![]() ![]() This seems extreme for a 48 VDC powered line, but the telephone company tests their lines by adding as much as another 200 volts, so you must guard against the worst case. It needs a maximum operating voltage of at least 250 Vrms. The MOV (metal-oxide varistor), or any similar transient voltage suppressor, is required due to the extreme lighting-induced voltage spikes that can travel on telephone lines (thousands of volts). The transformer provides the necessary isolation, while the capacitor blocks the DC voltage from the transformer. Positive (tip or green wire) tied to earth ground 4 so it measures 48 VDC (relative to ring or red wire)Ī single transformer and capacitor creates a POTS interface in a pinch however adding a few more components greatly improves the performance. POTS Characteristics ( typical) Bandwidth It operates in parallel with the telephone, with a circuit design that disturbs normal operation very little due to its high impedance input ( if the hold resistor is not needed). The phone patch allows connecting standard audio equipment to a phone line, while isolating the audio equipment from ring tone and line voltage. What is of concern here is how to add or remove audio from a normal telephone circuit without interfering with the operation of, or being harmed by, the telephone lines.Ī phone patch, or phone tap, is necessary to interface line-level analog audio to and from POTS. Teleconferencing applications require complex digital hybrids containing acoustic echo cancelling technology. Converting 2-wire local wiring to 4-wire long distance wiring requires a hybrid 2 and is not the subject of this note. Long distance lines separate transmit and receive audio paths and use 4-wire cable (two pairs). The system described here is typical for the United States. International harmonization is slowly changing this. Major differences are found in wiring practices and connectors, line impedances, and loop currents, signaling tones and safety regulations. The 48 VDC is current limited by series resistors (one in each leg), therefore phones "on-hook" (no current drawn) typically measure 48 VDC, while phones "off-hook" (current drawn) typically measure 6 - 8 VDC.īasically all phones work the same, yet many different systems coexist throughout the word. One pair of wires carries duplex 1 audio and 48 VDC for telephone operation. Analog Telephone OverviewĪn analog telephone line, at its simplest, is nothing more than a 600-ohm balanced line. (By contrast, if you work in the EU business place, then you will rarely run into POTS, since it has almost all been converted to ISDN.) Meanwhile, back in the USA: if you are lucky, the telephone system you get to work with will provide a line-level auxiliary analog audio feed, but if not, then this Note's for you. ![]() In the USA, in spite of all the hoopla about digital-this, digital-that and fiber-optic-whatever, the truth is that many small commercial paging and music-on-hold applications still involve interfacing with the plain old telephone service, or "POTS". Interfacing Audio and POTS (Plain Old Telephone Service) Dennis Bohn, Rane RaneNote 150 written 2002 last revised 6/09 ![]()
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