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"Interlaced" is the next portion of the definition of NTSC Video.

When television first was invented the only technology able to display video signals was the CRT, AKA Cathode Ray Tube. Early broadcast engineers needed to design a transmission and reception system that would satisfy several specific criteria:

1. It must function in harmony with our current supply system, which is 60Hz;
2. That would be economic to transmit;
3. Could produce an acceptable image on a CRT television without severe flicker.

However, with the CRT, problems surfaced, even at this frame rate. The first problem noted was that 24 to 30 frames per second weren't fast enough for the phosphor-based CRT to eliminate flicker. Phosphor isn't an instant on and off system like film running over a lamp on a film projector.

Engineers examined this problem and determined that since our electrical cycle is 60 cycles per second, video should also be 60 cycles, or in this case, frames per second. However, this decision created a yet another problem: bandwidth. During the early CRT era, 480 lines of resolution at 60 frames per second simply took up too much transmission bandwidth.

To remedy this, engineers decided to split each frame (or image) into two images called fields. The first field or — one half of a frame — would contain all the odd numbered scan lines from the image and the second field would contain all the even numbered scan lines.

To be more specific, the electron beam of a CRT starts at the top left where it scans the odd field then returns to scan the even field as shown in Figure 1.

Figure 1 - How a CRT draws an interlaced image

Figure 2 shows the odd and even fields of the actual images during the same process.

Figure 2. Odd and even fields of a single frame.

As time rolled on so did technology. By the advent of the modern computer, a new system of showing an image on a CRT was needed. Modern high resolution graphics were required to be shown at a faster rate to eliminate flickering. Hence, progressive scan displays were born.

A progressive scan display device draws the entire image at once instead of splitting the image into two fields. In this way a progressive scan image is shown at 60 Frames per second (60Fps) instead of the 60 fields per second (60fps) of standard NTSC video. We will go into greater depth about interlaced and progressive scan signals later as we cover DVD and HDTV.

It's important to note that NTSC video is often referred to by several different terms and specs. The most common are as follows:

1. 480i - Meaning 480 visible lines of resolution, where the i refers to interlaced format;
2. 525i - Meaning 525 total lines of resolution (only 480 lines visible) and again the i referring to interlaced format ;
3. Standard definition television - Meaning not High definition television and also not enhanced definition television

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