SE437102B - PROCEDURE FOR DETECTING DIGITAL MULTI FREQUENCY CODED INPUTS OF PCM TYPE AND DIGITAL RECEIVER FOR IMPLEMENTATION OF THE PROCEDURE - Google Patents

Description

H0 7714181-0 2 obtained by converting the analog signals appearing during the signaling phase on the link, into a central signaling equipment and vice versa.

Within these systems, the greatest difficulties occur during reception, because within time intervals predetermined for each signaling criterion, and always very short ones, a large number of operations must be performed with respect to the received signals in order to determine their significance, while during transmission the specific frequency pairs, which form the criterion to be transmitted (which is obviously already known), are generated time and time again.

Receivers of multi-frequency coded signals intended to be arranged in systems of the above-mentioned type are already known.

In a first type of receiver, the received PCM signal is converted back to a corresponding analog signal, which is then processed using conventional analog technology.

This solution has the serious disadvantage that it requires a double signal conversion (from analog to digital form at the station input and from digital to analog form at the receiver input), which obviously leads to a high circuit complexity. In addition, a circuit of this type does not take advantage of the time-sharing technology associated with time-sharing technology, which is characteristic of the digital station, since the receiver must include a separate processing unit for each connection that can be processed simultaneously. This makes the system more complex. Furthermore, the presence of separate units for each channel means that the device becomes more bulky and relatively expensive.

A second type of a known receiver is fully realized by means of digital technology by using digital filters as processing units for the received signals and threshold decision means for detecting the different frequency pairs.

A solution of this type, which is completely digital, is better than the analog-digital-hybrid solution described above, because it avoids double signaling conversion and allows processing of partially time-multiplexed channels. However, it has the disadvantage of a high circuit complexity in the part relating to signaling processing, which means that a large number of operations must be performed for a short time for each channel, so the possibility of time multiplexing is limited. To overcome these and other disadvantages, the present invention provides a method for detecting PCM-type digital multi-frequency coded inputs and for generating separate outputs corresponding to predetermined frequencies in these inputs. The new method is mainly characterized in that a received signal is correlated with each code-forming frequency, so that, in correspondence to the frequencies present in the received signal, output signals are generated which have an amplitude which at a selected time is greater than a threshold value, for which a fixed value or a value corresponding to a fraction of the mean value of the input signal is used depending on which is larger; that at the same time a correlation is performed between the magnitude of the received signal and a first predetermined constant, so that a first signal is derived which is proportional to said average value and which forms the basis for the determination of said threshold value; a sequence of amplitude measurements is performed on said output signals to detect whether only a single group of the signals generated in the first correlation has an amplitude exceeding said threshold value; that a sequence of time measurements is performed on said signal group in order to detect the duration of the received signal and to, on the basis of their longer duration, distinguish signals from disturbances, the cessation of the signal being recognized as the point where its amplitude falls to a predetermined fraction of its value at the time of detection; and that the information symbolized by the frequencies present in each valid signal is stored ready for further processing.

A receiver for performing the above method according to the invention preferably comprises: a group of correlators (CRI-CRm), each of which is arranged to correlate a received signal with one of the frequencies which form the code and to emit a signal at its output. (Y), the amplitude of which, in the event that the received signal contains the correction frequency in question, is substantially constant and at a selected time higher than a threshold value, for which a fixed value or a value corresponding to a fraction of the mean value of the size of the input signal is used depending on which is larger; at least one signal size sensing circuit (PCR1) having the same structure as the correlators and arranged to emit at its output a signal proportional to the mean of the magnitude of the received signal; a computer QJC), which is arranged to receive the signal from the signal size sensing circuit for determining the threshold value, and to compare the signals emitted from said correlators (CR1-CRm) with said threshold value and to perform time and amplitude measurements on these output signals in order to detect the amplitude and duration of the output signals in order to, on the basis of their longer duration, distinguish valid signals from Interference and then generate a a message containing the information represented by the frequencies in each signal as validly identified, and - a memory (ME2) arranged to receive and store such a message.

