BBC Research and Development Report 1996-04 : ISO/IEC MPEG-2 AUDIO: Bit-rate-reduced coding for two-channel and multichannel sound

17 Pages·1996·0.2 MB·English

by  G. Stoll, N.H.C. Gilchrist

BBC RD 1996/4
Research and
Development
Report
ISO/IEC MPEG-2 AUDIO:
Bit-rate-reduced coding for
two-channel and multichannel sound
G. Stoll, Dipl.-Ing.,(InstitutefürRundfunktechnik,Munich,Germany)
and N.H.C. Gilchrist, B.Sc.,C.Eng.,M.I.E.E.,AESFellow
Research & Development Department
Policy & Planning Directorate
THE BRITISH BROADCASTING CORPORATION
BBC RD 1996/4
*ISO/IEC MPEG-2 AUDIO:
Bit-rate-reduced coding for two-channel and multichannel sound
G. Stoll, Dipl.-Ing., (Institut für Rundfunktechnik, Munich, Germany)
andN.H.C. Gilchrist, B.Sc., C.Eng., M.I.E.E., AES Fellow
Summary
Since 1988 the ISO/IEC Moving Picture Experts’ Group (MPEG) has been developing
generic coding standards for video and audio, mainly for broadcast and multimedia
applications. The MPEG audio standard (ISO/IEC 11172-3) was finalised in November
1992. It follows a three-layer structure in order to fulfil the requirements of various
applications. Good audio quality at a bit rate of about 130 kbit/s per monophonic channel is
achieved.
The first objective of MPEG-2 audio was the extension from two to five channels, based on
standards and recommendations from international organisations such as ITU-R, SMPTE
andEBU. This was achieved in November 1994 with the approval of the ISO/IEC
document13818-3, sometimes termed ‘MPEG-2 Audio’. This standard provides high
qualitycoding of 5 full-bandwidth audio channels plus an optional low-frequency effects
(LFE or ‘sub-woofer’) channel, together with backwards compatibility to MPEG-1.
Backwards compatibility is the key to ensuring that existing 2-channel decoders will still be
able to decode compatible stereo information from multichannel signals. For audio
reproduction of surround sound, the loudspeaker positions left, centre, right, with left and
right surround are used (according to the 3/2-standard). The envisaged applications are
digital television systems such as dTTb, HDTV, HD-SAT, ADDT, as well as digital audio
broadcasting (DAB) and digital storage media. The second objective was the extension of
MPEG-1 audio to lower sampling frequencies to improve the audio quality at bit rates less
than 64 kbit/s per channel, in particular for commentary applications.
*
ThecorematerialofthisReportisanexpansionof thecopyrightpaperbyG.Stoll(IRT),writtenfortheAudioEngineer-
ing Society (AES) Special Publication:- CollectedPapersonDigitalAudioBit-rateReduction, and is published with
permission by the AES and the author.
IssuedundertheAuthorityof
Research & Development Department
Policy & Planning Directorate GeneralManager
BRITISH BROADCASTING CORPORATION Research&DevelopmentDepartment
1996
(cid:211) BritishBroadcastingCorporation
Nopartofthispublicationmaybereproduced,storedina
retrieval system, or transmitted in any form or by any
means,electronic,mechanical,photocopying,recording,
orotherwise,withoutpriorpermission.
