GB1566164A

GB1566164A - Method and apparatus for recognising and producing separated signals from multifrequency coded input signals

Info

Publication number: GB1566164A
Application number: GB5258577A
Authority: GB
Original assignee: CSELT Centro Studi e Laboratori Telecomunicazioni SpA
Current assignee: Telecom Italia SpA
Priority date: 1976-12-17
Filing date: 1977-12-16
Publication date: 1980-04-30
Also published as: AU507727B2; BR7707827A; FR2374808A1; TR19857A; AU3132077A; FR2374808B1; DE2756251B2; JPS53121408A; SE437102B; SE7714181L; DE2756251A1; DE2756251C3; MX144395A; AR216659A1; IT1072242B; JPS5731342B2

Description

(54) METHOD AND APPARATUS FOR RECOGNIZING AND PRODUCING SEPARATED SIGNALS FROM MULTIFREQUENCY CODED INPUT SIGNALS (71) We, CSELT -- CENTRO STUDI E LABORATORI TELECOMUNICAZIONI, S.p.A., of Via Guglielmo Reiss Romoli, 274, 10148 Torino, Italy, a joint stock company organized under the laws of Italy, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a procedure and a device for recognizing and producing separated signals corresponding with predetermined frequencies in digitalized multifrequency coded input signals.The invention is of particular use for signal exchange between a digital switching exchange and an exchange of a type arranged to operate with analogue signals When a digital exchange (i.e. an exchange which directly operates on PCM samples) is to be interconnected with traditional exchanges which process analogue signal and produce signals in their own code the digital exchange must be capable of recognizing the signals generated by the other exchanges and must be able to speak to them with the same signalling code.

Nowaways this situation is particularly interesting as new exchanges of digital type are being inserted in telephone networks and must operate with the pre-existing analogue exchanges.

When the signalling code of analogue exchanges is a multi-frequency code, in order to solve the problem of the conversation between the digital and analogue exchanges the present technique makes use of systems utilizing the switching network of the digital exchange as interconnection for sending PCM samples, obtained by converting the analogous signals present on the link during the signalling phase, to a centralized signalling equipment and viceversa.

Within these systems, the greatest difficulties are met with during reception, as within the times predetermined for each train of signals. said times being very short, a large number of operations has to be performed on the received signals in order to recognize their meaning, whilst during transmission the particular two frequencies forming the signal train (which are obviously already known) must be generated every time.

Receviers for multifrequency coded signals to be inserted in systems of the above mentioned type are already known.

In a first type of receiver, the received PCM signal is converted again into the corresponding analogue signal, which afterwards is processed by means of traditional analogue techniques.

This solution presents the serious disadvantage of requiring a double signal conversion (from analogue into digital at the input of the exchange, and from digital into analogue at the input of the receiver), that obviously gives rise to considerable circuit complexity. Moreover, a circuit of this kind prevents advantage from being taken of the capacity of operating in time-division which is a characteristic of digital exchanges, so that this receiver must comprise a separate processing unit for each of the junctions which may be processed at the same time.

That makes the system more complex; besides, the presence of separate units for each channel makes the apparatus cumbersome and expensive.

A second type of known receiver is arranged to operate completely by means of digital techniques by using digital filters as processing devices of the received signals as well as threshold decision devices for rescognizing the different pairs of frequencies of which the signals are composed.

A solution of this type, being completely digital, is betterthan the previously descirbed hybrid analogue-digital solution as it avoids double signalling conversion and it allows a partial time multiplexing of channels to be handled. However, it presents the disadvantage of a greater circuit complexity in the part relative to signal processing, entailing a large number of operations to be performed in a short time for each channel, and as a consequence the possibility of time multiplexing is limited.

To overcome these and other disadvan tages the present invention provides a method of recognizing and producing separated output signals corresponding with predetermined frequencies in digitized multifrequency coded input signals of PCM type, wherein the received digital signals are correlated with each of the frequencies forming the code so as to generate, in correspondence with the frequencies present in the received signals, output signals having an amplitude greater at a chosen instant than a threshold value proportional to the means value of the magnitude of the received signal and a duration longer than a predetermined minimum time; at the same time a correlation is made between the magnitude of said received signals and a first predetermined constant magnitude so as to generate a first signal proportional to said mean value and providing the basis for determining said threshold value; a first set of time and amplitude measurements is performed on said output signals to detect whether one group only of signals resulting from first correlation has an amplitude exceeding said threshold value; a second set of amplitude and time measurements is performed on said group of signals in order to detect the duration of the received signal and to discriminate, on the ground of their longer duration, real signals from disturbances, the end of the signal being recognized as the point where its amplitude declines to a fraction of its value at the instant of recognition and the information typified by the frequencies contained in each signal taken as valid is stored ready for further processing.

