Question E2B03:
How is an interlaced scanning pattern generated in a fast-scan (NTSC) television system?  

Choices:  
A: By scanning two fields simultaneously  
B: By scanning each field from bottom-to-top  
C: By scanning lines from left-to-right in one field and right-to-left in the next  
D: By scanning odd-numbered lines in one field and even-numbered lines in the next    

Correct Answer is  D: By scanning odd-numbered lines in one field and even-numbered lines in the next  

To create an interlaced image, NTSC splits each video frame into two fields:
- Field 1 scans the odd-numbered lines (1, 3, 5, …).
- Field 2 scans the even-numbered lines (2, 4, 6, …).
These two fields are displayed in rapid succession to form one complete frame, reducing flicker without doubling required bandwidth.

Why D is correct:
- Interlacing is defined by scanning odd-numbered lines in one field and even-numbered lines in the next. This halves the vertical detail per field but maintains perceived motion smoothness and reduces flicker.

Why the others are incorrect:
- A: “Two fields simultaneously” — NTSC uses a single scanning beam; fields are scanned sequentially, not at the same time.
- B: “Bottom-to-top” — Standard scanning is top-to-bottom; interlace concerns which lines are scanned, not the vertical direction.
- C: “Left-to-right then right-to-left” — Each line scans left-to-right; the return is a horizontal retrace during blanking. Direction doesn’t alternate between fields.

Useful facts and formulas:
- NTSC nominal lines per frame: $N_{\text{lines, frame}} = 525$
- Lines per field: $N_{\text{lines, field}} = 262.5$
- Frame rate: $f_{\text{frame}} \approx 29.97\ \text{Hz}$ (often simplified to $30\ \text{Hz}$)
- Field rate: $f_{\text{field}} = 2 f_{\text{frame}} \approx 59.94\ \text{Hz}$ (often simplified to $60\ \text{Hz}$)
- Horizontal scan frequency:
  $$f_H = N_{\text{lines, frame}} \times f_{\text{frame}} \approx 525 \times 29.97 \approx 15.734\ \text{kHz}$$

Rule of thumb: Interlaced NTSC = odd lines first field, even lines second field; 525 lines, ~30 frames/s, ~60 fields/s, ~15.7 kHz horizontal.