--- title: "Compare DSB-AM, DSB-SC and SSB" subject: "Communication Systems" chapter: 2 key: "semester_6/communication_systems:compare-dsbam-dsbsc-ssb" source: https://www.bctengineer.com/q/semester_6/communication_systems/compare-dsbam-dsbsc-ssb site: BCT Engineering --- # Compare DSB-AM, DSB-SC and SSB Communication Systems · Chapter 2: Amplitude Modulation - Asked in: 2071 Magh (old course), 8 marks - Video: [Comparison of DSB-FC, DSB-SC, SSB-SC and VSB in Communication Engineering by Engineering Funda](https://www.youtube.com/watch?v=f6F3kdnfg_k) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/compare-dsbam-dsbsc-ssb - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2 ## Question Compare DSB-AM, DSB-SC and SSB in terms of complexity, power and bandwidth efficiency. ## Short answer DSB-AM is simplest (envelope detector) but least efficient (max 33.3% power, BW=2W). DSB-SC uses coherent detection, 100% power efficient, but still BW=2W. SSB is most complex yet most efficient: 100% power efficiency and only half bandwidth (W), saving power and spectrum. ## Answer DSB-AM, DSB-SC and SSB are the three main forms of linear (amplitude) modulation. They trade circuit complexity against power and bandwidth efficiency. #### Expressions (single tone $A_m\cos\omega_mt$) $$ \begin{aligned} \text{DSB-AM: } & A_c[1 + \mu\cos\omega_mt]\cos\omega_ct \\ \text{DSB-SC: } & A_cA_m\cos\omega_mt\cos\omega_ct \\ \text{SSB: } & \frac{A_cA_m}{2}\cos(\omega_c \pm \omega_m)t \end{aligned} $$ #### Comparison | Criterion | DSB-AM (FC) | DSB-SC | SSB | |---|---|---|---| | **Transmitter complexity** | Simple (square-law, switching, high-level collector modulation) | Moderate (balanced / ring modulator) | High (sharp sideband filter or phase-shift network) | | **Receiver complexity** | Very simple envelope detector | Coherent detector + carrier recovery (Costas loop) | Coherent detector, needs accurate frequency | | **Carrier** | Transmitted | Suppressed | Suppressed | | **Power for $\mu=1$** | $1.5P_c$ | $0.5P_c$ | $0.25P_c$ | | **Power efficiency** | $\frac{\mu^2}{2+\mu^2}$, max 33.3% | 100% | 100% | | **Bandwidth** | $2W$ | $2W$ | $W$ | | **Bandwidth efficiency** | 50% | 50% | 100% | | **Noise performance (figure of merit)** | ≤ 1/3 (with envelope detection) | 1 | 1 | | **Typical use** | MW/SW broadcasting | Stereo FM, QAM, TV colour | HF voice, telephony FDM | #### Remarks - **Complexity:** DSB-AM is the cheapest, which is why it is used for broadcasting where there are millions of receivers and one transmitter. SSB is the most complex; filter method needs very sharp filters and the phase-shift method needs an accurate $90^\circ$ wideband shifter. - **Power efficiency:** DSB-AM wastes at least two-thirds of its power in the carrier. DSB-SC and SSB put all power into information-bearing sidebands. Compared with DSB-AM at $\mu=1$, DSB-SC saves 66.7% and SSB saves 83.3% power. - **Bandwidth efficiency:** SSB is best, using only $W$; DSB-AM and DSB-SC both need $2W$ since both sidebands carry the same information. - **Overall:** DSB-AM = simple but inefficient; DSB-SC = power efficient but not bandwidth efficient; SSB = power and bandwidth efficient but complex. VSB is a compromise between DSB and SSB. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. For mu = 1, which is the correct ordering of transmitted power in terms of Pc? A. All three need 1.5Pc B. DSB-AM 1.5Pc, DSB-SC 0.25Pc, SSB 0.5Pc C. DSB-AM 1.5Pc, DSB-SC 0.5Pc, SSB 0.25Pc D. DSB-AM 0.5Pc, DSB-SC 1.5Pc, SSB 0.25Pc Answer: C. The carrier is dropped in DSB-SC and one sideband in SSB. (Key point: Power at mu=1) 2. Which systems need bandwidth 2W? A. DSB-SC and SSB B. All three C. DSB-AM and DSB-SC D. SSB only Answer: C. Both DSB forms send both sidebands (2W); SSB sends one (W). (Key point: Bandwidth) 3. What is the maximum power efficiency of DSB-AM? A. 50% at mu = 1 B. 100% C. 33.3% at mu = 1 D. 66.7% at mu = 1 Answer: C. Efficiency = mu^2/(2+mu^2) = 1/3 at mu = 1. (Key point: Efficiency of DSB-AM) 4. Why is DSB-AM used for broadcasting despite poor efficiency? A. It needs the least bandwidth B. It needs no transmitter modulator C. Receivers are very simple (envelope detector) and there are millions of them D. It has the best noise performance Answer: C. Cheap receivers matter when there is one transmitter and many receivers. (Key point: Complexity) ## Related in Communication Systems - [Compare DSB-AM, DSB-SC, SSB, VSB; SSB by phase discrimination](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acompare-am-variants-ssb-phase) — 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 - [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 - [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 - [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 - [AM vs FM efficiency and complexity; coherent ASK error](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aam-fm-bw-efficiency-ask-error) — Communication Systems, chapter 9, asked 2× ## Related in other subjects - [AM wave power and bandwidth numerical](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aam-numeric-40-carrier-power) — Data Communication, chapter 5