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ISO/IEC MPEG-2 AUDIO:
Bit-rate-reduced coding for two-channel and multichannel sound
G. Stoll, Dipl.-Ing., (Institut f(cid:252)r Rundfunktechnik, Munich, Germany)
andN.H.C. Gilchrist, B.Sc., C.Eng., M.I.E.E., AES Fellow
1. INTRODUCTION ......................................................... 1
2. DIGITAL TELEVISION SOUND SERVICES .................................. 2
3. MAJOR DIGITAL AUDIO CODING SYSTEMS ............................... 2
3.1 Coding used for broadcasting and storage applications......................2
3.2 Coding formats used in the film industry.................................. 2
4. ISO/MPEG AUDIO: GENERIC CODING OF STEREO
AND MULTICHANNEL SOUND ............................................ 3
4.1 MPEG-2 audio: generic multichannel audio coding ......................... 4
4.1.1 Extension of MPEG audio to multichannel coding ............................ 4
4.1.2 Characteristics of the MPEG-2 audio coding system ....................... 4
4.1.3 3/2-stereo presentation performance ................................... 5
4.1.4 Downward compatibility ............................................ 6
4.1.5 Backward/Forward Compatibility with MPEG-1 ........................... 6
4.1.6 Low frequency enhancement channel .................................. 7
4.1.7 Associated services and configurability ................................ 7
4.2 Composite coding strategies for multichannel audio........................ 9
4.2.1 Redundancy reduction ............................................. 9
4.2.2 Common bit pool.................................................. 9
4.2.3 Dynamic crosstalk................................................. 9
4.2.4 Common masking threshold ......................................... 9
4.3 MPEG-2 extension to lower sampling frequencies ......................... 10
5. CONCLUSIONS ......................................................... 10
6. ACKNOWLEDGEMENTS ................................................ 11
7. REFERENCES .......................................................... 11
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ISO-MPEG-2 AUDIO:
A generic standard for the coding of two-channel and multichannel sound
G.Stoll,Dipl.-Ing.,(InstitutfürRundfunktechnik,Munich,Germany)
andN.H.C.Gilchrist,B.Sc.,C.Eng.,M.I.E.E.
11.. INTRODUCTION digital audio signal sampled at 48 kHz to be reduced
from 768 kbit/s (about 706 kbit/s for a sampling fre-
Digital audio was introduced to the consumer in the quency of 44.1 kHz) to approximately 128 kbit/s per
early 1980s with the Compact Disc (CD). The 16-bit mono channel, while preserving the subjective quality
PCM format of the CD is an accepted audio reproduc- of the original signal. This reduction in bit-rate coding
tion standard, although the bit-rate of about 706 kbit/s is possible because the coding adapts the quantising
per mono channel is rather high. Lower bit-rates are noise to the masking characteristics of the original
essential if there is only a limited capacity available audiosignal,andonlythosedetailsofthesignalwhich
for the storage or transmission of audio signals. Typi- willbe perceived bythelisteneraretransmitted.
cal application areas for low bit-rate coded digital
audio are: Fig. 1 shows the wide range of different bit-rates
needed for a number of techniques for mono and ste-
• Programme distributionand exchange reoaudiodigital coding.
• Digitalaudio broadcasting(DAB) When considering digital audio for television, it is
clearly necessary to determine what sound system
• Digital storage (e.g. archiving, studio record- should be adopted and what level of service will be
ingand consumerelectronics) provided to the consumer. Should a completely new
system be provided, or should an existing proposal be
• Interpersonal communications such as video- adaptedfordigitaltelevision?Are consumerstobeof-
conferencingand multimedia applications fered just mono and stereo, or should multichannel
sound (surround sound and multilingual services) be
• Enhanced-qualityTVsystems. made available? In either case, one must take account
of the consequentdemands on theprogramme channel
New coding techniques for high quality audio signals (i.e.the bit-raterequired).
use the properties of human sound perception by ex-
ploiting the spectral and temporal masking effects of For digital television, the choice of the audio coding
the ear. The objective is for the quality of the repro- systemwilldepend uponsuch thingsas:
duced sound to beas goodasthatobtainedwith16-bit
PCM at a sampling frequency of 44.1 or 48 kHz, • The available bit-rate
whilst using a minimal bit-rate for the coded signal
Such a source coding system was recently stand- • Compatibility with other services and hard-
ardised by ISO/IEC.1 It allows the bit-rate of a 16-bit ware (both within andbetweencountries)
stereo DAB
distribution
mono
cable
radio DAB
contrib-
video
ution
conference
audio
cellular mini-disc
conference
radio
CD
DAB
voice
mail ISDN NICAM
secure medium
voice audioquality
DCC DSR
2 4 8 16 32 64 128 256 512 1024 2048
kbits
Fig.1-Typicalapplicationsandbit-ratesfordifferenttypesofdigitalaudiocoding.
