1. What is Dolby Digital?
Dolby Digital is an advanced form of digital
audio coding that makes it possible to store and transmit high-quality digital
sound far more efficiently than was previously possible. First used in movie
theaters in 1992, it is the result of decades spent by Dolby Laboratories
developing signal-processing systems that exploit the characteristics of human
hearing.
2. What program sources
deliver Dolby Digital audio?
Dolby Digital audio is available via laser discs,
DVD-Video discs, DVD-ROM discs for computers, digital cable systems, direct
broadcast satellite (DBS) systems, and digital broadcast TV (DTV). They are
usually identified with the Dolby Digital logo.
3. Can I hear Dolby Digital
programs over a regular stereo or Dolby Surround Pro Logic system?
Yes, you can use most of the new Dolby Digital
program sources with your current playback system, because they incorporate
Dolby Digital decoders and provide conventional analog stereo outputs. However,
with many DVD-Video players, digital cable set-top boxes, and other sources, you
need a separate multichannel Dolby Digital decoder to experience the thrilling
5.1-channel surround sound used on many Dolby Digital programs.
4. What is
"5.1-channel" Dolby Digital?
At the option of their producers, Dolby Digital
programs can deliver surround sound with five discrete full-range
channels—left, center, right, left surround, and right surround—plus a sixth
channel for those powerful low-frequency effects (LFE) that are felt more than
heard in movie theaters. As it needs only about one-tenth the bandwidth of the
others, the LFE channel is referred to as a ".1" channel (and
sometimes erroneously as the "subwoofer" channel). Figure
4-1 illustrates a typical 5.1 playback system; see question
14 for further information on speakers, including subwoofers.
5. How does 5.1-channel Dolby
Digital differ from Dolby Surround?
5.1-channel Dolby Digital provides two surround
channels to Dolby Surround’s one for more precise localization of sounds and a
more convincing, realistic ambiance. Also, the surround channels cover the
entire audible range (20 Hz – 20,000 Hz), whereas the range of Dolby
Surround’s single surround channel is limited (100 Hz – 7,000 Hz). This
further heightens realism and gives sound mixers more creative freedom.
In addition, Dolby Digital’s multiple discrete
channels enable more sharply-delineated spatial effects, and its ".1"
LFE track makes it possible to reproduce low bass effects with stunning impact
(twice as loud as the other channels).
You can also find a chart
comparing Dolby Surround and Dolby Digital.
6. Does 5.1-channel Dolby
Digital make Dolby Surround obsolete?
No, Dolby Surround will be with us for as long as
stereo is with us. This is why all Dolby Digital decoder units also incorporate
a digitally-implemented Dolby Surround Pro Logic decoder.
Dolby Surround encodes four sound channels (left,
center, right, surround) onto the two tracks of any conventional stereo program
source. Dolby Digital soundtracks, on the other hand, can be carried only by
Laserdiscs and new formats such as DVD and DTV.
Dolby Surround-encoded programs can be played
back in mono, stereo, or with a Dolby Surround Pro Logic decoder that recovers
the four original channels. Dolby Surround sources include regularly-scheduled
TV programs (over a hundred in the U.S.), plus sports and special events; a
growing number of video games, CD-ROMs, and music CDs; and broadcasts, tapes,
and discs of thousands of movies.
7. Can I hear 5.1-channel
Dolby Digital programs over a regular stereo or Dolby Surround Pro Logic system?
Yes. All Dolby Digital decoders, whether
5.1-channel or two-channel, have a unique feature called "downmixing"
that assures full compatibility with any playback system. At your option, the
decoder will create "on the fly" from 5.1-channel programs a
two-channel, Dolby Surround-encoded mix for playback over a home theater system
with Dolby Surround Pro Logic decoding; a two-channel stereo mix for regular
stereo and headphone playback; or a mono mix for playback over a mono TV set.
See Figure
7-1.
