Abstract:
A circuit arrangement for a speech-controlled loudspeaker telephone station incorporates a pair of operational amplifiers for receiving voltages proportional to the microphone voltage and the loudspeaker voltage, and apparatus for rectifying and comparing the output voltages of the operational amplifiers, to supply control signals for selectively controlling operation of attenuators in the transmitting and receiving channels.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a circuit arrangement for a speech-controlled loudspeaker telephone station and in particular to a system using such station with individual channel amplifiers and with a speech direction recognition circuit. 
     THE PRIOR ART 
     In systems employing loudspeaker telephone stations, in order to avoid the possibility of positive feedback, either the transmitting device or the receiving device should be attenuated when the other is not. The control of the attenuators is directed by a speech direction recognition circuit, which determines the direction of transmission of communication, and controls the attenuators accordingly. 
     Such direction recognition circuits have a variety of requirements, which are partially contradictory. For example, it is desirable that no beginning syllables should be lost, and there should be an avoidance of interruptions between individual syllables. The possibility of the opposite station breaking into the conversion should be allowed, and there should be no faulty operation as the result of background noises. Past attempts to solve the problem have not resulted in completely effective solutions. 
     Since the signal arriving at a telephone receiver can fluctuate by a factor of 1:1000, it is necessary that a speech direction recognition circuit must work over a wide dynamic range, handling signals between 1 mV and 1 V. When the incoming signal is first rectified before producing a control signal of some kind, the threshold voltage of the rectifying system introduces an error, because such threshold voltage is typically a few tenths of a volt. When preamplification is employed, so that the signals with the lowest expected amplitude of 1 mV are raised to approximately 0.7 V, the threshold voltage is no longer a problem, but the maximum signal voltage could then rise to as high as 700 V, introducing additional problems. Attempts to solve this problem have employed the use of logarithmic control amplifiers, the output voltages of which are proportional to the logarithm of the amplified input signals. The logarithmic amplifiers are relatively expensive, however, and it is desirable to provide a simpler and less expensive circuit for speech direction recognition. 
     BRIEF DESCRIPTION OF THE INVENTION 
     It is a principal object of the present invention to provide a simple and economical circuit for speech direction recognition. 
     In one embodiment of the present invention, a pair of inverting operational amplifiers are employed for receiving the microphone and loudspeaker voltages, which amplifiers employ level-dependent negative feedback with separate feedback paths for the positive and negative-going half-cycles of the signals. A comparator, coupled to the outputs of the operational amplifiers, produces a signal for controlling the attenuators. 
     The use of the operational amplifiers makes the rectifier diodes essentially independent of threshold voltage, so that large level fluctuations can be accommodated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Reference will now be made to the accompanying drawing which shows a schematic diagram of an illustrative embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawing illustrates two inverting operational amplifiers V1 and V2, each of which has level-dependent negative feedback. Each amplifier has two negative feedback paths, one for the positive half-cycles of the output voltage of the operational amplifier, and the other for the negative half-cycles. A circuit including a diode D1 and a resistor R2 are provided for the positive half-cycles, and the diode D2 is provided for the negative half-cycles. 
     The microphone voltage is supplied to a terminal EM, and is passed through a capacitor C2 and a resistor R1 to the inverting input of the amplifier V1. In similar fashion, the remote signal, which is diverted from the loudspeaker voltage, is fed to the terminal EL, and passes through a similar resistor capacitor combination to the inverting input of the amplifier V2. The non-inverting inputs of both amplifiers are connected by a resistor R4 to a reference potential, and also to a bias supply described in more detail hereinafter. 
     When the output of the operational amplifier V1 is negative-going, current is drawn through the diode D2 so as to maintain the potential at the inverting input of the amplifier approximately equal to that on the non-inverting input. When the output of the operational amplifier is positive, feedback current flows through the diode D1 and the resistor R2 to the inverting input, in sufficient quantity to continue to maintain the voltage at the inverting input close to that of the non-inverting input. As a result, there is produced at the junction of the diode D2 and the resistor R2 a recitified and amplified signal corresponding to the input signal applied to the terminal EM. This is supplied to one input of a comparator V3 through a resistor R3. The input is also connected by a capacitor C1 to a reference potential, forming part of a low pass filter. 
     The circuit for the operational amplifier V2 is similar, and its output is connected to the other input of the comparator V3. The output of the comparator is available at terminal A, and is a binary output signal having one or another voltage level, depending on which of the two operational amplifiers V1 and V2 has a greater output. The attenuators are controlled in accordance with the control signal available at terminal A, by conventional means. The time constant of the speech direction recognition circuit is determined by the resistor R3 and the capacitor C1. The delay time is determined by the values of the capacitor C1 and the resistors R2 and R3. The gain of the amplifiers is determined by the ratio of the resistors R1 and R2. 
     The bias circuit incorporates an operational amplifier V4, the inverting input of which is connected to a terminal EH, to which is supplied a height regulating voltage. The non-inverting input of the amplifier V4 is connected to the tap of a potentiometer P2, which establishes a bias on the amplifier V4. The output of the amplifier V4 is connected by a diode D2 and a resistor R6 to the non-inverting input of the amplifier V1, and also to one end terminal of a potentiometer P1, the other end terminal of the potentiometer P1 being connected to the non-inverting input of the amplifier V2. The tap of the potentiometer P1 is connected by a resistor R5 to a source of positive potential. By adjusting the potentiometer P1, the relative bias supplied to the operational amplifiers V1 and V2 can be controlled. The specific value of bias supplied to these amplifiers is determined, in conjunction with the potentiometer P1, by the setting of the potentiometer P2 and the level of control voltage applied to the terminal EH. 
     A switch S is connected between the inverting input of the amplifier V1 and a reference potential, in order to, when closed, force the speech direction recognition circuit to the sending position, whenever that is desired. 
     From the foregoing, it is apparent that the present invention furnishes a simple and economical means for determining the direction of speech transmission, and for furnishing a signal which can selectively control attenuators in connection therewith. Various additions and modifications may be made in the apparatus of the present invention without departing from the essential features of novelty thereof, which are intended to be defined and secured by the appended claims.