This invention relates to telephone circuits of the type having a microphone amplifier, a speaker amplifier and a speech network coupled between the amplifiers for transmitting and receiving speech signals.

It will be appreciated that such telephone circuits are used both in ordinary telephones where the speech signals are transmitted and received via telephone lines and in radio telephones such as mobile, portable or cellular telephones where the speech signals are transmitted and received via radio channels. It will thus be further appreciated that although the invention will be described below with reference to ordinary telephones, it can apply in similar manner to radio telephones.

Telephones having microphones and loudspeakers tend to suffer from oscillation, the so-called Larsen effect due to feedback established by acoustic coupling from the loudspeaker to the microphone together with electrical coupling from the microphone to the loudspeaker due to inadequate isolation. It will be apparent that this may occur whether the microphone and loudspeaker are situated together in a handset, or in a telephone body, as in so called "speakerphones", or indeed if the microphone and loudspeaker are situated separately as in monitor phones where the microphone is in a handset for a single person to use but the loudspeaker is in the body for all to hear. In an ideal telephone system, the microphone signal coupled to the telephone line is not passed on to the loudspeaker or earphone because of a signal balancing system referred to as a hybrid balance circuit. The balance of the system depends, however, on the impedance presented by the particular telephone line. Since this may vary from line to line, the balance is often poor and oscillation can occur.

A well know method of controlling such oscillation is to reduce the gain of a variable gain stage coupled to the input of the loudspeaker amplifier when a signal is detected

in the microphone circuit and to reduce the gain of the microphone amplifier when no signal is detected in the microphone circuit. The maximum loop gain may thus be reduced to a point at which oscillation does not occur. However, this system causes serious degradation of simultaneous conversations since the gain reductions needed to prevent oscillation under worst case conditions may be very large, for example up to 30 dB.

It is thus an object of the present invention to alleviate this inconvenience by adjusting the gain reduction to only the degree of imperfection of the hybrid balance circuit. Accordingly, the present invention provides a telephone circuit comprising a microphone signal path including a microphone amplifier having an input for receiving microphone input signals and an output, a loudspeaker signal path including a loudspeaker amplifier having an output for providing loudspeaker output signals and an input, a variable gain stage having an output coupled to the input of the loudspeaker amplifier and an input, a speech network coupled between the microphone signal path and the input of the variable gain stage for receiving and transmitting speech signals, and a correlator having a first input coupled to the microphone signal path, a second input coupled to the loudspeaker signal path and an output coupled to the variable gain stage to control gain in dependence on the correlation between signals in the microphone and loudspeaker signal paths.

In a preferred embodiment, the correlator comprises means for producing quadrature-related signals representative of the signals at one of the amplifiers, means for multiplying each of the quadrature-related signals by the signal at the other amplifier to provide product signals, means for rectifying the product signals and means for adding the rectified product signals to provide a control signal for controlling the gain of the variable gain stage.

One embodiment of the invention will now be more fully described by way of example with reference to the drawing which is a simplified block diagram of a telephone circuit incorporating the invention.

Thus, the telephone circuit includes a microphone 1 coupled to a microphone amplifier 2 which is adapted to supply microphone input signals via the input terminal 105 of a telephone speech network 3 to a two-wire telephone line 22 for onward transmission. The speech network 3 also receives signals from the line 22 and couples them via a* variable gain stage 19 to the input of a loudspeaker amplifier 20 which drives a loudspeaker 21. The speech network also includes a hybrid balance circuit which, as discussed above, should preclude the coupling of the microphone signals to the variable gain stage 19.

However, if some microphone signals are coupled to the loudspeaker amplifier, there will be correlation between the signals present in the microphone and loudspeaker stages. By detecting correlation between these signals and assuming that the correlation arises predominantly from coupling of the microphone signal to the loudspeaker stage, the gain of the variable gain stage can be reduced in accordance with the degree of correlation detected.

Thus, in order to detect such correlation the microphone signals from the microphone amplifier 2 are supplied to a microphone signal level detection system comprising an amplier/rectifier block 4, RC-networks 5,6 and 8,9 and multiplier interface circuits 10 and 11. The microphone signal level detection system produces signals which are representative of the microphone signal with a quadrature and phase relationship and with relatively constant amplitude, as will be further described below. These phase and quadrature related signals are then multiplied by the loudspeaker signal, rectified and finally summed to give an indication of correlation between the microphone and loudspeaker signals.

