Untitled Document

How Camcorders Work
by Tom Harris

Table of Contents:
› Introduction to How Camcorders Work
› The Basics
› The CCD
› The Lens
› Formats

In most of the world, camcorders, or video camera-recorders, have been a familiar sight for nearly 20 years. People take them everywhere: to school plays, sports events, family reunions and even births! When you go to a popular tourist spot, you are surrounded by them. Camcorders have really taken hold in the United States, Japan and many other countries around the world because they are an extremely useful piece of technology that you can own for under $300 (or more than $100,000).
How can such a small device do so much? Particularly for anyone born before the 1980s, it's simply amazing that quality video cameras are now readily available as consumer items, and that they're so easy to use. In this article, we'll look inside these extremely popular devices to find out what exactly is going on. We'll explore traditional analog camcorders and also look at the technology used in digital camcorders.

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The Basics
A typical analog camcorder contains two basic parts:
1) A camera section, consisting of a CCD, lens and motors to handle the zoom, focus and aperture
2) A VCR section, in which a typical TV VCR is shrunk down to fit in a much smaller space.

The camera component's function is to receive visual information and interpret it as an electronic video signal. The VCR component is exactly like the VCR connected to your television: It receives an electronic video signal and records it on video tape as magnetic patterns (see How VCRs Work for details).
These two sections are easily seen in the following photos.

Camcorder with the outer shell removed

The camcorder's VCR unit

The camcorder's camera unit

The camera lens system

The camcorder's Charge Coupled Device (CCD)

The infrared autofocus mechanism

The motors that focus the camera lenses

A third component, the viewfinder, receives the video image as well, so you can see what you're shooting. Viewfinders are actually small, black-and-white or color televisions, but many modern camcorders also have larger full-color LCD screens. There are many formats for analog camcorders, and many extra features, but this is the basic design of most all of them. The main variable is what kind of storage tape they use.

Digital camcorders have all these same elements, but have an added component that takes the analog information the camera gathers and translates it to bytes of data. Instead of storing the video signal as a continuous track of magnetic patterns, it records the picture and sound as 1s and 0s. Digital camcorders are so popular because you can copy 1s and 0s very easily without losing any of the information you've recorded. Analog information, on the other hand, "fades" with each copy -- the copying process doesn't reproduce the original signal exactly. Video information in digital form can also be loaded onto computers, where you can edit it, copy it, e-mail it and manipulate it.

In the next section, we'll look at the heart of the camcorder, the semiconductor device that converts visual information into an electronic signal.

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The CCD
Like a film camera, a camcorder "sees" the world through lenses. In a film camera, the lenses serve to focus the light from a scene onto film treated with chemicals that have a controlled reaction to light. In this way, camera film records the scene in front of it: It picks up greater amounts of light from brighter parts of the scene, and lower amounts of light from darker parts of the scene. The lens in a camcorder also serves to focus light, but instead of focusing it onto film, it shines the light onto a small semiconductor image sensor. This sensor, a charge-coupled device (CCD), measures light with a half-inch (about 1 cm) panel of 300,000 to 500,000 tiny light-sensitive diodes called photosites.

CCD

Each photosite measures the amount of light (photons) that hits a particular point, and translates this information into electrons (electrical charges): A brighter image is represented by a higher electrical charge, and a darker image is represented by a lower electrical charge. Just as an artist sketches a scene by contrasting dark areas with light areas, a CCD creates a video picture by recording light intensity. During playback, this information directs the intensity of a television's electron beam as it passes over the screen.

Of course, measuring light intensity only gives us a black-and-white image. To create a color image, a camcorder has to detect not only the total light levels, but also the levels of each color of light. Since you can produce the full spectrum of colors by combining the three colors red, green and blue, a camcorder actually only needs to measure the levels of these three colors to be able to reproduce a full-color picture

In some high-end camcorders, a beam splitter separates a signal into three different versions of the same image -- one showing the level of red light, one showing the level of green light and one showing the level of blue light. Each of these images is captured by its own chip -- the chips operate as described above, but each measures the intensity of only one color of light. The camera then overlays these three images and the intensities of the different primary colors blend to produce a full-color image. A camcorder that uses this method is often referred to as a three-chip camcorder.

How the original (left) image is split in a beam splitter

This simple method produces a rich, high-resolution picture. CCDs are expensive and eat lots of power, however, so using three of them adds considerably to the manufacturing costs of a camcorder. Most camcorders get by with only one CCD by fitting permanent color filters to individual photosites. A certain percentage of photosites measures only levels of red light, another percentage measures only green light and the rest of the photosites measure only blue light. The color designations are spread out in a sort of grid (the Bayer filter below is a common configuration), so that the video camera computer can get a sense of the color levels in all parts of the screen. This method requires the computer to interpolate the true color of light arriving at each photosite by analyzing the information received by the other photosites in the vicinity. For a full explanation of this process, check out How Digital Cameras Work: Capturing Color.

