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The Eye of the Camcorder: Understanding the CCD

The Eye of the Camcorder: Understanding the CCD

The CCD chip, how does it work? We take an inside look at the technology.

The Charge Coupled Device has revolutionized the video camera. Because of its impact, videographers enjoy clean, crisp footage like never before.

Hidden deep inside your camcorder is a tiny sliver of silicon with the most important job of any component in the video world. We call this little wonder the Charge Coupled Device (or CCD), and its crucial role is to convert the light that forms visible images into an electrical signal. Put simply, the CCD is the eye of your camcorder--without it, your camcorder would be nothing more than a very expensive audio recorder.

Robots manufacture CCDs on large wafers of silicon in rooms cleaner than required for brain surgery. Understanding the details of making CCDs requires extensive study in electronics, transistor theory and several related fields. Grasping the principles behind how a CCD works requires nothing more than an interest in the topic. In this article, well concentrate on the basics of how CCDs function, with an eye toward those areas that actually affect the way you make video.

How it works
Despite its technological complexity, the CCD is really nothing more than a microscopic grid packed with hundreds of thousands of light sensors. Like the tiny photo-sensitive device that tells a night light when morning has arrived, the CCDs individual sensors (or "pixels") respond to the quantity of light hitting them. More light equals a stronger electrical charge in each pixel; less light makes for a weaker charge. The CCD pixel doesnt care what color the light is--it only responds to how much light falls on it.

The camcorders recording electronics read the charge value for each pixel every 1/60th of a second, combining all those thousands of charges into an image and laying it onto tape. Just after reading the light values from the pixels, the camcorder drains off the charges so the CCD can begin storing up a new image. Regardless of the camcorders actual shutter speed (more on this later), the camcorder keeps putting new images on tape at a rate of 60 per second.

Instead of reading individual pixel charges directly, the CCD transfers out a whole row of charges to the edge of the chip at the same time, bucket-brigade style. When one pixel gets oversaturated with light, its excessive charge spills down the chip and fouls up all the readings for its row. This characteristic of the CCD explains the vertical streaks of light you see when pointing your camcorder at a small, bright object.

You may be wondering how its possible for the CCD, which is packed with color-blind sensors, to make a color image. Thats a good question, one that has several different answers. In camcorders with a single CCD (these are most common by far), something called a mosaic color filter sits on top of the CCD. This covers the CCD with an alternating patchwork of colored lenses, usually cyan, magenta and yellow. Each pixel sits directly beneath a colored lens, and only responds to that specific color.

By keeping track of which pixels are responding to each color, the camcorder can derive a color signal from the CCD. Because a given color covers only a fraction of the pixels, the resulting color signal has a much lower resolution than the luminance (or brightness) signal. Our eyes are less sensitive to color detail than brightness detail, so the lower color resolution isnt really a problem. The other common way to discern color is to use multiple CCDs, an approach well discuss later in the article.

Resolution and Size
The size of the CCD sensor has numerous impacts on the operation of a camcorder. Low-light sensitivity, for example, depends largely on how efficient the CCDs tiny pixels are at gathering light. As with anything collecting energy, the size of the pixel makes a big difference in efficiency. Larger pixels gather more light, because they have a greater surface area.

Yet the trend in recent years has been towards smaller and smaller CCDs. 1/2-inch CCDs were common just a few years ago; these led to 1/3-inch chips and, finally, the increasingly common 1/4-inch CCD. Why this push for smaller CCDs? Primarily because smaller CCDs require smaller (and less expensive) lenses, and smaller lenses ultimately translate to cheaper and more compact camcorders. The sales trends are undeniable--people prefer smaller camcorders.

Manufacturers have several tricks in their bag to increase the sensitivity of the ever-shrinking CCD. The most common is to use a micro-lens on each pixel. This tiny glass canopy sits above the pixel and gathers light that would have otherwise missed the pixel. Sensitivity improves, allowing a 1/4-inch CCD to deliver low-light performance on-par with that of a 1/3-inch design.

Resolution is another performance characteristic affected by sensor size--this term refers to the amount of detail a recording system can faithfully capture and reproduce. In the case of the CCD, resolution is in direct proportion to the number of pixels packed on the surface of the sensor. More pixels mean more detail from the "cam" part of the camcorder.

As CCDs have shrunk, the demand for higher-resolution sensors has only increased. DV camcorders, for example, record more detail to tape than any consumer format in history, and require more densely populated CCDs. Manufacturers have accepted the challenge, turning out small CCDs with close to half a million pixels.

Strength in Numbers
As mentioned earlier, the mosaic color filter is just one way to coax color information out of a monochrome CCD. The other method involves splitting the light into its various color components and sending each part of the spectrum to a different CCD. Three-chip camcorders use this approach, dedicating a single CCD to a given color range.

Slicing the visible spectrum into three equal parts requires some pretty sophisticated optics. One common method uses a special prism block to divide the light and send it in three different directions. Three-chip camcorders from Sony and Panasonic use this technology. Another approach uses dichroic mirrors, which bounce only certain colors of light while letting the others pass through. Passing light through a series of such mirrors allows the camcorder to divide the spectrum into thirds.