It is also a particular object of the present invention to provide digital receiver for performing the method described which comprises: - a bank of correlators each operable to correlate the received signals with one of the frequencies forming said code and to supply at the output a signal whose amplitude (if the received signal contains the correlation frequency) is substantially constant and higher at a chosen instant than a threshold value proportional to the mean value of the magnitude of the received signal and whose duration is longer than a minimum predetermined time; at least one signal magnitude sensing circuit arranged to supply at the output a signal proportional to said mean value of the magnitude of the received signals;; - -a computer arranged to receive the signal from the signal magnitude sensing circuit and from said signal determines said threshold value and also to compare the signals outgoing from said correlators with said threshold value and to perform on said output signals time and ampli tude measurements in order to detect the amplitudes of the output signals as well as their duration so as to discriminate between valid signals and diturbances, and thereupon to compose a message containing the information typified by the frequencies contained in each signal recognized as valid, and a memory arranged to receive and store such message ready for further processing.

By using a bank of identical correlators advantage is taken of a digital system's capacity of operating in time division, while by using a microcomputer as the computer for check and measurement operations, reduced dimensions as well as greater construction simplicity are obtained, The characteristics of the present invention will become clearer from the following description of a preferred embodiment thereof taken in connection with the annexed drawings in which: - Fig. 1 is a schematic representation of the receiver according to the embodi ment to a digital exchange; - Fig. 2 is a block diagram of the embodi ment; - Fig. 3 is a graph showing the behavious with respect to time of the correlator outputs for a typical input signal.

With reference to Fig. 1, reference CN denotes as a whole a digital switching exchange, namely an exchange able to operate directly on PCM samples. Said exchange, as already stated, is connected to analogue exchanges (not shown in the drawing) through line g1, ge, . . ., gi.

During interexchange signalling phases, multifrequency coded signals will be present in such links. To make the description simpler and clearer reference will be made hereinafter to code CCITT R2, which employs a signalling system wherein signals are composed of the sum of two selected frequencies out of six predetermined frequencies.

Reference TPCM denotes a PCM terminal able to act as an interface between the analogue part of the network and the rest of the digital exchange CN, and more particularly able to convert analogue signals into PCM signals and vice versa.

The structure and the functions of devices like TPCM are well known to those skilled in the art, and so this device will not be further described.

Reference RC denotes the switching digital network of the exchange; references Fe, Fu schematically represent PCM groups respectively going in- and out of network RC, said groups being intended to interconnect the switching network and links gl, . . ., g". To simplify the drawing, links be- tween network RC and the connecting networks of other digital exchanges are not shown, as they are not necessary to the understanding of the invention.

Reference EL denotes the control processor of the exchange CN, connected to switching network RC through link 1.

A digital multifrequency signalling device, comprising sender TS and receiver RS is connected between network RC and processor EL to allow conversation between digital exchange and analogue exchanges.

Sender TS, connected on the one side to processor EL through connection 2 and on the other side to network RC through connection 3, is intended to generate in time division for each PCM channel correspond ings to links gls . . . gn, pairs of frequencies chosen out of a total of six, as required by CCITT R2 code.

Devices of this kind are known in the art and so will not be described hereinafter.

For instance, TS may be of the type described by J. Tierney, C. M. Rader, B. Gold in the paper titled "A digital frequency synthesizer", IEEE Transactions on Audio and Electroacoustics, Vol. AU-19, N.1, March 1971.

Receiver RS connected through connection 4 to the switching network RC, and through connection 5 to processor EL, has the task of recognizing the different signal trains and of sending the relative information to processor EL.

Receiver RS is the object of the present invention and will be described with further details hereinafter, with reference to Fig. 2.

