CA2050714A1 - Telephone line monitoring apparatus and method - Google Patents

Abstract

TELEPHONE LINE MONITORING APPARATUS AND METHOD

ABSTRACT

A fax monitoring apparatus comprises one or more line interface means to simultaneously detect and analyze audio signals produced on any given telephone line by two connected fax machines, analysis means to analyze and interpret the said audio signals to form a compressed fax image, expansion means, to expand the compressed fax image into a bit-mapped image, storage means, to store the compressed fax image and/or the bit-mapped image, and display means to display the bit-mapped image of the original fax transmission. A method for monitoring and analyzing the activity of a telephone line is also claimed.

Description

1638~1639/~/90 -1-TEIEPHQNE LD~E MQNlTQRlNG~ APPAR Aq~US AND ME~2 Field of the In~Tention The present invention relates to fax monitoring apparatus. More particularly, the invention relates to a monitoring apparatus which is capable of monitoring fax transmissions which take place between two fax transmitting apparatus connected through a telephone line.

In another aspect, the invention relates to the monitoring of telephone lines, and more particularly to a method by means o f which it is possible to identify the type of act*ity associated with a telephone line which is being monitored, and to record such activity.

Baçk~round of ~e ln~entiQn Fax transmissions have become more and more popular, and the volume of information transmitted through fax machines is increasing steadily. This has caused a number of problems, for instance, in that unauthorized transmission of material may occur from a given location, without leaving any trace, or fax machines may be used for criminal activities to bypass police wiretapping of normal telephone lines.

The present invention has as an object -to pro~ide a fax monitoring machine which may monitor, record, store and retrieve -fax transmissions which intervene between any two ~ax machines. Thus, it is one object of the present invention to provide a machine which may be located between any two fax machines which are communicating, may .

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1638tl6391EI/90 receive information transmitted from both fax machines during communication, and may permit to follow a transmission and analyze the material transmitted.

If the apparatus of the invention is used in open analysis of transmission, e.g., in an industrial location, it is not overly important to make sure that the transmitting and receiving fax machines do not detect that the transmission is being followed (however, it is of course necessary to avoid disturbances of the line which may cause a malfunction of the machines). On the other hand, in many other applications, such as for police wiretapping, it is important that the transactions of transmitting and receiving fax machines be monitored and recorded by the monitoring apparatus without making it evident that wiretapping exists. In all cases, it is desirable to avoid any interference with the two communicating machines. It is therefore necessary to provide an interface which is transparent" to the handshake, and which does not leave any record of its presence, either on the transmitting or the receiving machine, ;md which cannot be easily discovered by line-analyzing apparatus. Such a monitoring apparatus interface must be activated by the activity taking place on the telephone line between the two monitored machines, must record the transmission intervening bet,ween the two machines, but must not interfere with the transmitted signals or transmit any signal which act*ely takes part in the handshake between the apparatuses.
Providing such an interface is one of the purposes of this invention. It should be noted, however, that such a transparent interface will not prevent the operator from voluntarily interfering with the transmission, ,, ,,,.,~

1638/1639/l~V90 e.g., to stop the transmission of a non-authorized message, iP
appropriate interference means are provided. r Thus, it is another object of the invention to provide a fax monitoringapparatus which can effectively monitor transmission of fax messages occurling between two fax apparatuses, without leaving any trace of its presence, either on the transmitting or the receiving apparatus, and without being easily detectable by wiretapping-detecting apparatus.

It is well known in the art to connect two telephone lines, for instance for policing purposes, and to eavesdrop on telephone conversations.
However, when monitoring a plurality of lines at the same time, it is necessary to provide means for the automatic identification of activity starting and ending, and for the recording of telephone activity in an automatic manner, since a single operator is not capable of following a large number of telephone lines. In the art there are known devices which can be connected to telephone monitoring and recording apparatus for this purpose, which identify the starting and ending activity of a given telephone line by following the changes in voltage of the line. As it will be understood by a skilled person, using the telephone line, e.g., by lifting the receiver, results in changes of voltage which drops, when the line is used, from its previous value to a lower value.
The problem with this type of monitoring is that different telephone lines show different voltages, depending on the type of line, area, position of connection to the telephone line, and so on. Therefore, although nominal voltages of about 48V should exist on normal telephone lines, in many instances the actual value is much lower, and sometimes even . ~ .
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higher. Furthermore, the amount of voltage drop, after the receiver is lifted, is highly dependent on the distance between the receiver and the exchange central. For this reason devices known in the art are provided with adjusting means, normally a potentiometer, for adjusting the reference voltage before initiating monitoring of a telephone line~

