Question E2B05:
Which of the following describes the use of vestigial sideband in analog fast-scan TV transmissions?  

Choices:  
A: The vestigial sideband carries the audio information  
B: The vestigial sideband contains chroma information  
C: Vestigial sideband reduces the bandwidth while increasing the fidelity of low frequency video components  
D: Vestigial sideband provides high frequency emphasis to sharpen the picture    

Correct Answer is  C: Vestigial sideband reduces the bandwidth while increasing the fidelity of low frequency video components  

To calculate why vestigial sideband (VSB) is used in analog fast-scan TV:

- Full double-sideband AM of video would require twice the video bandwidth, which is inefficient.
- Pure SSB would distort very low-frequency (near-DC) video components.
- VSB keeps one full sideband and a “vestige” of the other, preserving low-frequency fidelity while cutting bandwidth.

Why the correct answer (C) is correct:
- VSB transmits the full upper sideband and only a small portion (vestige) of the lower sideband.  
- This reduces RF bandwidth compared with double-sideband, yet maintains proper reproduction of low-frequency video (brightness) content by satisfying the Nyquist condition near DC.

Why the others are incorrect:
- A: Audio is not in the vestigial sideband. In broadcast TV, audio is on a separate FM subcarrier; in amateur ATV it’s typically separate as well.
- B: Chroma (color) is a subcarrier within the video (e.g., NTSC at 3.58 MHz), present in the transmitted sideband(s). It is not uniquely carried by the “vestigial” portion.
- D: High-frequency emphasis (peaking) is a separate video processing technique (pre-emphasis/aperture correction), not the function of VSB filtering.

Useful rules of thumb and formulas:
- If baseband video extends to bandwidth $B_\text{video}$:
  - Double-sideband AM: $B_\text{RF} \approx 2B_\text{video}$
  - Single-sideband AM: $B_\text{RF} \approx B_\text{video}$
  - Vestigial sideband AM: $B_\text{RF} \approx B_\text{video} + \Delta$, where $\Delta$ is the vestigial width (a small fraction of $B_\text{video}$)
- Example (NTSC-like): video to about $4.2\,\text{MHz}$  
  - DSB would be about $8.4\,\text{MHz}$  
  - VSB uses one full sideband to $4.2\,\text{MHz}$ plus a vestige of the other (≈ $0.75$–$1.25\,\text{MHz}$), yielding a channel around $6\,\text{MHz}$
- Key concept: Including a vestige of the suppressed sideband avoids low-frequency amplitude/phase distortion (meets the Nyquist slope near the carrier), thus preserving fidelity of low-frequency video components.