With an arrangement of this kind in digital exchange CN, switching network RC may be directly utilized, as already said, as interconnecting medium for sending to a centralized signalling device the PCM samples obtained from the conversion of the analogue signals present during the signalling phase on links g1, .... . gn.

With reference to Fig. 2, CRI, CRi, CRm denote a bank of correlators (6 in the case of code CCITT R2) identical to each other and each tuned to one of the frequencies forming the utilized signalling code. Such correlators, upon receiving through con nection 4 the PCM samples relative to multifrequency signalling, are able to supply at the output, on connections 6-1, 6-i, 6-m, signals Yl, Yi, Ym representing the spectral contents of the input signal relative to the correlation frequency. The bank of correlators CRI, Cri, CRm compose the units processing the received signals, that is the units able to convert PCM samples of multifrequency signals into signals which can be recognized by logic circuits.

Each correla'torcomprises a pair of multipliers Ml, M2 able to effectuate the product between the input signal and functions Si = sin 2rr fi . nT and ci = cos 27r fi .

respectively, where fi (i = 1, 2 . . ., m) is the correlation frequency of generic correlator CRi, T is the sampling period and n an integer number. Functions si and ci are derived from pairs of generators, not shown in the drawing, analogous to those composing sender TS (Fig. 1).

References Fl, F2 denote two digital lowpass filters of identical characteristics, operating on the signals outgoing from M1, M2 they receive through connections 8 and 9.

Such filters can consist for instance of a plurality of serially connected identical cells having a structure as described in the Applicants' Italian Patent 980804. Such filters have the basic characteristics of being formed by adders only, multiplications being reduced to pure shifts. Moreover, the repetitiveness allows, if it is the case, to implement physically one cell only, and to utilize it in time-division to perform the function of the entire filter.

Reference SV denotes a so-called vector added which upon receiving through connections 10, 11 the in-phase and quadrature components of the signal at frequency fj, filtered respectively in F1 and F2, is able to calculate the magnitude of the vector sum of such components. Such magnitude is the output Yi of correlator CRi.

Circuit SV may be of any type known in the art, for instance it may be of the kind described by F. Braun and H. Blaser in the paper titled "Digital hardware for approximating to the amplitude of quadrature pairs", Electronic Letters, Vol. 10, N. 13, 27th June 1974.

Reference MAG denotes a circuit arranged to sense the magnitude of signal PCM entering the receiver. Circuits of this kind are well known in the art and so the structure of MAG will not be described in detail.

References PCR1, PCR2 denote a pair of circuits, hereinafter called "pseudo-correlators", which have the same structure and basically operate in the safe way as correlators CRI, CRm, the only difference consisting in that the two multipliers of each pseudo-correlator multiply by a constant factor the magnitude of the input PCM signal they receive from circuit MAG through connection 12. In this way, on connections 7-1, 7-2 outgoing from PCR1, PCR2 there will be present two signals which are proportional to the mean value of - the input signal. These signals will be utilized for purposes which will be more particularly specified hereinafter in the description of the receiver operation.More particularly, the two multiDlicative constants, relative to PCR1 and PCR2 respectively, have close absolute values and are of opposite sign so that signals, having close magnitudes will be present on outputs 7-1, 7-2.

The operations of the correlators and of the pseudo-correlators are timed by suitable signals related to the characteristics of the input PCM samples and supplied by time base BT.

Reference ME1 denotes a conventional read-write memory, which stores the values of the signals present on outputs 6 and 7 of the correlators and of the pseudo-correlators repectively, MEl is addressed in the writing phase by time base BT synchronously with the said characteristics of the PCM samples entering the receiver. During the reading phase it is addressed asynchronously by a micro-computer C of any known type forming the part of the receiver which is intended to recognize and interpret the input received by RS.

Connection 13 schematizes the path for transferring the contents of ME1 to microcomputer ,aC, whilst link 14 schematizes the path for sending from sC to ME1 the address signals.

Moreover, micro-computer FC receives from time base BT the time signals necessary to its operation, that is a basic time and a so-called real time.

The functions of these times are well known in the micro-computer technique and, as they are not significant for the purposes of the invention, will not be described in detail.

Reference ME2 denotes a second reads write memory, wherein messages generated by micro-computer ,aC and intended for processor EL (Fig. 1) of exchange CN are stored.

