Multiplex communications – Duplex – Transmit/receive interaction control
Patent
1997-07-02
2000-06-20
Kizou, Hassan
Multiplex communications
Duplex
Transmit/receive interaction control
370290, 379410, H04B 152
Patent
active
060785675
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the measurement of signal quality over telecommunications links, and in particular to the detection of interference.
2. Related Art
More particularly, this invention relates to the detection of spurious signals generated on a second channel as a result of signals being transmitted on a first channel, a situation known generally as `crosstalk`. The spurious signal, once detected, may be measured and cancelled.
The invention is particularly suited to detecting echo. This occurs in a two-way telecommunications link. A signal travelling in a first direction gives rise to a spurious signal travelling in the opposite direction. If this spurious signal returns to the original source of the signal, it will appear as an echo.
The echo effect may be caused in one of several ways. It may occur as a result of acoustical feedback between the earpiece and mouthpiece of a telephone. It may occur as a result of reflections caused by impedance mismatches. It may occur as a result of cross-coupling between the paths in 4-to-2 wire hybrid points; these are the points where the two-way traffic carried over the two-wire connection from a telephone termination is separated into two separate channels (a so-called four-wire connection). This invention is suitable for detecting echo effects at points in the system where signals in the two directions are carried over two separate channels.
The result of any such echo effect is that a speaker will receive his or her own speech, delayed by a short period. The magnitude of the delay is largely determined by the distance the signal has to travel, with a smaller contribution from signal processing delays. The distances travelled by signals in intercontinental calls can introduce delays readily detectable by human observers: the round trip distance over the earth's surface between one point on the earth's surface and its antipodes is 40000 km (approximately 140 light-milliseconds--since landlines do not necessarily follow the shortest route the practical distance is greater than this). The round trip distance between two points on the earth's surface via a geostationary satellite is approximately 1/2 light-second (150000 km). International call-diversion and other network services can create even longer paths.
Delays of this order of magnitude, as well as being annoying, also confuse the speaker who can find it impossible to continue speaking. It is therefore desirable to detect when echo is occurring so that remedial action can be taken. This remedial action may involve taking the faulty circuit out of use until it can be repaired, or limiting the use of the faulty circuit to uses where the echo causes less problems, such as short-distance calls (in which the echo delay is too short to be troublesome) or to one-way transmission such as facsimile transmissions. Methods also exist for cancelling the echo signal by combining it artificially with a complementary signal derived from the outgoing signal to generate a zero output. However, all these systems require prior knowledge that an echo exists, and something of its characteristics, notably Its delay time and its attenuation.
It is known to transmit test signals over a telecommunications link in order to detect the presence of echoes. This system can only be used on lines which are not currently in use, because traffic on the line would interfere with the detection of the test signal echo. It is also Known to use trained human observers to monitor live conversations, but this method is labour-intensive, subject to human subjectivity, and also has implications for the privacy of the speakers.
In-service non-intrusive measurement systems are known which use least mean square (LMS) adaptive filter systems to measure the delay and echo strength from the conveyed impulse response. Modern digital signal processors can support about 650 filter coefficients: at a sampling rate of 8 kHz this equates to a maximum detectable echo path of about 80 ms. To detect longer e
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Bai Jianxiong
Milner Simon
Sheppard Philip J.
Traill Douglas M.
British Telecommunications PLC
Kizou Hassan
Slavitt Mitchell
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