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• The need for reconfiguration of the transmit- centre channel) was equivalent to an additional one
ted signals gradeofimprovement,andfromthree channel(3/0)to
surround sound (3/2) was equivalent to an additional
• The possible need to provide performance gradeimprovement.
margins for multiple coding and decoding of
the signalbytandem-connectedcodecs
33.. MAJOR DIGITALAUDIO CODING
• Theservices tobe offered totheconsumer. SYSTEMS
Two further MPEG-2 topics – ‘Video coding’2 and
33..11 Codingusedforbroadcastingand
‘Overview of the systems layer’3 – are available as
storageapplications
companionR&DReports.
The main work in development and standardisation,
including extensive subjective evaluation, has taken
22.. DIGITALTELEVISIONSOUNDSERVICES
place on low bit-rate audio codecs within the various
international groups (MPEG, ITU-R, EBU, Grand
Members of specialist international groups* have
Alliance etc.). This work has led to conclusions and
spent much time evaluating the service options that
system proposals that are applicable to the different
can be considered for multichannel sound. Although
constraints withinwhicheachgrouphasbeenworking.
the discussions have centred mainly upon the options
for an HDTV service,they have consideredothertele-
High quality audio coding schemes already have, and
vision service situations and audio-only applications.
will continue to have, many applications in the areas
The two main options considered have been the ones
of recording, computer multimedia, telecommunica-
forsurroundsoundandformultilingualservices.
tion,radio and televisionbroadcasting,cable and film.
Table I provides an overview of the major digitaltwo-
The surround sound option has been considered as an
channel and multichannel audio coding systems
important one for several reasons. Firstly, surround
already in use or proposed for various applications.
sound is already available within the cinema industry,
Thesystemsare presentedinalphabetical order.
and to some degree also, within the broadcasting in-
dustry – where a stereo transmission format is
Three of the coding schemes mentioned are used
available and some source material is already in ana-
alreadyin the areaof homerecording.=Inthe telecom-
logueDolbyStereo**form(e.g.filmsoundtracksand
munications area, ITU-T standards, such as G711 and
some sports events). Secondly, if surround sound is to
G722, arebeingreplacedbyMPEG-1audio LayerII and
be introduced into a digital broadcasting environment,
Layer III coding. Computer multimedia uses mainly
something operationally more suitable than the ana-
MPEG-1 audio Layer II. The older digital radio satel-
logue method of combining channels needs to be
lite services, such as the German DSR-system, use
developed. Thirdly, domestic equipment manufactur-
simpleblock-compandingtechniques,8butmorerecent
ers participating within specialist collaborative
satellite and terrestrial digital radio services within
projects***have expresseda strong beliefthatsignifi-
Europe use Layer II. The AT&T PAC system9 is pro-
cant improvements in the sound system will be
posedforoneof theUSAdigitalsystems.
essential in the marketingof new, moreexpensivetele-
visionsystems.
In television broadcasting, NICAM10has been introduced
in some European countries to provide digital stereo
Subjectivetests,carriedoutspecificallytoquantifythe
sound. For future television systems, the European
subjective benefit of the different forms of reproduc-
broadcasters show a clearpreferencefor theMPEG-11
tion,4,5 have shown that going from mono (1/0) to
and MPEG-211 interrelated standards, whereas the
stereo (2/0) was equivalent to one grade improvement
North American broadcasters are currently preferring
on the ITU-R 5-point quality grading scale.6 Going
two differing standards, namely MPEG-1 (DirecTV)
from stereo (2/0) to three channel (3/0) (i.e. with a
andDolbyAC-312(GrandAlliance).
