DOLBY DIGITALaboutQUESTIONS

Frequently askedFrequently asked

QUESTIONSabout DOLBY DIGITAL

Table of Contents1. What is Dolby Digital? ............................................................................................................................................ 1

2. What program sources deliver Dolby Digital audio? ......................................................................................... 1

3. Can I hear Dolby Digital programs over a regular stereo or Dolby Surround Pro Logic system? .............. 1

4. What is “5.1-channel” Dolby Digital? .................................................................................................................... 1

5. How does 5.1-channel Dolby Digital differ from Dolby Surround? .................................................................. 1

6. Does 5.1-channel Dolby Digital make Dolby Surround obsolete? .................................................................... 2

7. How can I hear a 5.1-channel Dolby Digital soundtrack over

a regular stereo or Dolby Surround Pro Logic system? .................................................................................... 2

8. Do all Dolby Digital programs provide 5.1 channels? ....................................................................................... 2

9. Do all DVD-Video discs have Dolby Digital soundtracks? ................................................................................ 2

10. How can I determine the soundtrack’s format? ................................................................................................. 2

11. Do all program sources with Dolby Digital decoding provide 5.1-channel sound? ...................................... 3

12. How do I convert my current Dolby Surround Pro Logic system to 5.1-channel playback? ....................... 3

13. Will I still be able to play Dolby Surround program material? ......................................................................... 4

14. Can I use my current speakers? ............................................................................................................................ 4

15. What kind of speakers should I get for an all-new Dolby Digital system? ...................................................... 4

16. Should I use bipolar, dipolar, or tripolar surround speakers? ......................................................................... 4

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? ...................................................... 5

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? ............................................ 5

19. Why does my Dolby Digital decoder provide time delays? .............................................................................. 5

20. How to I set the surround time delays?............................................................................................................... 5

21. How do I set the center channel time delay? ..................................................................................................... 5

22. What is the “Midnight Mode” on my new Dolby Digital A/V receiver? ........................................................... 6

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? .......................................................................... 6

24. What equipment do I need to play Dolby Digital Laserdiscs? ......................................................................... 6

25. What equipment do I need to receive satellite broadcasts with 5.1-channel Dolby Digital sound? .......... 7

26. How can I receive high-definition (HD) satellite TV transmissions with Dolby Digital sound? .................. 7

27. Can I use my Dolby Digital home theater system for the new digital TV broadcasts? ................................ 7

28. How can I receive digital cable programs with Dolby Digital sound?............................................................. 7

29. Can I purchase DVD-Video discs with Dolby Digital soundtracks in Europe? ............................................... 7

30. I’m thinking about getting a PC with a DVD-ROM drive. Will it give me 5.1-channel Dolby Digital? .......... 8

31. I’ve heard about surround-sound systems that use only two speakers. Do they really work? .................. 8

32. Is it possible to get surround sound over headphones? .................................................................................. 9

33. Does Dolby plan to introduce higher data-rate versions of Dolby Digital? ................................................... 9

34. I’ve heard that Dolby Digital isn’t as good as some competing systems. Is this true? ................................ 9

35. Will Dolby Laboratories provide 5.1-channel music CDs? .............................................................................. 10

36. Can I play the new DVD-Audio discs on my DVD-Video player or my computer’s DVD-ROM drive? ....... 10

How Dolby Digital Works ............................................................................................................................................. 11

Dolby Digital vs. Dolby Surround comparison chart ............................................................................................... 12

Dolby Surround/Dolby Digital delivery paths chart ................................................................................................ 13

Frequently Asked Questionsabout Dolby Digital

olby Digital multi-channel audio is thenext step forward insonic realism and

listener involvement. It hasalready changed the way youexperience your favorite films inmovie theatres, and is now doingthe same for video programmingat home.

1. What is Dolby Digital?

Dolby Digital is an advancedform of digital audio coding thatmakes it possible to store andtransmit high-quality digitalsound far more efficiently thanwas previously possible. Firstused in movie theatres in 1992, itis the result of decades ofexperience by Dolby Laboratoriesin developing signal processingsystems that exploit thecharacteristics of human hearing.

2. What program sourcesdeliver Dolby Digitalaudio?

Dolby Digital audio is avail-able from laser discs, DVD-Videodiscs, DVD-ROM discs for com-puters, digital cable systems,direct broadcast satellite (DBS)systems, and digital broadcast TV(DTV). Discs are usually markedwith the logo.

3. Can I hear DolbyDigital programs over aregular stereo or DolbySurround Pro Logicsystem?

Yes, you can use most of thenew Dolby Digital programsources with your current play-back system, because they haveDolby Digital decoders built in toprovide conventional analogstereo outputs.

However, to experience thethrilling 5.1-channel surroundsound used on many Dolby Digitalprograms, you typically need aseparate unit, such as an A/Vreceiver, that incorporates a 5.1-channel Dolby Digital decoder.

4. What is “5.1-channel”Dolby Digital?

At the option of their pro-ducers, Dolby Digital programscan deliver surround sound withfive discrete full-range channels—left, center, right, left surround,and right surround—plus a sixthchannel for those powerful low-frequency effects (LFE) that arefelt more than heard in movietheatres. As it covers only aboutone-tenth the audible bandwidthof the other channels, LFE isreferred to as a “.1” channel.Figure 1 illustrates a typical 5.1

playback system; see question 14on page 4 for further informationon speakers, includingsubwoofers.

5. How does 5.1-channelDolby Digital differ fromDolby Surround?

5.1-channel Dolby Digitalprovides two surround channelsto Dolby Surround’s one for moreprecise localization of soundsand a more convincing, realisticambience. Also, the surroundchannels cover the entire audiblerange (20–20,000 Hz), whereasthe range of Dolby Surround’ssingle surround channel islimited to 7,000 Hz. This furtherheightens realism and givessound mixers more creativefreedom.

