The Signal Path

For most of us, the operations of everyday electronic devices like televisions, VCRs and camcorders are mysterious. Though many of us may have a vague idea of how a picture gets from lens to tape, most of us just take for granted that a picture appears on the screen when we fire up our camcorders and hit the record button.

As a Videomaker reader, however, you likely have more than a passing interest in the way a camcorder works. For example, you may be curious about why your pictures sometimes look clear and pristine, and why they sometimes look murky and grainy.

In this article, well take a look at how a camcorder turns the light that enters the lens into a signal that your television can interpret. Well follow that signal through the camcorder, out onto the cable and into whatever kind of gear you choose to wring it through. Well cover some technical ground, but dont worry – we wont go too deep into the specifics. Well just try to get you to the point where you have a better understanding of the video signal and the electronic pathways, both inside and outside your camcorder, that make the magic of video happen.

First Comes Light and Sound

Before your camcorder can create a signal to represent moving images and sound, it must first gather the light and sound that wed ordinarily see and hear with our eyes and ears. Instead of eyes, the camcorder has a lens. In practice, the lens works in much the same way your eye does: it gathers the light that bounces off the subject and focuses it into a sharp image on the camcorders CCD (Charge Coupled Device), much like the light that enters your eye is focussed on your retina. Instead of ears, the camcorder has a microphone, which also works something like its human equivalent. The microphone, like a human ear, picks up variations in air pressure and converts them to an electrical signal.

The two devices that convert events in the natural world into an electrical signal – the CCD and the microphone – belong to a class of objects known as transducers. Theyre called transducers because they transduce, or change, energy from one form into another. Thus, light energy and air pressure become fluctuations in electrical current, which your video equipment interprets and re-creates into pictures and sound.

The CCDs Role

As mentioned above, the Charge Coupled Device (CCD) sits at the center of the camcorders image-making apparatus. Its made up of hundreds of thousands of light-sensitive pixels arranged in a rectangular grid. Each of these pixels stores up an electrical charge in proportion to the amount and duration of light hitting it. Every 60th of a second (half a frame, or a single field, of video), the camcorder reads these charges and combines them to create a signal.

If a camcorder only measured the amount and duration of light hitting the CCDs pixels, then wed still be shooting pictures in black and white. In other words, a CCD is by nature a color-blind device. Camcorders extract color information from monochrome sensors in one of two ways. These different approaches to color extraction split the camcorder field into two camps.

Single-CCD camcorders use a single CCD sensor to handle all of the image-making duties. Such a camcorder derives color information from the sensor by covering it with an array of colored lenses called a mosaic color filter. This means if you could look very closely at the face of the CCD, youd see that it is covered in red, green and blue lenses. With the aid of these lenses and some clever electronic processing, the camcorder can derive both a brightness (luminance) and color (chrominance) signal from the single CCD chip.

The other, and far superior, method of extracting color is the three-chip design. Three-chip camcorders use a trio of CCDs, each specializing in a certain color. By using a complex prism block or arrangement of mirrors and filters, a 3-CCD camcorder splits the light coming through the lens into three color components. Light from each of the colors (red, green and blue) goes to its own sensor. The camcorder combines the output of these three chips to create a full-color video signal.

Single-CCD imaging systems are smaller, lighter, less complex and cheaper. Three-CCD systems, though larger and more costly, generally deliver color that is more accurate at a higher resolution. Three-CCD designs may also deliver a hard-to-define improvement in image depth and realism. Three-CCD camcorders often have better lenses than their single-chip counterparts, in order to keep up with the increased resolution and color representation.

Recent years have seen a trend toward smaller and smaller image sensors in camcorders, from 1/2-inch to 1/3-inch to todays tiny 1/4-inch designs. A smaller CCD doesnt just mean a smaller sensor assembly; it means a smaller lens as well. Every aspect of a lens design points back to how large an image it needs to create. If a lens needs to bathe a 1/4-inch sensor in light instead of a 1/2-inch sensor, designers can shrink the lens assembly considerably. This translates to smaller, cheaper and more compact camcorders.

Since a CCDs sensitivity is proportional to the surface of each pixel, a smaller sensor will be less sensitive to light if all other variables are held equal. In reality, however, the variables arent held equal. CCD manufacturers have found ways to gather more light onto a smaller sensor. This gives todays smaller CCD designs low-light sensitivity on par with that of larger sensors.

Sensor resolution also plays a factor in image quality, up to a point. Once a sensors resolution exceeds that of the recording system and tape format, theres little to be gained by increasing the sensors pixel count. A 270,000-pixel CCD delivers ample resolution for a standard format like 8mm or VHS. Will a 470,000-pixel sensor result in sharper images in these formats? Probably not. Where extra pixels can be put to good use is with digital zoom and image stabilization.

Signal Processing

As weve seen, the job of the CCD is to gather the light, measure it and turn it into an electronic signal. Once that job is complete, there are still a few hoops that the signal has left to jump through before it gets recorded onto tape or leaves the camcorder through the output jacks.