* Eureka95;EuropeanMultichannelExpertsGroup(EUMEG);‘Grand 33..22 Codingformatsusedinthefilmindustry
Alliance’in the USA; MPEG; Task Group10-1oftheinternational
Telecommunication Union’sRadiocommunicationSector (ITU-R).
A dilemma for the film industry is currently posed by
** ‘Dolby Stereo’ is a Trademark of Dolby Laboratories Licensing the existence of four different and completely incom-
Corporation. patible formats for digital audio. Two of the systems,
***HD-SAT (High Definition Television – Satellite); dTTb (Digital
Terrestrial Television Broadcasting; HDTV-T (High Definition
Television–Terrestrial);Eureka1187ADTT(AdvancedDigitalTele-
=
visionTechnologies);USSB(UnitedStatesSatelliteBroadcasting); ATRACsystemforSony’sMidiDisc,7MPEG-1audiocoding1(LayerI
DSS(DigitalSatelliteSystem);DirecTV. isusedforPhilip’sDCC, andLayer video-CD).
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Sony’s SDDS (Sony Dynamic Digital Sound) and The fact that the film industry has not selected
DTS from Digital Theatre Systems (a six channel sur- one digital audio system poses a dilemma for broad-
round system with the audio stored on a CD-ROM), casters, because transcoding between the film
are associated with major film studios. DTS is associ- sound format and the broadcast format cannot be
ated with Universal-MCA and SDDS with Sony avoided, whichever digital audio system is chosen for
Pictures(Columbia and TriStar).The third,DolbyAC-3, broadcasting.
also known as SR-D or Dolby Stereo Digital, is inde-
pendent. The fourthsystemfordigitalfilmsound,used
mainly in France and based on MPEG-1 Audio 44.. ISO/MPEG AUDIO:GENERIC CODING OF
Layer II, is Cinema DSP. If a single standard does not STEREO AND MULTICHANNELSOUND
emerge, this situation will pose rather a predicament
forstudiosand cinemas alike. From 1988 to 1992 the International Organisation for
Standardisation(ISO) hasbeendevelopingandprepar-
First of all, most of the cinema chains want to have ing a standard on information technology – coding of
digital multichannel sound for their auditoria, but they moving pictures and associated audio for digital stor-
will not want to purchase three or four systems for age media up to about 1.5 Mbit/s.1 The ‘Audio
each of their premises.MGM/United Artists are direc- Subgroup’ of MPEG had the responsibility for devel-
ting theirattention towardsDTS.NewLineCinemahave oping a standard for generic coding of PCM audio
expressed a qualified endorsement of DTS, because signals with sampling rates of 32, 44.1 and 48 kHz at
theatre owners are more likely to favour DTS because bit-rates in a range from 32 kbit/s to 192 kbit/s per
of the cost, compared to Dolby AC-3. Sony Pictures mono channel and 64to384kbit/sforstereosignals.
has stated that all films where Sony has control over
thedistribution will bein SDDS.However,evenCastle Two mechanisms can be used to reduce the bit-rate of
Rock,which usesSonyfordistribution,isreleasingsome audio signals. One mechanism removes the redundant
pictures in DTS, and Paramount recently announced information from the audio signal. The other removes
an agreement for five films in DTS. Although DTS the irrelevancy of the audio signal by taking advantage
mayseemto bethefront-runner,itisnotaclearvictor. of psychoacoustic phenomena, like spectral and tem-
This is a situation which will probably exist for some poral masking. Only with both of these techniques,
timeyet. exploiting redundancy and the masking effects of the
Table1:
Majordigitalaudiocodingsystemsforsinglechannel,
two-channelandmultichannelapplications.
