Question E2B06:
What is vestigial sideband modulation?  

Choices:  
A: Amplitude modulation in which one complete sideband and a portion of the other are transmitted  
B: A type of modulation in which one sideband is inverted  
C: Narrow-band FM modulation achieved by filtering one sideband from the audio before frequency modulating the carrier  
D: Spread spectrum modulation achieved by applying FM modulation following single sideband amplitude modulation    

Correct Answer is  A: Amplitude modulation in which one complete sideband and a portion of the other are transmitted  

Correct answer: A — Amplitude modulation in which one complete sideband and a portion of the other are transmitted.

Explanation:

1) Why A is correct
- Vestigial sideband (VSB) is an AM-derived technique where you transmit one full sideband plus a small “vestige” (portion) of the other sideband.
- Purpose: reduce bandwidth (compared to full AM) while preserving low-frequency content and allowing simpler demodulation.
- Classic use: analog TV video (NTSC) and digital TV (ATSC 8-VSB).

2) Why the other choices are incorrect
- B: “One sideband is inverted” describes an ssb-suppressed-carrier variant with frequency inversion (sometimes used in secure voice), not VSB. VSB is not defined by inversion, but by a partial sideband.
- C: Filtering an audio sideband before applying FM doesn’t make sense; NBFM is achieved by small deviation relative to modulating frequency, not by “removing an audio sideband.” VSB is an AM technique, not FM.
- D: “Spread spectrum by FM after SSB” is neither standard spread spectrum nor VSB. VSB is not a spread spectrum method and does not require FM.

3) Useful formulas and rules of thumb
- If the modulating signal has bandwidth $B_m$:
  - Double-sideband AM (DSB-AM) bandwidth: $$B_{\text{AM}} = 2B_m$$
  - Single-sideband (SSB) bandwidth: $$B_{\text{SSB}} = B_m$$
  - Vestigial sideband (VSB) bandwidth: $$B_{\text{VSB}} = B_m + B_v$$
    where $B_v$ is the small “vestige” width (a fraction of a full sideband).
- Example (NTSC video): one full upper sideband, about 1.25 MHz of the lower sideband as the vestige, enabling operation within a 6 MHz channel.