Under the general heading of signal processing, we include all processes that result in the massaging of the video and/or audio signals. These processes include titling, special effects and gain, among others. Lets take a look at each of these in turn.

Whenever you use the camcorders titler, or even its time and date stamp, youre interrupting the video signal and making changes (adding alphanumeric characters). This creates an opportunity for noise to enter the signal.

In-camera effects also interrupt the video signal and perform subtle changes in your recorded video. Most in-camera special effects are achieved by digitizing the signal one frame (or field) at a time and manipulating it while its reduced to a string of numbers. All, however, add a little bit of noise to the signal.

Some camcorders have a feature called Gain Up, which boosts the voltage level of the entire signal in order to make it brighter. The purpose of Gain is to allow shooting in low-light conditions, but it usually adds a considerable amount of noise to the signal.

The lesson to learn here is simple: every time you manipulate a video signal, no matter how subtly, more noise is added to your image. After the camcorder processes the signal, it is ready to be recorded onto tape.

Magnet Magic

To record the signal onto a videotape, your camcorder uses magnets. It consists of a drum that contains separate heads that record video, audio and control information on the tape. Videotape, if you did not already know, is plastic, with a magnetically active coating. When the heads come into contact with the tape, they organize the particles on the tape in separate tracks using the magnets. A camcorder or VCR uses at least two record heads, one for each field of video in a frame. Many use four or more heads to record, usually to provide better pause and still modes. As you know from comparing footage shot at standard play (SP) and long play (LP) settings in a camcorder, the faster a tape rolls across the record heads, the better the picture you will get. The faster speed allows more space on the tape for a given quantity of signal. To maximize the amount of space the heads can use in writing a signal, videotapes use a system known as helical scanning.

Helical scanning works like this: the tracks on the tape are laid in a diagonal fashion (see Figure 3a). The drum that contains the record heads is also set up at an angle. As the tape passes the heads in the drum, a fresh part of tape is always ready for recording. Playback is similar, but without having the heads reorganize the magnetic particles on the tape. When playing back, the playback heads only read the tracks. It then converts the tracks on the tape to another video signal that can go out of the camcorder.

Outside the Camera

When the signal leaves the camcorder, it becomes even more susceptible to noise. It enters a cruel world filled with stray electromagnetic radiation, seemingly coming from all directions at once.

Think of it: the signal travels down a long wire, which is really nothing more than a big antenna that picks up whatever fluctuations might be cruising through the spectrum at any given moment. Shielding on your video and audio cables is helpful, but it doesnt eliminate the problem entirely.

At this point, your video signal is probably still quite watchable; though a little degraded from the original shape it was in when it came off the camcorders CCD. Home videographers, however, are notorious for subjecting their video signals to every form of cruelty known to man before letting anybody view them. They add titlers, Special Effects Generators (SEGs), computers and other devices to the signal path, and often, in the attempt to create a dazzling video, they end up introducing a great deal of noise to their signal. When they are finished, their pictures look grainy and their audio sounds muffled and indistinct.

To keep this from happening to your signals, you can take several precautions. Probably the simplest one is avoid putting too many devices in the path of your signal. For example, you should only attach titlers and SEGs when theyre needed. And you should avoid signal processors altogether, because in their attempt to beef up the quality of the signal, they invariably create the opposite effect by adding noise.

Dont think youre off the hook if you edit with a computer. Any digital system that uses analog video inputs (including composite and S-video inputs) subjects your images to increased noise not to mention compression artifacts. In such situations, use S-video connections whenever possible to minimize the damage. Heres why: ordinary RCA-style video cables are composite cables, which means they carry a signal thats a mixture, or a composite, of the black-and-white and color video information. Most types of video equipment (including your camcorder) process the black-and-white portion of the signal separately from the color portion of the signal. In order to send a composite signal down the cable, the two portions of the signal must go through a process known as modulation. Similarly, before the equipment at the other end can interpret the signal, it must be demodulated. Each time you modulate or demodulate the signal, guess what happens? Thats right: more noise.

S-video cables keep the color and the black-and-white portions of the video signal separate. This means you dont have to worry about the added noise that comes from modulation and demodulation. You do, however, still have to worry about interference that results from stray EMR encountering the cable, so shielded S-video cable is well worth the investment.

FireWire cables, on the other hand, carry a digital signal, which is highly resistant to noise. By transmitting a purely digital signal, you can end up with a picture thats nearly as pristine as it was when it came off the CCD. This means you can transmit your video signal from a digital camcorder into a computer, add all sorts of fancy titles and effects, then dump it back out to digital tape with virtually no loss in signal quality.

If youre operating entirely in a digital environment, connecting your gear via FireWire, then you have very little to worry about. Your video and audio signals, being digital, will be able to withstand multiple copies without succumbing to the evils of noise.

In Summary

When all is said and done, theres a lot more to your camcorder than meets the eye. Even the most basic model contains a highly sophisticated imaging system that is nothing short of a scientific marvel.

Videomaker
The Videomaker Editors are dedicated to bringing you the information you need to produce and share better video.

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