CMOS vs CCD (page 2)

The Color-Coordinated Camcorder

Image sensors only detect and record the intensity of the light hitting them, recording the grayscale image; they don't actually record in color themselves. In a single-chip camcorder design, a color filter array sits on top of the image sensor. Somewhere between the surface upon which light hits the image sensor and the encoded output of the image processor, the color is extracted in accordance with the pattern of the tiny color filters. The data stream passes through the codec utilized by the camcorder (generally DV, MPEG-2 or AVCHD) to be compressed before it's recorded to whatever media the camcorder is using, be it tape, a hard drive, a disc, a memory card, a network connection, etc.

In the case of higher-end camcorders with multiple image sensors, each sensor captures a single primary light color via a prism inside the camera. The prism splits the light entering the camcorder's lens into red, green and blue and then sends those colors off to the matching image sensor, where the intensity of each color is encoded. The need for the three individual sensors, one for each primary color, is yet another reason why 3-chip pro cameras are more expensive than consumer-grade cameras.

Regardless of whether you have a small CMOS-based camcorder with a single image sensor, or a 3-CCD pro unit with a separate and complex high-end image processor, these chips do a whole lot of work that used to be done by many mechanical parts and a lot of chemicals in film-camera days. But they are not without their own problems.

Rolling Shutter, Smearing CCD

No, that's not the name of a rollicking martial arts film, but it does highlight some of the trade-offs of each technology.

To build on our analogy at the beginning of the article, shooting directly into overexposed bright lights with a CCD camera can result in smearing-which often manifests itself as glaringly obvious vertical streaks, very different from the way that lights bloom when photographed using film.

Most CMOS cameras use a rolling electronic shutter to capture an image sequentially in thin rows from top to bottom in the course of a single frame. This is in contrast to the more traditional global shutter of the CCD camera, where the sensor captures an image in its entirety.

The CMOS camera's rolling shutter can impose a different type of distortion: skew. This can cause distortion of the vertical lines in a shot during a quick pan. (Picture a whip pan shot of a New York skyscraper, for an extreme example.) But for those who primarily shoot weddings or news-gathering talking-heads footage, neither of these issues will occur very frequently. However, action-oriented videographers, take heed.

Choosing a Video Camera

Choosing between CMOS and CCD is only one element among many that must be considered when shopping for a new video camera. The size of the lens and whether or not the camcorder accepts interchangeable lenses for wide angle, telephoto, macro and other specialized applications is another. Is the camera equipped with XLR inputs for use with professional mics? Do those XLR inputs provide phantom power for clip-on lavalier mics? How flexible are the camcorder's controls, for those times when you don't wish to run the camera in automatic mode?

As with any professional equipment, from musical instruments to microscopes, poor technique will produce bad results, even if you are using the most expensive camcorder. Conversely, good technique can produce acceptable - sometimes even great - images on even the most inexpensive consumer cameras.

So should you choose CCD over CMOS? We asked Hahn Choi, who is one of the behind-the-camera talents at the streaming HD video web startup PJTV.com (where, in full disclosure, I am an occasional on-air talent). Choi said, "When it comes to quality, my personal opinion is that it doesn't make a difference, because of compression that's being used with most HD cameras. Whether it's HDV or AVCHD, it's still compression, and that'll affect the video quality. With the processing cameras do, too, it's tough to say one is better than the other." However, although many higher-end manufacturers, including Red Digital Cinema, are beginning to use CMOS chips, if you can afford it, 3-CCD is the way to go. This is especially true if your material is ultimately designed to make its way to broadcast TV.

But so much material is being shot these days for the web, for video aggregation sites such as YouTube and as blogs and big media websites go multimedia. If that's the case, given the factors Choi mentioned, a good CMOS camcorder will suffice for many purposes.

Ed Driscoll is a freelance journalist covering home theater and the media.