Section One: The Digital Video Process
Step One: Encoding or Digitizing the Video

The analog signal, which can be a direct broadcast or a videotape, such as a VHS cassette, must be digitized. An encoder card accepts an analog signal through a cable into an interface card and feeds the signal into the encoding hardware and software to encode the video into digital form.

Encoding is a simple concept: the video analog signal is encoded, or represented, in digital bits that can be read and operated upon by a computer processor. All digital files — whether a textual document, an image, a program, or a video — are representations of information in bits.

One meaningful element in graphical digital media is the pixel, or picture element, which is a two-dimensional base unit of programmable color, also represented in bits. A pixel is a logical element, so that its representation can vary based on factors such as bit depth and screen resolution. The color expressed by a pixel is a blend of some component of the red, green or blue color spectrum. The human eye has photoreceptor cone cells that respond to color in the three spectra, so three mathematical representations of color — red, green, and blue — are all that are needed for digital color reproduction. Like all other digital information, colors are represented by bits. More bits (8-bit, 16-bit, 24-bit, etc.) allow more precise definition of the exact blend or hue from the red, green, blue color spectrum. Analog colors are translated to pixels in the RGB digital color space. Digital video uses a non-linear variation of RGB called YCbCr, where Cb represents luminance, or brightness, and Cr represents chrominance (chromaticity), or "pure" color, in the absence of brightness.

The number of pixels displayed on your computer screen, along the horizontal axis and the vertical axis, is defined as the spatial resolution. Broadcast-quality digital video (CCIR 601) is commonly displayed with 720 x 480 resolution.

Video is more than color, however. Video requires multiple frames showing redundant information and fractional changes to create the illusion of motion across time and space. At some point in your childhood, you probably duplicated the illusion of motion by drawing stick figures on cards and flipping them to create your own "cartoon." The more redundancy between frames, the smaller the change from one frame to the next, the smoother and more continuous the illusion of motion on your television or movie screen. Video encoding algorithms take advantage of this redundancy to compress video, encoding only the difference between frames. This process is known as temporal compression. The decoder at the client end stores the information that does not change from frame to frame in the buffer to refresh the displayed frame as needed.

To convert analog video to digital, each frame must be digitized using encoding hardware and software. Encoding systems can be as inexpensive as a $300 card fitting into an available slot on a multipurpose microcomputer to a $10,000+ stand-alone system, which requires a dedicated microcomputer. A good tape deck and analog monitor are also usually required for the encoding process, depending on the requirements of the selected video card/encoding system.

A video-encoding card accepts analog inputs from a VCR or a video camera and converts the analog format into a digital video file. Encoding hardware and software vary greatly in cost and therefore support a wide range of functionalities, including, as the cost increases, higher quality output, separate input for video and audio, faster encoding, multiple file and batch file processing, analog output (e.g. digital video back to analog videotape), uncompressed conversion and, at the present time, a range of encoding formats, including M-JPEG, MPEG-1, editable MPEG, MPEG-2, Video for Windows/ActiveMovie, and QuickTime.

Video cards are available for proprietary formats such as Intel's Indeo©. Video cards create digital files that can be opened by editing software, such as Adobe Premiere©. Video editing packages allow you to make changes to digital video, such as adding credits or special effects, cutting or adding frames, merging digital video clips, and outputting the created movie to a range of digital file formats, for playback in a variety of ways, through the use of incorporated software or plug-ins.

When digitizing video, each frame must be converted to digital form. In addition, the audio track accompanying the digital video must be converted and synchronized to the video for playback. A straight digitization with no compression requires more bandwidth and processing power than desktop computers can handle. Currently, compression must be employed to convert analog audio and video so that it is usable at the desktop. Compression reduces redundant information so that meaning is not lost but file sizes are reduced to manageable form. When you "rent" a movie in a hotel room, you are seeing a slightly-compressed (MPEG-2) version of a movie you might have seen in uncompressed analog form a few months earlier at a movie theater.

Uncompressed digital video can be created on high-end platforms and broadcast in real time or stored for later use. The Society of Motion Picture and Television Engineers (SMPTE) and the Institute of Electrical and Electronics Engineers (IEEE) develop and manage standards for uncompressed digital video. These standards include CCIR-601 for television broadcast digital video, in the resolution of PAL, NTSC, and SECAM; SMPTE 259M for the transport of CCIR-601; and SMPTE 292M for the transport of high definition television (HDTV).

The Moving Picture Experts Groups, known collectively as MPEG, are responsible for developing and maintaining digital video and audio encoding standards to address a wide range of commercial and educational needs. MPEG employs established procedures for the development, adoption, testing and review of digital multimedia standards. Standards are published and made freely available to commercial developers, although reasonable costs for some technologies may apply. MPEG standards are international standards that insure video encoding systems will create standardized files that can be opened and played at any desktop with a standards-compliant decoder. MPEG encoding standards are discussed in more detail in Section 2.

In the past few years, digital video cameras have become available, in commercial and consumer-quality models. A high performance serial bus, IEEE P1394, popularly known as FireWire, was developed by Apple Computer but now supported by many vendors to support data transfer rates of 100, 200 or 400 Mbps.

These high transfer rates mean that digital video can be transported directly from the digital source (camera, DVD, etc.) into the microcomputer with no processing delays. FireWire streams video data off a hard drive in real time without computer assistance. FireWire supports up to 63 devices on a single bus and allows 1,023 buses to be bridged together FireWire allows devices to be connected in a star, tree or daisy chain pattern. Addressing is dynamic and allows devices to be connected without rebooting the computer. FireWire transfer speeds, currently at 100-400 Mbps, will increase to 800 Mbps/multi-Gbps in the next release — 1394B. The high transport speeds can result in latency problems, requiring significant buffering capacity, as can a heavily-loaded PCI bus, but these problems will abate as FireWire integration becomes the norm, and microcomputers are designed for FireWire integration.

One issue with digital video camera content creation is that the resulting digital video files are generally in formats not currently supported by digital video client/server systems. When investigating digital video cameras, insure that a method exists to output the file to a standard format (AVI-to-MPEG or directly to MPEG) — whether through an editing program such as Adobe Premiere or a transcoding system such as Heuris.