Multiplex communications – Wide area network – Packet switching
Patent
1982-02-12
1985-04-09
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
179 84VF, H04J 312
Patent
active
045106010
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a digital frequency receiver which in particular allows the identification of voice-frequency signals in telecommunications signalling codes. It can be used in particular in time-division telephone exchanges to receive interchange signals and to receive the signalling transmitted by subscriber keypad sets.
In time-division telephone exchanges, digital receivers are often connected to circuits and lines by the connection network and by a time multiplex link such as a PCM 32-time slot European standard PCM link. The receiver analyses the signals which it receives in these 32 time slots on a time-sharing basis, operating in conjunction with a logic decision circuit.
Many types of known receivers are based on the theory of linear filters defined by finite difference equations. A receiver of this type is described in French Pat. No. 1,603,175 corresponding to U.S. Pat. No. 3,683,162. Such a receiver has the disadvantage of requiring one band-pass filter per frequency to be recognized and it is difficult to adapt it to multiple codes.
Another type of receiver described e.g. in French Pat. No. 2,269,261 consists of a unit which performs frequency translation of the received signal and which detects low-frequency signals.
This is the principle on which the present invention is based. The translation performed is equal to the frequency to be detected. A low-pass filter allows the amplitude of the DC component of the modulated signal to be determined.
For a given received signal X(t), low-pass filtering of the complex signal energy contained in the spectrum of X(t) in the neighbourhood of the frequency which corresponds to the angular frequency .omega..
In practice, two circuits--a modulation circuit and a filter circuit--are used in parallel:
for multiplying by cos .omega.t and low-pass filtering; and
for multiplying by sin .omega.t and low-pass filtering. The quadratic mean of the two responses is taken.
A primary object of the invention is to produce a compact receiver controlled by a processor and capable of processing a large number of multifrequency codes.
SUMMARY OF THE INVENTION
The invention provides a digital frequency receiver connected to a processor for the recognition of multifrequency code signals received in signalling time slots and transmitted to the receiver in digital form by at least one PCM time multiplex link, the receiver including means for modulating the received signals with signals whose frequencies correspond to the code used and which are supplied by a digital generator; means for low-pass filtering the modulated signals; and means for analysing the energy contained in the spectrum of the signal received in the neighbourhood of the code frequencies.
According to one feature of the invention, the receiver includes:
a time base synchronized with the PCM connection to supply in each time slot phase signals which allow the circuits of the receiver to be enabled successively;
means for testing with a processor, including a programmable energy-analysis unit, a generator control memory and injection and sampling means; and
two groups of low-pass filter cells each corresponding to one of the quadrature components, each cell being constituted by a recurrence calculation circuit which generates based on a finite difference equation and by a memory circuit used to store the intermediate values which correspond to each time slot and to each frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention described hereinafter by way of example will enable a proper understanding of the features of the invention with reference to the accompanying drawings:
FIG. 1 is a general block diagram of a receiver of the invention and of the associated processor.
FIG. 2 is a waveform diagram of the main clock signals used.
FIG. 3 illustrates a modulator circuit with a digital generator.
FIG. 4 illustrates schematically a digital low-pass filter used in the receiver of the invention.
FIG. 5 illustrates a circuit which takes the quadr
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F. Melindo et al., "A Digital Receiver for Multifrequency Signals from Push-Button Telephone Sets", CSELT Rapporti Technici, vol. VI No. 2, Jun. 1978, pp. 105-111.
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Delpit Philippe
Duplessis Philippe
Renaudin Yves
Chin Wellington
Compagnie Industrielle des Telecommunications Cit-Alcatel
Olms Douglas W.
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