---
title: "Compare DSB-AM, DSB-SC, SSB, VSB; SSB by phase discrimination"
subject: "Communication Systems"
chapter: 2
key: "semester_6/communication_systems:compare-am-variants-ssb-phase"
source: https://www.bctengineer.com/q/semester_6/communication_systems/compare-am-variants-ssb-phase
site: BCT Engineering
---

# Compare DSB-AM, DSB-SC, SSB, VSB; SSB by phase discrimination

Communication Systems · Chapter 2: Amplitude Modulation

- Asked in: 2076 Bhadra (CS I), 2+6 marks
- Video: [Comparison of Different Amplitude Modulation Techniques: AM, DSB-SC, SSB-SC, VSB-SC | COM SYS | H...](https://www.youtube.com/watch?v=NwdyReXLsyI) — R K Classes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/compare-am-variants-ssb-phase
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2

## Question

Compare the performance of DSB-AM, DSB-SC, SSB-AM, VSB. Describe the process of generation of SSB-AM wave using phase discrimination method.

## Short answer

DSB-AM bandwidth 2W, efficiency ≤33%; DSB-SC 2W, 100% efficient; SSB bandwidth W, 100% efficient; VSB slightly >W. Phase-shift method for SSB: two balanced modulators fed with m(t) and its Hilbert transform m̂(t) using 90° phase-shifted carriers; summing or differencing outputs gives LSB or USB.

## Answer

#### Performance comparison

| Parameter | DSB-AM (FC) | DSB-SC | SSB | VSB |
|---|---|---|---|---|
| Bandwidth | $2W$ | $2W$ | $W$ | $W + f_v$ (slightly > $W$) |
| Carrier | Sent | Suppressed | Suppressed | Suppressed or sent (TV) |
| Power efficiency | ≤ 33.3% | 100% | 100% | ~100% (without carrier) |
| Detection | Envelope | Coherent | Coherent | Coherent or envelope (with carrier) |
| Generation | Simple | Simple | Complex | Moderate |
| Low-freq/DC message | Yes | Yes | Difficult | Yes |
| Receiver cost | Lowest | High | Highest | Moderate |
| Use | AM broadcast | Stereo, QAM | HF voice | TV video |

#### SSB generation by phase discrimination (phase-shift) method

```text
          +-->[Balanced mod 1]--v1--+
          |         ^               |
 m(t) ----+   Ac cos wc t           v
          |         |              (+/-) ---> SSB
          |     [-90 deg]           ^
          |         |               |
          +-[-90]->[Balanced mod 2]-v2
         m^(t)   Ac sin wc t
```

1. Balanced modulator 1 multiplies $m(t)$ with $A_c\cos\omega_ct$.
2. A wideband $-90^\circ$ phase shifter (Hilbert transformer) produces $\hat m(t)$; balanced modulator 2 multiplies it with the $-90^\circ$ shifted carrier $A_c\sin\omega_ct$.
3. The outputs are subtracted for USB or added for LSB.

**Derivation** with $m(t) = A_m\cos\omega_mt$:

$$
\begin{aligned}
v_1 &= A_mA_c\cos\omega_mt\cos\omega_ct = \tfrac{A_mA_c}{2}[\cos(\omega_c-\omega_m)t + \cos(\omega_c+\omega_m)t] \\
v_2 &= A_mA_c\sin\omega_mt\sin\omega_ct = \tfrac{A_mA_c}{2}[\cos(\omega_c-\omega_m)t - \cos(\omega_c+\omega_m)t] \\
v_1 - v_2 &= A_mA_c\cos(\omega_c+\omega_m)t \quad \text{(USB)} \\
v_1 + v_2 &= A_mA_c\cos(\omega_c-\omega_m)t \quad \text{(LSB)}
\end{aligned}
$$

General: $s(t) = A_c[m(t)\cos\omega_ct \mp \hat m(t)\sin\omega_ct]$.

**Advantages:** no sharp filter; works at any carrier frequency; sideband switched by changing sign. **Disadvantage:** the $90^\circ$ shifter must be exact over the entire audio band, otherwise the unwanted sideband is only partly cancelled.

## Check yourself

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

1. What is VSB bandwidth compared with SSB and DSB?
   A. 2W
   B. W + fv, slightly more than SSB and much less than DSB
   C. Exactly W
   D. 3W
   Answer: B. The vestige adds fv to W. (Key point: VSB bandwidth)

2. What is the maximum power efficiency of DSB-AM (FC)?
   A. 100%
   B. 50%
   C. 66.7%
   D. 33.3%
   Answer: D. At mu = 1, mu^2/(2 + mu^2) = 1/3. (Key point: Efficiency of DSB-FC)

3. What is the main disadvantage of the phase discrimination method?
   A. The 90° shifter must be accurate over the whole band or the unwanted sideband is only partly cancelled
   B. It needs a very sharp filter
   C. It cannot handle low frequencies
   D. It transmits the carrier
   Answer: A. Phase error leaves a residue of the unwanted sideband. (Key point: Phase shifter)

4. How is the sideband chosen in the phase method?
   A. By changing the carrier frequency
   B. By changing the filter bandwidth
   C. By subtracting (USB) or adding (LSB) the modulator outputs
   D. By squaring the message
   Answer: C. v1 - v2 gives USB and v1 + v2 gives LSB. (Key point: Sideband selection)

## Related in Communication Systems

- [Noise performance of DSB-AM, DSB-SC and SSB-SC](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-dsbam-dsbsc-ssbsc) — Communication Systems, chapter 9
- [Compare DSB-AM, DSB-SC and SSB](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acompare-dsbam-dsbsc-ssb) — Communication Systems, chapter 2
- [SSB vs AM and DSB-SC; balanced modulator](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Assb-vs-am-balanced-modulator) — Communication Systems, chapter 2
- [DSB-AM vs SSB; ring modulator for DSB-SC](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adsbam-ssb-compare-ring-modulator) — Communication Systems, chapter 2
- [Noise performance of DSB-SC and SSB-SC](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-dsbsc-ssbsc) — Communication Systems, chapter 9
- [DSB-AM, DSB-SC and SSB spectra; synchronous detection](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adsbam-dsbsc-ssb-spectrum-synchronous) — Communication Systems, chapter 2