[Opcom]

Two hypothetical progressive scan cases where the horizontal pixel resolution and the number of H lines in a complete image on the CRT is the same as with an interlaced system:

If the 525 H lines are scanned progressively 60 times a second, the term "field rate" would have no meaning except as a V deflection rate and the frame rate would double from 30Hz to 60Hz, the H freq. would double from about 15.75KHz to about 31.5 KHz, and the required pixel rate or pixel clock, a form of bandwidth representation, would also double if the horizontal resolution were to remain the same. One solution would be to use more RF bandwidth.

The above would be similar to the non-interlaced 640x480 VGA 'standard' running about 60Hz V and 31.5KHz H.

If the 525 H lines are scanned progressively 30 times a second, the term "field rate" would have no meaning except as a V deflection rate and the frame rate would be the same at 30Hz, the H freq. would remain the same at about 15.75KHz, and the required pixel rate or pixel clock, a form of bandwidth representation, for the same H resolution, would remain the same.

There would likely be an annoying flicker due to the 30Hz vertical rate. It would be like a 24 frame film theater except that a CRT display is a bit brighter so the effect would be more pronounced, but offset by a 30Hz V rate. One solution would be to use be longer phosphors.


Therefore the interlacing scheme represents a compromise solution taking into account the relationship between these two factors:
1.) the pixel clock frequency
and
2.) the refresh rate for a given volume of pixels.

In most analog video systems, MHz equals pixels per interval.
resolution = pixels / time
bandwidth = information / time

The NTSC scheme interlaces half the image every 1/60 second. it takes 1/30 second to present the information. The first case above presents all of the information in 1/60 second. the second case presents the information in 1/30 second but in a progressive manner.

The author is correct in what he seems to have meant, but not in what was said. He may have not explained it completely or properly. It is possible that the clarity of his statement relies upon information presented elsewhere in the volume.

If what I have said is wrong, I am willing to consider rebuttals or corrections.