Micro-computer ,aC addresses in writing the memory ME2, whilst the addressing in reading is operated by processor EL.

Messages to be memori7ed and writing addresses are supplied to ME2 through a connection 15. The output of ME2 is connection 5 which is also the output of the receiver.

Fig. 3, as already said illustrates graphicallv the output of the correlators (with the ordinate expressed in decibels), represented as function of the time (denoted in millisecond on the abscissa) for an input signal corresponding to a signal train in CCITT R2 code. In the case represented in the drawing, the received signal train comprises the two frequencies f = 1500 Hz and f4 = 1740 Hz, and has a duration denoted by waveforms f2, f4.

The six heavy lines Y1 ..., Y6 represent the homonymous outputs of the six correlators of a receiver according to the invention arranged to operate on signals in CCITT R2 code. The substantially horizontal dotted line represents a threshold value whose functions will be seen from the description of the operation.

As clearly seen, the outputs of the two correlators tuned on the frequencies composing the received signal have, the terminating and operating transients of the signals expected, an amplitude substantially constant and much greater than the amplitude of the output signals of the other correlators, as well as being greater than the threshold value.

The operation of the described device is as follows: when a multifrequency coded signal, combing from one of the links g g,, (Fig. 1) and converted into PCM signal by terminal TPCM appears on connec tion 4 at the input of receiver RS, the correlators CR1, . . ., CRm (Fig. 2) start the correlation with the reference frequencies, by multiplying the arriving samples by the samples of functions si and c1 by filtering the products and by calculating the amplitude of the signal resulting from the two products.

At the same time the magnitude of the PCM signal obtained by means of circuit MAG, is supplied to the pseudo-correlators PCR1 and PCR2 which calculate its mean value.

Values of signals Yl . . ., Ym and of two signals outgoing from PCR1 and PCR2, are stored in memory ME1. Microprocessor zC will then explore sequentially the memory cells corresponding to said signals for recognizing operations.

As already said with reference to Fig. 3, the two signals Y corresponding to the two frequencies forming the received signal train have a substantially constant and high amplitude during a time longer than a predetermined minimum time while the outputs of the other correlators will have amplitudes extremely variable in time, and, the transients expected, always lower than the amplitudes of the first two signals.

Then micro-computer ,uC compares the output signals of each correlator CR1 CRm either with the fixed threshold memor- ized in the same micro-computer, or with an adaptive threshold consisting of a fraction of the output of one of the pseudo-correlators according to which of the two thresholds has a higher value. More particularly, as shown in Fig. 3, the fixed threshold is utilized before the instant tl, after this instant the utilized threshold is the adaptive one, corresponding to a fraction of the signal provided by PCR1 or PCR2 (Fig. 2). As the signals applied by the two pseudo-correlators have basically the same amplitude, either may be utilized in the same way to determine the adaptive threshold.

By making suitable amplitude and duration measurements, micro-computer yC is able to establish the presence of a valid signal, owing to which only two signals Y, and always the same for a given input signal, will have an amplitude higher than the threshold for a significant time interval beginning at instant tl.

The instant at which the input signal is recognized is shown in the diagram of Fig.

3 by the dotted vertical line 0.

By these procedures valid signals may be discriminated from possible disturbances or non-valid signals owing to their insufficient duration: in fact these latter give rise to signals having a considerable time varying amplitude, and such that the threshold is reached for very short time intervals with respect to the time interval taken as significant for recognition.

A second series of time and amplitude measurements allows the micro-computer sC to recognize the instant of termination of the input signal. Such an instant is denoted in Fig. 3 by vertical dotted line R, and is determined by recognizing the instant at which the amplitude of both valid signals Y, is reduced to a fraction of the value taken at instant 0.

In the case of Fig. 3, the only signals which correspond to the requirements necessary to recognize a valid signal are signals Y2, Y4 corresponding to frequencies f-2, f4 of the used code. The drawing shows also that the entire recognizing procedure from the instant at which only signals Y2, Y4 exceed the threshold to the instant in which the termination of the input signal takes place occupies a time slightly longer than the duration of the input signal.

