| 摘要 |
<p>Method and apparatus for detecting whether load coils are attached to a telephone line. A stimulus waveform having multiple frequency components is applied to the line. The current and voltage at the near end of the line are coherently sampled and transformed to the frequency domain. The frequency spectra are used to compute auto and cross power spectra of the current and voltage. These power spectra are then used to compute the impedance on the line as well as a coherence function that indicates the extent to which the computed impedance was influenced by noise. If the coherence values indicate that the computed impedance is sufficiently reliable, load coils are detected by finding peaks in the magnitude of the impedance function or sign changes in the phase of the impedance function. Calibration, offset adjustments and ensemble smoothing are used to increase the accuracy of the results. The computation is fast because computing the spectra avoids the needs for individual measurements at multiple frequencies. The computation is accurate because it is not sensitive to noise.</p> |
