Ask The Guru

Mar 1, 2000

ASK THE GURU: 3CCD cameras vs. 1 CCD cameras

Q: What is it that makes a 3CCD camera video camera better than a 1CCD camera???

A: A CCD can't detect color; only brightness of the light striking it. So, to measure color you have to put a color filter in front of the CCD. There are basically three ways to do this:

Three CCDs:
Split the light into 3 beams and use 3 CCDs, one each with a Red, Green, Blue filter in front of it.
Pro: Much more accurate color definition, particularly where there are sudden and dramatic changes in color within the scene (at edges of objects for example, but even across the entire scene.) Interestingly, since compression algorithms tend to introduce artifacts at edges, I wonder how much the DV compression algorithm negates the advantages of the 3 chip design? (I have no idea, just wondering.)
Con: Prism and CCDs add cost, and complexity, and reduce light sensitivity.

One CCD, Multiple passes:
Use one CCD and take three pictures, putting a different color filter in front of the CCD for each picture. This technique is used for some expensive digital studio cameras.
Pro: Less cost and complexity than a 3 chip camera, but
Con: Can't be used for video, and even in studio settings only really works for still objects.

Once CCD, Patterned filter
Place a pattern of filters over the pixels in a single CCD. Each photosite in the CCD will be covered by a single red, green or blue filter. By arranging these filters in a pattern it is possible to estimate the color of a pixel by combining the color measurements of the surrounding pixels. The pattern most often used is G-R-G-B, called the Bayer pattern after Bryce Byer of Kodak. It is biased toward producing accurate green colors because the eye is more attuned to Green than other colors. Half the photosites measure Green, while a quarter measure Red, and the other quarter measure Blue. Other patterns are sometimes used, and Canon has developed a filter pattern of cyan, yellow, green and magenta (for their still digital cameras).
Pro: Less cost and can be used in video and for fast moving subjects
Con: Color is being estimated here, so there will be differences between a single CCD camera and a three chip camera, again, particularly were color is changing rapidly in the frame (I'm not talking about changes over a sequence of frames.)

Most still digital cameras use a single CCD. For still pictures, resolution-I think-is as much a concern as the accuracy of color. Since video cameras really only need a comparatively small image (compared to the 3 million pixel CCDs appearing now in still cameras) it would seem logical that manufacturers could use a large CCD, then resample the image down to the video size, improving the accuracy of the image further. Perhaps producing an image as good as a three chip camera. This may or may not be possible at the moment; I don't know whether the extra processing necessary to do this is a problem.

Does anyone know how Sony's new DV camera with the large chip creates it's video image? Does it just throw away every other line of the captured image to create the smaller video frame?

But back to the question: does a one chip versus 3 chip CCD camera really make a difference? It seems to, but then it might be hard for us commoners to tell. Try and find a three chip and a one chip camera with exactly the same optics, electronics and CCDs. Then you could compare the images and see the difference. Unfortunately, it's not that easy because most cameras differ in more ways than just the number of CCDs.

I believe that with any feature it's important not to simply believe the marketing hype. Look at the actual cameras and compare them, then make your decision. If you think the image produced by the camera you have is great, what do you care if it has one chip or three? But I also believe it's true that 3 chip cameras do produce an image that's more accurate than similar one chip cameras. But then I have a TRV-900 so I have to believe that...

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