Process and apparatus for control of a hands-free telephone set operating in alternation between sending and receiving
The present invention concerns so-called hands-free telephone sets, operating in alternation, and more particularly, a process and an apparatus to initiate the reversal of an inverter controlling the placement of such telephone sets in the receiving mode or the sending mode. A telephone set is said to be hands-free when it is not necessary to hold its microphone and speaker respectively before the mouth and the ear of the user. 
     However, in such a case, there is no acoustic screen between the microphone and the speaker, and very rapidly, a Larsen effect results. 
     Various solutions have been investigated to obviate this drawback. The French Pat. No. 2,466,150, for example, described a system enabling the set to operate in duplex. Such solutions are generally quite costly, and are not really justified, because two people in conversation very rarely speak simultaneously. 
     A simple solution consists in having the telephone set operate in simplex. However, in this case, an automatic unit, if possible, must decide the direction of transmission of signals. This unit must in practice be the result of a compromise, since the signal emitted by the microphone also must contain ambient local noise as well as speech, while the signal received by the speaker must also contain line noises, as well as speech, with an attenuation ratio which is not predictable. 
     The present invention has the object of palliating these problems so as to enable good functioning of a telephone set in alternation between sending and receiving, whatever the quality of the signals sent or received, whatever the relative position of the microphone and speaker, and whatever the acoustic characteristics of the telephone set and the unit in which it is installed. 
     Reference will be made below to an inverter for controlling the placement of the telephone set in the reception state or the transmission state. It is obvious that this expression defines a function which can be carried out by any means, for example, by controlling a maximum attenuation of the signal on the line not in use. 
     In summary, in accordance with the present invention, the position of the inverter is a function of the sign of the difference between the amplitude values of the analog signal emitted, and the analog signal received. Preferably, the received signal is drawn from downstream of the inverter (from the received-signal output side of the inverter), while the emitted signal is drawn from upstream of the inverter (from the emitter-signal input side of the inverter), the amplitude values being provided by integrators. 
     Preferably, a delay will be introduced in the receiving position of the inverter, while a second received signal is drawn upstream of the inverter (the received-signal input side of the inverter) to cancel the delay in certain cases. 
     By preference, a continuous signal of low amplitude will be added to one of the integrated signals, so that the inverter will be in a determined condition at rest. 
     Further, the integrated received signal is preferably also utilized to control the gain of the integrator of the emitted signal under certain conditions, particularly to reduce the influence of the integrated emitted signal during a brief instant at the start of each newly received signal. 
     Again by preference, a second integrated emitted signal is added to the integrated received signal, with a larger time constant, in order to control the threshold of reversal of the inverter. 
    
    
     The invention will be better understood, and other goals, advantages, and characteristics of the invention, will be made more clear by the following description of an apparatus implementing the process, which description is presented in a non-limitational sense, and to which is appended a sheet of drawings. 
     The single FIGURE represents schematically an apparatus conforming to the present invention. 
    
    
     Referring to the FIGURE, there is schematically represented one of the two wires 1 connecting the telephone set to an autocommutator, not represented. The telephone unit includes a microphone 2 and a speaker 3, connected respectively to signal amplification circuits 4,5 and to other circuits 6,7 of the telephone set. 
     An inverter 8 is schematically represented by a coil and two interrupters 9,10. The coil controls the opening and closing, or closing and opening, of the interrupters 9,10, which are arranged in series in the sending and receiving lines, respectively. 
     It will be understood that, to avoid any Larsen effect, one of the interrupters must be open when the other is closed. 
     Quite obviously, such an interrupter can be realized electronically by means of switches as well as by means of controlled attenuation circuits. 
     The emitted signal is drawn upstream of the inverter 8--that is, at the emitted-signal input side of the inverter or, more specifically, the input side of interrupter 9. A first received signal is drawn downstream of this inverter, at the received-signal output side of the inverter or, more specifically, the output side of interrupter 10; and a second received signal is drawn upstream of this inverter (input side of interrupter 10). The emitted signal is applied to an integrator 11 and to an integrator 12, while the first received signal, drawn downstream, is applied to an integrator 13, and the second received signal, drawn upstream, is applied to an integrator 14. 