By using a group of identically similar correlators, the high capacity of digital systems that work with time-sharing technology is advantageously utilized, while by using a microcomputer for control and measurement operations, less space is required and a simpler structure is obtained.

The invention will be described in more detail in the following by means of a non-limiting exemplary embodiment and the figures in the accompanying drawings, where Fig. 1 shows a schematic representation of the connection of a receiver according to the invention to a digital station, Fig. 2 shows a block diagram of the invention, and §ig_§ show a diagram illustrating how the correlator outputs vary in time for a typical input signal.

In Fig. 1, CN denotes an entire digital switching station, namely a station which can operate directly on PCM samples. This station shall, as already mentioned, be connected to analogue stations (not shown in the figure) via links g1, gz. During signaling phases between the stations, multi-frequency ... gn. coded signals to appear on such links. To simplify the description, reference is made in the following to the code CCITT R2, which represents a predetermined type of signaling, where the signals consist of the sum of two frequencies of six.

The designation TPCM denotes a PCM terminal, which is arranged to act as an interface between the analog part of the network and the digital station CN and which can thereby convert analog signals into PCM signals and vice versa.

The construction and operation of devices such as the TPCM terminal are well known to a person skilled in the art and will therefore not be described in more detail.

The designation RC refers to the digital switching network in the station and the designations Fe and Fu indicate schematically displayed PCM files, which are received at and exit from the network RC and which connect the switching network with the links g1-gn. To simplify the figure, the links between the network RC and the switching network of other digital stations are not shown, as they are not necessary for the understanding of the invention.

The designation EL indicates a control process unit in the station CN, which unit is connected to the switching network via a link 1.

A digital free frequency signaling equipment, consisting of a transmitter TS and a receiver RS, is connected between the network RC and the process unit EL to enable communication between the digital station and analogue stations.

The transmitter TS, which on the one hand is connected to the process unit EL via a connection 2 and on the other side is connected to the network RC via a connection 3, is arranged to in time division multiplex for each PCM channel corresponding to the links g1- generate pairs of frequencies of six frequencies in accordance with the code CCITT R2.

Devices of this kind are previously known and will therefore not be described in more detail in the following. For example, the transmitter TS may be of the type described by J. Tierney, C.M. Rader and B. Gold in an article entitled "A digital frequency synthesizer" in the publication IEEE Transactions on Audio and Electroacoustics, AU-19 (1971): 1 (March).

The receiver RS, which via a connection H is connected to the switching network RC and via a connection 5 is connected to the process unit EL, has the task of detecting the different signaling criteria and transmitting the associated information to the process unit EL.

The receiver RS is the subject of the present invention and will be described in more detail below with reference to Fig. 2.

With an arrangement of this type in a digital station CN, the switching network RC can be used directly, as already mentioned, as a connection medium for sending to a central signaling means the PCM samples obtained during the conversion of the during the signaling phase on the links g1- gn behavioral analog signals.

In Fig. 2, CR1, ... CRi, ... CRm denote a group of correlators (6 at the code CCITT R2), which are identically equal and each is tuned to one of the frequencies represented by the utilized the signaling code. Such correlators are arranged that upon receipt of the P ”M samples, which relate to multi-frequency signaling, via the connection H, at the output, on connections 6-1, the spectral content of the input signal with respect to the correlation ... 6- i, ... Sm emit signals Y1, ... Yi, ... Ym as the representation frequency. The group of correlators CR1, ... CRi, ... CRm are the units that process the received signals, ie units that are capable of converting PCM samples of multi-frequency signals into signals that can be detected by logic circuits.

Each correlator comprises a pair of multipliers M1, M2, which are arranged to generate the product between the input signal and functions si = SíD2WFi ° nT and oi = cos 2¶'fi-nT, respectively. where fi (i = 1,2, ... m) is the correlation frequency of a general correlator CRi, T is the sampling period and n is an integer. The functions si and oi are derived from pairs of generators (not shown in the figure), which correspond to those included by the transmitter TS (Fig. 1).

