Number of Wires
E&M defines 8 wires:
- E: Originally "Earth", now called "Ear"
- M: Originally "Magneto", now called "Mouth"
- SG and SB: Signal Ground and Signal Battery
- T and R: Tip and Ring - the ground and battery of the receive voice pair
- T1 and R1: the ground and battery of the transmit voice pair
4-wire E&M uses a 4-wire (2-pair) transmission path for the voice signal. 2-wire E&M uses a single pair for both transmit and receive voice signal. This is much inferior to 4-wire E&M as the 2-wire interface uses hybrid transformers which reduce signal quality and can introduce echo.
Read more about this topic: E And M Signaling
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