These devices known in the art suf~er from several considerable disadvantages. First of all, it is necessary to adjust the reference voltage for each monitored telephone line every time the device is allocated to a different monitored line. This is laborious and time-consuming.
Furthermore, even when the same telephone line is monitored, fluctuations in the actual voltage of the line exist, so that the actual value may be different from the reference value which has been set by the operator, which results in faulty fùnctioning of the system, missing required activations of the recording apparatus, etc.

It is therefore another object of the present invention to provide a method which overcomes the aforesaid disadvantages, and which can be used for accurately monitoring the activity of a telephone line, withaut the need for manual settings of reference values.

It is still a further object of the invention to provide an apparatus which can be used for the aforesaid purposes.

It should be no-ted that by "telephone line" it is intended to include all lines which are adapted to be used for transmitting telephone conversations, including lines which are dedicated to telecopying ~ ,. . :, ; . - .

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163~3/163~/lEI/90 r apparatus or computer transmission or the like. Thus, -the outcome of the monitoring of a given telephone line could be a recording oE a fax transmission or a voice conversation, or any other type of data trans.mission, and the actual nature of the transmitted conversation is not important for the purposes of this invention.

The fax monitoring apparatus according to the invention co~nprises:

- One or more line interface means to simultaneously detect and analyzeall audio signals produced on any given telephone line by two connected fax machines;

- Analysis means to analyze and interpret the said audio signals to form a compressed fax image;

- Expansion means to extend the compressed fax image into a bit-mapped image;

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- Storage means, to store the compressed fax image an~Jor the bit-mapped image;

- Display means to display the bit-mapped image of the original fa~
transmission; and - Optionally, printing means to provide a hard copy of the transmissionor part thereof.

- - ~, , . -~638~1639/~I191) -6-According to one embodiment of the inventiorl, the storage means comprise a temporary memory or bu~fer, and one or more hard disks.
Furthermore, in another embodiment of the invention, the storage means further comprise one or more digital cartridge recorders and, optionally, one or more floppy diskette drive and/o:r optical disk drive. Of course, any other mass storage media can be employed, as becomes available from time to time.

According to a preferred embodiment of the invention, the line interface means comprise a non-interfering line interface device comprising a circuit consisting of two symmetrical main branches connected to an isolation amplifier, two or more protectors being connected parallel to the isolation amplifier, the said two protectors being connected to the ground and the total impedance of each branch before the connection to the protector line being substantially smaller than the impedance of the branch after the said con~ection. Preferably the ratio of the ~maller to the greater impedance of each branch is about 1:5.

According to another preferred embodiment of the in~ention a separate modem is provided for each monitored telephone line. This is different from other existing apparatus in which it is attempted to exploit a single modem for several telephone lines. It has been found that using the same modem for a number of lines is not convenient, because one modem can $ake care OI a single telephone line at a time, and when two different telephone lines are operating at the same time, and fax transmissions are transmitted thereon, one single modem cannot handle both.

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The apparatus according to the present invention can further be used tomonitor and record mixed voice/fax transmissions. Additionally, the apparatus can be exploited for monitoring and recording telephone transactions which are entirely voice transactions. This, as will be apparent to a person skilled in the art, is very important because in many instances fax transmissions are preceded or followed by voice conversations, or a telephone line which is in principle dedicated to ~ax transmissions can be used for voice conversations. According to one embodiment of the invention, this is achieved by adding an analogue-to-digital transducer connected in parallel to each modem, to transform audio signal transmitted through the telephone line into digital storable information.

