AUDIO OPTIONS FOR DVD-VIDEO

May 1, 2003 12:00 PM, Gary Hall

           
 

DVD-Video includes a bewildering array of options for audio. DVD is the first medium in which not only the format but the resolution and data rate of audio are variable. To make things more confusing, various formats can be combined in different sections of the same disc or be offered as parallel streams to be selected by the viewer. (Only one stream plays at a time, by the way.)

Stereo audio can be included on a DVD-Video disc in data-compressed or linear PCM (uncompressed) formats. Audio data-compression standards provided by the specification include Dolby AC-3; MPEG-2, Layer 2; or DTS formats. In order to conserve space and bandwidth for video, which generally requires much higher rates, most commercial DVD titles use audio-data compression.

Uncompressed stereo PCM audio is supported in DVD-Video at resolutions of 16, 20, or 24 bits and sampling rates of 48 or 96 kHz (any combination of these resolutions and sampling rates can be used). Note that a sampling rate of 44.1 kHz, the standard for CD, is not supported in DVD. That means that if you are preparing audio releases on both CD and DVD-Video, you will have to do sampling-rate conversion at some point.

Stereo audio may also be prepared using data compression. Although options for DTS stereo and MPEG audio exist in theory, Dolby's AC-3 format predominates. For multichannel (5.1) surround, audio is always data compressed in DVD-Video. Dolby AC-3 is also the dominant format for 5.1 on DVD and is widely supported by the authoring tools. AC-3 encoders are readily available and are included with many midrange systems.

Many aficionados feel that the DTS-audio compression format offers better sound, although it uses more bits. Sadly, DTS is supported only by the most expensive professional DVD authoring tools, and DTS encoders (hardware or software) are more expensive and harder to find than those for AC-3. Also, not all DVD playback systems can decode DTS audio. Discs with DTS audio generally include alternate versions in AC-3 and/or linear PCM.

A certain amount of confusion exists regarding which types of audio the DVD-Video spec requires. The spec states that a stream of either uncompressed linear PCM or data-compressed audio must be included with each track of a disc (there can be up to eight streams), and that if audio is in data-compressed format only, then at least one stream must be AC-3.

Though the DVD-Video specification provides for the use of MPEG-2, Layer 2, audio compression (not MP3), the wording is ambiguous, and it turns out that many DVD players do not play MPEG audio at all. As a result, that audio format for DVD is, for all intents and purposes, dead.

All audio formats for DVD-Video consume a defined amount of data bandwidth (bit rate). DVD has a finite data-transfer rate of 9.8 Mbps, and under no circumstances can the total transfer rate of audio and video be allowed to exceed that number. DVD-Video allows for multiple streams of audio on disc, and these streams may be in different formats or have different content. (Commentary tracks, for example, are very popular for movie releases.) If multiple audio streams are used, the total bit rate of audio plus that of any accompanying video must remain under the magic figure of 9.8 Mbps. The table “DVD-Video” lists the bit rates for various formats of audio used in DVD-Video. As you might expect, the bits rates for uncompressed PCM are much higher than those for data-compressed audio, especially at higher resolutions and sampling rates.

DVD-Video

The total bit rate for audio and video cannot exceed 9.8 Mbps, the maximum transfer rate defined by the DVD-V specification. Video bit rates are generally much higher than those for audio.