This invention relates to a loudspeaker telephone set, that is, a telephone set which enables the user to engage in a hands-free telephone conversation. A conventional loudspeaker telephone set is shown in block diagram form in FIG. 1. Microphone 1 converts voice signals into electrical signals, which are amplified by microphone amplifier 2. Transmit attenuator 3 is controllable by means of a control signal to change the level of the outgoing signal, that is, the signal generated by microphone 1, between a high level (during transmission) and a low level (during reception). The outgoing signal after leaving attenuator 3 is amplified by power amplifier 4 and enters hybrid circuit 5. Hybrid circuit 5 is a type of directional coupler. Since it is desired that the outgoing signal from the microphone be applied only to telephone line 20 and not to the loudspeaker, and that the incoming signal from telephone line 20 be applied to the loudspeaker but not to the microphone, the hybrid circuit is inserted to maintain isolation between the incoming and outgoing signals. An incoming signal from telephone line 20, after passing through hybrid circuit 5, is amplified by receiving amplifier 6 and attenuated by receive attenuator 7, which functions in the same way as does transmit attenuator 3. Receive attenuator 7 changes the attenuation of the incoming telephone signal, in accordance with a control signal, between a high level (during reception) and a low level (during transmission). The attenuated incoming signal is then amplified by power amplifier 8 and converted into sound by loudspeaker 9.
As with a handset telephone, it would be desirable in the case of a loudspeaker telephone set to be able to speak and listen to a party at the same time; however, loudspeaker telephone sets are limited to simplex communication because of feedback, which results in a howling sound, caused by two factors: an imperfect isolation in the hybrid circuit and a non-zero coefficient of coupling between the microphone and the loudspeaker. Several amplifiers are placed in the outgoing signal path between microphone 1 and telephone line 20 in order to raise the outgoing signal to a satisfactory level. Several amplifiers are also placed in the incoming signal path between telephone line 20 and loudspeaker 9 to amplify the incoming signal to a satisfactory level to be heard. Consequently, the loop gain for the closed loop circuit including the microphone, the outgoing signal path, the hybrid circuit, the incoming signal path, the loudspeaker, and the acoustic coupling between the loudspeaker and the microphone, is far greater than 1 (0 dB). In the worst case, when the impedance mismatch in the hybrid circuit is at a maximum and the acoustic coupling between the loudspeaker and the microphone is the worst, the loop gain might reach 40 dB and cause howling. In order to prevent this, during transmission the gain of the incoming signal is decreased, while during reception the gain of the outgoing signal is decreased. Whether the telephone is transmitting or receiving, the attenuation of attenuators 3 and 7 is adjusted so that the loop gain remains below 0 dB.
A change in status from transmission to reception is accomplished automatically by voice switching. Transmitting detector 10 is connected to the output of microphone amplifier 2 to rectify the outgoing signal and detect its level. Similarly, receiving detector 11 is connected to the input of power amplifier 8 to rectify the incoming signal and detect its level. Switching signal generator 12 compares the level of the outgoing signal with the level of the incoming signal and generates a control signal to adjust the attenuation of attenuators 3 and 7. Switching signal generator 12 includes a time constant circuit, not shown, as its last stage. The time constant circuit causes the control signal to change gradually when the status of the loudspeaker telephone changes between transmission and reception.
In such a simplex communication system, when the status of the telephone set changes between transmission and reception, the level of the received voice sound typically changes by 40 dB. Thus, a sound level which is loud enough to hear during reception suddenly and markedly changes to such a low level that it cannot be heard at all when the status of the loudspeaker telephone changes from reception to transmission. A similar large (40 dB) change occurs in the other direction when the status of the telephone set changes from transmission to reception. This marked change in the sound level is very noticeable to the user and often gives him or her an unpleasant sensation, the degree of which depends upon the particular person. It is said generally that most people do not notice an unpleasant change if the difference between the transmission and reception sound levels is kept below 20 dB.