The apparatus can be activated by any suitable method which permits to recognize the activity of the telephone line. One such method which is novel and forms a part of the present invention comprises:

- continuously sampling the voltage of the telephone line to be monitored;
- calculating the mean value of the voltage from a plurality of samplings taken during a given time interval;
- comparing the said mean value of the voltage with a reference voltage defimed as the mean voltage obtained during the previous time interval;
- replacing the value of the re~erence voltage with the said mean value;

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:1638/1639/H190 - if the di:~rence between the last calculated :mean value and -the re-~erence value exceeds a predetermined value:

(a) stopping the replacement of the ref`erence voltage with the mean calculated value of the voltage;
~ b) activating signal recording means to record signals transmitted through the telephone line;
(c) continuously comparing the calculated mean value with the reference value, and if the diff!erence between the two compared values is below a predetermined minimal value:
1) deactivating the signal recording means; and 2) resuming the replacement OI the value of the reference voltage with the calculated mean value of the voltage, and the subsequent steps.

The signal recording means referred to above can be, in this case, an apparatus according to the invention.

According to a preferred embodiment of the invention, the method further comprises recording and analyzing the telephone number which is dialed by a monitored telephone line, which method includes the steps o ~ 1) identifying dialing beginning by comparing the calculated mean value of the voltage and the reference voltage, and determin;ng whether the difference is below a predetermined ma~imal value;

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16381163~/EI/90 (2) counting the number of pulses of high voltage occurring within about 01 second from one another ti a predetermined tolerance), to determine the dialed digit;
t3) identify~ng the end of one dialed digit by identifying a low-voltage interval of a length exceeding about 0.1 second ~-~ a predetermined tolerance);
(4) recording the said pulses of high voltage or the equivalent digits of the dialed number in the signal recording means.

As in many instances mixed dialing, including pulse and tone signals, is employed, e.g., to activate a telephonic secretary, a preferred embodiment of the invention further comprises providing tone-detecting means to identify and record tone-modulated codes and/or digits.

While the maximal difference between the calculated mean value of the voltage and the reference value can be changed, on the basis of various considerations which are within the scope of the person skilled in the art, it is preferred to initiate recording of telephone line activity when the calculated mean value is equal to or less than 80 - 85% of the reference voltage.

The mean calculated voltage is obtained, as said above, by averaging a large number of samplings taken during a very short period of time. In a preferred embodiment of the invention, the mean calculated voltage is the arithmetic mean of 200 to 6,000 samplings, preferably about 1,000 samplings taken during a time interval of about 1 second.

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1638~1639~I/90 The activity of a telephone line may be recorded on a variety of recording media. In a preferred embodiment of the invention the recording means comprise digital record;ng apparatus, and the activity of the telephone line is saved in the form of a digital record.

The invention filx ther encompasses a device ~or monitoring and analyzing the activity of a telephone line, which device comprises:

- sampling means for continuously sampling the voltage o~ the telephone line;
- calculating and comparing means for calculating and comparing mean values of the line voltage. These calculating and comparing means may be, e.g., a microcomputer or a microp-rocessol~;
- activating means for activating signal recording means. These activating means may, again, comprise a microprocessor or a microcomputer; and - signal recording means to record the activity of the telephone line.

Encompassed by the present invention is also a method of monitoring ~ax communication, which method comprises the steps of:

a) providing line interface means connected to a telephone line to be monitored;

b) providing activating rneans to detect line activity comprising one o~
start/stop act*ity, ringing and dialing pulse and DTMF signalling;

c) providing interpreting means activated by the said activating ,-,-. . .

~638116~9/EI/90 means;

d) interpreting fax transmissions by means of the interpreting means when line activity is detected;

e) assembling the interpreted fax transmission into compressed fax images;

f) storing the compressed fax images in memory means;

g) retrieving the stored compressed fax images and expanding them to bit-mapped images of the original message; and h) displaying the bit-mapped images on display means, or prin-ting them out on printing means.

In a preferred embodiment of the invention, the display means comprise a graphic screen, whereby information can be displayed on the screen.
The screen can also be exploited to look for information to be printed on the printer.

