GB1232974A - - Google Patents

Abstract

1,232,974. Loud - speaking telephones. DOWNHAM ELECTRONICS Ltd. 13 June, 1968 [16 June, 1967], No. 27952/67. Heading H4R. A control system in a voice switched loudspeaking telephone comprises first and second variable attenuators, one each in the transmitting and the receiving channel, controlled photo-electrically such that the presence of a signal in any channel reduces the attenuation in that channel and increases the attenuation in the other channel. As shown, the attenuators are located between the hybrid and the transmitting and receiving amplifiers respectively and each comprise a resistor R12 (or R28) shunted by a photo-cell PC1 (or PC2) and isolated by transistors VT3 and VT4 (or VT7 and VT8). The photo-cells PC1 and PC2 are each controlled by a lamp LP2 or LP1 respectively connected in the outputs of a comparator comprising transistors VT13 to VT16. In the quiescent condition the attenuators provide equal attenuation. A signal taken from the emitter of transistor VT4 is amplified at VT12 and rectified and applied to the comparator via diodes D1 and D2 in such a manner that the base of VT14 is driven positive and the base of VT15 is driven negative. Thus transistors VT13 and VT14 conduct further and transistors VT15 and VT16 conduct less increasing the brightness of lamp LP1 and reducing that of lamp LP2. The resistance of photo-cell PC2 is thus decreased increasing the attenuation in the receiving channel, while the resistance of the photo-cell PC2 increases, decreasing the attenuation in the transmitting channel. If an incoming signal is detected at the emitter of the transistor VT7 in the receiving channel this is amplified by transistors VT17 and VT18. Diodes D3 and D4 rectify this signal which is applied to the comparator in a sense opposite to that of the outgoing signal. A circuit comprising transistors VT103 to VT106 and associated components including a capacitor C103 amplifies and extends the duration of an incoming signal, and is used to make the comparator more sensitive to signals incoming from the remote telephone, this being necessary if the remote telephone is also of the loud-speaker type.