Patent GB-1470418-A (GB)
Title: Loudspeaking telephone circuit
IPC: H04M9/10
Filed: 1974-10-14
Abstract: 1470418 Loudspeaking telephones NIPPON TELEGRAPH & TELEPHONE PUBLIC CORP 14 Oct 1974 [18 Oct 1973] 44496/74 Heading H4R In a loudspeaking telephone circuit having variable loss devices 39 and 40 in the go and return paths controlled in part by the output of a comparator 49 fed with rectified and filtered signals derived from the go and return lines, and in part by the output of a control comparator 48 or 50, as stored in a memory circuit 51 or 54, the control comparator is fed direct with the rectified and filtered signal from one line and an attenuated signal from the other line. In a preferred embodiment, Fig. 1, the main comparator 49 provides the directional switching signal while control comparators 48 and 50 effectively respectively determine the level of anti side tone leakage and acoustic coupling from loud-speaker to microphone and modify the maximum loss in the variable loss circuits so that the total loss is no more than is required to prevent howling. Under transmit conditions with the worst side tone condition to be catered for the resistance attenuator 44, 45 reduces the transmit signal level control signal y t  to comparator 48 to the same level as the permissible side tone signal level y R , there is no output from comparator 65, and no output from memory 51 to modify the maximum attenuation provided from variable loss circuit 40. If the side tone level reduces due to better side tone balance being achieved y R  will reduce, the comparator output will go positive and the positive signal will be stored on the capacitor 53 and the attenuation inserted in the variable loss circuit 40 will be reduced by an amount corresponding to the improvement in side tone level. In a similar fashion if the acoustic coupling between the loud-speaker 7 and microphone 1 reduces, comparator 5 will produce an output which is stored in memory 54 and reduces the attenuation inserted by the variable loss circuit 39. In addition the output of memory 50 controls a variable loss device 41 which provides a negative feedback loop effect to cause the arrangement to stabilize at a point where the change in loss provided by the circuit 41 balances the reduction in acoustic coupling. A reset circuit 300, 301 is provided for each memory unit activated by the output from the respective comparator going negative, but the reset being disabled by the main comparator output, in the case of reset circuit 300 when the apparatus is in the receive condition and in the case of reset circuit 301 when the apparatus is in the transmit condition, which are conditions which would normally provide negative output from the respective comparator circuits. In a first modification Fig. 4 (not shown) the control variable loss circuit 41 is replaced by a variable gain amplifier (410) in the feed to the rectifier filter 42 while a second modification Fig. 10 (not shown) uses variable amplifiers 200 and 203 controlled by the switching signal from the main comparator and the comparator 50 and arranged between the microphone and loudspeaker and the respective rectifier filters. Control of gain or loss in the various circuits is effected by varying the source drain impedances of f.e.t.'s connected either in the input or feed back paths of operational amplifiers, Figs. 6 and 11 (not shown).