In order to further illustrate the invention, Figs. 1 to 7 are provided, in which:

- Fig. 1 is a block diagram of the information flow in a device according to an embodiment of the in~ention;

.. , 16381~639/~V90 - Fig. 2 is a schematic view of an apparatus according to one embodiment of the invention;

- Fig. 3 is a schematic representation of part of the receiving part of an apparatus according to one embodiment of the invention;

- Fig. 4 is a diagram showing a line interface according to the invention;

- Figs. 5 and 6 each show an alternative system set-up, in block diagram; and - Fig. 7 shows the aotivity of a telephone line frorn which a number isbeing dialed.

In Fig. 1, it is seen that the incoming lines are fed to the line interface to be described with reference to Fig. 4. Under this protection the signals are fed through decoding means, which decode the modem signals or translate voice modulation into digital data and transfer the signals to memory means in the apparatus. These memory means are the temporary memory means, from which the information is transferred to storage media such as hard disks and tape recordersO When it is desired to view the information and analy~e it, th~ fax information is retrieved from the hard disk or the tape, is expanded and is sent to view means, e.g., a screen or a printer. Similarly, voice information can be retrieved for and sent to appropriate voice regeneration apparatus.

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Turn;ng now to Fig. 2, an apparatus according to one embodiment of! theinvention is indicated by numeral 1. This apparatus comprises interface means (not shown), floppy disk drive 2, two digital casette recorders 3 and 3', and two hard disks, 4 and ~ casette compartment ~, is also shown, which of course is not essential. The apparatus is provided with two screens, a graphic screen 6, on which fax images are displayed, and a messages screen 7, which is used by the operator for messages, for instance for working with the memory means, to retrieve specific fax messages by their stored names, and for other control activities. An additional floppy disk drive 8 is provided, which is used to start up the program which controls the operation of the device. The operation of the apparatus is controlled through a keyboard 9, which is used to input commands for the various operations of the apparatus.

Thus, when a fax message is detected and is sent by the line interface means to the decoding means, after having been decoded it is stored in the hard disks 4 and 4', and in the digital casette recorders, 3 and 3'.
Normally, storage from the temporary memory buffer is done in the hard disks, and later the information is transferred to the digital casette. However, it is possible to indicate which kind of informakion is to be maintained on the hard disk for an early review, and which is to be sent to the digital casette for long-term storage.

When the operator wishes to view a specific fax message, he gives the appropriate commands through the keyboard 9 and the~message appears on the graphic screen 6. Alternatively, or additionally, the message may be automatically printed out by a line printer, not shown : ' `~ '" i " ~
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1638~1639/~I/90 in the figure for the sake of simplicity. The line printer should preferably be a laser printer or another comparably high-speed and heavy duty device.

The operator will identify different messages through the messages screen, e.g., by telephone numbers, time of transmission, or any other indication. Thus, for instance~ ;f the operator sees on the message screen 7 that a given ~a~ apparatus is transmitting, he will also see on the messages screen how and where the fax transmission is be;ng stored, will be able to retrieve the fax transmission, view it on the gr~phic screen 6, and if desirable, will be able to print it out on a printer.

As will be readily understood by the skilled person, the mode of operation described above is unique inasmuch as it permits to monitor a very large number of telephone lines, to store fax messages transmitted therethrough on long-term storage media, to review the messages at will, and to provide hard copies of these messages. This is achieved by the unique combination of elements and the method of operation of the apparatus of the invention. Of course, the invention can conveniently be exploited also when a single line is monitored.
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In Fig. 3 the receiving part of the apparatus of Fig. 2 is schematically shown, which comprises a plurality of modems 11, each of the said modems being connected to a telephone line 12, only two such modems and telephone lines being shown, for the sake of simplicity. Parallel to each modem there is connected an analogue-to-digital transducer 13, which has the purpose of translating ~oice messages into digital data.

