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Author Topic: SCART Switch Stephen Furley Film God Posts: 3059 From: Coulsdon, Croydon, England Registered: May 2002 posted 05-24-2006 05:14 PM                     quote: Joe Redifer Component does not transmit green at all, and it does not compress blue or red to my knowledge. RGB uses the same bandwidth for all three signals, but Y,Pr,Pb uses reduced bandwidth for the colour difference signals, so the total babdwidth required for the signal is less. Also, of course, the Y and colour difference signals are a necessary intermediate step in encoding to just about any of the composite formats, PAL, NTSC, SECAM, FAM and various oher experimental ones which never caught on. It would be possible in theory to use any two colour difference signals; I believe that the reason that green is not used in practice is that it is the smallest signal, and therefore the most vulnerable to noise. You can do all sorts of odd things with colours; some of the early single Vidicon tube colour cameras used a stripe filter which was not red, green and blue, as might be expected, but white, green and cyan. the green stripes provided the green signal, cyan-green gave the blue, nd white-cyan gave the red. The reason for doing it this way was that the stripe filter transmitted roughly twich as much light as a RGB one would have done.  |  IP: Logged Paul Mayer Oh get out of it Melvin, before it pulls you under! Posts: 3836 From: Albuquerque, NM Registered: Feb 2000 posted 05-24-2006 05:54 PM                        [Edit: Oops, doubled with Stephen but I'll leave it anyway.] I've never looked closely but yeah, there should be no visible difference between an image maintained in RGB through the entire signal path versus the same image in component, at least when using real-world sources. I'm sure there are some CG test images out there capable of revealing a difference. Component does indeed take advantage of reduced bandwidth for the two color-difference signals though. That's why it was created - to save bandwidth over RGB. Many years ago the NTSC did studies which showed that the human eye sees fine detail only in B&W. That's what lead to the component system - transmit the luminance and detail using full bandwidth just once; the color can get away with half bandwidth since the eye will never notice the lack of detail in color. In a way, video images are kinda like comic book or graphic novel drawings - detail is done with great effort using a fine black ink pen while the color can be applied using a brush.  |  IP: Logged Joe Redifer You need a beating today Posts: 12859 From: Denver, Colorado Registered: May 99 posted 05-24-2006 07:26 PM                        Green is not used in component because it takes up far more bandwidth than red or blue. The "color difference" takes up less bandwidth than the actual color as well, since it is just the difference. Green is determined by the display. Green is REALLY important. Check out the following uncompressed examples I made: Here is the full uncompressed picture (sized down for the forum) I took of Red Rocks. This picture has never been compressed. Size = 399KB The same picture, but the BLUE element has been reduced to 1/4 of its original size, and the green and red remain full size. Still looks fairly good, eh? Size = 352KB Now the RED element has been reduced to 1/4 of its original size, but blue and green are both still full resolution. Looks a bit worse, but still almost acceptable to most people. Size = 337KB Here the GREEN has been reduced to 1/4 its original size, but both the red and blue remain full resolution. Look how crappy it looks! The eye really depends on the color green. Size = 344KB Just for fun I made this picture where the green is at full res, but the red is 1/3 of its original size and the blue is a miniscule 1/8 of its original size! Looks like crap but it STILL looks better than when only the green is compressed by a factor of four. Size = 221KB. Moral of the story: Your eyes suck at blue.  |  IP: Logged Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 05-24-2006 10:00 PM                        With component you actually do get the full color bandwidth (that's why it's better than Y/C (S-Video) and composite). Sets recover green because having Y, R-Y, B-Y and knowing that Y = 0.30R + 0.59G + 0.11B, one can then manipulate R-Y, B-Y to arrive at: G-Y = -0.508(R-Y) - 0.186(B-Y) However, NTSC composite and Y/C don't actually use R-Y and B-Y directly, but rather I and Q which are linear combinations of the former: I = 0.74(R-Y) - 0.27(B-Y) Q = 0.48(R-Y) + 0.41(B-Y) This gives an I which ranges from red to cyan, and a Q that ranges from magenta to green, with I having 3 MHz of bandwidth and Q having 1 MHz (so the difference in bandwidth isn't simple red/blue/green). The benefit of Y/C (S-Video) is circumventing the set's Y/C separator which separates luma from chroma in the composite signal for further processing. Normally in a composite signal the luma and chroma bands overlap, causing dot-crawl if the chroma isn't properly filtered out of the luma or loss of detail if they're just simply band-limited to separate them. The comb filter is the better, more expensive, more complex solution. And then the benefit of component is circumventing the color decoder altogether. As in the equations above, YPrPb is a linear combination of RGB and they can be converted to the other with simple analog circuits, no bandwidth cuts necessary.  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 05-25-2006 08:50 AM                        A "rule of thumb" is that we get 60% of our visual information in the green, 30% in the red, and only 10% in the blue portions of the spectrum. That's why in color print film, the magenta dye layer is the topmost imaging layer (modulates green light), and the yellow dye layer (modulates blue light)is at the bottom. Also why in a "Bayer" filter array in a digital camera, you have more green sensitive pixels: Patent for "Bayer Filter" Used in Digital Cameras quote: United States Patent 3,971,065 Bayer July 20, 1976 Inventors: Bayer; Bryce E. (Rochester, NY) Assignee: Eastman Kodak Company (Rochester, NY)  |  IP: Logged

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