The DVD-Audio format is not yet specified. The International Steering Committee announced it expects to have a final draft specification by December 1997. This means DVD-Audio products may show up around 1999.
The following details are for audio tracks on DVD-Video. Some DVD manufacturers such as Pioneer are developing audio-only players using the DVD-Video format.
A disc can have up to 8 audio tracks (streams). Each track can be in one of three formats:
Two additional optional formats are supported: DTS and SDDS. Both require external decoders.
The .1 refers to a low-frequency effects (LFE) channel that connects to a subwoofer. This channel carries an emphasized bass audio signal.
All five audio formats support karaoke mode, which has two channels for stereo (L and R) plus an optional guide melody channel (M) and two optional vocal channels (V1 and V2).
Discs containing 525/60 (NTSC) video must use PCM or Dolby Digital on at least one track. Discs containing 625/50 (PAL/SECAM) video must use PCM or MPEG audio on at least one track. Additional tracks may be in any format. The DVD Forum has clarified that only stereo MPEG audio is mandatory for 625/50 discs, while multichannel MPEG-2 audio is recommended. Since multichannel MPEG-2 decoders are not yet available, most 625/50 discs include Dolby Digital audio.
For stereo output (analog or digital), all NTSC players and all PAL players (so far) have a built-in Dolby Digital decoder which downmixes from 5.1 channels (if present on the disc) to Dolby Surround stereo (i.e., 5 channels are matrixed into 2 channels to be decoded to 4 by an external Dolby Pro Logic processor). Both Dolby Digital and MPEG-2 support 2-channel Dolby Surround as the source in cases where the disc producer can't or doesn't want to remix the original onto discrete channels. This means that a DVD labelled as having Dolby Digital sound may only use the L/R channels for surround or plain stereo. Even movies with old monophonic soundtracks may use Dolby Digital - but only 1 or 2 channels.
The downmix process does not include the LFE channel and may compress the dynamic range in order to improve dialog audibility and keep the sound from becoming muddy on average home audio systems. This can result in reduced sound quality on high-end audio systems. Some players have the option to turn off the dynamic range compression. The downmix is auditioned when the disc is prepared, and if the result is not acceptable the audio may be tweaked or a separate L/R Dolby Surround track may be added. Experience has shown that minor tweaking is sometimes required to make the dialog more audible within the limited dynamic range of a home stereo system, but that a separate track is not usually necessary. If surround audio is important to you, you will hear significantly better results from multichannel discs if you have a Dolby Digital system.
Linear PCM is uncompressed (lossless) 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. The maximum bitrate is 6.144 Mbps, which limits sample rates and bit sizes with 5 or more channels. It's generally felt that the 96 dB dynamic range of 16 bits or even the 120 dB range of 20 bits combined with a frequency response of up to 22,000 Hz from 48 kHz sampling is adequate for high-fidelity sound reproduction. However, additional bits and higher sampling rates are useful in studio work, noise shaping, advanced digital processing, and three-dimensional sound field reproduction. DVD players are required to support all the variations of LPCM, but some of them may subsample 96 kHz down to 48 kHz, and some may not use all 20 or 24 bits. The signal provided on the digital output for external digital-to-analog converters may be limited to less than 96 kHz or less than 24 bits.
Dolby Digital is multi-channel digital audio, compressed using AC-3 coding technology from original PCM with a sample rate of 48 kHz at 16 bits. The bitrate is 64 kbps to 448 kbps, with 384 being the normal rate for 5.1 channels and 192 being the normal rate for stereo (with or without surround encoding). The channel combinations are (front/surround): 1/0, 1+1/0 (dual mono), 2/0, 3/0, 2/1, 3/1, 2/2, and 3/2. The LFE channel is optional with all 8 combinations.
MPEG audio is multi-channel digital audio, compressed from original PCM format with sample rate of 48 kHz at 16 bits. Both MPEG-1 and MPEG-2 formats are supported. The variable bitrate is 32 kbps to 912 kbps, with 384 being the normal average rate. MPEG-1 is limited to 384 kbps. Channel combinations are (front/surround): 1/0, 2/0, 2/1, 2/2, 3/0, 3/1, 3/2, and 5/2. The LFE channel is optional with all combinations. The 7.1 channel format adds left-center and right-center channels, but will probably be rare for home use. MPEG-2 surround channels are in an extension stream matrixed onto the MPEG-1 stereo channels, which makes MPEG-2 audio backwards compatible with MPEG-1 hardware (an MPEG-1 system will only see the two stereo channels.)
DTS is an optional multi-channel (5.1) digital audio format, compressed from PCM at 48 kHz. The data rate is from 64 kbps to 1536 kbps. Channel combinations are (front/surround): 1/0, 2/0, 3/0, 2/1, 2/2, 3/2. The LFE channel is optional with all 6 combinations.
SDDS is an optional multi-channel (5.1 or 7.1) digital audio format, compressed from PCM at 48 kHz. The data rate can go up to 1280 kbps.
A DVD-5 with only one surround stereo audio stream (at 192 kbps) can hold over 55 hours of audio. A DVD-18 can hold over 200 hours.