In addition, Dolby Digital’smultiple discrete channels enablemore sharply delineated spatialeffects, and its “.1” LFE trackmakes it possible to reproducelow bass effects with stunningimpact (twice as loud as theother channels).

On page 12 you will find achart comparing Dolby Surroundand Dolby Digital, and on page 13a diagram of how each works atthe mixing, mastering, andplayback stages.

D

1

Figure 1: Dolby Digital can deliver up to 5.1 discrete sound channels for the ultimate in surround sound.

Dolby Digitalprogram source (DVDplayer, DTV receiver,digital cable box,etc.)

Leftsurround

Audio electronics withDolby Digital decoding

LFE

Rightsurround

Left

Center

Right

2

6. Does 5.1-channelDolby Digital makeDolby Surroundobsolete?

No, Dolby Surround will bewith us for as long as stereo iswith us. This is why all DolbyDigital decoder units also incor-porate a digitally implementedDolby Surround Pro Logicdecoder.

Dolby Surround encodes foursound channels (left, center,right, surround) onto the twotracks of any conventional stereoprogram source, either analog ordigital. Dolby Digital soundtracks,on the other hand, can be carriedonly by digital formats such asDVD and DTV.

Dolby Surround-encodedprograms can be played back inmono, stereo, or with a DolbySurround Pro Logic decoder thatrecovers the four original chan-nels. Dolby Surround sourcesinclude regularly scheduled TVprograms (currently over 100 inthe U.S.), plus sports and specialevents; a growing number ofvideo games, CD-ROMs, andmusic CDs; and broadcasts,tapes, and video discs of thou-sands of movies.

7. Can I hear 5.1-channelDolby Digital programsover a regular stereo orDolby Surround Pro Logicsystem?

Yes. All Dolby Digitaldecoders, whether 5.1-channel ortwo-channel, have a uniquefeature called “downmixing” thatassures full compatibility withany playback system. On 5.1-channel programs, the decoderscan create several types of mixes“on the fly.” These include a two-channel, Dolby Surround-encoded mix for playback over ahome theater system with DolbySurround Pro Logic decoding; atwo-channel stereo mix forregular stereo and headphoneplayback; or a mono mix forplayback over a mono TV set (seeFigure 2).

This makes it possible forproducers of 5.1-channel pro-grams to provide only the one5.1-channel mix on the disc or

other source. The decoder in theplayback system does the rest,automatically conforming thesignal to the particular playbackcircumstances. Note: downmixingis not used for laser discsbecause they contain threedifferent soundtracks: stereoPCM for a conventional stereo orDolby Surround mix; one FMtrack for a mono mix; and one FMtrack for a 5.1-channel DolbyDigital mix. Frequently thesemixes are prepared at differenttimes, even by different mixers,so they can sound quite differentfrom each other.

8. Do all Dolby Digitalprograms provide 5.1channels?

No, Dolby Digital soundtrackscan provide anything from monoto full 5.1-channel surroundsound. DVD-Video discs ofmovies can even carry multipleversions of the soundtrack thatdiffer in the number of channels.A disc might contain a 5.1-channel sound mix with thedialogue in one language, a DolbySurround-encoded two-channelmix in another language, and amono track with the director’scomments or other supplemen-tary information. The defaultsoundtrack will vary from disc todisc, so always check the DVDdisc’s Language menu for thechoices offered.

9. Do all DVD-Video discshave Dolby Digitalsoundtracks?

Discs distributed in countriesthat use the NTSC broadcast TVstandard (such as the U.S.) mustprovide at least one soundtrackin either of two formats: DolbyDigital or PCM (the format usedon CDs). In countries using thePAL TV standard (including muchof Europe), the discs mustinclude at least one soundtrack inany one of three formats: DolbyDigital, PCM, or MPEG. As a resultof these requirements and theoptions they permit, not all discshave Dolby Digital soundtracks.

Dolby Digital, however, is theonly universal multichannelstandard, and every DVD playersold worldwide incorporatesDolby Digital decoding. Thereforethe vast majority of discs doinclude at least one Dolby Digitalsoundtrack.

10. How can I determinethe soundtrack’s format?

The logo on a DVD-Video Disc or other programsource indicates that it has oneor more Dolby Digital-encodedsoundtracks, but it does notindicate the number of channels.To help identify the latter, Dolby

Figure 3: Many DVD-Video discs and otherDolby Digital sources display symbolsidentifying the soundtrack’s channel format.

Figure 2: Dolby Digital decoders automaticallyconfigure the signal to the particular playbackcircumstances.

Figure 4: Some Dolby Digital decoders feature a display activated by theformat data on Dolby Digital soundtracks.

Mono Stereo Dolby Surround 5.1-channel

Laboratories is encouraging theuse on program packaging of thechannel format symbols shown inFigure 3.

In addition, to preventconfusion at playback, dataidentifying the originalproduction format of a program’ssoundtrack is incorporated rightinto the Dolby Digital datastream. Some Dolby Digitaldecoder units use this data todisplay on the control panel thetype of soundtrack you’relistening to (see Figure 4).

11. Do all programsources with DolbyDigital decoding provide5.1-channel sound?

As with Dolby Digital pro-gram material, the phrase “DolbyDigital” on a component such asa DVD player means only that itincorporates Dolby Digitaldecoding; it does not indicate thenumber of channels.

Many program sources,including DVD players and digitalcable set-top boxes, provide abuilt-in two-channel Dolby Digitaldecoder with analog stereooutputs (see question 7). For 5.1-channel playback, most unitshave a separate digital outputthat provides the undecodedDolby Digital signal (data stream)for connection to an externalunit, such as an A/V receiver, thathas its own 5.1-channel DolbyDigital decoder. See Figure 5.

