Patent GB-1329958-A (GB)
Title: Telecommunications exchange systems
IPC: H04B3/20
Filed: 1971-10-04
Abstract: 1329958 Automatic exchange systems; echo suppressors SIEMENS AG 4 Oct 1971 [14 Oct 1970 (2)] 46028/71 Headings H4K and H4R In a system wherein one exchange sometimes acts as a transit exchange and completes transit connections therethrough on a 4-wire basis, means are provided for disabling the echo suppressors connected to the ends of the junctions incoming thereto and outgoing therefrom so as to avoid possible disturbances such as interruptions and syllable clipping caused by the unwanted operation of these suppressors particularly in connections set up via earth satellites. First arrangement. -In the transit exchange shown in Fig. 3 where switches 7 and 8 represent the exchange selectors for the Go and Return directions in respect of a connection utilizing incoming junction EL2, SL2 and outgoing junction SL3, EL3, echo suppressors 2 and 3 are normally provided. Considering suppressor 3, the speech level in the Go direction on S3 is compared with the speech level in the Return direction on EL3 by means of speech detectors 34, 35 and comparison gate 36. The latter responds if the difference in levels exceeds a value which is greater than that one would expect if suppressor 3 was connected on its S3, E3 side to a 4/2 wire converter. In other words the detector is arranged to operate only when the difference between the signal levels in the two directions is very great and certainly much greater than is the case in a terminal exchange. A further criterion is made that the detectors are more sensitive than those used in the echo suppressors for switching in the Go direction of transmission. As a result of the comparison, and assuming the compared signals last long enough i.e. they are not transients, the suppressor is turned off by means of the inhibit input 33. It will be appreciated that suppressor 3 can only be inhibited by means of speech signals from the subscriber connected to the outgoing junction SL3, EL3, the other suppressors 2 being inhibited in response to the speech of the subscriber on the incoming junction. The suppressors are restored following the absence of speech signals in both directions for a predetermined period. Second arrangement.-In this case the inhibition of one suppressor automatically causes inhibition of the other. This is achieved by gating the output of tone generator G3 on to the return side E3 of suppressor 3-assuming this to be rendered inoperative first. Consequently tone receiver T2 responds thereby to inhibit suppressor 23 and incidentally to gate the output of its generator G2 on to the Go side E2 of suppressor 2. The tone frequency may be different for the two directions and it may be of the order of 5000 Hz. The suppressors are not restored until release of the connection when switches 7, 8 go home. Further arrangements (Figs. 4 and 5, not shown).-The speech detectors may be dispensed with and the generator G3 (of Fig. 3) is connected directly to E3 in all echo suppressors at the outgoing end of the junctions. Consequently whenever a tandem call is set up the transit exchange suppressors are cut out (as described for the second arrangement) as soon as switches 7, 8 are set. In order to prevent the suppressors in the terminal exchanges being cut out inadvertently with this arrangement, it is arranged that the output level of the generators is sufficiently low that the tones therefrom will not be conveyed effectively via the terminating 4/2 wire hybrids. It is suggested that the frequency of the tone for the Go path (left to right in Fig. 3) is 10 Kc/s whereas that for the Return path is 12 Kc/s so as to further prevent the Return tone (which is the only really relevant one so far as break through at a terminal exchange is concerned) from being passed through the hybrid. The suppressors may incorporate switched pads e.g. 32 in the receive path in addition to the traps e.g. 31 in the transmit paths. In all the arrangements utilizing tone signalling the effective directions of transmission thereof may be reversed by suitable connection of the generators and receivers to the speech paths.