For Dummies

Audio Stuff

All DVDs must have at least one Dolby Digital or PCM soundtrack to ensure high quality. Many DVDs also include a DTS soundtrack, but this is not a requirement. For those disks that have a DTS soundtrack, they must also include an alternative Dolby Digital or PCM soundtrack. The most common mediums are explained below:

• Dolby Digital - Dolby Digital (formerly called AC-3) carries from 1 to 6 channels of compressed digital audio, with an optional ".1" low-frequency effects (LFE) channel. The Dolby Digital track does not have to include 5.1, or 6.1 channels. It may be mono or stereo, and the stereo may or may not be Dolby Surround encoded (for playback on a system with a Dolby Pro Logic decoder). Most DVDs are encoded with Dolby Digital 2.0 Stereo, Dolby Digital 5.1 Surround, or both. The built-in, two-channel Dolby Digital decoder in every DVD player handles multichannel audio by "downmixing" it to two channels using Dolby Surround. This allows the analog stereo outputs to be connected to just about anything, including TVs and receivers with Dolby Pro Logic capability.
• Dolby Surround - A rear channel and a center channel are combined onto a two-channel signal. This can be played back on any stereo system, in which case the rear and center channel sounds remain mixed in with the left and right channels. When a Dolby Surround signal is played on a multi-channel audio system that knows how to process it, the extra channels are extracted to feed center speakers and rear speakers. The current technology used to decode a Dolby Surround encoded signal is Dolby Pro Logic II.
• DTS (Digital Theater Systems) - DTS is an audio encoding format similar to Dolby Digital. It requires a decoder, either in the player or in an external receiver. DTS audio is less compressed than Dolby Digital, having a typical data rate of 768 kbps, which is about 60% more than Dolby Digital. The DTS track is normally mixed for 5.1 surround and is typically louder than Dolby Digital. This can give the impression of superior sound to Dolby Digital, but this is hotly debated. I have found DTS to almost always be superior sounding.
• PCM (Pulse Code Modulation) - Uncompressed digital audio, the same format used on CDs. It can be sampled at 48 or 96 kHz with 16, 20, or 24 bits/sample. (Audio CD is limited to 44.1 kHz at 16 bits.) There can be from 1 to 8 channels. PCM is also often referred to as LPCM, or Linear Pulse Code Modulation.
• Dolby Pro Logic II - Four channels - front left, center, and front right, plus a stereo surround channel for the two rear speakers. This provides multichannel surround sound from any stereo source - CDs, tapes, videocassettes, TV broadcasts, etc. A Movie mode optimized for movies or programs with Dolby Surround soundtracks, and a Music mode, which creates a rich and enveloping surround ambience from stereo sources such as CDs.
• SRS Circle Surround - Circle Surround encoding allows up to a 6.1 surround mix to be delivered over any two-channel medium, and provide a surround experience over any surround sound system. One of the most powerful features of Circle Surround technology is that it is 100 percent compatible with all playback environments, from one-speaker mono to two-speaker stereo or multi-speaker surround sound systems.

Video Stuff

Video is stored on a DVD in 4:3 (standard) format or 16:9 (widescreen) format. Standard televisions have a width-to-height ratio of 4 to 3 (1.33 times wider than high). Widescreen televisions have a ratio of 16 to 9 (1.78 times wider than high). Most DVDs are specially designed to support widescreen displays.

Movies released in theaters normally have aspect ratios of 1.85, or 2.40. These obviously don't match the standard (1.33) and widescreen (1.78) television sizes, so one of the following processes must be used to make them fit:

• Letterbox - The video is presented in its theatrical aspect ratio, which is wider than both standard and widescreen TVs. Black bars, called mattes, are used to cover gaps created at the top and bottom of the screen. The size of these black bars depends on the original aspect ratio of the film, and the format of your TV. Video that is letterboxed before being encoded can be flagged so that the player will tell a widescreen TV to automatically expand the picture.
• Pan & Scan - The letterbox sized picture zoomed to cover the entire screen, and the sides of the picture get chopped off. If an important part of the image was cut from the visible portion on the screen, then it is panned to show that piece.
• Anamorphic 16x9 Enhanced - Provides a higher resolution image for widescreen televisions because less of the picture is wasted on letterbox mattes. The picture is stretched to use the full height of the display, and their are no black bars on the top and bottom. If you play an anamorphic enhanced DVD on a standard television, it will be presented in letterbox format, unless you have the option to display in pan & scan mode. If anamorphic video is shown unchanged on a standard 4:3 display, people will look unnaturally tall and skinny. The setup options of DVD players allow the viewer to indicate whether they have a 16:9 or 4:3 TV.

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