Telephone companies expect to see an impedance of 600-900 ohms (total including wires, phone(s), and chokes, on the lines when viewed from the Central Office. Filtering, etc assume that impedance will not differ greatly from that value. If it does, users may experience low signals, distorted audio, excessive echo, whistling, etc. In some case, parallel or serial resistance at the customer site may solve other problems at the expense of some audio volume. Lines are designed to be balanced, and any unbalance can result in poor noise rejection, crosstalk, RFI, etc. The telephone company may insert "loading coils" to improve frequency response. They may be willing to remove the loading coils if the coils interfere with modem usage.
In the US, the two phone wires are generally red ("Ring") and Green ("Tip"). Yellow and black wires may also be present. If so, they may be a almost anything: a second line (Yellow is "ring"); Ground (Black); DC light power for "Princess" phones, etc.
Telephone ringers are operated by an AC voltage sent down the same lines used for talking. Ring voltage is 40-150 volts 15-68Hz -- 20HZ in the US. The normal DC voltage may or may not be present during ring. Ring frequencies and on-off periods differ from country to country. Older telephones used mechanical ringers isolated by .47 ufd capacitors -- which will still work. Newer phones use a semiconductor warbler isolated by 1ufd. The capacitors are generally rated at 250v. The ringer circuit is resonant and alters the low frequency characteristics of the line as seen from the Central Office. Telephone companies use this fact to determine when equipment is added or removed on the customer premisses. That doesn't work if the ringer is disabled. Ringer devices have a Ringer Equivalence number that adds for every device at the customer premisses. A US phone company will generally guarantee delivery of enough current to drive a total REN of 5.0. US telephones are designed not to ring when brief transients occur on the phone line as when a phone is hung up.
Telephones themselves use either a passive or active network to separate and process incoming and outgoing audio. Phase shifting is used to limit the amount of outgoing audio fed back to the earpiece. Passive networks will work -- albeit weakly -- at low voltages. Active devices generally require at least 3 or 4 volts for operation. In principle, as many as three telephones can be off hook simultaneously on a "20ma" line and still work, but active (IC or transistorized) devices frequently won't work in such a configuration. Passive networks may use Varistors to limit maximum audio volume. active devices frequently include automatic volume control. While all phone systems are basically similar, there are differences. Telephone sets designed for one country's phone system may work poorly or not at all in another country.
Return To Index Copyright 1994-2002 by Donald Kenney.