This makes it possible for producers of
5.1-channel programs to provide only the one 5.1-channel mix on the disc or
other source. The decoder in the playback system does the rest, automatically
conforming the signal to the particular playback circumstances. Note: downmixing
is not used for laser discs because they contain three different soundtracks:
stereo PCM for a conventional stereo or Dolby Surround mix; one FM track for a
mono mix; and one FM track for a 5.1-channel Dolby Digital mix. Frequently these
mixes are prepared at different times, even by different mixers, so can sound
quite different from each other.
8. Do all Dolby Digital
programs provide 5.1 channels?
No, Dolby Digital soundtracks can provide
anything from mono to full 5.1-channel surround sound. DVD- Video discs of
movies can even carry multiple versions of the soundtrack that differ in the
number of channels. A disc might contain a 5.1-channel sound mix with the
dialogue in one language, a Dolby Surround-encoded two-channel mix in another
language, and a mono track with the directors’ comments or other supplementary
information. The default soundtrack will vary from disc to disc, so always check
the DVD disc’s Language menu for the choices offered.
9. Do all DVD-Video discs have
Dolby Digital soundtracks?
Discs in countries such as the U.S. that use the
NTSC broadcast TV standard must provide at least one soundtrack in either of two
formats: Dolby Digital or PCM (the format used on CDs). In countries using the
PAL TV standard, including much of Europe, the discs must include at least one
soundtrack in any one of three formats: Dolby Digital, PCM, or MPEG. As a result
of these requirements and the options they permit, not all discs have Dolby
Digital soundtracks.
Dolby Digital, however, is the only universal multichannel
standard, and every DVD player sold worldwide incorporates Dolby Digital
decoding. Therefore the vast majority of discs do include at least one Dolby
Digital soundtrack.
10. How can I determine the
soundtrack’s format?
The Dolby Digital logo on a DVD-Video Disc or
other program source indicates that it has one or more Dolby Digital-encoded
soundtracks, but it does not indicate the number of channels. To help identify
the latter, Dolby Laboratories is encouraging the use on program packaging of
the channel-format symbols shown in Figure
10-1.
In addition, to prevent confusion at playback,
data identifying the original production format of a program’s soundtrack is
incorporated right into the Dolby Digital signal. Some Dolby Digital decoder
units use this data to display on the control panel the type of soundtrack
you’re listening to. See Figure
10-2.
11. Do all program sources
with Dolby Digital decoding provide 5.1-channel sound?
As with Dolby Digital program material,
"Dolby Digital" on a component such as a DVD player indicates that it
incorporates Dolby Digital decoding, but not the number of channels.
Many program sources, including all DVD players
and digital cable set-top boxes, provide a built-in, two-channel Dolby
Digital decoder with analog stereo outputs (see question 7).
For 5.1-channel playback, most units have a separate digital output that
provides the undecoded Dolby Digital signal (data stream) for connection to an
external unit, such as an A/V receiver, that has its own 5.1-channel Dolby
Digital decoder. See Figure
11-1.
Some DVD players do provide a built-in Dolby
Digital 5.1-channel decoder with multiple analog outputs that can be connected
directly to a "Dolby Digital-ready" receiver, as described in question
12, for 5.1-channel sound. However, you will not be able to use the internal
Dolby Digital decoder for other program sources, and these players don’t offer
the full range of options provided by external Dolby Digital decoder units. On
the other hand, they are an inexpensive way to upgrade to 5.1-channel Dolby
Digital initially, and provide a digital output so that you can later bypass the
built-in decoder and use an external one.
12. How do I convert my
current Dolby Surround Pro Logic system to 5.1-channel playback?
If your current system consists of separate audio
components, you will need a 5.1-channel Dolby Digital decoder and a six-channel
preamplifier (available separately or combined in one convenient unit). The
preamplifier’s outputs will plug into your existing power amplifiers. See Figure
12-1.
If you have an older, integrated A/V receiver
with Dolby Surround Pro Logic that does not provide external inputs to its
built-in amplifiers, you have two choices. You can replace it with a new
receiver incorporating a 5.1-channel Dolby Digital decoder, or add an external
Dolby Digital decoder unit that includes center, left surround, and right
surround amplifiers to use in place of those in your receiver (which will
continue to power the left and right front speakers). See Figure
12-2.