Halfsamplingrate
Codingscheme Type Mono Stereo Multichannel
mono/stereo
APT13 aptx-100
AT&T PAC
AT&T MPAC
Dolby AC-2
Dolby AC-3
Block
DSR
companding
MPEG-1 Layer I
MPEG-1 Layer II
MPEG-1 Layer III
MPEG-2 Layer I
MPEG-2 Layer II
MPEG-2 Layer III
Block
NICAM
companding
Linear
PCM16BIT
representation
SONY ATRAC
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human ear, can a significant reduction of the bit-rate, The second objective of MPEG-2 Audio was the ex-
down to about 200 kbit/s per stereophonic signal, be tension of MPEG-1 Audio to lower sampling
obtained. frequenciesto improvethe audioqualityformono and
conventional stereo signals for bit-rates at or below
Different layers of the coding system with differing 64 kbit/s per channel; in particular, for commentary
degrees of encoder and decoder complexity and per- applications. This goal has been achieved by reducing
formance are described in the audio part of ISO the sampling frequency to 16, 22.05 or 24 kHz, with a
Standard 11172 (MPEG-1).1 The idea behind the consequent reduction in the audio bandwidth to 7.5,
three-layer concept was to have a universal coding 10.5 or 11.5 kHz. Compared with MPEG-1, the only
scheme for many applicationswith totallydifferentre- differences in the encoder and decoder, other than the
quirements. These would include consumer recording, change in the clock rate, are changes in the encoder
professional recording, the combined recording and and decoder tables of bit-rates and bit allocation. The
processing of audio and video, telecommunications encoding and decoding principles of MPEG-1 Audio
and broadcasting. A comprehensive description of the Layers I, II and III are fully maintained. Table II
whole MPEG-1 audio coding standardwith, a detailed shows themainareas of MPEG-2audio work.
explanation of its layer concept, can be found else-
where.14
4.1.1 Extension of MPEGaudioto
multichannelcoding
44..11 MPEG-2audio:genericmultichannel
audiocoding At present, multichannel audio is known primarily
from the cinema. However, even in consumer applica-
The first objective of MPEG-Audio phase 2, some- tions, multichannel audio has been available for the
times known as MPEG-2 Audio, was thedevelopment last few years (e.g. Dolby Surround with domestic
ofa standard forthelow bit-ratecoding ofmultichannel televisionand videocassetterecorders).Withthe intro-
audio; using perceptual coding methods which can be duction of Advanced or High Definition Television
used to transfer high quality digital surround sound (ADTV, HDTV) having improved resolution and in-
and/or multilingual audio information on channels creased picture size, to give an improved visual
with limited capacity.15 The MPEG-2 audio standard11 perspective more like a cinema, improved realism
was approved by the MPEG committee in November fromthe audio is appropriate.A way to achieve this is
1994. It takes account of standards and recommenda- to use morethantwoaudio channels.Recently,ITU-R,
tions from international organisations such as ITU-R, SMPTE and EBU have started to standardise the lis-
SMPTE (the Society of Motion Picture and Television tening arrangement for loudspeaker reproduction of
Engineers)and the EBU.One importantrequirement is multichannelaudio.Anadvantageofthissystemisthe
backwards compatibility tomono,stereoordual-chan- relatively large area over which satisfactory listening
nel audio programmes coded in accordance with can be experienced. There is, however, the disadvan-
ISO/IEC 11172-3.1 To fulfil this requirement, the tage that a relatively high bit-rate is needed for the
coded signal must be such that an ISO/IEC MPEG-1 digital audio signals to be transmitted or recorded.
audio decoder is able to correctly decode the basic With the applicationofthe codingsystem describedin
stereo information from the multichannel programme. this paper, economical digital storage or transmission
The basic stereo information needs to be kept in the of the multichannel audio is possible. In addition to
frontalleft-andright-channelcomponents,whichcon- ADTV and HDTV, a number of multimedia applications
stitute an appropriate downmix of the audio will adopt multichannelaudio if good performancecan
informationinallchannels. be obtainedeconomicallyat lowdata rates.
