04-21-2007, 09:08 PM
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Doin fine
Industry Role:
Join Date: Oct 2005
Posts: 24,983
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Quote:
Originally Posted by WarChild
Looks to me like he said NTSC has to be 29.95 frames per second. He never said HD couldn't be 29.95 frames per second. It was you that said NTSC having to be 29.95 was "categorically false".
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Yup and I am categorically correct.
Quote:
Interlacing
Above I stated that the frame rate of NTSC was 29.97 fps. Well, in a way, that was a lie. In fact, there is no such thing as a frame in NTSC video. NTSC is made up of fields, not frames. A field is made up of every other horizontal scanline. One field contains all the odd numbered scanlines, then the next field will contain all the even numbered scanlines. A frame is actually just two consecutive fields viewed at the same time. This process is called interlacing.
Therefore, NTSC actually runs at 59.94 fields per second (again, this figure is often simplified as 60 fields per second). A common misconception is that frames are recorded and then split into fields, while in fact it is fields that are recorded and then combined into frames. This is important because it means that a still NTSC frame is not actually "still." There is a temporal difference of 1/60th of a second between fields, so in fact half of the frame's scanlines were recorded 1/60th of a second after the other half. Most of the times, this time difference is so small as to not be noticeable so the frame looks like a still. However, in some cases with very fast action or when very fine details appear in one field but not another, artifacts of the interlacing can be seen.
Some digital cameras now offer progressive scan, in which the camera records all the scanlines in a frame sequentially, creating entire frames rather than interlaced fields. This is helpful if the video is meant to be viewed on a computer or a projector, both of which display video in a similar progressive fashion and on whom interlacing can be even more noticeable. However, all analog TVs display fields, meaning that even video recorded progressively will be interlaced.
In case you are curious, the reason NTSC is interlaced in the first place is that when the standard was agreed upon way back in the 50's, the phosphers used on television screens were not good at maintaining their brightness. If the image was shown progressively, by the time the scanlines at the bottom of a frame were being drawn, the top of the image had already begun to fade. When displayed at 30fps, this created a flickering image that was unwatchable. By only redrawing half the scanlines at a time, the image could be refreshed before it had begun to fade.
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