Some DVD players do pro-

vide a built-in Dolby Digital 5.1-channel decoder with multipleanalog outputs that can beconnected directly to a “DolbyDigital-ready” receiver, as

described in question 12, for 5.1-channel sound. However, you willnot be able to use the internalDolby Digital decoder for otherprogram sources, and theseplayers don’t usually offer the fullrange of options provided byexternal Dolby Digital decoderunits. On the other hand, they arean inexpensive way to upgrade to5.1-channel Dolby Digital initially,and provide a digital output sothat you can later bypass thebuilt-in decoder and use anexternal one.

12. How do I convert mycurrent Dolby SurroundPro Logic system for 5.1-channel playback?

If your current systemconsists of separate audiocomponents, you will need a 5.1-channel Dolby Digital decoderand a six-channel preamplifier

(available separately orcombined in one convenientunit). The preamplifier’s outputswill plug into your existing poweramplifiers (see Figure 6-a).

If you have an older,integrated A/V receiver withDolby Surround Pro Logic thatdoes not provide external inputsto its built-in amplifiers, you havetwo choices. You can replace itwith a new receiver incorporatinga 5.1-channel Dolby Digitaldecoder, or add an externalDolby Digital decoder unit thatincludes center, left surround,and right surround amplifiers touse in place of those in yourreceiver (which will continue topower the left and right frontspeakers). This same solutionalso works for upgrading aconventional stereo system (seeFigure 6-b).

Figure 5: Most DVD-Video players providestereo (two-channel) analog audio outputs (left),plus at least one Dolby Digital output (right).The unit above has two different kinds of DolbyDigital outputs, co-ax and optical; see question18 on page 4 for advice on which to use whenyou have the choice.

3

Figure 6. Converting to 5.1-channel playback.

Figure 6-a.

Figure 6-b.

Figure 6-c.

Figure 6-d.

4

If you have a newer “DolbyDigital-ready” A/V receiver withmultichannel inputs for anexternal Dolby Digital decoder,you can add a decoder anytime,using the receiver’s built-in DolbySurround Pro Logic decoder inthe meantime for satisfying hometheater sound. You can alsoconnect the multichannel outputsof a DVD-Video player that hasbuilt-in 5.1-channel Dolby Digitaldecoding to this kind of receiver.An external decoder is stillneeded, however, for otherDolby Digital program sources(see Figures 6-c and 6-d, andquestion 11).

Note: be sure that the newdecoder unit has a sufficientnumber of digital inputs toaccommodate all the DolbyDigital program sources youexpect to use (DVD-Video player,digital cable set-top box, digitalTV receiver, etc.). Also, if youplan to play Dolby Digital laserdiscs, be sure that the unit alsoincludes the necessary RFdemodulator. (Only laser discsrequire this feature; see ques-tion 24.)

13. Will I still be able toplay Dolby Surroundprogram material?

Yes, all Dolby Digitaldecoders include Dolby SurroundPro Logic decoding, so you cancontinue to enjoy DolbySurround-encoded programs afterconverting your system.

14. Can I use my currentspeakers?

Yes, you can use your currentspeakers, because theconfiguration for Dolby Digital’sfive main channels is essentiallythe same as that for DolbySurround Pro Logic (refer toFigure 1 on page 1).

A unique feature called BassManagement lets you route lowbass sounds to those speakers inyour system that are best able toreproduce them. For example,you might wish to route the LFEsignals to the front left and rightspeakers, if they have full basscapabilities, or to a separatesubwoofer. You can also redirect

the surround channels’ low bass,and so continue to use thelimited-bandwidth surroundspeakers you got for DolbySurround Pro Logic playback. Oryou could use small satellitespeakers for the five mainchannels and route all bass to asubwoofer. You could even go sofar as to add a subwoofer to eachmain channel.

For information on speakerplacement, visit www.dolby.com/ht/sound/sound2.html. Thisinformation can also be found onpage 6 of the booklet “DolbySurround: The Sound of HomeTheater,” published by DolbyLaboratories.

15. What kind ofspeakers should I get foran all-new Dolby Digitalsystem?

The ideal Dolby Digitalplayback system would useidentical full-range speakers forthe left, center, right, and eachsurround channel. If this isimpractical, be sure that theoverall tonal characteristic, ortimbre, of all the speakers issimilar. This is equally importantfor both Dolby Surround ProLogic and Dolby Digital playback.

Most speaker manufacturersoffer timbre-matched hometheater speakers. Many also offersystems combining matchedcompact speakers for the fivemain channels with one or moreseparate subwoofers for thenondirectional bass; such a“satellite” system can prove aneffective, space-saving solution.

16. Should I use bipolar,dipolar, or tripolarsurround speakers?

Surround speaker placement,room acoustics, and personalpreference are as important asthe speakers’ radiatingcharacteristic. These factors varygreatly, so Dolby Laboratoriescannot recommend a particularspeaker for home theater use.

Bipolar, dipolar, and tripolarsurround speakers use speakerdrivers aimed towards the frontand back of the room to achieve adiffuse soundfield like thatcreated by the multiple surroundspeakers used in movie theatres.Your room’s acoustics will effectthese designs more thanconventional direct-radiatingspeakers, which, when placedproperly, can also make effectivesurround speakers.

Figure 7: Pin configuration for DB-25 connectors.

