1. A method of identifying an electrode fault in a system designed to measure electrical activity of a subject, the method comprising: delivering a carrier signal to the subject by connecting a first electrode to the subject; sensing a combined signal having an electrical-activity portion and a carrier-signal portion by attaching at least one signal sensing electrode to the subject; processing the combined signal detected by the sensing electrode by dividing the electrical activity portion from the carrier signal portion; calculating an impedance value for the at least one signal sensing electrode; and determining whether the calculated impedance for the at least one sensing electrode exceeds a predetermined value.
2. A method as claimed in claim 1 , further comprising reducing high-frequency noise of the electrical activity portion of the combined signal.
3. A method as claimed in claim 1 , wherein processing the combined signal detected by the sensing electrode includes filtering the combined signal in a low-pass filter with a first zero point frequency, and wherein the carrier signal has a frequency substantially the same as the first zero point frequency.
4. A method as claimed in claim 3 , wherein the low-pass filter is a finite-impulse response filter.
5. A method as claimed in claim 4 , wherein the low-pass filter has a cut-off frequency of about 150 Hz.
6. A method as claimed in claim 4 , wherein calculating an impedance value involves using the carrier-signal portion.
7. A method as claimed in claim 1 , wherein the combined signals are sensed by attaching multiple sensing electrodes to the subject.
8. A method of identifying an electrode fault in a system designed to measure electrical activity of a subject, the method comprising: delivering a carrier signal to the subject by connecting a first electrode to the subject; sensing a combined signal having an electrical-activity portion and a carrier-signal portion by attaching at least one signal sensing electrode to the subject; filtering the combined signal detected by the at least one signal sensing electrode with a low-pass filter having a zero point frequency; and calculating the frequency of the filtered carrier-signal portion based on the zero-point frequency of the low-pass filter.
9. A method as claimed in claim 8 , further comprising: identifying an impedance value for the at least one signal sensing electrode; and determining whether the calculated impedance for the at least one sensing electrode exceeds a predetermined value.
10. An apparatus for detecting an electrode fault, the apparatus comprising: a signal processor; a carrier signal generator; at least one physiological activity electrode to sense a signal and coupled to the signal processor; a carrier signal electrode coupled to the signal processor and the carrier signal generator; and a filter to remove a carrier signal from the signal sensed by the at least one physiological electrode, wherein the filter is a low-pass filter, and wherein the low-pass filter is a finite impulse response filter with a first zero point frequency.
11. An apparatus as claimed in claim 10 , wherein the low-pass filter is used to determine the characteristics of the carrier signal.
12. An apparatus for detecting an electrode fault, the apparatus comprising: a carrier signal generator operable to generate a carrier signal having a frequency; and a signal processor operable to be coupled to at least one physiological activity electrode and a carrier signal electrode, the at least one physiological activity electrode operable to sense a signal having an electrical activity portion and a carrier signal portion, the signal processor having a filter operable to reduce high frequency noise in the signal and to separate the carrier signal portion from the electrical activity portion, the filter having a first zero point frequency that is substantially the same as the frequency of the carrier signal.
13. The apparatus as claimed in claim 12 , wherein the filter is a finite impulse response filter with corner frequency of 150 Hertz.
14. The apparatus as claimed in claim 12 , wherein the carrier signal electrode is connected to the right leg of a subject.
15. The apparatus as claimed in claim 12 , further comprising: a visual alarm activated by an electrode fault signal from the signal processor.
16. The apparatus as claimed in claim 12 , further comprising: an audible alarm activated by an electrode fault signal from the signal processor.