If you have a newer "Dolby
Digital-ready" A/V receiver with multichannel connectors for an external
Dolby Digital decoder, you can add a decoder anytime, using the receiver’s
built-in Dolby Surround Pro Logic decoder in the meantime for satisfying
home-theater sound. You can also connect the multichannel outputs of a DVD-Video
player that has built-in 5.1-channel Dolby Digital decoding to this kind of
receiver. An external decoder is still needed, however, for other Dolby Digital
program sources (see Figures 12-3
and 12-4,
and question 11).
Note: be sure that the new decoder unit has a
sufficient number of digital inputs to accommodate all the Dolby Digital program
sources you expect to use (DVD-Video player, digital cable set-top box, digital
TV receiver, etc.) Also, if you plan to play Dolby Digital laser discs, be sure
that the unit also includes the necessary RF demodulator (only Laserdiscs
require this feature; see question 24).
13. Will I still be able to
play Dolby Surround program material?
Yes, all Dolby Digital decoders include Dolby
Surround Pro Logic decoding, so you can continue to enjoy Dolby Surround-encoded
programs after converting your system.
14. Can I use my current
speakers?
Yes, you can use your current speakers, because
the configuration for Dolby Digital’s five main channels is essentially the
same as that for Dolby Surround Pro Logic (refer to Figure
4-1).
A unique feature called Bass Management lets you
route low bass sounds to those speakers in your system that are best able to
reproduce them. For example, you might wish to route the LFE signals to the
front left and right speakers, if they have full bass capabilities, or to a
separate subwoofer. You can also redirect the surround channels’ low bass, and
so continue to use the limited-bandwidth surround speakers you got for Dolby
Surround Pro Logic playback. Or you could use small satellite speakers for the
five main channels and route all bass to a subwoofer. You could even go
so far as to add a subwoofer to each main channel.
For information on speaker placement, click
here.
15. What kind of speakers
should I get for an all-new Dolby Digital system?
The ideal Dolby Digital playback system uses
identical full-range speakers for the left, center, right, and each surround
channel. If this is impossible, be sure that the overall tonal characteristic,
or timbre, of all the speakers is similar. This is equally important for both
Dolby Surround Pro Logic and Dolby Digital playback.
Most speaker manufacturers offer timbre-matched
home theater speakers. Many also offer systems combining matched compact
speakers for the five main channels with one or more separate subwoofers for the
non-directional bass; such a "satellite" system can prove an
effective, space-saving solution.
16. Should I use bipolar,
dipolar, or tripolar surround speakers?
Surround speaker placement, room acoustics, and
personal preference are as important as the speakers’ radiating
characteristic. These factors vary greatly, so Dolby Laboratories cannot
recommend a particular speaker for home theater use.
Bipolar, dipolar, and tripolar surround speakers
use speaker drivers aimed towards the front and back of the room to achieve a
diffuse soundfield like that created by the multiple surround speakers used in
movie theaters. Your room’s acoustics will effect these designs more than
conventional direct-radiating speakers, which, when placed properly, can also
make effective surround speakers.
17. My new Dolby Digital
decoder has a multi-pin "DB-25" connector, but my audio components
only have phono connectors. What do I do?
The DB-25 interface was developed by Lucasfilm
THX to save space on the rear panels of home theater products. If your
components use a mix of connectors, cables with a DB-25 (25-pin) connector on
one end and multiple phono connectors on the other are available from home
theater equipment retailers. If you wish to make your own adapter, see Figure
17-1 for the DB-25 pin configuration.
18. My DVD player has two
digital outputs, one optical and one coaxial, and my Dolby Digital decoder has
both kinds of inputs. Which should I use?
Under most conditions, optical and coaxial
digital connections work equally well. Under some rare circumstances, however,
coaxial cables, particularly very long ones, can pick up radio frequency (RF)
interference generated by household appliances, or nearby high-tension power
lines or broadcast towers.
