--- title: "DSB-AM, DSB-SC and SSB spectra; synchronous detection" subject: "Communication Systems" chapter: 2 key: "semester_6/communication_systems:dsbam-dsbsc-ssb-spectrum-synchronous" source: https://www.bctengineer.com/q/semester_6/communication_systems/dsbam-dsbsc-ssb-spectrum-synchronous site: BCT Engineering --- # DSB-AM, DSB-SC and SSB spectra; synchronous detection Communication Systems · Chapter 2: Amplitude Modulation - Asked in: 2072 Asoj (CS I), 5+3 marks - Video: [Introduction to Amplitude Modulation | Double Side Band Suppressed (DSB-SC) Carrier Explained](https://www.youtube.com/watch?v=3wBlB-TFwkQ) — ALL ABOUT ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/dsbam-dsbsc-ssb-spectrum-synchronous - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2 ## Question Compare the basics of DSB-AM, DSB-SC and SSB modulations with their respective spectrum? Why DSB-SC detection is known as synchronous detection, explain with required details? ## Short answer DSB-AM transmits carrier and both sidebands (BW=2W, envelope detection). DSB-SC suppresses carrier (BW=2W, coherent detection). SSB transmits one sideband (BW=W, coherent). DSB-SC detection is synchronous because the receiver must regenerate a carrier locked in frequency and phase to the suppressed carrier; any phase error reduces output by cosφ, and 90° error gives zero output. ## Answer #### Comparison of DSB-AM, DSB-SC and SSB For message spectrum $M(f)$ band-limited to $W$: | Point | DSB-AM (FC) | DSB-SC | SSB | |---|---|---|---| | Expression | $A_c[1 + k_am(t)]\cos\omega_ct$ | $A_cm(t)\cos\omega_ct$ | $\frac{A_c}{2}[m\cos\omega_ct \mp \hat m\sin\omega_ct]$ | | Carrier | Present | Absent | Absent | | Bandwidth | $2W$ | $2W$ | $W$ | | Efficiency | ≤ 33.3% | 100% | 100% | | Detection | Envelope | Coherent | Coherent | **Spectra** ```text DSB-AM: carrier LSB | USB ///// | \\\\\ -------+-----+----+----+-----+------> f fc-W fc fc+W DSB-SC: LSB (no carrier) USB ///// \\\\\ -------+-----+----+----+-----+------> f fc-W fc fc+W SSB (USB): USB \\\\\ ------------------+----------+------> f fc fc+W ``` #### Why DSB-SC detection is synchronous DSB-SC has no carrier, and its envelope is $|m(t)|$, not $m(t)$; the carrier phase reverses each time $m(t)$ crosses zero. So an envelope detector would give $|m(t)|$ (distorted). The message can be recovered only by multiplying with a local carrier that is **synchronised in frequency and phase** with the transmitter carrier, hence "synchronous detection". ```text s(t) --->( X )---> LPF ---> vo(t) ^ cos(wc t + phi) (from carrier recovery) ``` $$ v_o(t) = \text{LPF}\{A_cm(t)\cos\omega_ct\cos(\omega_ct + \phi)\} = \frac{A_c}{2}m(t)\cos\phi $$ - $\phi = 0$: output $\tfrac{A_c}{2}m(t)$, ideal. - $\phi \ne 0$: attenuation by $\cos\phi$; at $\phi = 90^\circ$ the output vanishes (quadrature null). - Frequency error $\Delta\omega$: output $\propto m(t)\cos\Delta\omega t$, a beating distortion. Hence exact synchronisation is required, provided by a pilot carrier, squaring loop or Costas loop. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which row correctly matches the three schemes? A. DSB-AM: no carrier; DSB-SC: carrier present; SSB: 2W B. DSB-AM: bandwidth W; DSB-SC: 2W; SSB: 2W C. DSB-AM: carrier and 2W; DSB-SC: no carrier, 2W; SSB: no carrier, W D. All three need coherent detection Answer: C. DSB-AM keeps the carrier with 2W; DSB-SC and SSB suppress it, and SSB halves the bandwidth to W. (Key point: Comparison table) 2. Why can an envelope detector not demodulate DSB-SC? A. The envelope is |m(t)| and the carrier phase reverses at zero crossings of m(t) B. The bandwidth is too small C. The carrier is too strong D. The diode cannot conduct Answer: A. The envelope of DSB-SC is |m(t)|, not m(t), so output would be distorted. (Key point: Why not envelope detection) 3. Why is DSB-SC detection called synchronous? A. The signal is sampled at a fixed rate B. The local carrier must be synchronised in frequency and phase with the transmitter carrier C. The message is synchronised with noise D. Detection uses a clock only Answer: B. The detector multiplies by a local carrier that must match the transmitted carrier. (Key point: Meaning of synchronous) 4. A phase error phi in the local carrier gives output proportional to: A. sin(phi) B. cos(2 phi) C. tan(phi) D. cos(phi), vanishing at 90 degrees Answer: D. vo = (Ac/2) m(t) cos(phi); at phi = 90 degrees the output is zero. (Key point: Effect of errors) ## 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 - [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 - [Detection gain of SSB-SC vs DSB-SC](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adetection-gain-ssbsc-dsbsc) — Communication Systems, chapter 9, asked 2× - [Detection gains of DSB-FC, DSB-SC and SSB](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adetection-gain-dsbfc-dsbsc-ssb) — Communication Systems, chapter 9 - [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-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