The amplifier/rectifier block has an inverting input terminal 102 and a non-inverting input terminal 101. The microphone signal is received from the output node 100 of microphone amplifier 2 at the non-inverting input 101 and linear and rectified output signals are produced at terminals 104 and 103 respectively. The linear output terminal 104 is coupled to the inverting input terminal 102 so that the linear output signal at terminal 104 is substantially equal to the signal voltage at node 100. A signal current determined by this voltage and the impedance of an RC-network 8,9 thus flows through the RC-network which is coupled to a ground reference. Quadrature and phase related signals representative of the input signal are thus developed across resistor 8 and capacitor 9. The rectified output terminal 103 is coupled via an RC-network 5,6 to a node 106. The RC-network 5,6 is composed of an integrating capacitor 5 coupled between output terminal 103 and ground reference, and a resistor 6 coupled between output terminal 103 and node 106. Thus, the integrated, rectified current representative of the level of microphone signals is available at node 106 from which it is passed to the control inputs 112,113 of multiplier interface circuit 10 and 11 respectively.

Multiplier interface circuit 10 has input terminals 110 and 111 coupled to received a phase signal P developed across resistor 8 and multiplier interface circuit 11 has input terminals 114 and 115 coupled to receive a quadrature signal Q developed accross capacitor 9. The multiplier interface circuits are designed to produce output signals having an amplitude which is dependent on that of the signal applied at their differential inputs (110,111 and 114,115) divided by that of the signal applied at their control inputs 112,113. Since the control input signal is derived from the average microphone signal level it will be understood that the average output signal will be largely independent thereof. The output signals from the multiplier interface blocks at output terminals 131 and 132 are thus versions of the microphone signal with a quadrature phase relationship but

normalised to have a relatively constant amplitude over a wide range of microphone signal levels. Speech network 3, being coupled to the telephone line 22 provides the signal received from the line, at output terminal 118 from where it is coupled to the input terminal 119 of variable gain stage 19 having input, output and control terminals (119,120 and 121 respectively). The ^* output terminal 120 of the variable gain stage 19 is coupled both to the loudspeaker amplifier 20 and to first input terminals 122,123 of multiplier circuits 12 and 15 respectively. Multipliers 12,15 have second input terminals 124 and 125 respectively coupled to the output terminals 131 and 132 of multiplier interface circuits 10 and 11 respectively. The loudspeaker signal is thus taken from output 120 of the variable gain stage and used to multiply the normalised phase signal P and normalised quadrature signal Q output from multiplier interface circuits 10 and 11 respectively to produce product signals at the outputs 126 and 127 of multipliers 12 and 15 respectively. The output of multiplier 12 is thus the product of the loudspeaker amplifier input signal and the phase signal P derived from the microphone signal. The average output will thus depend on the phasing and correlation between these signals.

Similarly, the output of multiplier 15 is the product of the loudspeaker amplifier input signal and the quadrature signal Q derived from the microphone signal. The average output of multiplier 15 will thus also depend on the phasing and correlation between these signals. If the loudspeaker signal contains a frequency originating from the microphone, then it will be correlated with signals P and Q and will produce a non-zero average output from one or both multipliers depending on the phase relationships. The output terminals 126 and 127 of multipliers 12 and

15 are coupled via integrating capacitors 14 and 17 respectively to a ground reference and via resistors 13 and

16 respectively to the respective inputs 128 and 129 of a control signal processor 18 which has an output 130 coupled to the control signal input 121 of the variable gain stage 19. Thus,

the product signals at the outputs 126 and 127 of multipliers 12 and 15 respectively are filtered by respective RC networks 13,14 and 16,17 and then fed to inputs 128 and 129 of the control signal processor 18. Here they are rectified and then summed to produce the control signal used for controlling the gain of the variable gain stage 19. The invention thus provides that the gain of the loudspeaker stage will be little affected by microphone signals if the hybrid balance circuit is close to ideal. Simultaneous conversations will therefore be unimpaired. However for imperfect hybrid balance, some microphone signal will be coupled to the loudspeaker stage so that significant correlation will be found between the signals in the microphone and loudspeaker stages when a microphone signal is present. The loudspeaker stage gain will then be reduced, but only to a measure dependent on the correlation, so as to produce a minimum impairment of simultaneous conversations.

When the hybrid balance and acoustic coupling are such that oscillation could occur, and no other microphone signal is present to cause the loudspeaker stage gain to be reduced, then this will produce correlated microphone and loudspeaker stage signals which will reduce the loudspeaker stage gain. The oscillation can thus be limited to a low amplitude and may be permitted to occur only in short ^• bursts. It will, of course, be apparent that although in the described embodiment the phase and quadrature related signals are determined from the microphone signal and multiplied by the loudspeaker signals the correlator could alternatively be turned around so that the quadrature and phase related signals are determined from the loudspeaker signal and multiplied by the microphone signal.