FR2750818A1 - Transmission line signal extraction and separation device - Google Patents

Abstract

The device has a transformer core, with the transmission line placed in this. The effect of the transformer (12) on the line is to maintain the series line impedance at a constantly zero or negligible value. The transformer secondary is connected to a very low impedance amplifier to produce an output signal. The voltage is measured through line connections of two resistors of high enough impedance so as to not interfere with the line. They are connected via a very low impedance isolation transformer to a very low or zero impedance amplifier to avoid low-pass filtering and interfering capacitances. The signals are reconstructed by standard summer and difference circuits and equaliser and echo cancellation circuits removes reflections and echoes.

Description

DEVICE FOR EXTRACTING AND SEPARATING TRANSPORTED SIGNALS
THROUGH A WIRE LINE
The present invention relates to the observation of the signals carried by a transmission line used in bidirectional simultaneous mode. This management mode is that normally used for telephone lines when they carry analog or digital signals. Observing the signals carried by these lines is often necessary to ensure that no illicit use is made of them. The most common embodiment is the telephone pulse counter set up at the subscriber's home to allow him to check his consumption. This application remains but, while it was very easy to implement on analog telephone lines, it becomes much more complex with digital telephone lines such as Numeris in France and may require the insertion of a specific device with cut-off. on the line. Indeed, in the case of analog telephony, the charging information was carried by very specific signals and very easy to identify without ambiguity or risk of errors. On the other hand, in the case of Numeris, all the signaling information which travels on the lines and in particular that relating to consumption and charging, are encapsulated in a message exchange protocol and their analysis requires the complete analysis of all transported signals. The accumulation of several applications of the same kind risks significantly degrading the operation of the telephone line.

To help resolve this problem. the present invention provides a device for intercepting signals carried by the line in both directions of transmission allowing any subsequent processing on these signals without intervening in any way whatsoever on the operation or characteristics of the line. Such a device constitutes a single interface with the telephone line and to which several other devices can be connected to perform several treatments on the extracted signals. In addition, such a device is unable to replace the normally connected equipment and make telephone calls itself.

The device presented operates regardless of the analog or digital nature of the signals transmitted. In fact, the signals transmitted over a telephone line are always analog in nature. We only speak of digital signals when the use of a modem is necessary to represent them by means of analog signals capable of being transported by the physical medium of the telephone line. This is the case for data transmissions or image transmissions by facsimile on analog support and all telephone, facsimile and data signal transmissions on digital media such as Numeris. On analog media the modems used are amplitude, frequency or phase modulation devices at bit rates from 50 to 28,800 bits per second. On digital media, these are generally modems operating in baseband at bit rates of 144, 160 or 2048 kbits per second, to name just the most common products today.

The wired telephone lines mentioned above can be in the form of double pairs of conductors, each of these pairs ensuring the transmission of signals in one direction or the other, or in the form of single pairs simultaneously ensuring both directions signal transmission. Of course, telephone lines can borrow other types of support such as coaxial cables, waveguides, optical fibers, radio cables, radio waves, but the device presented here only applies to the case of use of telephone pairs. Note in passing that a telephone pair can be in a degraded situation, comprising only one conductor and using an electrical return common to several signals. The second conductor of the pair is then to be considered as the electrical symmetric of the first with respect to the common return. This particular arrangement, sometimes encountered in junctions between different pieces of equipment, does not prevent the use of the device presented here.

Among the types of modulation presented above, it is generally the slowest which are likely to be exploited on bidirectional pairs. The increase in the transmitted bit rate makes this type of operation more difficult because of the disturbance brought to reception by the echoes caused by the signals transmitted when they are transmitted on the same physical medium. The limit is now 160 kbits per second, but it is likely to be gradually reduced.

As mentioned above, the device presented can be applied to all the types of transmission mentioned above, without being an exhaustive list, future transmission methods will certainly have characteristics allowing the same application.

However, it is in the case of the use of modems or digital signals as they have been presented above that the use of the device is the most advantageous and especially when using bidirectional single pairs.