Pin Assignment Pin Assignment1 Left positive 14 Left ground2 Center positive 15 Center ground3 Right positive 16 Right ground4 Subwoofer positive (or L sub) 17 Subwoofer ground5 Left surround positive 18 Left surround ground6 Right surround positive 19 Right surround ground7, 8 Reserved (optional R sub) 20, 21 Reserved9, 10 Shield drains 22 Shield drain11 Remote trigger C 23 Remote trigger C ground12 Reserved 24 Logic ground13 Remote trigger A (power) 25 Remote trigger A ground

Pin Assignment Pin Assignment1 Left positive 14 Left negative2 Center positive 15 Center negative3 Right positive 16 Right negative4 Subwoofer positive 17 Subwoofer negative5 Left surround positive 18 Left surround negative6 Right surround positive 19 Right surround negative7, 8 Reserved 20, 21 Reserved9, 10 Shield drains 22 Shield drain11 Remote trigger C 23 Remote trigger C ground12 Reserved 24 Logic ground13 Remote trigger A (power) 25 Remote trigger A ground

Balanced

Unbalanced

17. My new Dolby Digitaldecoder has a 25-pin D-sub connector, but myaudio components onlyhave phono connectors.What do I do?

The DB-25 interface wasdeveloped by Lucasfilm THX tosave space on the rear panels ofhome theater products. If yourcomponents use a mix of connec-tors, cables with a DB-25 (25-pin)connector on one end andmultiple phono connectors onthe other are available fromhome theater equipment retail-ers. If you wish to make your ownadapter, see Figure 7 for theDB-25 pin configuration.

18. My DVD player hastwo digital outputs, oneoptical and one coaxial,and my Dolby Digitaldecoder has both kindsof inputs. Which should Iuse?

Under most conditions,optical and coaxial digitalconnections work equally well.Under some rare circumstances,however, coaxial cables, particu-larly very long ones, can pick upradio frequency (RF) interferencegenerated by householdappliances, or nearby high-tension power lines or broadcasttowers. Coaxial is less expensive,so is a good place to start.

Note: some DVD players andDolby Digital decoders haveeither a coaxial or an opticalconnector. Be sure that theunits you purchase both use thesame type.

19. Why does my DolbyDigital decoder providetime delays?

Because the surround speak-ers in a home theater system areusually closer to viewers thanthe front speakers, both DolbyDigital and Dolby Surround ProLogic playback require a split-second electronic delay of thesurround channels to optimizearrival time relationships.

In the case of Dolby Digital,the delay ensures that thelistener hears sound from thesurround and front speakerssimultaneously. For DolbySurround Pro Logic, which hasless inherent channel separationthan Dolby Digital, a slightlylonger (by 15 milliseconds) delayensures that sound from thesurrounds arrives just aftersound from the front. This delayreduces the audibility of front-channel sounds that might leakinto the surround speakers.

Some Dolby Digital decoderunits also provide an adjust-able center-channel delay toensure that the viewer hearssound from all three frontspeakers simultaneously.

20. How do I set thesurround time delays?

The time delays in yourDolby Digital decoder are presetto provide good results in mosthome theater installations. If youwish to fine-tune theadjustments, consult thedecoder’s instruction manual.

Most units allow you toadjust the surround delay ineither the Dolby Digital or DolbySurround Pro Logic mode.

Because the relationship betweenthe two delays is fixed, you onlyneed to set the delay in onemode. The decoder will automa-tically provide the appropriatedelay whenever you switch to theother mode.

To establish the propersurround delays for your system,you need to know the distancesbetween your viewing positionand a front speaker (A) and asurround speaker (B) as shown inFigure 8. Then, using Figure 9(Dolby Pro Logic mode) or 10(Dolby Digital mode), find thedistance to the surround speakeron the graph’s vertical axis, andthe distance to the front speakeron the horizontal axis. Linesdrawn from these points willintersect on the graph at therecommended delay setting.Many decoders simplify thisprocess by allowing you to sim-ply enter these distances into thesystem for automatic adjustment.

21. How do I set thecenter channel timedelay?

Adjusting the center channeldelay has a subtle effect, sounder normal circumstancesthere is no need to worry aboutit. If you nevertheless wish to setit very precisely, read on.

You need no center delay (setit at “0”) if your seating area issmall with a few viewers at anytime, and if you can place thethree front speakers equidistantfrom it as shown in Figure 11-a. Ifthe center speaker must beplaced more in line with the leftand right speakers, as shown infigure 11-b, thereby bringing itcloser to the viewers than the left

5

Figure 8: Use dimensions A and B to calculatesurround delay using either chart to right; settingdelay for one mode will provide correct delay forthe other.

Figure 9: Calculating surround delay withdecoder set to Dolby Surround Pro Logic mode.

Figure 10: Calculating surround delay with decoderset to Dolby Digital mode.

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decoders have come up withnames like “Midnight Mode” forthis feature (see Figure 12).

The amount of compressionis not arbitrary, but is decided inadvance by the soundtrack’sproducers, and coded right ontothe soundtrack. Some DolbyDigital decoders let you selectvarious amounts of the availablecompression (e.g., 50%, 75%,100%), while others provide only100% when the compressionmode is selected.

23. I’m tired of having toadjust the volume everytime the programchanges or I switchsources or TV channels.Can Dolby Digital help?

Yes. Conventional programsources often force you toreadjust the volume when youchange channels or play adifferent video. With Dolby Digitalprogram sources, on the otherhand, a feature called DialogueNormalization lets you setplayback volume and forget it.

Dialogue Normalizationautomatically adjusts the volumewhen you change Dolby Digitalprograms so that the level of the

dialogue remains constant. Itdoes not alter the dynamic range,only the overall playback level,based on data about the averagesoundtrack loudness that iscoded into the program signal(see Figures 13 and 14).

With Dialog Normalization,you can “channel surf” whenwatching digital TV withouthaving to adjust the volume eachtime you change channels. Andyou can play an evening’s worthof DVD-Video discs without evertouching the volume control.