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~638/16391EI190 Memory means 14 are provicled, for intermediate storage of data received from modem 11 or A-to D transducer 13, which can be divided in a plurality of sub-units, or which can consist of separate and connected memory units. The intermediately stored information from intermediate memory means 14 subsequently flows to central or main elaborating means 1~, not described in detail here and which have been described with reference to Figs. 1 and 2.

As will be apparent to a person skilled in the art, the modem 11 is only capable of recognizing fa~ transmissions and there~ore it will be activated only when the information transmitted on the telephone line is a fax transmission. If speech is transmitted on the telephone line, then the A/D transducer 13 will be activated, and modem 11 will not be operative. Information will flow from the telephone line to intermediate memory means 14 via the A/D transducer 13.

Referring now to l?ig. 4, the line inter~ace device shown in the figure is connected to telephone line 12 through connections 12a and 12b. The circuit which constitutes the line interface has two identical branches generally indicated by 16 and 16', which are connected to the bases of an isolation amplifier 17. Connected and parallel to the isolation amplifier 17 there is a protecting branch 18, on which there are located two protectors 19, between which there is found the connectioT~ to the ground. An additional impedance, indicated by numera~ 20, is also found in parallel to the isolation amplifier. As will be seen from the figure, the total impedance of the part of branch 16 which is found between cormection 12a and protecting branch :18 is much smaller than 1~;3811639~I/90 the remaining impedance of the same branch, being 1 MQ, the remaining impedance being, in this particular embodiment of the invention, 4.7 MS2. It should be understood that the dimensions of the impedances in this circuit are dictated by the various requirements of the circuit. On the one hand, it is necessary to provide -the highest possible impedance, to render the interface as much non-detectable as possible. On the other hand, the protectors 19, together with the res;stance found before them7 will constitute a filter to the signal coming from the telephone line, because the protectors include a capacitance.
Thus, it is necessary to di~ride the resistance along the branch of the circuit to maintain a high level of non-detectability, while avoiding too high a ~ltering effect of the various components. One suitable isolation amplifier is, e.g., the AD202 ampli~ler, manufactured by Analog Devices, Inc.

The reduction of the signal from the telephone line resulting from the arrangement of the circuits shown in Fig. 4 is about 50:1. This enables khe line voltage analyzing circuitry to deal with low level signals.
However, since the audio signals are also attenuated by the same ratio it is necessary to amplify further the audio signal after the isolation amplifier. According to one embodiment of the invention, an additional amplifier, which need not be an isolation ampli~er, is provided a~ter the isolation amplifier 17, as indicated in broken lines by numeral 21 in Fig.
4.

An alternative embodiment is shown in Fig. 5 in block diagrams.
According to this embodiment there are provided two separate branches, , 1638/1639~H/90 connected to the telephone line at connections 22 and 2~'. The upper branch contains an attenuator 23, an isolation amplifier 24, and line voltage analysis means, 25. The lower branch contains two capacitances, 26 ancl 26', an isolation amplifier 27, and an AID
transducer.

The telephone line conveys to the interface of Fig. 5 two comblned signals: the DC line voltage (up to about 200 V when ringing) and AC
information (about 1 V), carrying audio (speech and fax) information.

After attenuation at the attenuator 23, which as said is of the order of 50:1, the voltage can be transmitted through the isolation amplifier. At this point the DC voltage can be analysed, but the A(: voltages are too low for analysis. For this reason, in this embodiment of the invention there is provided the lower branch, and the DC voltags is blocked by capacitances 26 and 2~', while the audio signal is transmitted to a second isolation amplifier 27, and then to an A/D device 28.

Alternatively, the system may operate as in Fig. 6, and the signal attenuated by attenuator 29 can be passed, after isolation amplifier 30, to a second branch and to amplifier 31, where the AC signal is amplified by a factor of 50 and then fed to the A/D transducer 32.

The A/D transducer does not discriminate between speech and Çax signals, but the modem can identify fax messages. Thus, it is possible to control the operation of the A/I:) device according to the recognition of fax signals based on the modem.

