Ham Radio Extra Class Practice Test

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

1. By scanning two fields simultaneously

2. By scanning each field from bottom to top

3. By scanning lines from left to right in one field and right to left in the next

4. By scanning odd numbered lines in one field and even numbered lines in the next

The correct answer is: By scanning odd numbered lines in one field and even numbered lines in the next

An interlaced scanning pattern in a fast-scan television system, such as NTSC, is generated by scanning odd numbered lines in one field and even numbered lines in the next. This technique essentially alternates the lines that are drawn on the screen in each field. In the first field, the television scans and displays all of the odd-numbered horizontal lines (e.g., lines 1, 3, 5, etc.), completing that half of the image. The subsequent field then scans and displays the even-numbered lines (e.g., lines 2, 4, 6, etc.). This interlacing method allows for smoother motion and helps to reduce flicker, as the viewer sees a full frame made up of two fields in quick succession, effectively doubling the perceived rate of refresh. Interlacing is a key feature in traditional analog television systems, enabling them to convey the necessary information in a lower bandwidth compared to non-interlaced formats. The resulting image appears smoother and provides a more continuous look than scanning every line in sequential order, which is characteristic of progressive scanning.