Detecting faults in transmission lines employing an echo cancellation signal

• US 4,859,951 A
• Filed: 10/21/1987
• Issued: 08/22/1989
• Est. Priority Date: 10/22/1986
• Status: Expired due to Fees

• Current Assignee: British Telecommunications PLC (BT Group PLC)
• Original Assignee: British Telecommunications (BT Group Plc) (BT Group PLC)
• Inventors: Cole, Nigel G., Adams, Peter F.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US07/111,049
• Time in Patent Office: 671 Days
• Field of Search: 324/534, 324/532, 324/533, 324/521, 324/58.5 B, 324/58 B, 379/3, 379/26, 379/406, 379/410, 379/411, 370/231, 370/32, 364/507, 364/522
• US Class Current: 324/534
• CPC Class Codes: G11C 27/02 Sample-and-hold arrangements G11C27/04 takes precedence
• CPC Class Codes: H04B 3/23 using a replica of transmitted signal in the time domain, e.g. echo cancellers
• CPC Class Codes: H04B 3/231 Echo cancellers using readout of a memory to provide the echo replica
• CPC Class Codes: H04B 3/493 Testing echo effects or singing

Abstract

A variation in the cancellation coefficents of an echo cancelling modem indicate a change in the impedance of a line. In an echo canceller with T spaced samples (i.e. producing a cancellation signal at the data rate) the resolution is too poor to provide satisfactory identification of faults. A set of coefficients is stored and the sampling phase adjusted to produce a second set. This may be repeated producing eight sets in the preferred embodiment. A higher resolution picture of the line impedance is built up allowing comparisons with an earlier or optimum set of values. The test facility may be included in an operating modem providing line test facilities when not transmitting data.

Claims

1. Fault detection apparatus for detecting faults in transmission lines comprising:

transmitter means for supplying a sequence of output pulses to a wire,

receiver means for receiving echoes of said pulses from said wire using a sample clock rate of 1/T and an adaptive cancellation processor arranged to produce a cancellation signal in response to each output pulse,

said apparatus further including a fault detection processor,

said fault detection processor being arranged to receive N sets of cancellation coefficients from said cancellation processor, each of said sets of cancellation coefficients being derived by adjusting the sample clock phase of the receiver means and hence of the cancellation signal by T/N,

said fault detection processor interleaving said N sets of coefficients to derive an overall coefficient set representing line characteristics at N times sample rate 1/T,

said overall set of coefficients being compared with a stored reference set to determine whether a fault has occurred on the line.

2. Fault detection apparatus according to Claim 1 in which the coefficients from each of said N sets are interleaved and displayed graphically.

3. Fault detection apparatus according to claim 1 in which said phase is adjusted seven times and N=8.

4. A method of detecting faults in transmission lines comprising steps of:

supplying a sequence of output pulses to a wire,

receiving echoes of said pulses from said wire and producing a cancellation signal in response to each output pulse,

adjusting the sample clock using a sample clock rate 1/T phase used to produce the cancellation signal (N -

     1) times to produce N sets of cancellation coefficients,

interleaving said N sets of cancellation coefficients to produce an overall set of cancellation coefficients at N times the sample clock rate and comparing said overall set of coefficients to determine whether a fault has occurred on the line.

5. A method according to claim 4 in which the phase of the cancellation signal is adjusted relative to the phase of the output pulses.

6. A method according to claim 4 in which the relative phase is adjusted seven times to produce eight sets of cancellation coefficients.

7. A method according to claim 6 in which the coefficients are interleaved and displayed graphically.

8. Transmission line fault detecting apparatus comprising:

transmitter means for launching repetitive pulses onto a transmission line;

receiver means connected to receive resulting pulse echoes from said transmission line and to calculate a set of echo cancellation coefficients using supplied clock pulses of controllable phase and a transversal filter with tap spacings of T seconds; and

a fault detection processor means for controllably changing the phase of said clock pulses by an increment of less than T seconds between successive cycles of calculation and storage of said cancellation coefficients thereby providing plural sets of stored coefficients which can be interleaved into a composite set having a resolution interval of less than T.

9. Transmission line detecting method comprising:

launching repetitive pulses onto a transmission line;

receiving resulting pulse echoes from said transmission line and calculating a set of echo cancellation coefficients using supplied clock pulses of controllable phase and a transversal filter with tap spacings of T seconds; and

controllably changing the phase of said clock pulses by an increment of less than T seconds between successive cycles of calculation and storage of said cancellation coefficients thereby providing plural set of stored coefficient which can be interleaved into a composite set having a resolution interval of less than T.