Which of the following best describes interlaced scanning in a picture tube?

Question

Which of the following best describes interlaced scanning in a picture tube?

1. A. Scanning lines progressively from top to bottom in one pass
2. B. Scanning odd lines first and then even lines in two passes (Correct answer)
3. C. Scanning all lines simultaneously in a single pass
4. D. Scanning half of the lines in the first pass and the other half in the second pass

Correct Answer

Option B — Scanning odd lines first and then even lines in two passes

Detailed Solution & Explanation

The correct answer is Scanning odd lines first and then even lines in two passes.

Key Points

• In interlaced scanning, each frame is divided into two fields: the first sweeps all the odd-numbered lines, and the second fills in the even-numbered lines.
• The two fields interleave to build one complete frame, so the frame is transmitted in two passes.
• The purpose is to halve the bandwidth while keeping the flicker rate high enough for the eye to perceive smooth motion.

Additional Information

• With a 25 frames-per-second system, interlacing produces 50 fields per second, comfortably above the threshold at which flicker becomes visible.
• Progressive scanning — the alternative offered in Option A — draws every line in sequence in a single pass and is what modern digital displays use.
• The standard 625-line system carries 312.5 lines per field, the half-line being what allows the two fields to interleave correctly.