     Such integrators may be constituted, for example, by an operational amplifier, whose input and output terminals are shunted by a diode provided with a resistance in parallel, and with the output terminal connected to ground through a condenser, in practice delivering a continuous rectified signal which is a function of the peak value of the input signal. 
     The time constant of the integrators 12, 13 and 14 is relatively small (less than 10 milliseconds), while that of the integrator 11 is relatively large (5 to 10 seconds). The output terminal of the integrator 12 is connected to one terminal of a comparator 15, to the other input terminal of which is applied the sum of the output signals of the integrators 11 and 13, and, possibly, a continuous signal of low amplitude, intended to raise slightly the threshold of the comparator 15, and thus to place the inverter 8 in the receiving condition when there is no signal and the set is at rest. The output of the comparator 15 is connected to a delay circuit 16 presenting a sufficient time constant in one direction so that the inverter will not reverse between syllables, for example of the order of 200 milliseconds, and very small in the other direction, so the the time of passage from rest to the emitting condition will be very short, thus avoiding the loss of the first syllable of the first word pronounced before the microphone 2. The output terminal of the integrator 14 is connected to an input terminal of a second comparator 17, to the other input terminal of which is applied the signal issuing from the delay circuit 16. 
     The change of sign of the output signal of this second comparator 17 can thus be utilized directly to control the reversing of the inverter 8. 
     The output terminal of the integrator 14 is also connected to an integrator 18, whose output signal serves to diminish the gain of the amplifier 5 when the signal received on the line 1 is too high; this control is moderated by the output signal of the integrator 11. 
     To describe the functioning of this apparatus, it is first necessary to specify that the two integrators 12 and 13 are indispensible. In fact, with a relatively short time constant, they enable comparison at each instant of the amplitude of the signals in the reception and emission lines. 
     Taking as a hypothesis that at the initial moment there is no signal on the line 1, the inverter is placed at rest in the reception position. This is obtained by means of a continuous signal added to the received signal integrated at 13 so as to raise the threshold of the comparator 15. 
     If a signal appears in reception, it is integrated at 13, and the sign of the output signal of the comparators 15 and 17 does not change. 
     If a signal appears in emission, it is integrated at 12, and is compared at 15 with the integrated received signal. If it presents a sufficient amplitude, the output signal of the comparator 15 changes sign, and is applied to the second comparator 17. Since it presents an amplitude greater than that of the integrated received signal, the signal issuing from the second comparator changes sign, which commands reversal of the inverter 8. At this instant, the interrupter 9 is closed, and the interrupter 10 is opened. The integrator 13 no longer receives a signal. 
     If there is no longer a signal emitted, the output signal of comparator 15 changes sign again due to the earlier mentioned continuous signal applied to the comparator. This change of sign is transmitted to the comparator 17 after a certain delay necessary to avoid re-reversal of the inverter 8 if the absence of emitted signal corresponds to a normal pause between two syllables. if, before the end of the time constant of the delay circuit 16, a significant signal appears on the line 1, not due to the microphone 2, the output signal of the integrator 14 enables more rapid reversal to reception. To this end, the signal applied to the integrator 14 is drawn upstream of the inverter 8. 
     The role of the integrator 11, whose time constant is relatively large, is to increase the threshold of the comparator 15 as a function of the ambient noise around the microphone 2. 
     In effect, the signal issuing from it is substracted from the signal issuing from the integrator 12, and makes it possible to return to the reception position if necessary. 
     Finally, a distant caller does not risk being cut off due to the acoustic connection between the speaker 3 and the microphone 2, because the signal issuing from the integrator circuit 13 is utilized to lower the gain of the integrator 12 during a brief instant at the start of each newly received signal, so that the output signal of the comparator 15 cannot change sign inopportunely. This system must be extremely rapid, in order to intervene before the return of the echo from the speaker 3 to the microphone 2. 
     Although only a single preferred mode of the invention has been shown and described herein, it is obvious that various modifications may be made by those skilled in the art without departing from the framework of the present invention as specified in the claims which follow.