August 2001

Digital Video/Versatile Disc (DVD) Basics

While DVDs may look just like CDs, they are more complex and can store much more data. For instance, a CD is basically a collection of audio tracks that are individually accessible via a table of contents. DVDs, on the other hand, can be encoded with multiple audio tracks (including various audio formats), full-length video, subtitles, alternate camera angles, and menus that can be programmed to access any part of the disc or even different versions of a film through seamless branching, in addition to many other features.

DVD players offer excellent picture quality and discrete multichannel audio that is superior to that of VHS tape. They also have the ability to playback standard CDs. With the widespread availability of DVDs for rent and for sale, and with street prices for entry-level models from all the major manufacturers starting at around $200, there really is no reason for not owning a DVD player.

The DVD format -- video specifications

DVDs store video information in a digital format known as MPEG-2. This is a variable bit rate, digital compression scheme that typically allows for just over two hours of high-quality video to be stored on one side of a single-layered DVD. Depending on how much the video is compressed, a single-sided and single-layered DVD can contain much more or much less than two hours of video with increased compression allowing for more running time, but resulting in a lower-quality picture. The length of video that can be stored on a DVD is also affected by the amount of storage space taken up by other features, especially multiple audio tracks, which use up a lot of the available space. DVDs can be dual layered and can even be recorded on both sides of the disc, effectively quadrupling their storage capacity.

One of the drawbacks of presenting widescreen versions of films on video is that a lot of the storage capacity of the media is wasted on reproducing the black bars at the top and bottom of the screen resulting in reduced vertical resolution of the film itself. However, for presentation on 16:9 widescreen televisions, many DVDs are "anamorphically" enhanced to increase the resolution of the picture. This means that all of the storage capacity of the DVD is used to store the actual picture information in its widescreen form and none of it is used on the black bars, which are not displayed on 16:9 televisions anyway. When these DVDs are played back on standard 4:3 televisions, the DVD player actually compresses the picture and creates the black bars at the top and bottom of the screen at the time of playback.

Most DVD players can output three types of video signals: composite, S-video, and component. Composite video utilizes RCA jacks and is the most common and least sophisticated type of video connection; it has the lowest quality picture. S-video utilizes a four-pin connector, which separates the chroma (color) and luminance (brightness) signals for a technically superior interface. An ever better method is component video, which further separates the color signals and is normally transmitted over three cables with RCA connectors. An enhancement to the component signal is "progressive scan," which fully scans the entire frame of video on each pass of the scanning beam. This is contrasted by the usual practice of "interlacing," where only alternating lines of the picture are scanned on the first pass of the scanning beam and the remaining lines are filled in on the next pass. Unfortunately, many televisions that have component video inputs are not capable of accepting the more advanced progressive-scan signals.

Audio specifications

The standard audio format of DVDs is Dolby Digital (DD) which can contain up to 5.1 discrete channels of sound (left, center, right, left surround, right surround, and low frequency effects). While DD was initially developed to deliver 5.1 audio soundtracks, DVDs can contain DD soundtracks with anything from a single channel up to the full palette of 5.1 channels. They can also contain standard two-channel PCM (the format used on CDs) audio sampled at 16, 20, or 24 bits and a sampling rate of 48 or 96kHz, as well as multichannel DTS movie soundtracks.

There are up to four types of audio outputs that can be present on the back of a DVD player. The two types of digital outputs which carry the audio data to a receiver or processor for decoding are TosLink (optical) and coaxial (typically RCA-type connectors). There are also stereo analog outputs with left and right RCA jacks that can be connected to televisions, two-channel receivers, integrated amplifiers, etc. Some DVD players also have six analog outputs (one for each of the 5.1 channels) and perform their own decoding of DD or DTS soundtracks internally for connection to a "digital ready" receiver or processor (i.e., one that has six-channel analog inputs).