24. What equipment do Ineed to play DolbyDigital laser discs?

Dolby Digital laser discplayers do not have a built-inDolby Digital decoder; theyoutput the Dolby Digital datastream from laser discs on aradio-frequency (RF) carrier thatrequires demodulation prior toDolby Digital decoding. If youwish to play the Dolby Digitalsoundtrack on laser discs,therefore, be sure that the discplayer has a “Dolby AC-3 RF”output, and that your DolbyDigital decoder unit includes anRF demodulator specifically forlaser disc use. Not all decoderunits provide the demodulator,because no other Dolby Digitalprogram source, including DVD,requires it.

If you have an older laserdisc player and don’t want todecode the Dolby Digital tracks

and right speakers, add 1 ms ofdelay for each foot it is closer.For example, if dimension C inFigure 11-b is one foot less than Lor R, set the time delay to 1 ms.

If your seating area is wide,with several viewers in a row asin a movie theatre, it’s best toplace the left, right, and centerspeakers all in line, as shown inFigure 11-c, and use no centertime delay.

Finally, if it is necessary toplace the center speaker furtherback than the left and rightspeakers (not illustrated), selecta –1 ms delay (for a one footdifference) or –2 ms delay (fortwo feet), if these options areprovided. If they are notprovided, use “0” delay.

22. What is the“Midnight Mode” on mynew Dolby Digital A/Vreceiver?

Dolby Digital soundtrackscan have a very wide dynamicrange between soft and loudsounds. At a full playback level,this can be thrilling, providing atruly theatrical experience,particularly for movies.

Late at night, however, theloud sound effects found on somemovie soundtracks could disturbyour family or neighbors. But ifyou turn down the volume to“keep the peace,” the dialoguewill be hard to hear, and subtlelow-level effects may get lostaltogether.

What you’d really like to do isturn down the volume on just theloud effects, turn up the volumeon quiet sounds, and keep thedialogue at the same level. This isexactly what Dolby Digital’sDynamic Range Control featuredoes. To help explain its function,manufacturers of Dolby Digital

L C R

L = C = R:Center delay = 0.

L C R

L = R > C, small seating area:Center delay (ms) = L or R – C (ft.).

L C R

L = R > C, wide seating area:Center delay = 0.

Figure 11-a. Figure 11-b. Figure 11-c.Figure 11: Setting center channel time delay.

Figure 12: With Dynamic Range Control on, the fulldynamic range (A) of the program is reduced (B), aspredermined by the producers.

Figure 13: The average level of conventionalprogram sources, and even individualprograms, varies greatly.

Figure 14: Dolby Digital’s Dialog Normal-ization maintains a consistent playback levelfor all programs keyed to their dialogue level.

on newer discs, they will playperfectly well on your olderplayer. On laser discs, the DolbyDigital track is encoded on whatused to be the right FM analogtrack, leaving the standard PCMdigital stereo tracks intact forconventional stereo and DolbySurround Pro Logic playback. Theleft FM analog track may containa mono version of the sound-track, commentary, or othermaterial (see Figure 15).

One final note: laser discswith 5.1-channel Dolby Digitalsoundtracks were originallyidentified as Dolby SurroundAC-3, “AC-3” being the technicalterm for the digital audio

coding upon which DolbyDigital is based. Today thesimpler term “Dolby Digital” isused instead to identify laserdiscs, like other formats, thathave Dolby Digital soundtracks.

25. What equipment do Ineed to receive satellitebroadcasts with 5.1-channel Dolby Digitalsound?

To receive the 5.1-channelsoundtrack on broadcasts suchas Dolby Digital-encoded theatri-cal films, you’ll need a satellitereceiver that provides a DolbyDigital output (the generic“Digital” output on conventionalsatellite receivers does nothandle Dolby Digital signals). Aswith other Dolby Digital compo-nents, you connect the receiver’sDolby Digital coaxial or opticaloutput to a digital input on yourDolby Digital decoder unit.

Standard stereo audio,usually Dolby Surround-encoded,is broadcast simultaneously, soyou need a new satellite receiveronly if you wish to receive theDolby Digital soundtrack.

26. How can I receivehigh-definition (HD)satellite TV transmissionswith Dolby Digitalsound?

To receive high-definitionprogramming transmitted viasatellite, you will need an HDsatellite receiver, and possibly anew, larger dish. Most receiversare equipped like most DVD-Video players, that is, with abuilt-in, two-channel Dolby Digitaldecoder with analog stereooutputs, and a Dolby Digitaloutput for 5.1-channel sound withan external 5.1-channel DolbyDigital decoder unit.

27. Can my Dolby Digitalhome theater systemdecode the new digitalTV broadcasts?

Yes, it will. Whether high-definition (HD) or standard-definition (SD), all digitaltelevision (DTV) broadcasts inthe U.S., Canada, and othercountries that have adopted theATSC television standard(www.atsc.org) are transmittedwith Dolby Digital. Dolby Digitalaudio is also an option forbroadcasts in areas such asEurope using the DVB digital TVstandard. All such broadcasts arecompatible with existing DolbyDigital decoders.

Like other Dolby Digitalprogram material, the audioformat of DTV broadcasts variesfrom mono to full 5.1-channelsurround sound. Because mostTV facilities have to gear up formultichannel audio productionand distribution, many DTVprograms are being broadcastinitially in two-channel DolbyDigital (often Dolby Surround-encoded for four-channelplayback with Dolby SurroundPro Logic decoding).

DTV sets and tuners alsovary with respect to the numberof audio channels they provide.

Some have built-in two-channelDolby Digital decoders, whileothers have 5.1-channel DolbyDigital decoders for connectionto a Dolby Digital-ready A/V re-ceiver (see question 12). Othersinclude not only Dolby Digitaldecoding but also amplifiersand speakers.