The different amplitude and time measurements, the comparisons with thresholds and so on are carried out by the micro-computer taking into account the features required by the utilized signalling code. Such measurements present a certain degree of complexity because the part of the logic before the micro-computer is extremely simple. In any case, notwithstanding this complexity, their realization is a problem of conventional technique, and so they will not be described in detail.

Once the recognition operations are over, micro-computer sLC (Fig. 2) composes a message containing the results of the processing operation, and such a message is stored in ME2, ready to be used by processor EL (Fig. 1).

While performing the operations concerning signal recognition, micro-computer C (Fig. 2) performs also diagnostic operations on the entire receiver by utilizing output signals from the pseudo-correlators PCRI, PCR2.

For these operations advantage is taken of the fact that the multiplication constants of the two pseudo-correlators have absolute values close to one another but of opposite sign 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 (that is multipliers, filters, and vector adder) are excited in a different way even if they give-results of a similar amplitude.

Micro-computer iC compares signals outgoing from PCR1 and PCR2 with one another: a malfunction of the receiver will be evidenced by the presence of different amplitude values of said signals, and will be immediately sent to processor EL (Fig. 1) for the necessary action.

WHAT WE CLAIM IS:- 1. Method of recognizing and producing separated output signals corresponding with predetermined frequencies in digitalized multifrequency coded input signals of PCM type, characterized in that the receiver signals are correlated with each of the frequencies forming the code so as to generate, in correspondence with the frequencies present in the received signals, output signals having an amplitude greater at a chosen instant than a threshold value -proportional to the mean value of the magnitude of the received signal and a duration longer than a predetermined minimum time; at the same time a correlation is made between the magnitude of said received signals and a first predetermined constant magnitude so as to generate a first signal proportional to said mean value and providing the bases for determining said threshold value; a first set of time and amplitude measurements- is performed on said output signals to detect whether one group only of signals resulting from the first correlation has an amplitude exceeding said threshold value; a second set of amplitude and time measurements is performed on said group of signals in- order to detect the duration of the received signal and to discriminate, on the ground of their longer duration, real signals from disturbances, the end of the signal being recognized as the point where its amplitude declines to a predetermined fraction of its value at the instant of recognition; and the information typified by the frequencies contained in each signal taken as valid is stored ready for further processing.

2. Method according to claim 1, characterized in that a correlation is carried out between the amplitude of the input signals and a second predetermined constant mag- nitude having an absolute value substantially identical to the value of the first one but of opposite sign, in order to generate a second signal proportional to the mean value of said magnitude and having an amplitude basically identical to the amplitude of said first signal; the amplitudes of said two signals proportional to the mean value are compared to each other, and if they are not identical, an alarm signal is sent to the exchange processor.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

will have an amplitude higher than the threshold for a significant time interval beginning at instant tl.

The instant at which the input signal is recognized is shown in the diagram of Fig.

3 by the dotted vertical line 0.

By these procedures valid signals may be discriminated from possible disturbances or non-valid signals owing to their insufficient duration: in fact these latter give rise to signals having a considerable time varying amplitude, and such that the threshold is reached for very short time intervals with respect to the time interval taken as significant for recognition.

A second series of time and amplitude measurements allows the micro-computer sC to recognize the instant of termination of the input signal. Such an instant is denoted in Fig. 3 by vertical dotted line R, and is determined by recognizing the instant at which the amplitude of both valid signals Y, is reduced to a fraction of the value taken at instant 0.

In the case of Fig. 3, the only signals which correspond to the requirements necessary to recognize a valid signal are signals Y2, Y4 corresponding to frequencies f-2, f4 of the used code. The drawing shows also that the entire recognizing procedure from the instant at which only signals Y2, Y4 exceed the threshold to the instant in which the termination of the input signal takes place occupies a time slightly longer than the duration of the input signal.

The different amplitude and time measurements, the comparisons with thresholds and so on are carried out by the micro-computer taking into account the features required by the utilized signalling code. Such measurements present a certain degree of complexity because the part of the logic before the micro-computer is extremely simple. In any case, notwithstanding this complexity, their realization is a problem of conventional technique, and so they will not be described in detail.

Once the recognition operations are over, micro-computer sLC (Fig. 2) composes a message containing the results of the processing operation, and such a message is stored in ME2, ready to be used by processor EL (Fig. 1).

