Plain old telephone systems (POTS) telephone line consits of one wire pair which carries full duplex audio and the operating current for the telephone. The telephone connected to line is powered from current limited 48V power source, so phones on-hook, should measure around 48 volts DC. Practically the opearating voltages of telephone systems can vary from 24V to 60V depending on the application, although 48V nominal voltage is the most commonly used. Telephone applications often require and use positive grounding in the central office, where the positive conductor of the 48V power supply is connected to earth ground. The telecommunication industry began the positive ground convention in the 1940s and many telecomm companies still employ the traditionally positive grounded system.
This means that generally when telephone is on-hook, one telephone line wire is quite near to the ground potential and other one carries -48V. When telephone is put off-hook the voltage beween wires going to telephone drops down to the 3 to 9 volt range and typically acurrent of 20-60 mA will flow through the telephone. The remaining voltage drop occurs over the copper wire path and in the telephone central electronics.
Typical telephone line bandwidth is around 200-3200 Hz, which was what phone companies decided years ago be sufficient for speech intelligibility while allowing them to multiplex many calls over coax and twisted pair. The low end is rolled off early to stay away from the mains frequency (50 or 60 Hz) interference. The high end cut off is caused by the telephone transmission system (nowadays the audio is digitized at 8 kHz). The typical signal to noise ratio of a telephone line is approximately 45 dB or somewhat less. The average signal levels on telephone line is -9 dBm average speech.
The voice on a tip/ring pair is full duplex balanced audio which requires a two wire to four wire hybrid circuit or transformer to convert it into separate transmit and receive audio paths. Bulky and expensive hybrid transformers have been replaced in most telephones by ICs which perform the same function. In a telephone, the biggest contributor to poor audio quality is the handset microphone (it has be be cheap and withstand very hard use).
Telephone standards world is fragmented. Typically each country has its own standards because of both the historical roots of the phone service and the desire to protect the local phone market from outside competition.
Basically the telephone systems work in the same way in different countries, but there are are some differences which can mean that a devices designed for one country does not meet the regulations of other country and work poorly or not at all. The differences in local technical standards range from minor to severe and affect many of the signaling conditions on local loops. The most major differences are different wiring practices and connectors, different line impedances, different nominal loop currents, different signaling tones and different electrical safety regulations. Unfortunately nowadays many countries are harmonizing many standards across their boundaries so it is nowadays possible to designe devices which work well and meet the regulations in more than one country at the time. For example in Europe the European Commission adopted CTR 21 standard covers nonvoice equipments (for example FAX and MODEM) in more than 20 countries.
Computer can be used to perform lots of fuctions in modern telephony systems. This link collectains mostly information on linking computers and traditional telephone systems. There is a separate link section for Internet telephony.
Technical characteristics of tones for the telephone service are listed in ITU-T Recommendation Q.35 (1988). Infortunately that document is not freely available (can be ordered from ITU-T if you are willing to pay), so the information available in the following documents might be useful if you are looking for free information on telephone line signaling.
The following telephone circuits are mainly ment to give ideas for your own desigs. Many of the circuits are quite propably do not meet your local telephone network terminal equipment specs (they vary from country to country) and are quite propably poorly designed (this applies as well to many circuits in books and hobby magazines). Connecting non-approved equipment to public telephone network is illegal in many countries. If you connect non-approved circuit which do not meet the specs to the public telephone network the telephone do not work properly with them and they also cause safety hazards.
For information what is allowed to be connecte to telephone line the reader form USA should check FCC part 68 regulations and European reader should check NET4 (ETS 300 001) regulations. Good information on telephone equipment regulations can be found from Compliance Engineering Magazine website.
Intercom systems used in TV and stage productions are usually headset type intercoms connected to one line using party line arrangement. This kind of intercom systems are frequently referred to as 2-wire although this term is misleading since the stations are connected together using 2-conductor shielded microphone cable, and the shield is very much a conductor. This connector is typically used to carry the party line voice in one wire and the station operating power (usually 24-30V) in another. The ground is common for both of them.
There are several interconnect standards for headset intercom systems, none of which are directly compatible with the others. The most common standards are those originally devised by Clear-Com®, Telex® and RTS® . The Clear-Com standard is also used by Technical Projects Production Intercom, HM Electronics, and a number of English and European manufacturers.
Telephone bugging is an illegal practice. The following circuits are for educational use. They are illagal in many countries.
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