Patent GB-1507866-A (GB)
Title: System for controlling the speech direction in a duplex telephone circuit
IPC: H04M9/00
Filed: 1975-10-28
Abstract: 1507866 Two-way speech communication MAXMAN AG 28 Oct 1975 [29 Oct 1974] 44473/75 Heading H4R Speech direction switching in a duplex telephone system is effected by deriving and storing a regulating factor dependent on the envelope of the transmission channel signal and representative of the value by which that signal should be multiplied to cancel crosstalk components from the receive channel so that subsequent channel signal comparison for direction switching is based more accurately on the respective speech components; the stored regulating value being adjusted whenever its rate of change is less than a predetermined amount. The system may be applied to two stations interconnected by a two-wire line or to multiple stations in which incoming and outgoing switches are sampled in TDM and the speech direction is established by a switch ANP Fig. 5 interconnecting the GO and RETURN lines Lo, Li. The outgoing speech on Lo is compared in AK with a signal L + M which is the sum of the signal envelope stored in SR and either binary zero if the B#A direction is operative or a regulating value from RR if the A#B direction is operative. If the outgoing speech on Lo exceeds L + M, arithmetic unit AU1 adds 1 to SR but otherwise SR is decremented by 1. The gate A 1, however, prevents incrementing of SR during the A#B mode. SR therefore contains the envelope of the speech signal for a non-operational direction and the envelope minus the regulating signal for an operational direction. RR contains a regulating signal for each channel which corresponds to the difference between the envelopes for operative and inoperative channels due to frequency distortion and level charge in the receive/ transmit crosstalk. The results of transmit and receive channel signal comparison in DK are adjusted into RR and thence SR in such a sense as to tend to maintain the envelope of the operative channel one unit above the inoperative channel. If the inoperative channel envelope rises faster than RR can cope with, this is interpreted as a breakthrough attempt provided that state of affairs continues for at least two successive sampling periods. Adjustment of RR is blocked for an inoperative channel to prevent crosstalk charge being adjusted into RR. Register SDR stores the speech level charge between sample periods so that switching can only occur when the level in the inoperative channel is still rising over the same two periods that it exceeds the operative channel, thereby preventing false switching due to local acoustics. KSR operates under the same criteria as SDR and is responsible for operating the switch ANP. KFR delays KSR and KR reduces the sensitivity of DK to prevent a low value of RR, arising from coarse quantizing, causing false switching. In a non-TDM system (Fig. 7, not shown) RR and SR are combined in a single register (RSR) and the logarithmic voltage developed in a counter (PT). If automatic volume control is employed, compression of the crosstalk component is compensated by either adjusting the regulating valve (Fig. 9) or by compressing the operative channel speech signal (Fig. 10) by the same proportion.