CCDs: Capturing the Image

How often do you simply pick up a camcorder, turn it on, press the button, shoot video, then take the tape home and watch it, all without giving a moment’s thought to how it all works?

There are far too many technological wonders in our world for us to stop and wonder about the underlying science of each one. Nonetheless, it is a good idea to get some kind of basic understanding, if for no other reason than to make informed purchasing decisions. When shooting video, for example, it’s a good idea to know something about how the camcorder’s image-capturing chip, the CCD, turns focused light into an electronic signal.

In this article, we’ll take a brief look at CCD technology. We’ll look at how CCDs work, why some camcorders have more than one CCD, and which of a camcorder’s many systems rely on the CCD to operate.

What it Is

It isn’t difficult to understand how a film camera works: the shutter opens for a specific amount of time, allowing the camera lens to focus light onto the film, and the chemicals on the film react to the varying intensity and hue of the light, creating an image. A video camera, on the other hand, is a little harder for most of us to comprehend. While the principles are the same-light, focused by a lens, creates an image-the science of camcorders is completely different. Instead of film, a camcorder focuses light onto a tiny silicon chip, called a charge-coupled device (CCD), which then creates an electronic record of a moving image at 30 frames per second.

Each CCD has a certain number of picture elements (i.e. pixels), or tiny light-sensitive dots, that convert light photons into electrons. This process is a result of the photoelectric effect, which was first accurately explained by Albert Einstein. The details of his 1905 Nobel Prize-winning work are beyond the scope of this article, however. The camcorder takes these electrons and records them as a digital signal onto tape.

Size Sometimes Matters

In crude terms, it can be said that the more pixels the CCD has, the better it does its job of re-creating a detailed image of the world. You might think, logically enough, that a larger CCD would be better, since it can potentially have more pixels and thus more resolution. Typically, CCDs on home camcorders are a mere 1/4-inch in size or even smaller. Professional camcorders often have larger CCDs of 1/3-, 1/2- or even 2/3- of an inch. But bigger does not necessarily mean better. First, there is an upper limit to the resolution of the video recorded to tape. For NTSC Mini DV, it could be argued that 350,000 or so pixels (720×480) is sufficient to get a good image and anything more is overkill. Second, pixel density is important. Would you rather have a 1/2-inch CCD with 270,000 pixels or a 1/4-inch CCD with 680,000?

Size, however, does come into play. With all those pixels crammed into such a tiny space on a 1/6-inch CCD, the information from each pixel tends to get mixed up with that of the pixels around it (a concept in electronics called crosstalk). With a little more room, each pixel has the breathing space to keep its signal separate from the others. More importantly, larger CCDs mean a larger imaging plane, which roughly means that a camera can have larger (and better) optics, which translates into significantly better low-light performance. A larger imaging plane also means that the entire optical system can have a much narrower depth of field, which is artistically wonderful. This is one important difference between film cameras and video: consumer CCDs are about 5-6mm in size, while filmmakers typically work with a 35mm imaging plane.

Finally, the quality of the technology is important. Larger CCDs from the early 1990s are most likely not as good as the latest chips from 2003.

Effective Pixels

What can the consumer do to sort this all out? While pixel count is not the whole picture, it is a good starting point. The next time you look at a Videomaker camcorder buyer’s guide grid, look at the specs for the CCD. Note that some camcorders have more pixels than others. Theoretically, 350,000 pixels should be enough for Mini DV. It is and it isn’t. First, each CCD pixel does not translate in a one-to-one relationship to a pixel on your television screen. Second, CCDs are wildly complex to manufacture, and each one has a certain percent of pixels that are broken. For expensive medical and scientific-grade CCDs, that number is very small. The CCDs in a camcorder have a greater error tolerance.

Raw pixel counts in this age of megapixels do not tell the whole story. One number (that, unfortunately, some manufacturers provide and others don’t) is the Gross Pixel count and a smaller Video Effective pixel count. The Video Effective number is much more useful to us. The extra pixels in a megapixel CCD are not wasted, however, and are used for electronic image stabilization and digital still imaging.

Stabilize It

Most consumer camcorders are small handheld devices that have image stabilization technology to compensate for the motion that is inevitable with a handheld shot. As your hand wobbles in one direction, the camcorder detects the motion and moves the image equally in the opposite direction, resulting in a steady picture.

Economical electronic image stabilization (EIS; sometimes called digital image stabilization or DIS) works by sampling a smaller area of the CCD, then using the CCD to detect motion and moving the image across the chip to compensate. Clearly, it’s much better to have more pixels available when electronic image stabilization is in effect. If there are enough pixels, there is no picture degradation whatsoever when you turn on the image stabilization. If your camcorder’s CCD only has something in the realm of 210,000 pixels, you’ll probably see a noticeable drop in the quality of your picture when you turn on the EIS and the framing in your shot will crop in slightly.

Megapixels

While television technology (standard and high definition) limits the useful resolution of video, digital still pictures have no such limitation. This is why camcorder CCDs tend to have fewer pixels than the CCDs of modern digital still cameras. It’s also another reason why a camcorder with digital still picture capabilities might benefit from having a CCD with a high pixel count.

Digital still cameras have megapixel, or millions of pixel, CCDs. Inexpensive digital still cameras commonly have the ability to create pictures of three megapixels (2048×1536) or more. Contrast this with a DV or Digital8 camcorder, which only requires a maximum of 720×480 resolution (scarcely a megapixel) to make a video image. Early digital camcorders that included a still camera feature could not compete with the inexpensive digital still cameras on the market at the time because their CCDs simply did not have enough pixels. Today’s digital camcorders often include CCDs with enough pixels to shoot good video and take good still images.

Three Chips are Better than One

Many high-end professional and prosumer camcorders make use of three CCD chips instead of one to create a sharp video image that’s rich in color and detail. The reason three chips are better than one is due to the technology that’s required to create a color image from a single CCD. It’s a tricky bit of technology to separate the color information on a single CCD, one that can result in washed out colors that bleed outside the lines like a first-grader’s crayon scribbles in a coloring book.

Three-chip cameras solve this problem by devoting one CCD to each primary color in the additive color spectrum: one for red, one for blue and one for green. Before light strikes the CCDs, it passes through a prism, which divides the light into its component colors. The results are crisp, clean, vibrant colors that seldom bleed. If you’ve ever shot video with a three-chip digital camera, you’ll know what we mean when we say the results can be quite astonishing.

Chip Away

Now that you have a basic understanding of CCD technology, you can make more informed purchases the next time you’re out shopping for a camcorder. It’s not all about size and raw pixels, but you can still glean valuable information from marketing materials and manufacturer reported specifications. Even if you already own the camcorder of your dreams, knowledge of how the CCD works may help you decide when to turn on the image stabilization, and when to leave it off and use a tripod instead.

[Sidebar: OIS]
Optical image stabilization (OIS) is another type of stabilization and uses optical technology to compensate for shaky camera movements. Instead of electronically moving the image, optical image stabilization uses clever lens technology (a movable prism, to be exact) to shift the image. This is a more expensive approach to the problem, but it is very effective at stabilizing the image without losing picture quality.

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