1638/:1~9/H/90 In order to better understand the metllod of monitoring and analyzing the activity of a telephone line, reference is made to Fig. 7, which shows the activity of a telephone line from which a number is being dialed. The normal voltage of the line, when no activity exists, is indicated by Vl.
This is the higher voltage which, nominally, is 48V. When the receiver is lifted, the voltage drops very quickly to a lower value, V2, which happens at to. The value of V2 is normally between 10 - 30V.

In Fig. 7 we see that the receiver has been lifted at to, dialing has beglm at td~ and different digits are being dialed which reflect in pulses shown in the figure. The first digit is a 3, as can be seen by three dif~erent pulses of higher voltage. The time intervals tl and t2 are of 0.1 second or less, and interval t3 is greater than 0.1 second, indicating that the first group of pulses has ended. The first group includes three pulses, so that the first digit is 3, the second group includes two pulses, so that the second digit is 2. Further digits are not shown, for the sake of simplicity.

According to the method of the invention, the value of Vl is sampled continuously, and if no difference exists between two subsequently calculated values ~within a predetermined tolerance), the last calculated value becomes the reference value. When the receiver is lifted the difference Vl V2 exceeds the predetermined maximal difference, as V2 is less than 80 - 85% of Vl, and recording of the activity of the line begins.
This recording will include all that happens from the time to onwards, until such time as the voltage of the telephone line climbs again to a 16~8/~6~91~V90 value about the last r~corded value of Vl 1 and remains stable for a period of time of several seconds. At that time the recoraing of the activity of the telephone line stops.

It should be noted that, if desired, it is also possible to identify and record incoming calls. When the telephone is ringing at the monitored end, pulses of a higher and lower voltage are detected around the nominal Vl value without any lower voltage base-line, because the receiver has not been lifted. It is possible to start recording the activity of the telephone line beginning with incoming calls, in addition to with the lifting of the receiver. Then, when an incoming call is detected, if nobody lifts the receiver recording will stop when the pulses about the nominal Vl voltage stop and the voltage remains stable for a period of time of a number of seconds. On the other hand, if the receiver is li~ted, then the situation is like that depicted in Fig. 7, beginning at time to, and recording will continue as described above.

It should be noted that most telephone exchanges employ the line-reversal technique for outgoing calls to indicate the establishment of the connection with the called party. In this technique, the line changes its polarity at the instant in which the handset o~ the called party is lifted, while preserving its absolute value. Since while sampling the line voltage it is possible to determine its polarity, activating the recording means can be delayed until the connection with the called party has been established.

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1638/1639/~I/90 As will be appreciated by a person skilled in the art, the method of the invention is very convenient inasmuch as it "learns" the telephone line it is monitoring, it permits to adjust the threshold value to the actual value e~isting on the telephone line, even if this value changes with time during the day or during a number of days, it frees the operator from the need to control and manually set reference values, is reliable inasmuch as it is based on a very large number of samplings, and it is easy and simple to carry out, as it may utilize almost every normal microcomputer available on the market.

As will be apparent to a person skilled in the art, it is possible to provide very different arrangements of the various components of the apparatus of the invention, in order to achieve the same results. For instance, it is possible to provide a double interface, one for line voltage analysis and one for the audio transmissions, although this is a more expensive solution. Likewise, it is possible to employ dif~erent types of modems and analogue-to-digital transducers, to connect them in different ways, to change the order or type of elaboration of the va~ious signals, to provide additional storage and/or retrieval and/or viewing means.
Furthermore, it is possible to provide many modi~lcations in the monitoring and analysis method of the invention. For instance, it is possible to activate another system, e.g., a loudspeaker, rather than recording the con~ersation which is taking place on the telephone line, although this is normally not advantageous. Likewise, it is possible to connect devices which utilize the method of the invention to a variety of outlets and recording or activating devices, to obtain 1~8/1639~H190 -21 -dif~erent responses to an activity detected on the telephone line, all without exceeding the scope of the in~ention.

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Claims (16)

1. Fax monitoring apparatus comprising:

- one or more line interface means to simultaneously detect and analyze audio signals produced on any given telephone line by two connected fax machines;
- analysis means to analyze and interpret the said audio signals to form a compressed fax image;
- expansion means, to expand the compressed fax image into a bit-mapped image;
- storage means, to store the compressed fax image and/or the bit-mapped image;
- display means to display the bit-mapped image of the original fax transmission.