Most DTV receivers have aseparate digital output thatprovides the undecoded DolbyDigital signal for connection toyour A/V receiver or other unitwith 5.1-channel Dolby Digitaldecoding. As a result, you canadd a DTV receiver to yourDolby Digital home theatersystem at any time, without fearof obsolescence or costlyequipment duplication.

28. How can I receivedigital cable programswith Dolby Digitalsound?

Digital cable televisionsystems can provide you withanother source of programmingfor Dolby Digital playback. Thenumber of channels with DolbyDigital cable transmissions variesas with DTV broadcasts andother Dolby Digital programsources. All digital cable set-topboxes incorporate a two-channelDolby Digital decoder with stereoanalog outputs for compatibilitywith conventional stereo andhome theater systems, whilesome units also provide a digitaloutput for multichannel playbackvia an A/V receiver or other unitwith a 5.1-channel Dolby Digitaldecoder.

29. Can I purchase DVD-Video discs with DolbyDigital soundtracks inEurope?

Yes, Dolby Digital audio isused on DVD-Video discsthroughout the world (seequestion 9). A word of caution,though: while the audio andvideo technical standards are thesame for DVD-Video worldwide,discs purchased in one part ofthe world may not play at all inanother part of the world due to“regional coding.”

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Figure 15: Soundtrack configurations ofconventional laser discs (top) and those withDolby Digital audio (bottom).

8

Regional coding was devel-oped at the behest of the filmindustry, which often releasesmovies on video in North Americabefore they’ve even reachedmovie theaters in other parts ofthe world. Discs released in amarket like North America(Region 1) can be coded to playonly on players manufactured forthat market. Take a Region 1-coded disc to another part of theworld, and it won’t work in theplayers manufactured for thatmarket. Some discs, on the otherhand, are coded as Region 0,meaning they will play on all DVDplayers in all regions. Check thepackaging to find out if the dischas any regional limitations.

30. I’m thinking aboutgetting a PC with a DVD-ROM drive. Will it giveme 5.1-channel DolbyDigital?

That depends on the com-puter’s configuration and its DVD/audio card, so check the specifi-cations carefully to be sure youget the audio capability you want.

Most PCs can be connectedto external playback equipmentranging from amplified stereospeakers to a full 5.1-channelplayback system. The latterinclude not only full home theatersystems, but compact,multichannel PC speaker systemsincorporating amplifiers andDolby Digital and/or DolbySurround Pro Logic decoding. PCsbundled with stereo amplifiersand speakers may have a built-intwo-channel Dolby Digitaldecoder for the DVD-ROM playerthat will downmix 5.1-channelsoundtracks.

Audio outputs possible onPCs include analog poweramplifier outputs, line-leveloutputs for external amplifiers,headphone jacks, and RF carriers.They may also provideconventional PCM digital audiooutputs, and/or outputs thatprovide the undecoded DolbyDigital data stream via S/PDIF (IEC1937), USB, IEEE 1394, LAN,wireless link, or other formatconnections. PCs that offer a S/PDIF output that passes throughthe Dolby Digital stream, or

provide multichannel analogoutputs, may be identified withthe logo on the systemitself.

31. I’ve heard aboutsurround-sound systemsthat use only twospeakers. Do they reallywork?

Thanks to the increasingpower and dropping cost ofsignal-processing chips, soundthat appears to come fromaround the listener is nowpossible with just two speakers.

These audio processingsystems, often referred to as“virtual surround,” require thatthe listener be positioned in aspecific “sweet spot” in front ofthe two speakers. As a result,they cannot substitute formultiple speakers inhome theaterapplications. But forPCs, where a singleuser is seated directlyin front of the monitor,they can provide a cost-effective alternative to afull, multispeaker surroundsetup.

There are twofundamentalapproaches to “virtualsurround.” Somesystems take any stereoprogram source andcreate what might be calleda “pseudo-surround” effect.The results areinconsistent, however,and can create effectsnot at all intended bythe program producers. Theother approach has beendeveloped by Dolby Laboratoriesto reproduce the actual surroundinformation recorded onmultichannel Dolby Surround andDolby Digital program material,for a more accurate and consist-ent listening experience.

Called Virtual DolbySurround and Virtual DolbyDigital, these processes start bydecoding, respectively, theoriginal four or 5.1 channels ofthe program material. The centerchannel is then divided equallybetween the left and rightspeakers to create a phantom

center image, while the surroundsignals are processed by a“virtualizer” circuit and mixed inwith the main left and rightchannel signals. The virtualizerprocessor adds cues to thesurround signals based on howwe identify a sound’sdirectionality, thereby creatingvirtual surround images from justtwo speakers. Unlike amultispeaker home theatersystem, the center and surroundeffects are affected only by thelistener’s position relative to thetwo speakers, and not by roomacoustics (see Figures 16 and 17).

Dolby offers its ownvirtualizer technique, and alsotests and approves othervirtualizers for use in combina-

tion with Dolby Surround ProLogic and Dolby Digital decodingto create Virtual Dolby Surroundand Virtual Dolby Digital.Approved technologies include3D Phonic, Virtual Sonic, VMAx,A3D, Qsurround, N-2-2 DVS,TruSurround, Sensaura, RSX, andIncredible 3D Surround.

Products with Virtual DolbySurround and/or Virtual DolbyDigital include DVD players, PC/DVD packages, television sets,and multimedia speaker systems.

Figure 21: Signals from a Dolby Surround Pro Logic or Dolby Digitaldecoder are processed by a “virtualizer” circuit to produce the effect ofsurround sound with two speakers.

Dolby Surround orDolby Digital program

32. Is it possible to getsurround sound overheadphones?

Yes, it is now possible toachieve convincing surroundsound over conventional stereoheadphones from both 5.1-channel Dolby Digital and four-channel Dolby Surround-encodedprograms, thanks to a newdevelopment called DolbyHeadphone (see Figure 18).