The backwards compatibility requirement arises 4.1.2 Characteristics ofthe MPEG-2
because many integrated decoding chips for ISO/ audio codingsystem
MPEG audio and video are under development. The
audio decoders will handle only two audio channels, Agenericdigital multichannelsoundsystemapplicable
however. With backwards compatibility, a two-channel to television and sound broadcasting and storage, as
decoder will be able to deliver a basic stereo signal well as other non-broadcasting media, should meet
from the multichannel audiobitstream. several basic requirements and provide a number of
technical/operationalfeatures.Inadditiontotwo-channel
The main techniques for reducing the bit-rate of compatibility and interoperability between different
MPEG-2 multichannel coding, while maintaining the media, and downwards compatibility with sound for-
subjective quality of the input audio signals, are: sub- mats consisting of a smaller number of audio
band filtering, perceptual modelling of the ear, the channels, other aspects also need consideration.
sharing ofbitsbetweenchannelsfrom a commonpool, Multilingual services, ‘clean’ dialogue and dynamic
jointstereocoding,commonmaskingthresholdsandthe range compression are important, in order to serve the
introductionofdynamiccrosstalkbetweenchannels. widest possible range of applications. It is important,
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TableII:
MainworkareasofMPEG-2audio.
1 Backwards compatible multichannelsound
Extensionofthe ISO/MPEG-1standard uptofive audio multichannel atanytime.
channels,plusa low-frequencyenhancementchannel.
Up tosevenmultilingual channels,eitherwiththe same
Backwardscompatibility: AcurrentMPEG-1stereo or withhalfthe samplingfrequencyofthe main audio
decoderwillreproduce a compatiblestereo signal,i.e.a programme.
down-mixofall five channels,when suppliedwithan
MPEG-2bitstream.Boththe programme providerand the TheMPEG-2Audiostandard wasfinalisedin
the consumer can switch from two-channel to November1994.
2 Extensiontolowersamplingfrequencies
Three additional samplingfrequencies:16,22.05 and Excellentcommentaryqualityat48kbit/s,together with
24kHz. 16kbit/sforancillarydatain anISDNB-channel of
64kbit/s.
Reducedbandwidth (upto11.5 kHz)butimproved
coding gain,withbetter adaptation tothe masking Onlyminor changesareneededtothe ISO/MPEG
threshold,givingmuchbetter audioqualityatbit-rates standard (two tablesin the decoder).Thecurrent
below64kbit/sperchannel.Especiallysuitablefor encoderand decoderhardwareand softwarecan
commentaryapplications. easilybeupdated tosupportall sixsampling
frequencies
3 Non-backwards compatible multichannelsound
RequirementsdefinedduringMarch-July1994. Anaddendumtothe ISO/MPEG-2standard isdue to
befinalisedin July1997.
Collaborative effortwilllead tothe combination ofthe
bestalgorithmicelementsofall submissions.
as well, to obtain a level of audio quality close to that (the studio, storage media, contribution, distribution,
of the original signal; typically, a linearly-coded PCM emission links, and in the home). Fig. 2 shows the po-
audio signal with a resolution of at least 16 bits, with- sitions of the loudspeakers reproducing the 5-channel
outrequiringan unreasonablycomplexdecoder. signalina reference listeningarrangement.
MPEG-2 audio provides for a wide range of bit-rates For sound that accompanies pictures, the three frontal
from 32 kbit/s up to 1066 kbit/s. If the audio is sub- channels ensure directional stability and clarity of the
jected to only one coding/decoding process, quite a picture-relatedfrontal images, andaccords to common
low bit-rate may give acceptable results. It is of con- practice in the cinema.In particular, the centre channel
siderable importance to digital audio broadcasting is of high importancefor certain situations(e.g. where
(DAB), for example, to be able to use really low bit-
rates, because of the relatively low transmission
C
capacity.Higherrates,uptoabout180kbit/sperchan-
L R
nel may be necessary if there are a number of coding/
decoding processes or there is the need for some post-
screen
processing(e.g.a re-mix).
