Understanding Digital Video : factors influence image quality

       

 My early moviemaking career was stunted by the practical considerations of lugging around a fragile, expensive camera and the huge VCR to which it was attached. Twenty-five years later, though, the time is right to work with video-the tools and technologies available to amateurs today would seem magical if we could teleport them back to the disco era.

Video technology is crucial to many home and office computer users. Regardless of whether you're sending Grandma digital video of a child's birthday party, setting up a home security system, or preparing a video presentation for business purposes, your decisions about equipment, technology, and software are important to the final product's quality.  Before you dive into video, you need to understand some ba

sics about the medium. I provide enough information to get you started; in future columns, I'll cover techniques for taking better videos, review equipment and software for making and editing digital video, and discuss the technical details of various gear.

                                             Analog vs. Digital

In the past, video recording was an exclusively analog process; recorders placed video signals on a magnetic wire or tape by varying the amount of voltage that passed through a recording head. Analog is infinitely variable because you can change the voltage or frequency of the signal sent to the head to change the signals on the tape. VHS and Betamax formats are both analog.

Although analog technology has existed since 1956, the signal quality isn't great. While a broadcast NTSC signal might have about 330 lines of resolution (resolution is the number of horizontal lines that make up an image), a VHS recording of that same signal will produce only about 240 lines of resolution. (For more information about resolution, visit 

The difference in resolution explains why VHS recordings tend to look muddy and washed out compared with broadcast signals. In comparison, DVDs output 540 lines of resolution, although not all TVs can display that many lines. Resolution is why on most TVs, DVD images are sharper and more colorful than the same images recorded on videotape.

Betamax, Beta SP, and Super VHS (SVHS) formats use more bandwidth to record signals. More recorded bandwidth means a higher-fidelity signal. For example, SVHS has about 400 lines of resolution, a great improvement over VHS. However, these formats still use analog recording, which means you can't do much with them except record and play back.

In the mid-1980s, digital editing systems appeared; these systems use a frame buffer to capture individual frames or a frame sequence from an analog tape, then digitize the image so that you can edit it. After you edit the image, you can send it to an analog tape. At first, only television networks used digital editing products, but as digital technology improved, prices dropped and these systems became more widespread. Recording a video image in digital format lets you superimpose logos or text, and adding special effects is easy because you don't need to perform the special effects in realtime (i.e., while the video's actually playing); it's OK to take longer than 1 second to add effects to a 1-second video clip.

Digital images let editors randomly jump to any point in a video stream without rewinding or fast-forwarding a tape. One of digital's biggest advantages is that images don't lose their quality when you copy them. This advantage is crucial for editing and production companies, but it's important to home users as well.

Types of Digital Technology

As digital technology became less expensive and more versatile, electronics manufacturers developed standards for digital video recording. The first standard on the market was JVC's Digital VHS. DVHS offered more than 500 lines of resolution on what looked like a standard VHS tape; however, DVHS never caught on because VHS and SVHS were too entrenched in the home market.

Concurrently, a group of manufacturers developed the Digital Video (DV) standard, which uses digital recording to encode compressed video as a sequence of 720 x 480 frames. Several tape formats use the DV video-encoding scheme; Mini DV is the most popular. Mini DV tapes are slightly larger than two matchboxes stuck together; because the Mini DV tapes are so small, so are the cameras.

Sony, which already had a substantial investment in analog 8mm camcorder technology, also introduced a hybrid format, Digital8. D8 uses standard, widely available 8mm tapes, but it records in the DV format. D8 camcorders also can play back analog 8mm tapes, which is useful if you want to capture and edit existing analog video.

Recording Pictures

The digital camcorder element that actually captures an image is a Charge Coupled Device (CCD). Incoming photons strike cells (i.e., pixels) in the CCD, creating an electrical impulse that the camera records. The CCD's native resolution improves as the number of cells increases. CCD capability is often rated by CCD size, so a 0.5" CCD might have four times as many cells as a 0.25" CCD. The number of scan lines on the CCD is a better measure of picture quality. Most consumer camcorders capture colors by using sequential scanning of the CCD, once for each primary color (red, green, and blue). More sophisticated, and more expensive, units use three separate CCD elements, one for each primary color.