--- title: "VSB modulation, motivation and use in TV" subject: "Communication Systems" chapter: 2 key: "semester_6/communication_systems:vsb-motivation-tv" source: https://www.bctengineer.com/q/semester_6/communication_systems/vsb-motivation-tv site: BCT Engineering --- # VSB modulation, motivation and use in TV Communication Systems · Chapter 2: Amplitude Modulation - Asked in: 2070 Bhadra (CS I), 2+2+2 marks - Video: [Vestigial Sideband (VSB) Modulation Explained](https://www.youtube.com/watch?v=poYG4yNGlEk) — 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/vsb-motivation-tv - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2 ## Question What is vestigial side band modulation? What is the motivation behind using VSB? Why is VSB suitable for television transmission? ## Short answer VSB transmits one sideband almost completely plus a vestige of the other, with bandwidth between W and 2W. Its motivation is to save bandwidth while avoiding the sharp filter needed for SSB, preserving low frequencies. It suits TV because video has large bandwidth, important DC content, and allows cheap envelope detection. ## Answer #### What is VSB? **Vestigial sideband (VSB)** modulation transmits one sideband almost completely plus a small part (a "vestige") of the other sideband. It is obtained by passing a DSB signal through a VSB filter whose response has odd symmetry about $f_c$: $$ H(f-f_c) + H(f+f_c) = \text{constant for } |f| \le W $$ Its bandwidth is $B_T = W + f_v$, where $f_v$ is the vestige width ($W < B_T < 2W$). ```text |H(f)| 1 | ________________ | / 0.5| / <- odd symmetry | / about fc 0 +--/-----------------------> f fc-fv fc fc+fv fc+W ``` #### Motivation for VSB - **DSB** wastes bandwidth ($2W$), which is costly for wide-band signals. - **SSB** saves bandwidth but needs a filter with a very sharp cut-off at $f_c$. This is impossible when the message has large low-frequency and DC content, because there is no gap around $f_c$ to place the filter transition. - VSB is a compromise: bandwidth close to SSB, but the filter can have a gradual roll-off, so it is easy to build. Low frequencies, including DC, are kept without distortion. - With a carrier added, VSB can be detected with a simple envelope detector (with small distortion). #### Why VSB suits television 1. A video signal has a large bandwidth (about 4.2 MHz in NTSC, 5–5.5 MHz in PAL). DSB would need 8–11 MHz per channel; VSB needs only about 5.75 MHz (PAL video) and allows a 7–8 MHz channel including sound. 2. Video has important content at very low frequencies and DC (background brightness). SSB filtering would damage it; VSB preserves it. 3. The VSB filter's gradual slope is practical and gives little phase distortion, which matters for picture quality. 4. A carrier is sent with the VSB signal (VSB + C), so cheap TV receivers can use an envelope detector. 5. In PAL, the full upper sideband (5 MHz) and a 0.75–1.25 MHz vestige of the lower sideband are sent. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does VSB transmit? A. Only the carrier B. One sideband with no part of the other C. One sideband almost fully and a vestige of the other D. Both sidebands and carrier only Answer: C. A small vestige of the other sideband is retained. (Key point: VSB definition) 2. Why is SSB unsuitable for video? A. SSB needs more bandwidth than DSB B. SSB cannot be detected at all C. Video has no low frequencies D. Video has large low-frequency/DC content, needing an impossibly sharp filter at fc Answer: D. No gap exists around fc to place the filter transition. (Key point: Why not SSB) 3. What is the VSB bandwidth relation? A. BT = 2W B. BT = fv C. BT = W + fv, between W and 2W D. BT = W - fv Answer: C. fv is the vestige width. (Key point: Bandwidth) 4. Why is a carrier sent with VSB in TV? A. To reduce bandwidth B. To remove the vestige C. To avoid filters D. So cheap receivers can use envelope detection Answer: D. VSB + C permits simple envelope detection. (Key point: TV receivers) ## Related in Communication Systems - [VSB generation; VSB for TV; SSB applications](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Avsb-generation-tv-ssb-applications) — Communication Systems, chapter 2 - [E1 hierarchy; PCM of 4.2 MHz TV signal with 512 levels](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-pcm-tv-4-2mhz-512) — Communication Systems, chapter 4 - [PAM, PWM, PPM; PCM of 4.8 MHz TV signal with 512 levels](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apam-pwm-ppm-tv-4-8mhz) — Communication Systems, chapter 4 ## Related in other subjects - [Band applications and microwave transmission lines](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aband-applications-transmission-lines) — RF and Microwave Engineering, chapter 1