More DVD features and…

DVDs often contain multiple soundtracks recorded in different surround formats such as DD 5.1, Dolby Surround, and DTS, and may also have different languages, commentary tracks by the film’s creators, or isolated music scores. DVDs are also regionally coded, meaning that the disc is coded for a certain region so that they will only play back on players that are meant for that area. For example, Region 1 DVDs are sold in North America and will not play in players sold in Europe, Asia, or other parts of the world. Some DVDs such as those meant for domestic use and for export have no regional coding (actually they are coded as Region 0) so that they are compatible with players from all regions.

DVDs support closed captioning, but additionally include subtitles that are often offered in the choice of several different languages. There is also the provision for different video angles, but this a relatively obscure feature used mainly in adult videos and occasionally for instructional-type videos or special features. There are many other features supported by DVD such as parental lock, Macrovision, and others that are too numerous to mention in this column.

Note that not all DVDs support all of these features. Most of these features are optional and a DVD can be anything from a basic audio and video transfer of a movie to very complex special editions with several versions of the film and tons of extras.

What does it all mean?

All of the preceding information is good to know, but what do you really need to know when purchasing a DVD player? Surprisingly, not that much. In the few short years since its introduction, the DVD format has become fairly mature. In fact, most DVD players from the major electronics manufacturers (Panasonic, Sony, Pioneer, JVC, etc.), including the most basic and inexpensive models, have nearly every conceivable feature that one could expect. About the only features that these entry-level players do not normally possess are progressive-scan video outputs and six-channel (5.1) analog audio outputs. If you have a television capable of handling progressive-scan signals, then seeking out a player with progressive scan will be worth the effort, but otherwise just about any brand-name DVD player will produce a good picture and excellent sound.

One thing that you might consider when looking for a DVD player is multiple audio and video outputs. Although most players will have at least one of each type of audio and video output, some players will have more than one. This may be useful if you intend to connect the DVD player to more than one monitoring device at a time, such as a receiver/processor and a television. This is something you should consider prior to purchasing, as many of the more expensive players will have more output options.

Once you get your DVD player home, you should check its settings to make sure that it is functioning in the manner that is appropriate for your system. Although the default video setting is usually set for a 4:3 monitor, you should check it and adjust it accordingly, otherwise the picture will appear distorted. Many DVD players will also have the output of DTS signals disabled, so you will have to turn this on if you intend to watch DTS-encoded discs. If you do not do this, there will be no sound when attempting to playback a DTS DVD -- a common mistake.

Another problem that people frequently encounter is poor picture quality when connecting a DVD player to a television through the inputs and outputs of a VCR. This is caused by Macrovision copy protection that is present on many DVDs and is intended to prevent the unlawful taping of copyrighted material. When a video signal containing Macrovision is even passed through a VHS VCR, the picture will become unstable. Although there are devices that are designed to defeat Macrovision, routing a video signal through a VCR is not recommended, as it will tend to degrade the signal. Some DVDs will also default to Dolby Surround if you do not specifically select the Dolby Digital 5.1 (or DTS) soundtrack. Thus you may end up watching a movie in matrixed surround rather than discrete digital if you are not aware of this. Fortunately, most DVD players and receivers/processors will indicate what type of signal is being decoded on the display so you can easily check this during playback.

And as always, be sure to read the owner’s manual of your DVD player. There may be operational quirks or nonstandard features such as enhanced black level, conversion of DD signals to PCM, settings for the six-channel analog outputs, or picture zoom that are unique to your brand and model of player.

I have only touched the surface of this topic, but if you would like more detailed information on the DVD format, you can read Jim Taylor’s excellent book DVD Demystified (McGraw-Hill, 2000) or visit the DVD FAQ (Frequently Asked Questions), which is continually updated at www.dvddemystified.com/dvdfaq.html. These resources will be especially useful to those of you who are more technically inclined or if you have a very specific question about the DVD format.

...Roger Kanno
roger@hometheatersound.com

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