ADSL filters
Telephone wires were originally designed to carry "commercial " Speech" between your home and the telephone exchange. This uses a band of frequencies from 300 to 3400 hertz. this system is called PSTN (public switched telephone network).
ADSL uses frequencies very much higher than this speech band to carry fast data traffic. ADSL systems use typically frequencies between 25 kHz and around 1.1 MHz.
Because PSTN and ADSL systems operate at different frequencies, they can be carried though the same wire pair at the same when the operating conditions are right. Voice calls operate between 300Hz and 3.4KHz, and include also DC power (0-72V DC at on-hook condition, typically 0-60 mA current and lower voltage at on-hook) and rign voltage (typically 40-80 V AC at 20-25 Hz freuquency). The voice telephone system is matched to 600 ohm (or close to it) impedance at voice frequencies. ADSL technology operates between 26KHz and 1.1 MHz and is designed for around 100 ohms impedance. Because the two frequency spectrums do not overlap, it follows that both data and voice can be present at the same time on a single pair of copper wire. The different impedances have hostorical and technical reasons. The impedance of telephoen wiring is typically around 100-120 ohms at the frequencies ADSL system uses. The cable impedance is somewhat higher at voice frequency range, considerably higher than 100 ohms, and where where historical 600 ohms impedance comes to picture (cable might not be exactly 600 ohms for voice, but that's what devices are designed for).
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