If cost is a consideration, start with coaxial,
which is less expensive. If you then hear RF interference, you can try
relocating the cables, moving your components closer together so you can use
shorter cables, or, if all else fails, changing to costlier optical cable. If
cost is no object, using high-quality optical cables from the outset is probably
your best long-term choice.
Note: some DVD players and Dolby Digital decoders
have either a coaxial or an optical connector. Be sure that the
units you purchase both use the same type.
19. Why does my Dolby Digital
decoder provide time delays?
Because the surround speakers in a home theater
system are usually closer to viewers than the front speakers, both Dolby Digital
and Dolby Surround Pro Logic playback require a split-second electronic delay of
the surround channels.
In the case of Dolby Digital, the delay ensures
that the viewer hears sound from the surround and front speakers simultaneously.
For Dolby Surround Pro Logic, which has less inherent channel separation than
Dolby Digital, a slightly longer (by 15 milliseconds) delay ensures that sound
from the surrounds arrives just after sound from the front. This delay
reduces the audibility of sound leakage from the front to the surround speakers.
Some Dolby Digital decoder units also provide an
adjustable center-channel delay to ensure that the viewer hears sound from all
three front speakers simultaneously.
20. How to I set the surround
time delays?
The time delays in your Dolby Digital decoder are
preset to provide good results in most home theater installations. If you wish
to fine-tune the adjustments, consult the decoder’s instruction manual.
Most units allow you to adjust the surround delay
in either the Dolby Digital or Dolby Surround Pro Logic mode. Because the
relationship between the two delays is fixed, you only need to set the delay in
one mode. The decoder will automatically provide the appropriate delay whenever
you switch to the other mode.
To establish the proper surround delays for your
system, you need to know the distances between your viewing position and a front
speaker (A) and a surround speaker (B) as shown in Figure
20-1. Then, using Figure
20-2 (Dolby Pro Logic mode) or Figure
20-3 (Dolby Digital mode), find the distance to the surround speaker on the
graph’s vertical axis, and the distance to the front speaker on the horizontal
axis. Lines drawn from these points will intersect on the graph at the
recommended delay setting.
21. How do I set the center
channel time delay?
You need no center delay (set it at
"0") if your seating area is small with a few viewers at any time, and
if you can place the three front speakers equidistant from it as shown in Figure
21-1. If the center speaker must be placed more in line with the left and
right speakers as shown in Figure
21-2, thereby bringing it closer to the viewers than the left and right
speakers, add 1 ms of delay for each foot it is closer. For example, if
dimension C in Figure
21-2 is one foot less than L or R, set the time delay to 1 ms.
If your seating area is wide, with several
viewers in a row as in a movie theater, it’s best to place the left, right,
and center speakers all in line as shown in Figure
21-3 and use no center time delay.
Finally, if it is necessary to place the center
speaker further back than the left and right speakers (not illustrated), select
"–1 ms" (for a one foot difference) delay or "–2 ms"
(two feet) delay, if these options are provided. If they are not provided, use
"0" delay.
22. What is the "Midnight
Mode" on my new Dolby Digital A/V receiver?
Dolby Digital soundtracks can have a very wide
dynamic range between soft and loud sounds. At a full playback level, this can
be thrilling, providing a truly theatrical experience, particularly for movies.
Late at night, however, the loud sound effects on
movie soundtracks might not be appreciated by your family or neighbors. But if
you turn down the volume to "keep the peace," the dialogue will be
hard to hear, and subtle low-level effects may get lost altogether.
What you’d really like to do is turn down
the volume on just the loud effects, turn up the volume on quiet sounds,
and keep the dialogue at the same level. This is what a Dolby Digital
feature called Dynamic Range Control does. For lower-level listening, it applies
dynamic range compression that preserves low-level sounds, prevents dramatic
passages from getting too loud, and keeps dialogue intelligible. To help explain
its function, manufacturers of Dolby Digital decoders have come up with names
like "Midnight Mode" for this feature. See Figure
22-1.