The designations P1, P2 indicate two digital low-pass filters with identical characteristics, which filters operate on the signals output via connections 8 and 9 from the multipliers M1 and M2, respectively. Such filters may, for example, consist of a plurality of series-connected identical cells, which are constructed in accordance with the construction described in Italian patent specification 980 80k.

Such filters are characterized by the fact that they are only made up of adders, whereby multiplications are reduced to pure shifts. Furthermore, the repetition allows, where applicable, that only a single cell is physically realized and that it is used in time-sharing technology to realize the entire filter.

The designation SV refers to a so-called vector adder, which is arranged to calculate the size of the vector sum of such components when receiving the DC and cross-phase components of the signal at the frequency f1 via the connections 10 and 11 after filtering in the filters P1 and P2, respectively. This magnitude is represented by the output signal Yi from the correlator CRi.

The circuit SV may be of any known type, for example of the type described by F. Braun and H. Blaser in an article entitled "Digital hardware for approximating the amplitude of quadrature pairs" in the publication Electronics Letters, 10 ( 1974): 13 (June).

The term MAG refers to a circuit which is arranged to determine the magnitude of a PCM signal incoming to the receiver. "Circuits of this type are previously known, so the structure of the circuit MAG will not be described in detail.

The terms PCR1 and PCR2 refer to a pair of circuits, hereinafter referred to as "pseudo-correlators", which are constructed in the same way and function in essentially the same way as the correlators CR1-CRm. The only difference is that the two multipliers of each pseudo-correlator by a constant multiply the magnitude of the incoming PCM signal, which they receive from the circuit MAG via a connection 12. In this way, connections 7 are obtained. -1, 7-2 emanating from the circuits PCR1, PCR2 two signals 10 15 20 25 30 35 H0 7714181-0 to occur, which are proportional to the average value of the input signal.

These signals will be used for purposes which are explained in more detail below, in connection with the description of the receiver's mode of operation.

The two multiplication constants used for the PCR1 and PCR2 circuits, respectively, have adjacent absolute values and opposite characters, so that signals of adjacent magnitudes will appear at outputs 7-1 and 7-2.

The functions of the correlators and pseudo-correlators are rate-controlled by matched signals, which are related to the frame of the input samples and which are emitted by a time base BT.

The designation ME1 refers to a conventional read and write memory, which stores the values of the signals appearing at the outputs 6 and 7 of the correlators and the pseudo-correlators, respectively.

The memory ME1 is addressed during the writing phase of the time base BT in a synchronous manner with respect to the frame of the PCM samples received by the receiver. During the reading phase, it is addressed in an asynchronous manner by a microcomputer PC of any known type, which forms the part of the receiver RS, which is arranged to detect and interpret the signaling received by the receiver.

The connection 13 schematically represents the transmission path via which the contents of the memory MB1 are transmitted to the microcomputer pC, while the link 1U sohematically represents the transmission path via which the address signals are transmitted from the microcomputer pC to the memory ME1.

The microcomputer pC also receives from the time base the clock signals required for its function, ie a base clock signal and a so-called real-time clock signal.

The function control with these clock signals is well known in microcomputer technology and since it is not significant to the invention, it will not be described in detail.

The designation ME2 refers to a second read and write memory, in which messages, which are generated by the microcomputer pß and which are intended for the process unit EL (Fig. 1) in the station CN, are stored.

The microcomputer pC addresses the memory ME2 during the write phase, while the addressing during the read phase is performed by the process unit EL.

Messages to be stored as well as writing addresses are supplied to the memory ME2 via a connection 15. The output of the memory ME2 is connected to the connection 5, which also constitutes the output of the receiver.

Pig. 3 shows the output signals of the correlators (expressed in decibels in the ordinate direction) as a function of time (indicated in milliseconds on the abscissa axis) for an input signal corresponding to a signal criterion in the code CCITT R2 . In the case shown in the figure, the received criterion comprises the two frequencies fz = 1500 Hz and fu = 1740 Hz and has a duration determined by the waveforms fz, fu.