There are several advantages to three-chip cameras, the first being color accuracy. Because the three CCDs each "specialize" in a given color, the resulting image is much more true-to-life than when picked up by a single CCD. Color resolution also gets a boost, as each chip devotes all its pixels to a given color. This produces a red value from each pixel, for example, instead of from every third or fourth pixel as with a mosaic color filter.

There are some drawbacks to three-CCD camcorders. Because the light has to bounce through a prism block or labyrinth of mirrors before hitting a sensor, not all of it reaches its destination. A little bit of light is lost in the process, making some three-chip designs perform more poorly in low light than their single-chip brethren. Three-chip designs are also more complex than single-chip camcorders, increasing their price considerably. Adding an additional two sensors and prism or mirror block to the camera section also increases the size and weight of the camcorder. Though they can be quite compact, three-chip camcorders will never be as small as single-chip designs.

Speedy the Shutter
In a still camera, a mechanism called a "shutter" opens for a specified time to expose the film to light. A camcorder has no such apparatus between lens and sensor, but its CCD simulates the effects of a shutter electronically. Like a still camera, most camcorders offer a range of shutter speeds with various effects on the image.

When the camcorders record electronics read--then drain--the signal from the CCD, the pixels immediately begin building up charges to create a new image. With a standard shutter speed, the CCD has the full interval between record cycles (1/60th of a second) to build up a charge. At higher shutter speeds, the drain cycle forces the CCD to start from a clean slate at a later point--this gives the CCD less time to build up a charge. At a shutter speed of 1/120th of a second, the camcorder drains the CCD at the halfway point between record cycles. This gives the chip just 1/120th of a second to build up a new image. At a shutter speed of 1/10,000 of a second, the CCD charges are drained a scant 0.1 milliseconds before the image is read off the chip.

High shutter speeds offer the benefit of freezing even the fastest motion, but this capability comes at a price. Because the CCD has far less time to build up a charge, it requires more light to create an image. A shutter speed of 1/10,000th of a second exposes the CCD to light for just 1/166th as long as the normal shutter speed. As anyone who has tried to shoot at a high shutter speed indoors will attest, faster shutter settings require a great deal more light.

Some camcorders offer slow shutter speeds as well, usually including 1/30th, 1/15th, 1/12th and 1/8th of a second. As with high-speed shutter, the camcorder is still dutifully recording an image to tape every 60th of a second in these modes. Where low-speed shutter differs is in the way the camcorder drains the CCDs charges after each record cycle. Instead of completely purging the chip (as with the normal shutter speed), low-speed shutter only drains away a portion of the chips charges. This lets the image build up over a longer period, with pixel charges spilling over from one record cycle to the next.

In contrast to the crisp images of high shutter speeds, low-speed shutter gives motion a blurry, dreamy look. Because the chip has longer than 1/60th of a second to build up an image, slow shutter speeds make it possible to shoot in much lower light. At 1/8th of a second or slower, you can get recognizable images from moonlight or dim candlelight. Anything moving in the frame will give the effect away, but slow shutter speeds can be quite effective for stationary subjects bathed in low light.

Eye to Eye
The CCD is the very eye of your camcorder. Understand how it performs its image-gathering magic, and your videos will be more pleasing to the eye that really matters--the human eye.


Zoom Steady
Digital zoom and electronic image stabilization (or EIS) are two increasingly popular camcorder features intimately tied to the resolution of the CCD. Both discard the outer edges of the CCD image, enlarging a smaller portion to fill the screen. Since this forces the camcorder to make an image from fewer pixels (effectively throwing away some resolution), the pixel count of the CCD determines the resulting image quality.

Electronic image stabilization crops in on roughly 90 percent of the CCD image, expanding it to fit the screen. If the camcorder moves due to camera shake, the EIS circuit changes the inner portion of the pixels sampled to counteract the movement. When the active portion of the CCD reaches the edge of the sensor, EIS cant compensate for any further movement. The functions of EIS are easier to see than to explain.

With some camcorders, enabling EIS causes a noticeable drop in resolution thanks to the reduced number of pixels used to form the image. Manufacturers have compensated for this by equipping certain camcorders with CCDs that have pixels to spare. These ultra-high-resolution CCDs provide ample detail even when discarding 10 or 15 percent of their pixels for EIS. When you enable EIS with such a camcorder, theres no discernible change in image quality.

Digital zoom enlarges smaller and smaller portions of the CCD to simulate tighter focal lengths. At extremely high digital zoom settings (100x, for example) the image may consist of just a few thousand pixels instead of 400,000. The result is a coarse, blocky look, much like that of a mosaic special effect. CCDs with very high pixels counts will offer better digital zoom quality, but every chip has its limits. For the best results from a digital zoom, use it in moderation. If you cant afford any reduction in image resolution, dont use it at all.

Tags:  October 1998
Loren
Alldrin
Thu, 10/01/1998 - 12:00am