| 摘要 |
A twisted-pair telephone line (100) is connected to a conventional telephone (at 103) via a low-pass filter and (102) equipment (at 106) for receiving signals in an upper frequency range e.g. 25-1000 kHz via a high-pass filter (105). The low pass filter (105) is primarily passive (to allow passage of line power, ringing and signalling voltages) but, to improve matching to non-resistive line and telephone impedances, at least the main part of the filter T4, T5, C4A/B, C5A/B, C6A/B, C7A/B is flanked by impedance converters (300, 400) in which amplifiers IC2, IC4 having an appropriate transfer function feed voltages back into the line via transformers T3, T7 through which power, ringing and dialling may pass unimpeded.
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