The amount of compression is not arbitrary, but
is decided in advance by the soundtrack’s producers, and coded right onto the
soundtrack. For example, if there’s a loud sound coming from the left surround
that’s meant to startle the audience, the soundtrack mixers might indicate
less compression for that sound than other loud sounds when the Midnight Mode is
switched on.
Some Dolby Digital decoders let you select
various amounts of the available compression (e.g., 50%, 75%, 100%), while
others provide only 100% when the compression mode is selected.
23. I’m tired of having to
adjust the volume every time the program changes or I switch sources or TV
channels. Can Dolby Digital help?
Yes. Conventional program sources often force you
to readjust the volume when you change channels or play a different video. With
Dolby Digital program sources, on the other hand, a feature called Dialogue
Normalization lets you set playback volume and forget it.
Dialogue Normalization automatically adjusts the
volume when you change Dolby Digital programs so that the level of the dialogue
remains constant. It does not alter the dynamic range, only the overall playback
level, based on data about the dialogue that is coded into the program signal.
See Figures 23-1
and 23-2.
With Dialog Normalization, you can "channel
surf" when watching digital TV without having to adjust the volume each
time you change channels. Those extra-loud commercials will be tamed. And you
can play an evening’s worth of DVD-Video discs without ever touching the
volume control.
24. What equipment do I need
to play Dolby Digital Laserdiscs?
Dolby Digital Laserdisc players do not have a
built-in Dolby Digital decoder; they output the Dolby Digital data stream from
Laserdiscs on a radio-frequency (RF) carrier that requires demodulation prior to
Dolby Digital decoding. If you wish to play the Dolby Digital soundtrack,
therefore, be sure that the disc player has a "Dolby AC-3 RF" output
and your Dolby Digital decoder unit includes an RF demodulator specifically for
laser disc use. Not all decoder units provide the demodulator, because no other
Dolby Digital program source, including DVD, requires it.
If you have an older laser disc player and
don’t want to decode the Dolby Digital tracks on newer discs, they will play
perfectly well on your older player. On laser discs, the Dolby Digital track is
encoded on what used to be the right FM analog track, leaving the standard PCM
digital stereo tracks intact for conventional stereo and Dolby Surround Pro
Logic playback. The left FM analog track may contain a mono version of the
soundtrack, commentary, or other material. See Figure
24-1.
One final note: laser discs with 5.1-channel
Dolby Digital soundtracks were originally identified as Dolby Surround AC-3,
"AC-3" being the technical term for the digital audio coding upon
which Dolby Digital is based. Today the simpler term "Dolby Digital"
is used instead to identify laser discs, like other formats, that have Dolby
Digital soundtracks.
25. What equipment do I need
to receive satellite broadcasts with 5.1-channel Dolby Digital sound?
To receive the 5.1-channel soundtrack on programs
such as the Dolby Digital-encoded theatrical films broadcast by DIRECTV on its
pay-per-view channels, you need a new satellite receiver that provides a Dolby
Digital output (the "Digital" output on conventional satellite
receivers does not handle Dolby Digital signals). As with other Dolby Digital
components, you connect the Dolby Digital coaxial or optical output to a digital
input on your Dolby Digital decoder unit.
Standard stereo audio, usually Dolby
Surround-encoded, is broadcast simultaneously, so you need a new satellite
receiver only if you wish to receive the Dolby Digital soundtrack.
26. How can I receive
high-definition (HD) satellite TV transmissions with Dolby Digital sound?
To receive high-definition programming
transmitted via satellite, you will need an HD satellite receiver, and a new,
larger dish. Most receivers are equipped like most DVD-Video players, that is,
with a built-in, two-channel Dolby Digital decoder with analog stereo out puts,
and a Dolby Digital output for 5.1-channel sound with an external 5.1-channel
Dolby Digital decoder unit.
27. Can I use my Dolby Digital
home theater system for the new digital TV broadcasts?