The six solid lines Y1-Y6 represent the homonymous output signals from the six correlators in a receiver according to the invention, which are arranged to operate on signals in the code CCITT R2. The substantially horizontal dashed line represents a threshold value, the purpose of which will be explained below. As is clear, the output signals from the two correlators, which are tuned to the frequencies occurring in the received signal, apart from the uplink and downlink transients, have an amplitude which is substantially constant and much higher than the amplitude of the output signals from the other correlators. as well as the threshold value.

The puncturing method of the described device is as follows: When a multi-frequency coded signal, which is received on one of the links g1-gn (Fig. 1) and which is converted into a PCM signal in the terminal TPCM, appears on the connection 4 at the input to receive RS, a plurality of correlators CR1-CRm (Fig. 2) start their correlation with the reference frequencies by multiplying the incoming samples by the function samples si and ei, by filtering the products thus obtained and by determining the amplitude of the the resulting signal from the two products.

At the same time, the PCM signal size, which is determined by means of the circuit MAG, is applied to the pseudo-correlators PCR1 and PCR2, which calculate its average value.

The values of the signals Y1-Ym and of the two signals emanating from the pseudo-correlators PCR1 and PCR2 are stored in the memory ME1} The microcomputer pC then sequentially scans the memory cells corresponding to these signals in the detection operations.

As already mentioned in connection with Fig. 3, the two signals Y, which correspond to the two frequencies which form the received criterion, have a substantially constant and high amplitude for a time which is longer than a predetermined minimum time, while the output signals from the second correlators have amplitudes which vary considerably with respect to time and which, apart from the transients, are always lower than the amplitude of the first two signals.

The MíkP ° dëf0rn pß then compares the output signals from each correlator CR1-CRm either with the fixed threshold value stored in the same microcomputer, or with an adaptive threshold value consisting of a fraction of the output signal from one of the pseudo-correlators , depending on which of the two thresholds has the highest value. More specifically, as shown in Fig. 3, the fixed threshold value is used before the time t ', and after this time the utilized threshold value is the adaptive value, corresponding to a fraction of that emitted from the pseudo-correlator PCR1 or PCR2 (Fig. 2). the signal. Since the signals emitted from the two pseudo-correlators have essentially the same amplitude, both can be used in the same way to realize the adaptive threshold value.

By performing appropriate amplitude and duration measurements, the microcomputer pC can determine the behavior of a valid signal, whereby only two signals Y, and always the same for a given input signal, will have an amplitude higher than the threshold value below a significant time interval beginning at time t '. The time at which the input signal is detected is marked in the diagram according to Fig. 3 by the dashed vertical line 0.

Through these procedures, valid signals can be distinguished from any interference or invalid signals due to the insufficient duration of the latter. The invalid signals then show strongly time-varying amplitudes, so that the threshold value is reached only for very short time intervals in relation to the time interval used significantly for the detection.

With the help of a second sequence of time and amplitude measurements, the microcomputer PC can detect the switch-off time of the input signal. Such a time is marked in Fig. 3 by the dashed vertical line R and is determined by detecting the time at which the amplitude of the two valid signals Y is reduced to a fraction of the value at time 0.

In the case of Fig. 3, the signals Y2, YH, corresponding to the frequencies fz, fu in the code used, are the only signals which meet the requirements for the detection of a valid signal. Fig. 3 also shows that the whole detection procedure, from the time at which only the signals Y2, Yu exceeds the threshold value to the time at which the disconnection of the input signal is detected, lasts a time which is slightly longer than the duration of the input signal.

The different amplitude and time measurements, the comparisons with threshold values, etc. are performed by the microcomputer, taking into account the measures required for the signaling code used. Such measurements show a certain degree of complexity, since the logic existing before the microcomputer is extremely simple, but can nevertheless be realized with conventional technology, so they will not be described in further detail.

When the detection operations are over, the microcomputer pC (Fig. 2) generates a message containing the results of said operations, and this message is stored in the memory ME2 and is ready for use by the process unit EL (Fig. 1).

