3.6.2 Audio details of DVD-Video
Description
This article is from the DVD Formats FAQ, by jtfrog@usa.net (Jim Taylor) with numerous contributions by others.
3.6.2 Audio details of DVD-Video
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. Some DVD-Video discs contain mostly audio with only video
still frames.
A DVD-Video disc can have up to 8 audio tracks (streams). Each track can be
in one of three formats:
* Dolby Digital (formerly AC-3): 1 to 5.1 channels
* MPEG-2 audio: 1 to 5.1 or 7.1 channels
* PCM: 1 to 8 channels.
Two additional optional formats are provided: DTS and SDDS. Both require
external decoders and are not supported by all players.
The ".1" refers to a low-frequency effects (LFE) channel that connects to a
subwoofer. This channel carries an emphasized bass audio signal.
Linear PCM is uncompressed (lossless) digital audio, the same format used
on CDs and most studio masters. 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 bit rate is 6.144 Mbps, which
limits sample rates and bit sizes when there are 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
and less than 24 bits.
Dolby Digital is multi-channel digital audio, using lossy AC-3 coding
technology from original PCM with a sample rate of 48 kHz at up to 24 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). (Most Dolby Digital decoders support up to 640 kbps.) 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. For details see ATSC document A/52
<http://www.atsc.org/document.html>. Dolby Digital is the format used for
audio tracks on almost all DVDs.
MPEG audio is multi-channel digital audio, using lossy compression 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.) MPEG Layer III (MP3) and MPEG-2 AAC (aka NBC or
unmatrix) are not supported by the DVD-Video standard.
DTS (Digital Theater Systems) Digital Surround is an optional multi-channel
(5.1) digital audio format, using lossy compression from PCM at 48 kHz at
up to 20 bits. The data rate is from 64 kbps to 1536 kbps, with typical
rates of 768 and 1536. (The DTS Coherent Acoustics format supports up to
4096 kbps variable data rate for lossless compression, but this isn't
supported by DVD.). 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.
The DVD standard includes an audio stream format reserved for DTS, but many
players ignore it. According to DTS, existing DTS decoders work with DTS
DVDs. The DTS format used on DVDs is different from the one used in
theaters (Audio Processing Technology's apt-X, an ADPCM coder, not a
psychoacoustic coder). All DVD players can play DTS audio CDs, since the
standard PCM stream holds the DTS code. See 1.32 for general DTS
information. For more info visit <http://www.dtstech.com> and read Adam
Barratt's article for Movie Sound Page.
SDDS (Sony Dynamic Digital Sound) 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. SDDS is a theatrical film soundtrack format based on the
ATRAC compression format that is also used by Minidisc. Sony has not
announced any plans to support SDDS on DVD.
THX (Tomlinson Holman Experiment) is not an audio format. It's a
certification and quality control program that applies to sound systems and
acoustics in theaters, home equipment, and digital mastering processes. The
THX Digital Mastering program uses a patented process to track video
quality through the multiple video generations needed to make a final
format disc or tape, setup of video monitors to ensure that the filmmaker
is seeing a precise rendition of what is on tape before approval of the
master, and other steps along the way. THX-certified "4.0" amplifiers
enhance Dolby Pro Logic: crossover sends bass from front channels to
subwoofer; re-equalization on front channels (compensates for
high-frequency boost in theater mix designed for speakers behind the
screen); timbre matching on rear channels; decorrelation of rear channels;
bass curve that emphasizes low frequencies. THX-certified "5.1" amplifiers
enhance Dolby Digital and improve on 4.0: rear speakers are now full range,
so crossover sends bass from both front and rear to subwoofer;
decorrelation is turned on automatically when rear channels have the same
audio, but not during split-surround effects, which don't need to be
decorrelated.
Discs containing 525/60 video (NTSC) must use PCM or Dolby Digital on at
least one track. Discs containing 625/50 video (PAL/SECAM) must use PCM or
MPEG audio or Dolby Digital on at least one track. Additional tracks may be
in any format. A few first-generation players, such as those made by
Matsushita, can't output MPEG-2 audio to external decoders.
The original spec required either MPEG audio or PCM on 625/50 discs. There
was a brief scuffle led by Philips when early discs came out with only
two-channel MPEG and multichannel Dolby Digital, but the DVD Forum
clarified in May 1997 that only stereo MPEG audio was mandatory for 625/50
discs. In December 1997 the lack of MPEG-2 encoders (and decoders) was a
big enough problem that the spec was revised to allow Dolby Digital as the
only audio track on 625/50 discs.
For stereo output (analog or digital), all players have a built-in
2-channel Dolby Digital decoder that downmixes from 5.1 channels (if
present on the disc) to Dolby Surround stereo (i.e., 5 channels are phase
matrixed into 2 channels to be decoded to 4 by an external Dolby Pro Logic
processor). PAL players also have an MPEG or MPEG-2 decoder. 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 labeled 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. Sony players can optionally downmix to non-surround stereo.
If surround audio is important to you, you will hear significantly better
results from multichannel discs if you have a Dolby Digital system.
The new Dolby Digital Surround EX (DD-SEX?) format, which adds a rear
center channel, is compatible with DVD discs and players, and with existing
Dolby Digital decoders. The new DTS Digital Surround ES (DTS-ES) format,
which likewise adds a rear center channel, works fine with existing DTS
decoders and with DTS-compatible DVD players. However, for full use of both
new formats you need a new decoder to extract the rear center channel,
which is phase matrixed into the two standard rear channels in the same way
Dolby Surround is matrixed into standard stereo channels. Without a new
decoder, you'll get the same 5.1-channel audio you get now. Because the
additional rear channel isn't a full-bandwidth discrete channel, it's
appropriate to call the new formats "5.2-channel" digital surround.
The Dolby Digital downmix process does not usually 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. The
dynamic range compression (DRC) feature, often called midnight mode,
reduces the difference between loud and soft sounds so that you can turn
the volume down to avoid disturbing others yet still hear the detail of
quiet passages. Some players have the option to turn off DRC. 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.
Dolby Digital also includes a feature called dialog normalization, which
could more accurately be called volume standardization. DN is designed to
keep the sound level the same when switching between different sources.
This will become more important as additional Dolby Digital sources
(digital satellite, DTV, etc) become common. Each Dolby Digital track
contains loudness information so that the receiver can automatically adjust
the volume, turning it down, for example, on a loud commercial. (Of course
the commercial makers can cheat and set an artificially low DN level,
causing your receiver to turn up the volume during the commercial!) Turning
DN on or off on your receiver has no effect on dynamic range or sound
quality, its effect is no different than turning the volume control up or
down.
All five DVD-Video 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).
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.
Many people complain that the audio level from DVD players is too low. In
truth the audio level is too high on everything else. Movie soundtracks are
extremely dynamic, ranging from near silence to intense explosions. In
order to support an increased dynamic range and hit peaks (near the 2V RMS
limit) without distortion, the average sound volume must be lower. This is
why the line level from DVD players is lower than from almost all other
sources. And so far, unlike on CDs and LDs, the level is much more
consistent between discs.
For more information about multichannel surround sound, see Bobby
Owsinski's FAQ at <http://www.surroundassociates.com/safaq.html>.
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