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IP: The Appearance of TV Displays
From: Dave Farber <farber () cis upenn edu>
Date: Mon, 31 Aug 1998 08:32:09 -0700
Bill is an EXPERT on this game djf From: wfs () image mit edu (William Schreiber) The Appearance of TV Displays In the course of the Advanced Television Research Program at MIT, which I headed from 1983 until I retired in 1990, Russ Neuman, now at U Penn, directed an audience study at the Liberty Tree Mall in Danvers, Mass. Viewers were enticed out of the Mall with a small gift certificate and asked to look at TV and listen to audio and answer some questions. The operation was completely anonymous; the viewers did not know it was MIT and were not told what they were observing. Among other things, we compared "studio quality" 1125 I to NTSC with a variety of subject matter. (There was only a slight preference for HDTV and little willingness to spend much for it, but this is not the point I am trying to make here.) Even with very careful professional attention, it was very hard to get the picture contrast, brightness, and color close enough on the two displays so that an unbiased judgment could be made as to quality as dependent only on resolution and aspect ratio. (The two pictures, when compared side-by-side, were the same height but different widths.) I had the feeling that turning any knob that affected picture appearance on either display could have made either picture look better than the other. Note that we controlled for viewing distance and for many personal characteristics of the viewers. We also showed the pictures one by one and asked for a comparision with viewers' TVs at home. One conclusion from this part of the study was that picture quality at home was controlled primarily by analog channel impairments, not scanning standards or bandwidth. Everybody thought that the pictures we showed were much better than they had at home. Of all the quality factors studied, the most important was picture size. Most viewers preferred larger pictures even when they said the smaller picture was sharper. Temporal Aliasing Another part of the ATRP study was motion rendition. In a few days, I shall post citations to two PhD theses on this subject, which may be ordered from the MIT library. I have no doubt that, at typical motion speed, temporal aliasing is always present at any reasonable frame rate. Cinematographers know this and adjust camera technique appropriately. Video people, who are really using 60 fps in NTSC, can move the camera much faster without causing much damage to the picture. In work by Ed Krause, now at Imedia in SF, a demo was made testing various ways to render motion. One was a computer-generated "bouncing ball" demo, in which 3/2 conversion to NTSC was compared with motion-compensated interpolation. The latter won, hands down. In another, video was made of a close-up of a wristwatch, causing it to move horizontally with sinusoidally varying velocity. We compared frame repetition, motion-blur, and motion compensation. Again, the latter won easily. Motion blur did not appreciably reduced the motion judder of frame repetition. Note that when the eye is tracking motion on the TV screen, any attempt to reduce judder by blurring greatly reduces perceived sharpness. Some video is deliberately shot with very short exposure time per field just to retain sharpness of moving objects. ("You can see the stitching on the baseball!") Krause also converted both 24-fps and 12-fps simulated film to NTSC using motion compensation. The results were very good, altho the latter was not quite as good as the former, which was essentially perfect. Note that if motion compensated interpolation is to be used at the receiver without transmission of motion vectors, then the picture should not be blurred at all before transmission. In another thesis by Dennis Martinez, motion-compensated interpolation was used to convert any frame rate to any other frame rate. The subject was a closeup of a person's face while speaking. (The audio was also successfully converted with duration changes of 20% or so.) The results were essentially perfect with large changes in frame rate and corresponding changes in the speed of motion.
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