Yes, you can: whether high-definition (HDTV) or
standard-definition (SDTV), all digital television (DTV) broadcasts use Dolby
Digital audio in the U.S., Canada, and other countries that have adopted the
ATSC television standard (www.atsc.org).
Dolby Digital audio is also used in some countries like Australia in conjunction
with other transmission standards.
Like other Dolby Digital program material, the
audio format of DTV broadcasts varies from mono to full 5.1-channel surround
sound. Because most TV facilities have to gear up for multichannel audio
production and distribution, many DTV programs are being broadcast initially in
two-channel Dolby Digital (some-times Dolby Surround-encoded for four-channel
playback with Dolby Surround Pro Logic decoding). Movies prepared with
5.1-channel Dolby Digital soundtracks for theatrical release are the primary
source of multichannel Dolby Digital DTV programming initially.
DTV sets and tuners also vary with respect to the
number of audio channels they provide. Some have built-in two-channel Dolby
Digital decoders, while others have 5.1-channel Dolby Digital decoders for
connection to a Dolby Digital-ready A/V receiver (see question 12).
Others include not only Dolby Digital decoding but also amplifiers and speakers.
Most DTV receivers have a separate digital output
that provides the undecoded Dolby Digital signal for connection to your A/V
receiver or other unit with 5.1-channel Dolby Digital decoding. As a result, you
can add a DTV receiver to your Dolby Digital home theater system at any time,
without fear of obsolescence or costly equipment duplication.
28. How can I receive digital
cable programs with Dolby Digital sound?
Digital cable television systems can provide you
with another source of programming for Dolby Digital playback. The number of
channels with Dolby Digital cable transmissions varies as with DTV broadcasts
and other Dolby Digital program sources. All digital cable set-top boxes
incorporate a two-channel Dolby Digital decoder with stereo analog outputs for
compatibility with conventional stereo and home theater systems, and
fully-compliant units also provide a digital output for connection to your A/V
receiver or other unit for 5.1-channel playback with an external multichannel
Dolby Digital decoder.
29. Can I purchase DVD-Video
discs with Dolby Digital soundtracks in Europe?
Yes, Dolby Digital audio is used on DVD-Video
discs throughout the world (see question 9). A word of
caution, though: while the audio and video technical standards are the same for
DVD-Video worldwide, discs purchased in one part of the world may not play at
all in another part of the world due to "regional coding."
Regional coding was developed at the behest of
the film industry, which often releases videos of movies in North America before
they’ve even reached movie theaters in other parts of the world. Discs
released in a market like North America (Region 1) can be coded to play only on
players manufactured for that market. Take a Region 1-coded disc to another part
of the world, and it won’t work in the players manufactured for that market.
Some discs, on the other hand, are coded as Region 0, meaning they will
play on all DVD players in all regions. Check the packaging to find out if the
disc has any regional limitations.
30. I’m thinking about
getting a PC with a DVD-ROM drive. Will it give me 5.1-channel Dolby Digital?
That depends on the computer’s configuration
and its DVD/audio card, so check the specifications carefully to be sure you get
the audio capability you want.
Most PCs can be connected to external playback
equipment ranging from amplified stereo speakers to a full 5.1-channel playback
system. The latter include not only full home theater systems, but compact,
multichannel PC speaker systems incorporating amplifiers and Dolby Digital
and/or Dolby Surround Pro Logic decoding. PCs bundled with stereo amplifiers and
speakers may have a built-in two-channel Dolby Digital decoder for the DVD-ROM
player that will downmix 5.1-channel soundtracks.
Audio outputs possible on PCs include analog
power amplifier outputs, line level outputs for external amplifiers, headphone
jacks, and RF carriers. They may also provide conventional PCM digital audio
outputs, and/or outputs that provide the undecoded Dolby Digital data stream via
S/PDIF (IEC 1937), USB, IEEE 1394, LAN, wireless link, or other format
connections.
31. I’ve heard about
surround-sound systems that use only two speakers. Do they really work?
Thanks to the increasing power and dropping cost
of signal-processing chips, sound that appears to come from around the listener
is now possible with just two speakers.
