---
title: "Hybrid, balancing network and singing"
subject: "Telecommunications"
chapter: 2
key: "semester_8/telecommunications:hybrid-balancing-singing"
source: https://www.bctengineer.com/q/semester_8/telecommunications/hybrid-balancing-singing
site: BCT Engineering
---

# Hybrid, balancing network and singing

Telecommunications · Chapter 2: Transmission Media

- Asked in: 2073 Chaitra, 6+2 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/hybrid-balancing-singing
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2

## Question

Explain how hybrid transformer and balancing network together act as four wire / two wire termination set. How does it eliminate the singing problem?

## Short answer

The hybrid’s bridge transformer and balancing network Z_B ≈ Z_L separate the two-wire line from the four-wire GO and RETURN paths. When balanced, received currents cancel in the send winding, providing high transhybrid loss. This ensures total loop gain G₁+G₂−(L_A+L_B) < 0 dB, preventing oscillation (singing).

## Answer

A **hybrid** (four-wire/two-wire termination set) is a bridge transformer that joins the two-wire subscriber line to the four-wire transmission path (separate GO and RETURN pairs). Together with a **balancing network** it passes signals from the 2-wire line to the 4-wire GO path and from the 4-wire RETURN path to the 2-wire line, while keeping the RETURN path isolated from the GO path.

#### Construction and operation
```text
   4-wire RECEIVE (in)          2-wire line
   ---------+                  +---------- Z_L
            |   +----------+   |
            +---|  hybrid  |---+
                | (bridge  |
   4-wire SEND  |  transf.)|---+
   <--------+---|          |   |
            |   +----------+   +---------- Z_B
                              balancing network
```
The hybrid has a centre-tapped (differential) winding. One half connects to the two-wire line (impedance $Z_L$), the other half to the **balancing network** $Z_B$, which is designed to copy the impedance of the line over the voice band.

1. **2-wire to 4-wire (transmit):** speech from the subscriber enters the line winding and divides equally between the send winding (4-wire GO pair) and the receive winding; the part reaching the receive side is absorbed by the one-way RETURN amplifier output and does no harm. The GO pair therefore gets half the power (about 3 dB loss). The line and the balancing network are conjugate ports, so ideally none of this power goes into $Z_B$.
2. **4-wire to 2-wire (receive):** the incoming signal enters the centre tap and splits into two equal currents, one into $Z_L$ and one into $Z_B$. Since the currents flow in opposite senses through the two halves of the winding, their fluxes **cancel** in the send winding when $Z_B = Z_L$. So no receive signal leaks into the GO path; the subscriber still gets half of it (3 dB loss).
3. The loss from the receive port to the send port is called the **transhybrid loss**. With perfect balance it is very large (ideally infinite); in practice it is set by the **balance return loss** (transhybrid loss ≈ BRL + 6 dB for the two 3 dB hybrid losses)
$$
BRL = 20\log_{10}\left|\frac{Z_L + Z_B}{Z_L - Z_B}\right|\ \text{dB}
$$

#### Singing and how it is eliminated
A four-wire circuit with a hybrid at each end forms a closed loop: GO amplifier – far hybrid – RETURN amplifier – near hybrid – GO amplifier.

```text
        G1 (amplifier) -->
 hyb A ===================== hyb B
        <-- G2 (amplifier)
 leak through A (L_A)   leak through B (L_B)
```
If the hybrids leak, part of the signal goes round the loop. When the total loop gain is $\geq 0$ dB, i.e.
$$
G_1 + G_2 - (L_A + L_B) \geq 0\ \text{dB}
$$
the circuit **oscillates (sings)**, producing a howl. Smaller leakage that does not cause oscillation still produces **echo** and a hollow sound ("near-singing").

The balancing network prevents singing by making $Z_B \approx Z_L$, which keeps the transhybrid losses $L_A, L_B$ high. The loop then has a net loss (a **singing margin**, typically a few dB) and cannot oscillate. Further aids are compromise networks for varying line lengths, a fixed net loss inserted in the four-wire circuit, and echo suppressors/cancellers on long circuits.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the role of the balancing network Z_B?
   A. To amplify the receive signal
   B. To copy the line impedance over the voice band so the receive signal cancels in the send winding
   C. To filter noise
   D. To raise the line voltage
   Answer: B. Z_B approximates Z_L, giving balance and high transhybrid loss. (Key point: Balancing network role)

2. What determines the transhybrid loss in practice?
   A. The amplifier gain only
   B. The length of the trunk
   C. The balance return loss, 20log|(Z_L+Z_B)/(Z_L-Z_B)|
   D. The number of subscribers
   Answer: C. Perfect balance gives infinite loss; in practice the BRL sets it. (Key point: Transhybrid loss)

3. The circuit sings when:
   A. G1 + G2 - (L_A + L_B) >= 0 dB
   B. G1 + G2 - (L_A + L_B) < 0 dB
   C. L_A and L_B are large
   D. Z_B differs from Z_L by a small amount
   Answer: A. Loop gain at or above 0 dB makes the four-wire loop oscillate. (Key point: Singing condition)

4. How does the balancing network prevent singing?
   A. By reducing amplifier noise
   B. By keeping Z_B near Z_L so hybrid leakage is low and the loop has net loss (singing margin)
   C. By removing the hybrids
   D. By increasing the amplifier gain
   Answer: B. High transhybrid loss gives a net loop loss and so no oscillation. (Key point: Preventing singing)

## Related in Telecommunications

- [Wired vs wireless media; four-wire and hybrid](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awired-wireless-four-wire-hybrid) — Telecommunications, chapter 2, asked 2×
- [Media types, four-wire system and hybrid](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amedia-types-four-wire-hybrid) — Telecommunications, chapter 2
- [Crosstalk causes and hybrid transformer](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acrosstalk-hybrid-transformer) — Telecommunications, chapter 2
- [Noise and two-wire to four-wire converter](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Anoise-two-four-wire-converter) — Telecommunications, chapter 2
- [Two-wire vs four-wire communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atwo-vs-four-wire) — Telecommunications, chapter 2
- [Talker and listener echo in four-wire circuit](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afour-wire-echo-numerical) — Telecommunications, chapter 2

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