Dolby Headphone is a signalprocessor that recreates thesound of a real five-speakersurround system playing multi-channel material in an actuallistening room. It works by elec-tronically imparting to eachaudio channel—two on stereoprograms and up to five onsurround programs—the uniquesonic signature of a correspond-ing speaker properly placed in acarefully-defined acousticenvironment. The processorcombines the signatures into twocomposite channels that deliverto your ears, via conventional

stereo headphones, the sum ofthe direct and reflected soundsfrom each virtual speaker in thevirtual room.

With conventional head-phone listening, by contrast, thesound is fed directly to youreardrums without these spatialcues. The result is an unnaturallyflat image, with left and rightsounds directly beside you, andcenter sounds within your head.This “in-head” effect is not onlyunrealistic, but can also be tiring.

Dolby Headphone works withboth stereo and multichannelprogram sources, and can beincluded in just about any pro-duct with a headphone jack,including set-top boxes, VCRs,DVD players, conventional anddigital TV receivers, PCs andlaptops, and A/V surrounddecoders and receivers.

33. Does Dolby plan tointroduce higher data-rate versions of DolbyDigital?

While all Dolby Digital decod-ers are capable of handling datarates as high as 640 kb/s, the 448kb/s rate used on DVDs (themaximum allowable) providessound quality that criticallistening tests have consistentlyconfirmed as on a par withoriginal master tapes. It wouldrequire the introduction of a newdelivery format before the optionto use more than 448 kb/s wouldeven be considered.

34. I’ve heard that DolbyDigital isn’t as good assome competing systems.Is this true?

No, it’s not. In fact, most suchstatements are based on theassumption that “more is better,”and not on proper listening tests.

To understand what’s goingon with respect to Dolby Digitaland competing systems, you firstneed to know something about“perceptual” digital audio coding.Conventional digital audio, likethat used on CDs, encodes theaudio using 16-bit PCM (pulsecode modulation), which ex-presses the value of the originalanalog waveform in each channelto one of 65,536 possible levels,44,100 times per second. Thisalready takes up a lot of storageand transmission space forplain stereo, so providingmultiple channels of PCM forsurround sound has beenimpractical, particularly formedia that also has to carrydata-hungry digital video.

As a result, Dolby Digital andother coding systems (such asMPEG, DTS, and ATRAC) havebeen developed to save space bytransmitting only the data that’snecessary to portray the originalsound based on what we canactually hear, as opposed todelivering the original PCM databit-for-bit. As a result, these aresometimes known as “percep-tual” or “lossy” coding systems.

A complex coding scheme, oralgorithm, processes the audiosignal based on the principles ofpsychoacoustics, the science ofhow we perceive sound. Thecoder takes advantage of noiseand frequency masking to allowcapturing the audio with justenough precision to make sure itsounds the same as the original.The more powerful the algorithm,the more precisely the encodercan shave bits without affectingthe perceived sound quality. Thisis why some coder technologiessuch as Dolby Digital can soundjust as good as others like DTSwhich use from two to four timesas much data.

Dolby Digital’s algorithm,technically known as Dolby AC-3,is based upon Dolby Laboratories’

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Figure 18: Dolby Headphone enables convincingsurround sound over conventional stereoheadphones by electronically imparting the uniquesonic signatures of speakers in a listening room.

Figure 17: While five speakers are necessary for home theater (left), Virtual Dolby Surround and Virtual DolbyDigital (right) provide convingincing surround sound for an individual listener from only two speakers.

decades of research into how weperceive sound, and delivers 5.1channels of audio with up to 24-bitprecision at an even higher (48kHz) sample rate than conven-tional CDs. As a result, DolbyDigital achieves very high soundquality at the low data ratesnecessary for uncompromisedpicture quality on such formats asDVD-Video discs and DTV digitaltelevision broadcasts.

35. Will Dolby provide5.1-channel music CDs?

Nobody is more enthusiasticabout multichannel sound thanDolby Laboratories. However, weacknowledge the importance ofstandardization and compat-ibility, and the reluctance ofproducers to release, andretailers to stock, more than oneversion of a given program. As aresult we will not release CDswith Dolby Digital soundtracks,or encourage others to do so.

5.1-channel music CDs arenonstandard; that is, they have touse some form of audio codingother than the PCM audiospecified by the CD “Red Book”audio specification. They are alsoincompatible in that they cannotbe heard at all without a special

external decoder, and in manycases require a speciallyequipped CD player as well.

Newer formats like DVD-Videodiscs, on the other hand, havebeen developed from the outset tosupport multichannel digitalaudio. For greater compatibility,these new formats can carry morethan one kind of soundtrack. Inaddition, as explained in question7, sources with Dolby Digitalsoundtracks are inherentlycompatible with mono, stereo,and Dolby Surround Pro Logicplayback systems.

For older stereo formats,including CD, there’s tried-and-true Dolby Surround, a fullycompatible format enabling two-channel soundtracks to beencoded with four-channelsurround sound (see question6). There are currently morethan 700 CD titles encoded withDolby Surround.

When it comes to music-only recordings with multi-channel digital audio, a newkind of DVD disc called DVD-Audio has been developed (seequestion 36).

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36. Can I play the newDVD-Audio discs on myDVD-Video player or mycomputer’s DVD-ROMdrive?

Yes…and no.The new DVD-Audio discs can

have two zones, one to carry high-resolution PCM audio, and theother, which is optional, to carryvideo programs, such as an artistinterview, music videos, or musicrecordings with still pictures.

