Nuts and Volts - September/October 2018

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.