While performing the operations concerning signal recognition, micro-computer C (Fig. 2) performs also diagnostic operations on the entire receiver by utilizing output signals from the pseudo-correlators PCRI, PCR2.

For these operations advantage is taken of the fact that the multiplication constants of the two pseudo-correlators have absolute values close to one another but of opposite sign 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 (that is multipliers, filters, and vector adder) are excited in a different way even if they give-results of a similar amplitude.

Micro-computer iC compares signals outgoing from PCR1 and PCR2 with one another: a malfunction of the receiver will be evidenced by the presence of different amplitude values of said signals, and will be immediately sent to processor EL (Fig. 1) for the necessary action.

WHAT WE CLAIM IS:- 1. Method of recognizing and producing separated output signals corresponding with predetermined frequencies in digitalized multifrequency coded input signals of PCM type, characterized in that the receiver signals are correlated with each of the frequencies forming the code so as to generate, in correspondence with the frequencies present in the received signals, output signals having an amplitude greater at a chosen instant than a threshold value -proportional to the mean value of the magnitude of the received signal and a duration longer than a predetermined minimum time; at the same time a correlation is made between the magnitude of said received signals and a first predetermined constant magnitude so as to generate a first signal proportional to said mean value and providing the bases for determining said threshold value; a first set of time and amplitude measurements- is performed on said output signals to detect whether one group only of signals resulting from the first correlation has an amplitude exceeding said threshold value; a second set of amplitude and time measurements is performed on said group of signals in- order to detect the duration of the received signal and to discriminate, on the ground of their longer duration, real signals from disturbances, the end of the signal being recognized as the point where its amplitude declines to a predetermined fraction of its value at the instant of recognition; and the information typified by the frequencies contained in each signal taken as valid is stored ready for further processing.
2. Method according to claim 1, characterized in that a correlation is carried out between the amplitude of the input signals and a second predetermined constant mag- nitude having an absolute value substantially identical to the value of the first one but of opposite sign, in order to generate a second signal proportional to the mean value of said magnitude and having an amplitude basically identical to the amplitude of said first signal; the amplitudes of said two signals proportional to the mean value are compared to each other, and if they are not identical, an alarm signal is sent to the exchange processor.
3. Method according to claims 1 and 2,

chatacterized in that as threshold value for said amplitude measurements, a fixed value or a fraction of said mean value of the magnitude of the input signal is utilized according to which of two values is higher.
4. A digital receiver for performing the method according to claim 1, characterized in that it comprises: - a bank of correlators each operable to correlate the received signals with one of the frequencies forming said code and to supply at the output a signal whose amplitude (if the received signal contains the correlation frequency), is substantially constant and higher at a chose instant than a threshold value proportional to the mean value of the magnitude of the received signal and whose duration is longer than a minimum predetermined time; - at least one signal magnitude sensing circuit arranged to supply at the output a signal proportional to said mean value of t magnitude of the received signals;; - a computer arranged to receive the signal from the signal magnitude sensing circuit and from said signal determine said threshold value and also to compare the signak outgoing from said correlators with said threshold value and to perform oa said output signal time and amplitude measurements in order to detect the amplitudes d the output signals as well as their duration so as to discriminate between redid signals and disturbances, and thereupon to compose a message aontinig the information typified by the h neies contained in each signal recognized as valid, and a memory arranged to receive and store such message.
5. A receiver according to claim 4, characterized in that there is included a second circuit arranged to generate a second signal proportional to the mean value of the magnitude of the input signal and of amplitude basically identical to that of said first signal, and characterized also in that said computer is able to be effectuate a comparison between said two signals proportional to the mean value of the magnitude and to emit an alarm signal when the comparison detects a different amplitude between the signals.
6. A receiver according to claim 4, characterized in that tht correlators and the circuits arranged to compute the mean value of the magnitude, comprise low pass filters consisting of a plurality of identical, serially connected cells, said cells consisting only of adders and of means arranged to cause the shifting of the processed signal.
7. A method of recognizing and producing separated signals corresponding with predetermined frequencies in multifrequency coded input signals converted into digital form as claimed in claim 1 and substantially as described.
8. A digital receiver for recognizing and producing separated output signals corresponding with predetermined frequencies in digitalized multifrequency coded input signals of PCM type substantially as described with reference to the accompanying drawings.