Other sample rates and word
lengths can also be used such as 44.1, 48 or 88.2 kHz at either 16, 20
or 24 bit for one to six channels on any given program or track.
In addition, combinations of
sample rates and word lengths could be used. For example, a 5-channel
music mix could be produced with the front three channels, left, center,
and right delivered at 96-kHz/24 bit, and the left and right surround
channels delivered at 48-kHz/ 1 6 bit.
It is possible to put multiple
LPCM programs on a DVD Audio disc. For example, a 5.1 channel music mix
and a separate 2 channel stereo mix of the same program could be put on
the same disc using any of the combinations of sample rates and word lengths
described above.
The DVD Audio specification
mandates the capability of a 74-minute playing time for any given program
on the disc. This is the same as compact disc.
Meridian Lossless Packing
(MLP)
Like DVD Video players, DVD
Audio players have the capability of delivering approximately 9.6 Mbytes
per second of data from a disc. Some higher resolution LPCM audio streams
exceed this capacity. In other words, there is too much data at one time
to fit through the output of the player. For example, 5.1 channels of
96kHz/24-bit audio have more data than the DVD Audio player can play.
(This is not a problem with LPCM streams with lower sample rates or word
lengths such as 48-kHz/20 bit.)
Some higher resolution LPCM
data takes up a lot of space on a DVD Audio disc. So in addition to the
output capacity problem of a 5.1 channel 96/24 program, there exists a
potential storage problem. 74 minutes of 5.1 channels of 96kHz/24bit of
LPCM audio will not fit on the disc. (Again, this is not a problem with
LPCM streams of lower sample rates and word lengths.)
The WG4 and the ISC mandated
that only "lossless" audio coding could be used to allow higher
resolution LPCM streams (like 5.1 channels of 96 kHz/24 bit) to be stored
and delivered on DVD Audio discs and players. This decision was based
on the desire of the music industry to maintain the highest sound quality
and audio purity in the DVD Audio format.
The WG4 and ISC chose Meridian
Lossless Packing (MLP) as the solution for the delivery and storage problems
described above. When the use of "lossless" coding is necessary
on a DVD Audio disc for storage and delivery of higher resolution LPCM
streams, Meridian Lossless Packing will be used.
MLP is a completely "lossless"
digital coding system which more efficiently "packs" the LPCM
data streams so more data can be stored on the DVD Audio disc and output
from the players. Typically it provides about a 2 to I savings in capacity.
MLP is bit for bit accurate.
This means that when you compare the MLP packed digital audio data being
delivered to the original high resolution digital audio data, the two
are identical, as such, the sound is not changed. (One analogy would be
to look at MLP as similar to "PK Zip" on your computer hard
drive. It "lets the air" out of the data without changing it.
)
Since MLP is a "lossless"
system, it is different than "lossy" or perceptual audio coders
like Dolby Digital, DTS or MPEG. While these systems can sound quite good,
sonically close to an original master and provide greater savings in storage
and delivery, they change the sound. They are not bit for bit accurate
like MLP.
Early on, the ISC directed
the WG4 that "lossy" coding systems would not be mandated in
the DVD Audio format.
The use of MLP on a DVD Audio
disc is at the option of the producer. It is not necessary to use MLP
on programs with lower sampling rates and word lengths in order to deliver
74 minutes of multi-channel programs, although a producer may elect to
do so. However, MLP will have to be used in order to put higher resolution
stereo (like 192kHz/24bit) or multi-channel LPCM streams (like 5.1 channels
of 96kHz/24bit) on a DVD Audio disc at all and reach 74 minutes of playing
time.
MLP will make it possible for
producers to achieve the goal of putting up to 74 minutes of a 5.1 channel
96 khz/24 bit music program on a single layer DVD Audio disc along with
a separate two channel 96kHz/24 bit stereo mix of the same program. On
a dual layered disc, a producer and artist could offer a 5.1 channel 96/24
mix and a stereo 192kHz/24bit mix. This allows the producer and artist
to present their work at the highest quality possible with the greatest
flexibility and no aesthetic compromise.
MLP is invisible to the consumer.
At the time a DVD Audio disc is authored, higher resolution LPCM audio
will be "encoded" with an MLP encoder and the MLP streams are
placed on the DVD Audio disc. When the consumer plays the disc with MLP
encoded streams, the DVD Audio player will simply decode or "unpack"
the MLP stream and output it from the player. There will be no new buttons
to push, no outboard decoders. All DVD Audio players will be able to play
MLP encoded audio programs,
Other Zones and Features
The DVD Audio format also includes
a "video zone". This area of the disc and its use are optional.
