Full duplex on a single twisted pair

Full duplex on a single twisted pair

In the local loop both directions of speech travel on the same two wires at the same time. This is possible because the signals are added on the pair and separated by hybrid circuits at each end: an anti-sidetone/hybrid network in the telephone set and a hybrid (2W/4W converter) in the exchange line card. Each hybrid passes the outgoing signal to the line and the incoming signal to the receiver while largely blocking the outgoing signal from its own receive path (balanced bridge). Since the two directions are kept apart by balance, not by frequency or time, the loop works as full duplex.

Need for the two-wire to four-wire converter

• Amplifiers, repeaters, multiplexers and PCM codecs carry signals in one direction only, so trunk transmission is four-wire (separate go and return paths).
• The converter (hybrid) joins the two-wire loop to the four-wire trunk, separating the two directions, and avoids echo and singing caused by signal returning round the four-wire loop.

Operating mechanism

                    4-wire receive (from far end)
                         │  (amplifier)
                         v
              ┌──────────────────────┐
  2-wire  ────┤ T1             T2    │
  line   ─────┤  (hybrid transformer)│
  (Z_L)       │   centre-tapped      │
              │   windings           │
  Balance ────┤                      ├───> 4-wire transmit
  network Z_B └──────────────────────┘     (to far end)

Operation of the hybrid (bridge principle):

1. Receive → 2-wire: a signal arriving on the 4-wire receive pair is fed to the centre-tapped winding. It divides equally between the 2-wire line ($Z_L$) and the balance network ($Z_B$). Half the power reaches the line (3 dB loss).
2. No leakage to transmit pair: the two halves flow in opposite senses through the windings coupled to the transmit pair, so their magnetic effects cancel when $Z_B = Z_L$. Ideally no receive signal appears on the transmit side.
3. 2-wire → transmit: speech from the 2-wire line divides between the transmit and receive ports; the transmit port gets half (3 dB loss), and the part going to the receive port is absorbed by the amplifier output and does no harm.
4. Balance: in practice $Z_B$ only approximates $Z_L$ over the band, so a small signal leaks across. This is measured by the balance return loss

$$B = 20\log_{10}\left|\frac{Z_L + Z_B}{Z_L - Z_B}\right|\ \text{dB}$$

Poor balance causes echo, and if the loop gain round the four-wire circuit exceeds unity, singing (oscillation).