These audio processing systems, often referred to
as "virtual surround," require that the listener be positioned in a
specific "sweet spot" in front of the two speakers. As a result, they
cannot substitute for multiple speakers in home theater applications. But for
PCs, where a single user is seated directly in front of the monitor, they can
provide a cost-effective alternative to a full, multispeaker surround setup.
There are two fundamental approaches to
"virtual surround." Some systems take any stereo program source and
create what might be called a "pseudo-surround" effect. The results
are inconsistent, however, and can create effects not at all intended by the
program producers. The other approach has been developed by Dolby Laboratories
to reproduce the actual surround information recorded on multichannel Dolby
Surround and Dolby Digital program material, for a more accurate and consistent
listening experience.
Called Virtual Dolby Surround and Virtual
Dolby Digital, these processes start by decoding, respectively, the original
four or 5.1 channels of the program material. The center channel is then divided
equally between the left and right speakers to create a phantom center image,
while the surround signals are processed by a "virtualizer" circuit
and mixed in with the main left and right channel signals. The virtualizer
processor adds cues to the surround signals based on how we identify a sound’s
directionality, thereby creating virtual surround images from just two speakers.
Unlike a multispeaker home theater system, the center and surround effects are
affected only by the listener’s position relative to the two speakers, and not
by room acoustics. See Figures 31-1
and 31-2.
Dolby offers its own virtualizer technique, and
also tests and approves other virtualizers for use in combination with Dolby
Surround Pro Logic and Dolby Digital decoding to create Virtual Dolby Surround
and Virtual Dolby Digital. Approved technologies include 3D Phonic, Virtual
Sonic, VMAx, A3D, Qsurround, N-2-2 DVS, TruSurround, Sensaura, RSX, and
Incredible 3D Surround.
Products with Virtual Dolby Surround and/or
Virtual Dolby Digital include DVD players, PC/DVD packages, television sets, and
multimedia speaker systems.
32. Is it possible to get
surround sound over headphones?
Yes, it is now possible to achieve convincing
surround sound over conventional stereo headphones from both 5.1-channel Dolby
Digital and four-channel Dolby Surround encoded programs, thanks to a new
development called Dolby
Headphone.
Dolby Headphone is a signal processor that
recreates the sound of a real five-speaker surround system playing multichannel
material in an actual listening room. It works by electronically imparting to
each audio channel—two on stereo programs and up to five on surround
programs—the unique sonic signature of a corresponding speaker properly placed
in a carefully-defined acoustic environment. The processor combines the
signatures into two composite channels that deliver to your ears, via
conventional stereo headphones, the sum of the direct and reflected sounds from
each virtual speaker in the virtual room. See Figure
32-1
With conventional headphone listening, by
contrast, the sound is fed directly to your eardrums without these spatial cues.
The result is an unnaturally flat image, with left and right sounds directly
beside you, and center sounds within your head. This "in-head" effect
is not only unrealistic, but can also be tiring. Dolby Headphone works with both
stereo and multichannel program sources, and can be included in just about any
product with a headphone jack, including set-top boxes; VCRs; DVD players;
conventional and digital TV receivers; PCs and laptops; and A/V surround
decoders and receivers.
33. Does Dolby plan to
introduce higher data-rate versions of Dolby Digital?
While all Dolby Digital decoders are capable of
handling data rates as high as 640 kbps, the 448 kbps rate used on DVDs (the
maximum allowable) provides sound quality that critical listening tests have
consistently confirmed as on a par with original master tapes. It would require
the introduction of a new delivery format before the option to use more than 448
kb/s could even be considered.
34. I've heard that Dolby
Digital isn't as good as some competing systems. Is this true?
No, it's not. In fact, most such statements are
based on the assumption that "more is better," and not on proper
listening tests.
