FIG. 1 illustrates an example signal capable of being detected by the method and the device according to the invention. It is the well known signal corresponding to the "busy tone" transmitted by the telephone network in the following cases:
a called party is already in the process of communication, PA1 a party, whether called or calling, releases the line in the process of communication. PA1 the method is very sensitive to interference signals with a frequency in the frequency band of the signal to be detected. On the assumption that these signals are too numerous, one at least of the periods T.sub.ON or T.sub.OFF is not recognised, PA1 in the case of samples not recognised as belonging to a period T.sub.ON or T.sub.OFF, the information contained in these samples, although capable of being significant, is lost. There results at the very least a delay in the detection of the signal. PA1 a signal in which the frequencies of 1100.+-.38 Hz are absent, in which case the signal at the output 14 of the detection filter 18 is at a first level, PA1 and a signal having components at 1100.+-.38 Hz, in which case the output 14 is at a second level. This means that these components are from the telephone line 16, and, according to a conventional counting method such as presented above, this possibly means that the incoming signal is a CNG. PA1 to cause to correspond, to each sample of ranking i varying from 1 to ln, a state value B(i) representative of its level, and to calculate, over at least a first part of the sampling duration, a true correlation value SR, by cumulating, for each pair of samples of ranking i and i+Z, the instantaneous value of correlation between the state value B(i) and a state value B(i+Z), the samples of ranking i and i+Z being located at instants shifted by a period T, Z being the number of samples taken during a period T of the signal to be detected, PA1 to measure over at least a second part of the sampling duration the rate r of samples which level is representative of the presence of the said signal to be detected, this rate r being the ratio of the number of samples for which level is representative of the presence of the said signal to be detected over the total number of samples of said second part of sampling duration, PA1 to determine at least one so-called "first theoretical correlation average" value Sabs, and PA1 to compare the true correlation value SR with each theoretical average correlation value and deduce the presence or absence of the signal to be detected according to the result of this comparison.
This signal, which in France is defined by the standard PAA/1108, paragraph 6.6., is formed by the amplitude modulation of a carrier (of 440 Hz nominal value capable of varying between 406 Hz and 474 Hz) by an envelope 1a. This envelope appears in the form of a periodic square wave determining a periodic sending duration T.sub.ON of the carrier (0.5 sec) and a periodic "silence" duration T.sub.OFF (0.5 sec), the period T of the square wave then being 1 second.
As regards the busy tone, this may be mixed with an answering machine voice message (called "OGM") in the case where a party calls a subscriber who has a telephone answering machine connected on his line. This is because certain parties terminate their communication as soon as they hear an answering machine voice message. Now, according to the above-mentioned standard, the busy tone must be detected by the answering machine between 3 and 6 seconds after it has started to be sent, so that the answering machine can in its turn release the line.
In the prior art, through the U.S. Pat. No. 5,185,783, a device is known capable of detecting a signal such as that of FIG. 1 when it is mixed in a set of signals. Such a device comprises a filter capable of selecting only the frequency of the carrier (with its tolerance) in a set of signals sent on the telephone line and containing a signal such as that of FIG. 1 mixed with other signals. The signal at the output of the filter is sampled. The samples are processed as they are taken. By counting the samples and testing the crossing of a numeric threshold by the number of samples obtained during this counting, an attempt is made to determine the occurrence of a period T.sub.ON and then that of a period T.sub.OFF. The presence of the signal is validated when a period T.sub.OFF and a period T.sub.ON have been detected.
This method is not satisfactory for all cases and the inventor has notably observed the following disadvantages:
Another device illustrated in a simplified manner in FIG. 2 is also known. This device is intended, within a facsimile machine associated with an answering machine, to enable the detection of a so-called "CNG" signal, sent by a calling facsimile machine and identifying the calling facsimile machine as such. The CNG signal consists of the modulation of an 1100.+-.38 Hz carrier by an envelope similar to that of FIG. 1, the duration T.sub.ON being 0.5 seconds and the duration T.sub.OFF being 3 seconds. This device has a band rejection filter 10 connected to the output of a telephone answering machine 11. The output 15 of the telephone answering machine 11 is also connected to a switch 12 enabling the rejection filter 10 to be short-circuited. The rejection filter enables the frequencies of 1100.+-.38 Hz to be removed from the OGM voice message sent by the answering machine. The telephone line access 16 is connected on the one hand to a terminal of the switch 12, the second terminal of which is connected to the output 17 of the rejection filter 10, and on the other hand to a filter 18 detecting 1100.+-.38 Hz frequencies.
During the duration intended for the detection of the CNG signal which may be present in the signal arriving by the telephone line 16, the input 19 of the circuit 18 is capable of seeing the following signals:
When, at the end of this duration, the CNG signal has not been detected, the switch 12 is changed to the closed position, so that the band rejection filter 10 is no longer active.
The disadvantage of this device is to degrade the OGM voice message during the duration of the detection, insofar as a broad frequency band of around 80 Hz centred on 1100 Hz is removed from the OGM signal. Likewise, in the case where this device is used to detect the busy tone, the OGM message is degraded by the removal of a frequency band of around 70 Hz, centred on 440 Hz. Now this frequency band, which corresponds to the note "A", is very much present in the voice. Another disadvantage stems from the fact that, since this method requires dedicated components (a band rejection filter and an analogue switch), the implementation of the method is complicated and thus more costly.