The audio zone can be playedonly on the new DVD-Audio players(which typically will also play DVD-Video discs). The optional videozone, however, has the samespecifications as DVD-Video discs,so this portion will play on anyDVD-Video player and DVD-Videocapable ROM drive. This makes itpossible for producers toincorporate in the video zone aDolby Digital version of the sameprogram in the audio zone. If theydo so, you will be able to buy DVD-Audio discs as soon as they areavailable, and enjoy them withDolby Digital sound until such timeyou decide to get a new DVD-Audioplayer to “unlock” their higher-resolution audio and other uniqueprogram features.

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he digital audiocoding used oncompact discs (16-bitPCM) yields a total

dynamic range of 96 dB from theloudest sound to the noise floor.This is achieved by taking 16-bitsamples 44,100 times per secondfor each channel, which is oftentoo much data to store or trans-mit economically, especiallywhen multiple audio channels arerequired. As a result, new formsof digital audio coding—oftenknown as “perceptual coding”—have been developed that elimi-nate redundant data, therebyreducing data rates with aminimum of perceived degrada-tion of sound quality.

Lessons learned fromnoise reduction

Dolby Digital is the firstperceptual coding systemdesigned specifically to processmultichannel digital audio. Itbenefits from Dolby’s develop-ment of two prior successfulperceptual coding systems, andof what are in essence analogperceptual coding systems: thefull gamut of Dolby professionaland consumer noise reductionsystems. Indeed, DolbyLaboratories’ unique experiencewith audio noise reduction isessential to Dolby Digital’s datarate reduction, as the fewer thebits used to describe an audiosignal, the greater thebackground noise (known asquantizing noise).

Dolby noise reduction worksby lowering the noise when no

audio signal is present, whileallowing strong audio signals tocover or mask the noise at othertimes. Thus it takes advantage ofthe psychoacoustic phenomenonknown as auditory masking. Evenwhen audio signals are present insome parts of the spectrum,Dolby NR reduces the noise in theother parts so the noise remainsimperceptible. This is becauseaudio signals can only mask noisethat occurs at nearby frequencies.

Putting masking to work

Dolby Digital has beendesigned to take maximum advan-tage of human auditory masking.It divides the audio spectrum ofeach channel into narrowfrequency bands of different sizesoptimized with respect to thefrequency selectivity of humanhearing. This makes it possible tosharply filter coding noise, so thatit is forced to stay very close infrequency to the audio signalbeing coded. By reducing oreliminating coding noise wher-ever there are no audio signals tomask it, the sound quality of theoriginal signal can be subjectivelypreserved. In this key respect, aperceptual coding system likeDolby Digital is essentially a formof selective and powerful noisereduction.

With Dolby Digital, bits aredistributed among the filter bandsas needed by the particularfrequency spectrum or dynamicnature of the program. A built-inmodel of auditory masking allowsthe coder to alter its frequencyselectivity (as well as timeresolution) to make sure that a

How Dolby Digital Workssufficient number of bits is usedto describe the audio signal ineach band, thus ensuring noise isfully masked.

Pooling the bits

Dolby Digital also decideshow the bits are distributedamong the various channels froma common bit pool. This techni-que allows channels with greaterfrequency content to demandmore data than sparselyoccupied channels, for example,or strong sounds in one channelto provide masking for noise inother channels.

More bits for audio

Dolby Digital’s sophisticatedmasking model and shared bitpool arrangement are key factorsin its extraordinary spectrumefficiency. Furthermore, whereother coding systems have to useconsiderable (and precious) datato carry instructions for theirdecoders, or to carry the sameaudio in separate channels forcompatibility reasons, DolbyDigital can use more of the trans-mitted data to represent essentialaudio, which means inherentlyhigher sound quality.

Dolby Digital can process upto 24-bit digital audio signals overa frequency range from 20 Hz to20 kHz on the full-range channels,and from 20 Hz to 120 Hz on theLFE channel. Sampling rates of32, 44.1, and 48 kHz are support-ed at a variety of data ratescovering a wide range ofrequirements.

T

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Dolby Digital vs. Dolby Surround

Surround channels

Low frequency effects channel

Panning options

Channels

Other

Dolby Digital

Stereo, full range(20 Hz – 20,000 Hz).

Yes (20 Hz – 120 Hz).

Multiple.

L C R

LS RS

Six discrete; each channel cancarry a different signalsimultaneously.

Improved sound imaging via“time alignment,” i.e., making itsound like each speaker is thesame distance from the listener.

Appropriate compressionadjustments during low volumeplayback of dynamic moviesoundtracks (late at night, forinstance) to ensure low-levelprogram content is retained.

Decoders programmable to routelow bass only to channels in thesystem equipped with wide-rangespeakers or subwoofers.

A dramatic step forward inlistener involvement.Unprecedented creativeopportunities for programproducers, directors, recordingengineers, performers.

Dolby SurroundPro Logic

Mono, limited range(100 Hz – 7,000 Hz).

No.

Left to right, right to left, front toback, back to front.

L C R

S

Four derived; only one dominantsignal can be decoded at a time.

Economical way to achieve highperformance surround sound.

Surround sound from any non-encoded stereo source.

Compatible with all current andfuture two-channel formats.

Represents a major step forwardin sound over conventionalstereo. The most widely usedsurround format worldwide.

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Dolby Laboratories, Inc. 100 Potrero Avenue, San Francisco, CA 94103-4813 Telephone 415-558-0200 Fax 415-863-1373Wootton Bassett , Wiltshire SN4 8QJ England Telephone (44) 1793-842100 Fax (44) 1793-842101 www.dolby.com D o l b y a n d t h e d o u b l e - D s y m b o l a r e t r a d e m a r k s o f D o l b y L a b o r a t o r i e s . © 2 0 0 0 D o l b y L a b o r a t o r i e s , I n c . S 0 0 / 1 3 0 2 1 / 1 3 4 4 9