Patent GB-407884-A (GB)
Title: Improvements in or relating to four-wire telephone systems
IPC: H04B3/20
Filed: 1933-09-11
Abstract: 407,884. Two - - way transmission. SIEMENS & HALSKE AKT.-GES., Siemensstadt, Berlin. Sept. 11, 1933, Nos. 25171 and 25172. Convention dates, Sept. 10, 1932 and Sept. 13, 1932. [Class 40 (iv).] Intermittent blocking of a four-wire circuit with an echo suppressor ES1, ES2 near each end, due to substantially simultaneous arrival of speech in both directions within the transmission time of the circuit, is prevented by a further pair of echo-suppressors ES3, ES4 preferably at the electrical centre of the circuit. The intermediate pair are quick operating and are connected as shown, i.e. ES3 is in the same direction as ES1 and ES4 as ES2. The commencement of echo-suppression is determined by the intermediate suppressors and the hang over time by the terminal ones. When two four-wire circuits are connected in tandem, the usual arrangement of echo-suppressors (Fig. 3) has the intermediate pair 2, 3 the wrong way round and to obtain the advantage of the present invention, this pair is reversed as at 2<1>, 3<1>, Fig. 5, when the circuits are connected in tandem. Or, as in Fig. 6, alternative suppressors are provided at each terminal. The full-line suppressors are those in use when a circuit is used alone, but when two are used in tandem, suppressors 2<1>, 3<1> are used in place of 2, 3. Echo suppression may be by relay operation or by grid bias control and the switching of the echo-suppressors may be performed manually, or automatically by transmission of D.C. or A.C. over a speech path or over a control path. In the case of the more extensive network shown in Fig. 7, a number of toll centres EF are directly connected to a centre VF a number of which are directly connected to a regional centre DF connected to a centre WF which may be a world centre. There are also cross connections between centres of similar grade. The cross connections between EF and the series connection EF-VF are two-wire lines, the cross connections VFVF may be two-wire lines or echo suppressed four wire circuits, while the remaining connections to and between higher grade centres are four-wire circuits with echo-suppressors. In the first of the connections shown in Fig. 8, no change in the echo suppressors is required while in the second case, the suppressors at the DF terminals are reversed as previously described. In the third example, the suppressors in the middle portion DF-DF are in accordance with the invention, but the echo suppressors at the DF terminals of the VF-DF connections require to be reversed. The changes in the fourth and fifth examples will now be obvious. In general, the suppressors in the cross-connections, e.g. DF-DF require no change but in a series connection, e.g. VF-DF the suppressor at the higher grade terminal (DF) requires reversal. In the fourth example, the suppressors at only the WF terminals may be reversed while those at the DF terminals may be disconnected or disabled, while in the third and fifth examples, all the suppressors may be disabled except one at each end of the connection and the pair in the middle portion (DF-DF or WF-WF) of the connection.