If you've read How Digital Cameras Work, then all this has probably been familiar to you -- camcorders and digital still cameras both take pictures using CCDs. But since camcorders produce moving images, their CCDs have some additional pieces you won't find in digital camera CCDs. To create a video signal, a camcorder CCD must take many pictures every second, which the camera then combines to give the impression of movement.

If you've read How Television Works, you know that a television "paints" images in horizontal lines across a screen, starting at the top and working downward. TVs actually paint every other line in one pass (this is called a "field") and then paint the alternate lines in the next pass. To create a video signal, a camcorder captures a frame of video from the CCD and records it as the two fields. The CCD actually has another sensor layer behind the image sensor. For every field of video, the CCD transfers all the photosite charges to this second layer, which then transmits the electric charges at each photosite, one by one. In an analog camcorder, this signal goes to the VCR, which records the electric charges (along with color information) as a magnetic pattern on videotape. While the second layer is transmitting the video signal, the first layer has refreshed itself and is capturing another image.

A digital camcorder works in basically the same way, except that at this last stage an analog-to-digital converter samples the analog signal and turns the information into bytes of data (1s and 0s). The camcorder records these bytes on a storage medium, which could be, among other things, a tape, a hard disk or a DVD. Most of the digital camcorders on the market today actually use tapes (because they are less expensive), so they have a VCR component much like an analog camcorder's VCR. Instead of recording analog magnetic patterns, however, the tape head records binary code. Interlaced digital camcorders record each frame as two fields, just as analog camcorders do. Progressive digital camcorders record video as an entire still frame, which they then break up into two fields when you output the video as an analog signal. (To learn more about analog-to-digital picture conversion, check out How Digital Cameras Work: Digitizing Information and How Analog and Digital Recording Works.)

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The Lens
As mentioned previously, the first step in recording a video image is to focus light onto the CCD, using a lens.

To get a camera to record a clear picture of an object in front of it, you need to be able to adjust the focus of the lens -- that is, move the lens so it aims the light beams coming from that object precisely on the CCD. So, just like film cameras, camcorders let you move your lens in and out to focus light. Of course, most people need to move around with their camcorders, shooting many different things at different distances, and constantly refocusing is extremely difficult.

This is why all camcorders come with an autofocus device, usually an infrared beam that bounces off objects in the center of the frame and comes back to a sensor on the camcorder

Infrared autofocus mechanism

To find the distance to the object, the processor calculates how long it takes the beam to bounce and return, multiplies this time by the speed of light, and divides the product by two (because it traveled the distance twice -- to the object and back again). The camcorder has a small motor that moves the lens, focusing it on objects at this distance. This works pretty well most of the time, but sometimes you have to override it -- you may want to focus on something in the side of the frame, for example, but the autofocus is picking up what's right in front of the camcorder. To learn more about autofocus mechanisms, check out How Autofocus Cameras Work

Camcorders are also equipped with a zoom lens. In any sort of camera, you can magnify a scene by increasing the focal length of the lens (the distance between the lens and the film or CCD). An optical zoom lens is a single lens unit that lets you change this focal length, so you can move from one magnification to a closer magnification. A zoom range tells you the maximum and minimum magnification. To make the zoom function easier to use, most camcorders have an attached motor that adjusts the zoom lens in response to a simple toggle control on the grip. One advantage of this is that you can operate the zoom easily, without using your free hand. The other advantage is that the motor adjusts the lens at a steady speed, making zooms more fluid. The disadvantage of using the grip control is that the motor drains battery power.

Some camcorders also have something called a digital zoom. This doesn't involve the camera's lenses at all; it simply zooms in on part of the total picture captured by the CCD, magnifying the pixels. Digital zooms stabilize magnified pictures a little better than optical zooms, but you sacrifice resolution quality because you end up using only a portion of the available photosites on the CCD. The loss of resolution makes the image fuzzy.

One of the great things about a camcorder is that it can adjust automatically for different levels of light. It's very obvious to the CCD when an image is over- or under-exposed because there isn't much variation in the charges collected on each photosite. The camcorder monitors the photosite charges and adjusts the camera's iris to let more or less light through the lenses. The camcorder computer always works to maintain a good contrast between dark and light, so that images don't appear too dark or too washed out.