4.1.3 3/2-stereopresentation
performance
Ls Rs
As regards stereophonic presentation, specialist groups
of the ITU-R, SMPTE and EBU recommend a
5-channel system as the reference surround sound for-
listeningarea
mat.16 This arrangement has a centre channel C and
two surround channels Ls, Rs, in addition to the front
left and right stereo channels L and R. It is referred to
as ‘3/2-stereo’ (3 front/2 surround channels) and re-
quires the handling of five channels in all situations Fig.2-3/2referenceloudspeakerarrangement.
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link, or where a lower number of reproduction chan-
nelsis required.
3/2 3/1 3/0
4.1.5 Backward/Forwardcompatibility
withMPEG-1
For several applications, it is the intention to improve
2/1
2/2 2/0
the existing 2/0-stereo sound system progressively by
transmitting additional sound channels (centre, sur-
round) without making use of a simulcast operation.
Lo=L+xC+yLs
The multichannel sound decoder has to be back-
Ro=R+xC+yRs
1/0
wards/forwards compatible with the existing sound
typicalvaluefordownmixcoefficients: format.
x=y=(cid:214) 2
Backwards compatibility means that an existing
Fig.3-Downmixfrom3/2multichannelto1/0monowith
two-channel (low price) decoder should properly de-
MPEG-2audiocoding.
code the basic 2/0-stereo information from the
a dialogue requires stable localisationin the middle of multichannel bitstream (see section 4.1.6). This im-
the frontal area). Additionally, such a centre channel plies the provision of compatibility matrices using
enlarges the area of the listening zone. For audio-only appropriatedownmix coefficients, as showninFig.4.
applications three frontal channels have been found to
give a worthwhile improvement over two-channel Forwards compatibility means that a future mul-
stereophony.4,5 The addition of one pair of surround tichannel decoder should be able to decode the basic
channelstothethreefrontchannels improves realism. 2/0-stereobitstreamproperly.
Optionally,theremaybeanevennumberofmorethan Thecompatibility isrealisedbyexploitingtheancillary
two rear/sideloudspeakerswhichmayprovidealarger data-field of theMPEG-1audioframefortheprovision
optimum listening area. Since the locations of the ofadditionalchannels(seeFig.5).Thevariablelength
side/rear surround loudspeakers are largely non-criti- of the ancillary data field gives the possibility of
cal with respect to both direction and distance, they carrying the complete multichannel extension
should be accommodated readily in an existingliving- information. A standard two-channel MPEG-1 audio
room environment. decoder just ignores this part of the ancillary data
field.
4.1.4 Downward compatibility
One example of this strategy is the Digital Audio
A hierarchy of sound formats providing a lower Broadcastsystem (DAB,developedbytheEUREKA147
number of channels and reduced stereophonic presen- Consortium), which will not provide multichannel
tation performance(down to 2/0-stereo or even mono), sound in the first instance. In this case, themultichannel
and a corresponding set of downward mixing equa- sound system has to be backwards/forwards compat-
tions can be recommended,17 to provide downward iblewithanMPEG-1Audiodecoder.
compatibility, as shown in Fig.3. Useful alternative
lower level sound formats are 3/0, 2/2, 2/0, 1/0. These There will be other applications which do not require
may be used in circumstances where economic or backwards/forwards compatibility with existing 2/0-
channel capacity constraints apply in the transmission stereo sound formats. In these cases, the compatibility
matrixing transmission dematrixing
a * g Lsw 1/(a * g)
Ls X X Ls
a Lo 1/a
L X + + X L Fig.4-Thecompatibilitymatrix
(encoder)tocreatethecompatible
a * b Cw 1/(a * b ) basicstereosignal(Lo,Ro),andthe
C X X C inversematrix(decoder)torecreate
fiveaudiochannels.
a Ro 1/a
R X + + X R
a * g Rsw 1/(a * g)
Rs X X Rs
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