The "video zone" allows a producer to include a music video
or other type of limited length video programming along with the LPCM
audio programs on the same disc. The "video zone" on a DVD Audio
disc adheres to DVD Video specifications. Like DVD Video, the audio portion
in this zone could be PCM, Dolby Digital, MPEG or DTS. The "video
zone" could be important to producers who want their DVD Audio discs
to be compatible and play on DVD Video players and DVD ROM drives in computers.
For example, utilizing MLP, a producer could elect to put up to 74 minutes
of 5.1 channel 96/24 LPCM audio, and an additional 96/24 stereo mix of
the same program on the disc. They could also include a Dolby Digital
5.1 channel stream of the entire album (with a music video accompanying
one track) in the video zone.
This would create a disc where
the album could be enjoyed on any DVD Audio player, DVD Video player or
DVD ROM drives. (DVD ROM drives will only play DVD ROM or DVD Video discs.)
The DVD Audio format specification
will also likely include the use of an "optional audio coding"
zone. This alternative digital audio content could have other "lossy"
coded type digital audio such as Dolby Digital, DTS, MPEG, SDDS or other
digital audio formats to be invented in the future.
DVD Audio discs can also have
onscreen graphics areas and interactive areas and even web URL's can be
imbedded. This is where producers and artists can include album artwork,
liner notes and interactive lyric sheets where they could highlight a
word or phrase in a lyric which would take the listener to that section
of the song. These, like other optional features, of DVD Audio can be
used at the election of the producer.
"Smart Content"
Down Mixing
A producer and artist may decide
not to include a separate two channel stereo mix of the music program
in order to save space on the DVD Audio disc for other content. DVD Audio
provides the capability for multi-channel audio mixes on DVD Audio discs
(such as 5. 1) to fold down or "down mix" to two channels.
In DVD Audio this will be known as "Smart Content" (System Managed
Audio Resource Technique). Smart Content down mixing allows a multi-channel
mix to be heard on headphones or on a standard stereo system or portable.
This will also allow less expensive portable "Discman" type
headphone DVD players or car units to be available in the market. Use
of down mixing in these types of products make it possible for any DVD
Audio disc, regardless of the type of mix or content, to be played in
any DVD Audio player.
The Smart Content down mix
function in the DVD Audio format is not arbitrary. In fact, producers
and artists will have a degree of control over the parameters of how a
multichannel track down mixes to stereo. Producers will have control of
channel level, pans and other coefficients of the down mix which are saved
as control data and tell DVD Audio players exactly how to fold down a
multi channel mix to stereo. This gives the artist and producer the ability
to present a down mix exactly as they intend it to be heard.
Some Final Notes
As DVD Audio enters the market,
potentially four types of DVD players will exist; DVD Audio only players,
DVD Video only players, DVD Universal players and DVD ROM drives.
DVD Audio only players would
be similar to current CD players. They will only access and play the LPCM
audio streams on a DVD Audio disc. They would have no video outputs and
would be unable to play any content in the "video zone" or graphics
areas of the disc. These may be very high-end players that appeal to audiophiles
only interested in high-end audio playback or low-end players like DVD
Audio portables or headphone players.
DVD Video only players are
the players we can buy in stores today to play DVD Video discs. These
types of players will continue to flourish and grow in the market for
some time to come.
DVD Universal players will
be players that will play both DVD Audio and DVD Video discs. They will
have video outputs and be able to access any audio and video data on both
kinds of DVD discs. It is likely that eventually the market will gravitate
largely towards these kinds of players.
DVD ROM drives will read and
play DVD ROM data storage discs or interactive DVD ROM entertainment discs.
They will also play DVD Video discs and Dolby Digital audio streams.
Technology and costs will likely
cause the DVD format to evolve towards the manufacture and marketing of
DVD Universal players for the home as the defacto players. In time there
will not be a major economic impact implementing the technology required
to play DVD Audio formatted discs along with DVD Video technology in the
same player.
DVD Audio will also require
a new type of interface and connector to deliver highresolution LPCM multi-channel
digital audio. Current SPDIF (Sony/Philips Digital Interface) digital
outputs and connections are unable to deliver these new high resolution
streams. The WG4 has an ad hoc comrnittee defining these requirements
which will likely take form as an IEEE (International Electrical and Electronics
Engineers) PI 394 "firewire" connector. Most DVD Audio players
would include these connectors. Some players may also include internal
D/A converters and multi-channel analog outputs.
Special
thanks to the WG-4 and our MPGA Corporate Sponsors for this simplified
explanation, which will help all of us more efficiently understand and
implement this new format.