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Digital Formats
Digital camcorders differ from analog camcorders in a few very important ways. They record information digitally, as bytes, which means the image can be reproduced without losing any image or audio quality. Digital video can also be downloaded to a computer, where you can edit it or post it on the Web. Another distinction is that digital video has a much better resolution than analog video, typically 500 lines. There are two consumer digital formats in widespread use:

Formats
Analog Formats
Analog camcorders record video and audio signals as an analog track on video tape. This means that every time you make a copy of a tape, it loses some image and audio quality. Analog formats lack a number of the impressive features you'll find in digital camcorders. The main difference between the available analog formats is what kind of video tape the camcorder uses and the resolution. Analog formats include:

Standard VHS:
Standard VHS cameras use the same type of video tapes as a regular VCR. One obvious advantage of this is that after you've recorded something, you can pop the tape out and play it on most VCRs. Because of their widespread use, VHS tapes are a lot less expensive than the tapes used in other formats; they also give you a longer recording time. The chief disadvantage of standard VHS format is that the size of the tapes necessitates a larger, more cumbersome camcorder design. They have a resolution of about 230 to 250 horizontal lines, which is the low end of what's now available.

VHS-C:
VHS-C camcorders record on standard VHS tape that is housed in a more compact cassette. You can play VHS-C cassettes in a standard VCR, but you need an adaptor device that runs the tape through a full-size cassette. Basically, though, VHS-C format offers the same compatibility as standard VHS format. The smaller tape size allows for more compact designs, making VHS-C camcorders more portable. But the reduced tape size also means VHS-C tapes have a shorter running time than standard VHS cameras. In short play mode, the tapes can hold 30 to 45 minutes of video. They can hold 60 to 90 minutes of material if you record in extended play mode, but this sacrifices image and sound quality considerably

Super VHS:
Super VHS camcorders are about the same size as standard VHS cameras, because they use the same size tapes. The only difference between the two formats is that super VHS tape records an image with 380 to 400 horizontal lines, a much higher resolution image than standard VHS tape. You cannot play super VHS tapes on a standard VCR, but, as with all formats, the camcorder itself is a VCR and can be hooked up directly to your television or to your VCR to dub standard VHS copies.

Super VHS-C:
Basically, super VHS-C is to super VHS as VHS-C is to standard VHS: It's just a more compact version that uses a smaller size cassette.

8mm:
These camcorders use small 8mm tapes (about the size of an audio cassette). The chief advantage of this format is that manufacturers can produce more compact camcorders, sometimes small enough to fit in a coat pocket. The format offers about the same resolution as standard VHS, with slightly better sound quality. Like standard VHS tapes, 8mm tapes hold about two hours of footage, but they are more expensive. To watch 8mm tapes on your television, you have to attach your camcorder and use it as a VCR.

Hi-8:
Hi-8 camcorders are very similar to 8mm camcorders, but there are several important differences. For one, Hi-8 camcorders have a much higher resolution -- about 400 lines. Also, Hi-8 tapes are more expensive than ordinary 8mm tapes.

MiniDV:
MiniDV camcorders record on compact cassettes, which are fairly expensive and hold about 60 to 90 minutes of footage. The video has an impressive 500 lines of resolution, however, and can be easily transferred to a personal computer. DV camcorders can be extremely lightweight and compact -- many are about the size of a paperback novel. Another interesting feature is the ability to capture still pictures, just as a digital camera does.

Digital8:
Digital8 camcorders (produced by Sony exclusively) are very similar to regular DV camcorders, but they use standard Hi-8mm tapes, which are less expensive. These tapes hold up to 60 minutes of footage, which can be copied without any loss in quality. Just as with DV camcorders, you can connect Digital8 camcorders to your computer to download your movies for editing or Internet use.
Digital8 cameras are generally a bit larger than DV camcorders -- about the size of standard 8mm models.

DVD:
DVD camcorders are still relatively rare, as compared to MiniDV models, but their numbers are growing steadily. Instead of recording magnetic signals on tape, these camcorders burn video information directly onto small discs. The main advantage of this format is that each recording session is recorded as an individual track, just like the individual song tracks on a CD. Instead of rewinding and fast-fowarding through tape, you can jump immediately to each section of video. Other than that, DVD camcorders are pretty close to MiniDV models in performance. The picture is a little better on DVD models, however, and DVDs can store more footage. Depending on the camcorder's settings, a disc can hold 30 minutes to two hours of video

Memory card:
Memory card: There are now some digital camcorders that record directly onto solid-state memory cards, such as Flash memory cards, Memory Sticks and SD cards.

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Source: http://electronics.howstuffworks.com/camcorder.htm