Patent GB-335937-A (GB)
Title: Improvements in or relating to telephone transmission systems
IPC: H04B3/20
Filed: 1929-07-03
Abstract: 335,937. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). July 3, 1929. Two-way repeating systems.-Echo-suppressors,. which incapacitate a line for East-West transmission when West-East speech is passing, must not be operated by line noise and are generally designed to be insensitive to the maximum noise to which the line is liable; this involves however needless inefficiency for voice-echo suppression at times when the line is quiet. The invention comprises means for automatically adjusting the sensitiveness or effectiveness of the suppressor according to the level of noise, or (since a speaker raises his voice on a noisy line) according to the strength of the voice-currents. Discrimination between noise and voice depends on the steadiness of one and fluctuation of the other. Adjustment may be step-by-step (e.g., by relays of graded sensitiveness cutting in resistances) or continuous (e.g., by valve circuits which automatically vary their own sensitiveness). Numerous modifications are described, in some of which East, by raising his voice, can break in on the speech of West. Fig. 1. As the noise in the West-East line L1 increases, relays 12, 13, 14, connected to tappings of a potentiometer 11 in the output of a detector 4, are energized in turn and the sensitiveness of the suppressor relay 9 is reduced by shifting its point of connection to the potentiometer. The relays 12, 13, 14 are slow to energize but quick to release, so that a relay energized after an interval by increase of the current in the line L1 falls back at the next momentary reduction of the current. The relays 12, 13, 14 and the counter-direction relays 16, 17, 18 are interconnected so that both suppressor relays 9 and 7 have their sensitiveness equally affected. Fig. 2. The suppressor relay 8 is operated by the output of a detector valve 6; the alternating component of the output is transferred to the grid-leak input circuit of a valve 7 and reduces its output; this allows condensers 19, 20 to discharge and so increases the bias on an amplifier valve 5 which controls the valve 6; the combination therefore reduces its own sensitiveness as the input from the line L1 increases. The reduction of sensitiveness is gradual, depending on the discharge of condensers 19, 20 through a circuit of high resistance, but on a fall of input the restoration of sensitiveness is immediate, the condensers being at once re-charged. Connections 23 between the East and West suppressors ensure equal changes of sensitiveness in both. Fig. 3. Echo-suppression depends on East- West amplifier R1 being biassed to an ineffective condition when peaks of speech from West rise above the noise level. A detector A supplies polar relays 1, 2, of which 1 is relatively insensitive and does not respond to noise, while 2 is intermittently operated by noise. When speech arrives from West, relay 2 is energized continuously and at every speech-peak relay 1 is momentarily energized and adds a battery B1 to B3 to bias the amplifier R1 to ineffective condition. Between the energizations of relay 1 the extra bias is sufficiently maintained by the discharge of a condenser C1. A relay 3 in the charging circuit of C1 short-circuits the opposite condenser C1<1> to prevent the amplifier R from being insensitized. Fig. 4 (not shown). This is a modification of the arrangement of Fig. 3, in which the relays 1, 2 are supplied by separate detectors; relay 1 sensitizes the detector supplying relay 2, and relay 2 insensitizes the amplifier R1. Fig. 5 (not shown). In this modification speech from West operates through a detector on a series of relays of graduated sensitiveness which introduce shunts across the East-West line, so that the stronger the speech the more effective is the shunting-out of echoes. Noise on the East-West line will result in the West-East line being more or less shunted, but arrival of speech from West will shunt the noise on the East-West line and thereby undo the shunting of the West-East line. Fig. 7. Speech from West acts through a tuned amplifier C1 (tuned to favour speech in comparison with noise) on a detector D1 and rectifier 20. The detector D1 energizes a relay 9 to short a relay 12<1> and to remove a short from relay 12 which pulls up and shunts the East- West line. The rectifier 20 charges a condenser 21<1> and increases the bias on the opposite detector D2 to render it insensitive. Noise on the East-West line may have operated D 2,9<1>, 12<1> to shunt the West-East line, but West by speaking loudly enough can insensitize D2 to remove this shunt. East by speaking loudly enough can overcome the insensitizing bias on D2 and can insensitize D1 and so break in on West. These results are possible because in each line the tuned amplifier and the line shunt are separated by an amplifier or by an impedance network, so that the amplifier is not directly shunted. Fig. 8 (not shown). This is a modification of Fig. 7 in which the rectifier 20 insensitizes the tuned amplifier instead of the detector D2. Fig. 9. Speech from West acts through an amplifier A2 or detector D2 and rectifier Tl. The rectifier D2 operates a shunt 31 on the East-West line, while the rectifier T1 increases the bias on the opposite detector D1. Owing however to a battery B1 the rectifier T1 only begins to pass current and to increase the bias on D1 when West's speech exceeds the strength for which the shunt 31 provides sufficient safeguard. Fig. 10 (not shown). This is a simplified form of Fig. 9 in which the detectors Dl, D2 are dispensed with, the suppressor relays being operated directly from the rectifiers T1, T2, which mutually bias one another by a resistance common to their circuits.