations during the transmission.

For example, assuming that the antenna 200 receives signals from a first transmitter and the antenna 201 receives signals from a second transmitter, then

■ Kl is calibrated for filtering the transponders 21, 22 and 23 coming from the first transmitter;

^■ K2 is calibrated for filtering the transponders 28 and 29 coming from the first transmitter;

^■ K3 is calibrated for filtering the transponders 38, 39, 40, 41 and 42 coming from the first transmitter;

^■ K4 is calibrated for filtering the transponders 47, 48, 49 and 50 coming from the second transmitter;

^■ K5 is calibrated for filtering the transponders 59, 60, 61, 62, 63 and 64 coming from the second transmitter.

As a result, in this non-limiting example the transponders 21, 22 and 23 selected from the first transmitter will be amplified in the same way; this will also apply to the other transponders, arranged in groups, outputted by the other filtering units.

Although the present invention has been described herein with reference to the examples shown in Figs. 1 to 3, it is clear that these examples do not limit the protection scope of the invention as defined by the appended claims.

The man skilled in the art may therefore make changes to the above-described examples without departing from the protection scope of the present invention.

By way of example, the signals may be transmitted by terrestrial or satellite transmitters and be received by means of suitable apparatuses, i.e. satellite dishes or antennas.

The variable-gain amplifiers may be built in the filtering units.

Also, the number of variable-gain amplifiers may not match the number of filtering units, so that it is conceivable that the outputs of two filtering units (e.g. Kl and K2) are first added together and then supplied to the input of a common amplifier.

This latter example appears to be economically advantageous when a large number of transponders are attenuated in the same way and require therefore the same amplification. In this case there will be multiple filtering units for extracting the different transponders, but just one amplifier.

For the purpose of allowing the addition or removal of a variable-gain amplifier, the head-end station of the distribution system is designed in a modular manner by mounting the amplifiers on separate boards which can be inserted into suitable sockets. The connection between the filtering units and the amplifiers is modular as well, in that it uses connection pins.