In the situation which has just been mentioned, a telephone line, during the signal transmission phase which interests us, is a quadrupole presenting its characteristic impedance Z at each of its ends. The equipment connected to each end each emits its signal at an impedance equal to this characteristic impedance. The electric voltage and current present at any point on the line are therefore representative of the superposition of the signals emitted by the two end devices.

If U1 and U2 are the electrical voltages emitted by each end device, we can summarize the situation schematically by evaluating to:
I = (U1 -U2) / Z the value of the current flowing on the line and at
U = (U1 + U2) / 2
the value of the electrical voltage.

A differential device allows each device to extract from the line the signal it is supposed to receive. This task is possible because this equipment, in addition to the signals present on the line, also knows the signals that it emits itself. The immediate effects of the line on this operation are the time delay and the weakening that it imposes on the transfer of signals between the two end devices. The above equations remain valid assuming U1 and U2 affected by the delay and attenuation caused by the line sections separating the observation point from each of the end devices.

For additional equipment to have the possibility of extracting and separating the signals corresponding to each direction of transmission, it is necessary for it to have knowledge of the electric voltage and current at any point on the line, the equations presented above leading immediately to:
U1 = U + ZI and U2 = UZ.I
At this level the delay and the weakening imposed by the line do not change anything.

In reality, the situation does not present itself as perfectly because the characteristic line impedance and the emission impedances of the end equipment are never perfectly equal. The characteristic impedance of the line itself is not strictly constant at all points. At each point presenting a variation of this impedance, one observes a weakening of the transmitted signal, which is not annoying, and a reflection of this signal towards the end which emitted it, which is much more so. As these reflections are likely to be all the more numerous the longer the line, the signals received by each end are affected by an echo and a reverberation. These phenomena are well known to all specialists in the transmission of analog or digital information and are normally corrected by mechanisms of echo canceller for the former and equalization for the latter.

The signal extraction and separation device presented here does not escape these drawbacks, each signal U1 or U2 restored resulting in fact from the superimposition of different occurrences of the signals U1 and U2 emitted, these occurrences being themselves affected by different time delays. The application of the equations allowing to find U1 and U2 from U and I does not solve anything from a qualitative point of view, but it allows to do most of the work. In fact, it is exactly the same for the differential devices implemented in the end devices themselves. They do not eliminate the echoes, they only reduce them to a level allowing the echo cancellers that exist in the equipment to function properly.

The device presented here is in the same situation and makes use of the same methods of equalization and echo cancellation which are therefore not part of the presentation which is made here.

The problem posed by the implementation of the device presented here is the extraction of the electric voltage and of the current present at a point of a telephone line without interaction with the situation and the operation of this line. Figure 1 shows the general diagram of the device. The value U of the electric voltage present on the line (1) is obtained by means of the branch connection of a high input impedance amplifier provided with an isolation transformer (2). The value I of the electric current carried by the line is obtained by means of a low-impedance current transformer and an amplifier (3).

As described in figure 4, the uninterruptible insertion of the current transformer in series on the line is possible using a magnetic core (4) composed of two parts allowing the opening of the magnetic circuit and its closing on a winding (5 ) constituting the secondary and a small number of turns (6) formed with the wires of the line (1) to constitute the primary. The value of the voltage and the current, associated with the knowledge of the operating impedance of the line allows by sum and difference to determine the value of the signals conveyed in each direction.

The observation of the electric voltage does not pose a problem and is solved in a conventional manner by the use of an amplifier with high input impedance as shown in a non-exclusive manner in FIG. 2. An impedance of the order 40 times the characteristic line impedance is sufficient not to disturb the nature and the size of the transmitted signals.

In the diagram described in Figure 2, the use of passive resistors (7) and (8) for the input impedance of the amplifier (9) guarantees that this impedance remains and therefore that the operation of the line (1 ) is not disturbed when the device is connected but is out of operation. The amplifier itself operates with zero input impedance. It is an operational amplifier connected directly to the secondary of the transformer (10) and the value of its gain is determined by its loop resistance (11).