To understand what's going on with respect to
Dolby Digital and competing systems, you first need to know something about
"perceptual" digital audio coding. Conventional digital audio, like
that used on CDs, encodes the audio using 16-bit PCM (pulse-code modulation),
which expresses the value of the original analog waveform in each channel to one
of 65,536 possible levels, 44,100 times per second. This already takes up a lot
of storage and transmission space for plain stereo, so providing multiple
channels of PCM for surround sound has been impractical, particularly for media
that also has to carry data-hungry digital video.
As a result, Dolby Digital and other coding
systems (such as MPEG, DTS, and ATRAC) have been developed to save space by
transmitting only the data that's necessary to portray the original sound based
on what we can actually hear, as opposed to delivering the original PCM data
bit-for-bit. As a result, these are sometimes known as "perceptual" or
"lossy" coding systems.
A complex coding scheme, or algorithm,
processes the audio signal based on the principles of psychoacoustics, the
science of how we perceive sound. The coder takes advantage of noise and
frequency masking to allow capturing the audio with just enough precision to
make sure it sounds the same as the original. The more powerful the algorithm,
the more precisely the encoder can shave bits without affecting the perceived
sound quality. This is why some coder technologies such as Dolby Digital can
sound just as good as others like DTS which use from two to four times as much
data.
Dolby Digital's algorithm, technically known as
Dolby AC-3, is based upon Dolby Laboratories' decades of research into how we
perceive sound, and delivers 5.1 channels of audio with up to 24-bit precision
at an even higher (48 kHz) sample rate than conventional CDs. As a result, Dolby
Digital achieves very high sound quality at the low data rates necessary for
uncompromised picture quality on such formats as DVD-Video discs and DTV digital
television broadcasts.
35. Will Dolby provide
5.1-channel music CDs?
Nobody is more enthusiastic about multichannel
sound than Dolby Laboratories. However, we acknowledge the importance of
standardization and compatibility, and the reluctance of producers to release,
and retailers to stock, more than one version of a given program. As a result we
will not release "CDs" with Dolby Digital soundtracks, or encourage
others to do so.
5.1-channel music CDs are nonstandard; that is,
they have to use some other form of audio coding than the PCM audio specified by
the CD "Red Book" audio specification. They are also incompatible in
that they cannot be heard at all without a special external decoder, and in many
cases require a specially equipped CD player as well.
Newer formats like DVD-Video discs, on the other
hand, have been developed from the outset to support multichannel digital audio.
For greater compatibility, these new formats can carry more than one kind of
soundtrack. In addition, as explained in question 7, sources
with Dolby Digital soundtracks are inherently compatible with mono, stereo, and
Dolby Surround Pro Logic playback systems.
For older stereo formats, including CD, there's
tried-and-true Dolby Surround, a fully compatible format enabling two-channel
soundtracks to be encoded with four-channel surround sound (see question
6). There are currently more than 700 CD titles encoded with Dolby Surround.
When it comes to music-only recordings with
multichannel digital audio, a new kind of DVD disc called DVD-Audio has been
developed (see question 36).
36. Can I play the new
DVD-Audio discs on my DVD-Video player or my computer's DVD-ROM drive?
Yes…and no.
The new DVD-Audio discs can have two zones, one
to carry high-resolution PCM audio, and the other, which is optional, to carry
video type programs, such as an artist interview, music videos, or music
recordings with still pictures.
The audio zone can be played only on the new
DVD-Audio players (which typically will also play DVD-Video discs). The optional
video zone, however, has the same specifications as DVD-Video discs, so this
portion will play on any DVD-Video player and DVD-Video capable ROM
drive. This makes it possible for producers to incorporate in the video zone a
Dolby Digital version of the same program in the audio zone. If they do so, you
will be able to buy DVD-Audio discs as soon as they are available, and enjoy
them with Dolby Digital sound until such time you decide to get a new DVD-Audio
player to "unlock" their higher-resolution audio and other unique
program features.
All
Dolby Digital related content is Copyright © 2002 Dolby Laboratories, Inc. All
rights reserved.
Dolby and the double-D symbol are registered trademarks of Dolby Laboratories.
All other trademarks are the property of their respective owners.
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