2. Apparatus according to claim 1, wherein the storage means comprise a temporary memory or buffer and one or more hard disks.

3. Apparatus according to claim 2, further comprising one or more digital cartridge recorder and optionally one or more floppy diskette drive and/or optical disk drive;

4. Apparatus according to any one of claims 1 to 3, wherein a separate modem is provided for each monitored telephone line.

5. Apparatus according to claim 4, wherein an analogue-to-digital transducer is connected in series to each modem, to transform into digital storable information audio signal transmitted through the telephone line.

6. A non-interfering line interface device comprising a circuit consisting of two symmetrical main branches connected to an isolation amplifier, two or more protectors being connected in parallel to the isolation amplifier, the said two protectors being connected to the ground, the total impedance of each branch before the connection to the protector line being substantially smaller than the impedance of the branch after the said connection.

7. An interface according to claim 6, wherein the ratio of the smaller to the greater impedance of each branch is about 1:5.

8. A method of monitoring fax communication, comprising the steps of:

a) providing line interface means connected to a telephone line to be monitored;
b) providing activating means to detect line activity comprising one of start/stop activity, ringing and dialing pulse and DTMF signalling;
c) providing interpreting means activated by the said activating means;
d) interpreting fax transmission by means of the interpreting means when line activity is detected;

e) assembling the interpreted fax transmission into compressed fax images;
f) storing the compressed fax images in memory means;
g) retrieving the stored compressed fax images and expanding them to bit-mapped images of the original message; and h) displaying the bit-mapped images on display means.

9. A method according to claim 8, wherein the display means comprise a screen and/or a printer.

10. A method for monitoring and analyzing the activity of a telephone line, comprising:

- continuously sampling the voltage of the telephone line to be monitored;
- calculating the mean value of the voltage from a plurality of samplings taken during a given time interval;
- comparing the said mean value of the voltage with a reference voltage defined as the mean voltage obtained during the previous time interval;
- replacing the value of the reference voltage with the said mean value;
- if the difference between the last calculated mean value and the reference value exceeds a predetermined value:

(a) stopping the replacement of the reference voltage with the mean calculated value of the voltage;

(b) activating signal recording means to record signals transmitted through the telephone line;
(c) continuously comparing the calculated mean value with the reference value, and if the difference between the two compared values is below a predetermined minimal value:

1) deactivating the signal recording means, and 2) resuming the replacement of the value of the reference voltage with the calculated mean value of the voltage, and the subsequent steps.

11. A method according to claim 10, further comprising recording and analyzing a telephone number dialed by a monitored telephone line by the steps of:

(1) identifying dialing beginning by comparing the calculated mean value of the voltage and the reference voltage, and determining whether their difference is below a predetermined maximal value;
(2) counting the number of pulses of high voltage occurring within about 0.1 second from one another, to determine the dialed digit;
(3) identifying the end of one dialed digit by identifying a low-voltage interval of a length exceeding about 0.1 second;
(4) recording the said pulses of high voltage or the equivalent digits of the dialed number in the signal recording means.

12. A method according to claim 11, further comprising providing tone-detecting means to identify and record tone-modulated codes and/or digits.

13. A method according to any one of claims 10 to 12, wherein recording is initiated when the calculated mean value is equal to or less than 80 -85% of the reference voltage.

14. A method according to any one of claims 10 to 13, wherein the mean calculated voltage is the arithmetic mean of 200 to 5,000 samplings, preferably about 1000 samplings taken during a time interval of about 1 second.

15. A method according to any one of the preceding claims, wherein the recording means comprise digital recording apparatus.

16. A device for monitoring and analyzing the activity of a telephone line, comprising:

- sampling means for continuously sampling the voltage of the telephone line;
- calculating and comparing means for calculating and comparing mean values of the line voltage;
- activating means for activating signal recording means; and - signal recording means to record the activity of the telephone line.