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Cracking DVD Encoding
Posted Mar 3, 2004 Print Version     Page 1of 3 next »

Part of what makes today's consumer-friendly DVD authoring tools so accessible is their success in shielding users from encoding and other techie tasks. But while ignorance may be bliss, aspiring DVD authors can't do competent commercial work without a working understanding of the encoding process. What's really happening behind the scenes, and what do you need to know about encoding to produce top-quality results?

Today's entry-level DVD creation tools enable users with little or no expertise in video encoding to make pleasing, playable DVDs. They may be audio producers expanding into DVD, 3D animators building DVD into their workflow, video editors with backgrounds in tape or other uncompressed media, corporate trainers migrating analog content, or novices and hobbyists with no technical expertise at all. But they all can become DVD authors, after a fashion, with little or no knowledge of the work going on behind the scenes. The embedded intelligence of today's tools renders encoding—the process that makes video DVD-compatible—virtually invisible.

However inviting this invisibility may be, it's anything but informative. The quagmire of entry-level authoring tools is that anyone can get involved in encoding video for DVD, but in order to advance to greater levels of customization—let alone the professional look that some argue comes only with sophisticated hardware encoders—they must understand the technology and take some control of the process.

With entry-level DVD applications, it's easy to breeze through encoding with little thought or attention on the user end. The software is such that the encoding process happens automatically, as most tools have wizards and presets that save users from even the most rudimentary parameter management. Consequently, the complexities of the encoding process are often hidden from the users within the software.

For those aspiring to work at a professional level, it is necessary to understand the "ins and outs" of the encoding process to achieve advanced levels of video quality. So let's take a closer look at what is going on in your software that you might need to know for a studio-caliber film, but don't (necessarily) have to know to capture and record your daughter's piano recital.

Encoding and Compression
Video captures the sequential movement of a series of still pictures. During the process of compression the pictures are compared, and the elements that do not change (redundancies) over time become compressed information. Compression consists of two fundamental processes: sampling and quantization. The sampling process separates the image into isolated pieces of picture rudiments called pixels. These pixels exist as signal points in 2D space. They are assigned to specific digital values and converted into a finite range of bits and bytes. This process of reducing the infinite scales of pixels to discrete numeric values is called quantization. Once the pixels are systematically converted to numeric values, a digital video file is created.

Because DVD capacity is fixed, the longer the video, the more compression it usually has to undergo. There is an inverse relationship between the level of quality and amount of compression. As a video undergoes more compression, quality will decrease, if only nominally in some cases, because of the video information removed. The closeness of the original video to the recreated video depends upon the amount of compression it undergoes and the way that compression is applied. The efficiency of the compression process is contingent upon the relationship between quality and the given bit rate. Encoders with better compression efficiency can deliver higher quality at lower bit rates, although much of this is dependent on the quality of the video source, and the degree of redundancy inherent to the content itself.

The Importance of the Algorithm
The calculations of the encoding process utilize numeric notations that stand for the individual characteristics of an image. As previously mentioned, the sampling process forms a collection of the numeric values that symbolize every pixel within the frame sequence. According to Richard Diercks, principal of RADCO Media and author of Aquaria, the first commercially available DVD-18 title, these sequences of numbers support the entire encoding process. "Each encoder is driven by a series of algorithms that compress/convert/transcode the digital video stream into an MPEG-2-compliant format in what amounts to a plain old computer file," Diercks says.

Encoding takes into account the process of describing the changes between pictures. The numeric values of the digital video file serve to flag such differences between frames, and the unchanged elements are referred to as redundancies. "In simple terms, video compression algorithms try to take advantage of spatial and temporal similarities within video frames," says William Chien, director of product development at Pinnacle Systems.

Encoding is the process of converting uncompressed video to compressed digital video using various standards, or codecs (compression/decompression algorithms) such as MPEG, Real Video, Windows Media, and QuickTime, depending on the playback medium, the platform that will be used to deliver the content, and the bandwidth available. The process of encoding includes describing the changes between each picture of a frame sequence. Encoding has the greatest effect on final video quality, as it is the process that determines the amount of compression a video stream undergoes.

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