This arrangement limits the low-pass filtering effects due to the parasitic capacitance of the transformer (10) and to the high value of the connection impedance constituted by the resistors (7) and (8).

The observation of the current present on the line requires the insertion of an impedance in series in the line. An impedance of a value less than 2% of the characteristic impedance does not disturb the mask of the transmitted signals. The electrical diagram of Figure 3 gives a non-exclusive example of embodiment of such a device.

The current transformer (12) has a low number of turns in the primary and a large number of turns in the secondary. This arrangement allows him to reduce to the primary an impedance equal to the impedance seen by his secondary divided by the square of this ratio.

In the diagram described in FIG. 3, the secondary impedance of the transformer (12) is very low, the impedance reduced to the primary is therefore practically zero. The secondary impedance is, in fact, the practically zero input impedance of the amplifier (13), constituted, like the amplifier (9) of FIG. 2, of an operational amplifier connected directly to the transformer and looped by the resistor (14) which determines the value of its gain. The insertion of the current transformer (12) into the line (1) is easily done without interruption by spacing the two wires of the pair apart enough to form one or two turns (6) placed in a magnetic core (4) consisting of ( 2) half-pots or half-tori or any other similar shape (see Figure 4). The transformer secondary (5) is itself placed in the magnetic core before it is closed. This provision does not guarantee that the impedance brought back into the line remains sufficiently low when the device is not in operation. For this it is necessary to add between the accesses of the current amplifier a relay rest contact or a voltage limiter (not shown in the figure).

The signals U1 and U2 are obtained by means of conventional adder circuits as shown without exclusivity by the diagrams in FIG. 5.

An alternative solution consists in carrying out sampling and analog-to-digital conversion of these signals in order to then process them in the digital or computer device used for the subsequent analysis.

Claims (8)

1. Device for extracting and separating the signals present on a wired transmission line (1) characterized in that it is based on the simultaneous observation of the voltage U and of the electric current I at a single point on the line and without requiring the physical cutting of any wire thereof, in that the observation of the electric current present on the line proceeds from the use of a current transformer (12) consisting of a small number of primary turns (6) made without interruption by placing the line wire (s) in a magnetic core (4) composed of two separable parts, and in that the impedance brought back into series on the line by the current observation mechanism is constantly maintained at zero or a negligible value by the continuous presence of a zero or very low impedance at the secondary of the current transformer (12). 2. Device according to claim 1 characterized in that the observation of the voltage U of the line (1) proceeds from the connection of an amplifier whose impedance brought back in parallel on the line is constantly maintained at a sufficient value for not to disturb the operation of the line by the use of passive resistive components (7) and (8) and in that the galvanic isolation between line and voltage observation equipment is ensured by a transformer (10) operating at very low impedance to avoid the low-pass filtering effect of its stray capacitances. 3. Device according to claims 1 and 2 characterized in that the signals U1 and U2 emitted by each of the end devices are reconstructed from the signals U and I supplied by the voltage and current probes by means of adder circuits and conventional subtractor. 4. Device according to claims 1 and 2 characterized in that the signals U1 and U2 emitted by each of the end devices are reconstructed from the signals U and I supplied by the voltage and current probes by means of adder circuits and digital or computer subtractor. 5. Device according to claims 3 or 4 characterized in that it is associated with an equalizer and a conventional echo canceller so as to obtain signals U1 and U2 free of reverberation and echo. 6. Device according to all or part of claims 3 and 4 characterized in that it is used to extract and separate the signals of the two directions of transmission of an analog telephone link. 7. Device according to all or part of claims 3 and 4 characterized in that it is used to extract and separate the signals of the two directions of transmission of a telephone link operated in data transmission by modem. 8. Device according to all or part of claims 3 and 4 characterized in that it is used to extract and separate the signals of the two directions of transmission of a digital telephone link.