While the operations relating to signal detection are executed, the microcomputer pC (Fig. 2) also performs diagnostic operations with respect to the whole receiver by means of the outputs of the pseudo-correlators PCR1 and PCR2. For the latter operations, the fact that the two pseudo- the multiplication constants of the correlators have close absolute values but with opposite signs, and therefore, in binary representation, the expressions of the two constants will be complementary. In this way, the logic circuits of the two pseudo-correlators are driven in different ways, even if they give results with the same amplitude.

The microcomputer pC compares from the pseudo-correlators PCR1_gch PCR2 outgoing signals with each other, whereby a malfunction in the receiver is revealed as different amplitude values of said signals and will be immediately communicated to the process unit EL (Fig. 1) for performing required operations.

It should be noted that the above description relates only to a non-limiting exemplary embodiment of the invention and that several modifications and variants are possible within the scope of the invention.

Claims (5)

n 7714181-0 Claim m1. A method for detecting digital multi-frequency coded input signals of the PCM type and for generating separate output signals corresponding to predetermined frequencies in these input signals, characterized in that a received signal is correlated with each code-forming frequency, so that, corresponding to the frequencies present in the received signal, output signals are generated which have an amplitude which at a selected time is greater than a threshold value, for which a fixed value or a value corresponding to a fraction of the average value of the input signal is used depending of which is greater; that at the same time a correlation is performed between the magnitude of the received signal and a first predetermined constant, so that a first signal is derived which is proportional to said average value and which forms the basis for the determination of said threshold value; a sequence of amplitude measurements is performed on said output signals to detect whether only a single group of signals generated in the first correlation has an amplitude exceeding said threshold value; that a sequence of time measurements is performed on said signal group in order to detect the duration of the received signal and to, on the basis of their longer duration, distinguish signals from disturbances, the cessation of the signal being recognized as the point where its amplitude falls to a predetermined fraction of its value at the time of detection; and that the information, which is symbolized by the frequencies present in each valid signal, is stored ready for further processing. j Method according to claim 1, characterized in that a correlation is performed between the magnitude of the received signal and a second predetermined constant, which has an absolute value substantially identical to the absolute value of the first constant but which has the opposite sign, in order to generate a second signal, which is proportional to the average value of said magnitude and which has an amplitude which is substantially identical to the amplitude of said first signal; and that the amplitudes of the two signals proportionally to the average value are compared with each other, whereby in the event that they do not correspond to an alarm signal is sent to the process unit in a telephone exchange. 77141M ~ Û ~ '2 Digital receiver for performing the method according to claim 1 for detecting PCM-type digital multi-frequency input signals, characterized in that it comprises: - a group of correlators (CR1-CRm), each of which is arranged to correlate a received signal with one of the frequencies forming the code and to emit at its output a signal (Y), the amplitude of which, in the case that the received signal contains the correlation frequency in question, is substantially constant and at a selected time higher than a threshold value, for which a fixed value or a value corresponding to a fraction of the average value of the input signal is used depending on which is the largest; at least one signal size sensing circuit (PCR1) having the same structure as the correlators and arranged to emit at its output a signal proportional to the mean of the magnitude of the received signal; a computer (pC), which is arranged partly to receive the signal from the signal size sensing circuit for determining the threshold value, partly to compare the signals emitted from said correlators (CR1-CRm) with said threshold value and to these output signals perform time and amplitude measurements in order to detect the amplitude and duration of the output signals in order to, on the basis of their longer duration, distinguish valid signals from interference and then generate a message containing the information represented by the frequencies in each as validly identified signal; and - a memory (MB2) set up to receive and store such a message. . Receiver according to claim 3, characterized in that a second circuit (PCR2) is arranged to generate a second signal, which is proportional to the average of the magnitude of the received signal and which has an amplitude which is substantially identical to the amplitude of the first signal; and that the computer (yC) is arranged to perform a comparison between said two signals proportional to the average of the magnitude of the received signal and is arranged to send an alarm signal when the comparison shows different amplitudes of these signals. _ Receiver according to claim U, characterized in that said correlators (CR1-CRm) and said first and second circuits (PCR1, PCR2) comprise low-pass filters (P1, P2), consisting of a plurality of identical, series-connected cells, which are only by adders and by means for shifting the processed signal. '-