Evaluación del Modelo de Estructural

The creators of DVD realized that in order to succeed, DVD had to be more than just a roomier CD or a more convenient laserdisc. Hollywood started the ball rolling by requesting a digital video consumer standard that would hold a full-length feature film, had better picture quality than existing high-end consumer video with wide-screen aspect ratio support, contained multiple versions of a program and parental control, supported high-qual- ity surround audio with soundtracks for at least three languages, and had built-in copy protection. Then the computer industry added its require- ments of a single format for computers and video entertainment with a common cross-platform file system, high performance for both movies and computer data, compatibility with CDs and CD-ROMs, compatible record- able and rewritable versions, no mandatory caddy or cartridge, and high data capacity with reliability equal to or better than CD-ROM. Later on, Hollywood decided that it wanted a copy protection and locking system to control release across different geographic regions of the world.

The designers threw in a few more features, such as multiple camera angles and graphic overlays for subtitling or karaoke,and DVD was born. Unlike CD, where the computer data format was cobbled on top of the dig- ital music format, the digital data storage system of DVD-ROM is the base standard. DVD-Video is built on top of DVD-ROM using a specific set of file types and data types. A DVD-ROM can contain digital data in almost any conceivable format, as long as a computer or other device can make use of it. DVD-Video, on the other hand, requires simple and inexpensive video players, so its capabilities and features are strictly defined.

From the beginning, the plan was to create a separate DVD-Audio for- mat based on input from the music industry. Requirements included copy- right identification and copy protection, compatibility with DVD-ROM and DVD-Video, CD playback (including an optional hybrid DVD/CD format that could play in CD players), navigation and random access similar to DVD-Video but also usable on players without an attached video display, slideshow features, and of course, superior sound quality. DVD-Audio sup- ports a subset of DVD-Video features, although eventually every new DVD- Video player also will play DVD-Audio discs. Apart from audio-only players such as small, portable devices, the distinctions between formats and play- ers eventually should disappear.

The following sections present the features of DVD-Video. The term

player also applies to software players on computers, as well as other devices such as videogame consoles that have the ability to play DVD-Video discs. DVD-Audio features are mentioned when relevant.

Over 2 Hours of High-Quality Digital Video

and Audio

Over 95 percent of Hollywood movies are shorter than 2 hours and 15 min- utes, so 135 minutes was chosen as the goal for a digital video disc. Uncom-

pressed, this much video could take up 255 gigabytes.2_{DVD uses MPEG-2}

compression to fit high-resolution digital video onto a single disc. The MPEG-2 encoding system compresses video in two ways: spatially, by reducing areas of repetitive detail and removing information that is not perceptible, and temporally, by reducing information that does not change over time. Reducing the video information by a factor of almost 50 enables it to fit in less than 5 gigabytes. Unfortunately, compression can cause unwanted effects such as blockiness, fuzziness, and video noise. However, the variable data rate of DVD enables extra data to be allocated for more complex scenes. Carefully encoded video is almost indistinguishable from the original studio master.

As mentioned earlier, the 135-minute length (or the absurdly precise 133-minute length) is a rough guideline based on estimates of average video compression and number of audio tracks. The length of a movie that can fit on a standard DVD depends almost entirely on how many audio tracks are available and how heavily the video is compressed. Other factors come into play, such as the frame rate of the source video (24, 25, or 30 frames per sec- ond), the quality of the original (soft video is easier to compress than sharp film grain, and clean video is easier to compress than noisy or dirty video), and the complexity (slow, simple scenes are easier to compress than rapid motion, frequent changes, and intricate detail).

In any case, the average Hollywood movie easily fits on one side of a DVD. This overcomes one of the big objections to laserdisc—that you had to flip the disc over or wait for the player to flip it over after each hour of playing time. For DVD-Audio the quality was bumped up to the next level with double the PCM sampling rates of DVD-Video and lossless packing to increase play- ing times without lossy compression. A single-layer DVD-Audio disc can play

Chapter 4

158

2_{Digital studio masters generally use 4:2:2 10-bit sampling, which at 270 Mbps eats up over 32}

74 minutes of super-fidelity multichannel audio or over 7 hours of CD- quality stereo audio.