interlaced, which means each field contains every other
line of a frame and a complete frame is constructed by
displaying alternating fields.
The fields are labeled as either an odd field or an even
field. The field rate is 59. 94 Hz, making the frame rate
29. 97 Hz. Each field contains 262.5 lines, which consist of
video and synchronization signals. I think I heard another,
“huh?” Yes, fields contain half lines and when added
together, the entire frame is 525 lines. This does actually
make sense once you see how the electron beam traverses
the screen in Figure 4.
So, our NTSC picture has a vertical resolution of 525
lines, which is not too bad for an old analog signal! Not
quite. Some of those lines are vertical synchronization
signals which consist of a series of pre-equalizing pulses,
vertical synchronization serrations, post-equalizing pulses,
and blank lines. Figure 5 should clear all that up for you.
There are at least 20 vertical synchronization lines in
each 262.5 line field, which leaves 242.5 active video lines
per field, or 485 active video lines per frame. To be honest,
What about horizontal resolution? Some quick public
math: For a 29. 97 Hz frame rate and 525 vertical lines, we
have about 63. 56 µs per line. However, approximately 10. 9
µs of each line consists of the horizontal synchronization
signal, leaving 52. 66 µs of active video. To generate 640
horizontal pixels in that time, we would need a 12. 15 MHz
pixel clock. To keep the math easy, we round that up to
12. 5 MHz and black out the extra pixels. Figure 6 shows
the profile of an active video line with the appropriate
synchronization signals.
Alright! We squeaked a 640x480 resolution out of
NTSC! Well, sort of. I know, you’re getting sick of the curve
balls. Although it’s true we can output that many active
pixels, the unfortunate reality of NTSC video is that there
are areas of a picture that may or may not be shown. The
two basic restrictions are the action-safe area and the title-safe area. Long story short, we will use action-safe because
you should be able to see that region on most modern TVs.
Figure 5. Vertical synchronization lines
(SMPTE 170M-2004).
72 September/October 2018