data compression and multilevel signaling techniques, channel bandwidths can be reduced actually,
comparedwithanalogsystems. Inbroadcastsystems,thereducedbandwidthrequirements,together
with the error robustness of the coding algorithms, will allow an efficient use of available radio and
TV channels as well as “taboo” channels currently left vacant because of interferenceproblems.
MPEG Standardization Activities
Ofparticularimportancefordig ital audioisthestandardizationworkwithintheInternational
OrganizationforStandardization(ISO/IEC),intendedtoprovide international standardsfor audio-
visual coding. ISO has set up a Working Group WG 11 to develop such standards for a wide range
of communications-based and storage-based applications. This group is called MPEG, an acronym
for Mov ing Pictures Experts Group.
MPEG’s initial effort was the MPEG Phase 1 (MPEG-1) coding standards IS 11172 supporting bit
rates of around 1.2 Mb/s for video (with video quality comparable to that of today’s analog video
cassette recorders) and 256 kb/s for two-channel audio (with audio quality comparable to that of
today’scompact discs) [8].
The more recent MPEG-2 standard IS 13818 provides standards for high quality video (including
High Definition TV) in bit rate ranges from 3 to 15 Mb/s and above. It provides also new audio
features including low bit rate digital audio and multichannel audio [9].
Finally,thecurrentMPEG-4workaddressesstandardizationofaudiovisualcodingforapplications
rangingfrommobileaccesslowcomplexitymultimediaterminalstohighqualitymultichannelsound
systems. MPEG-4willallowforinteractivityanduniversalaccessibility,andwillprovideahighdegree
of flexibility and extensibility [10].
MPEG-1, MPEG-2, and MPEG-4 standardization work will be described in Sections 40.3 to 40.5
of this paper. Web informat ion about MPEG is available at different addresses. The official MPEG
Web site offers crash courses in MPEG and ISO, an overview of current activities, MPEG require-
ments, workplans, and information about documents and standards [11]. Links lead to collec-
tions of frequently asked questions, listings of MPEG,multimedia, or digital video relatedproducts,
MPEG/Audio resources, software, audio test bitstreams, etc.
40.2 Key Technologies in Audio Coding
Firstproposalstoreducewidebandaudiocodingrateshavefollowedthoseforspeechcoding. Differ-
encesbetweenaudioandspeechsignalsaremanifold;however,audiocodingimplieshighersampling
rates, better amplitude resolution, higher dynamic range, larger variations in powerdensity spectra,
stereophonic and multichannel audio signal presentations, and, finally, higher listener expectation
ofquality. Indeed, the high quality of the CD withits 16-b per sample PCM format has madedigital
audio popular.
Speech and audio coding are similar in that in both cases quality is based on the properties of
human auditory perception. On the other hand, speech can be coded very efficiently because a
speech production model is available, whereas nothing similar exists for audio signals.
Modestreductionsinaudiobitrateshavebeenobtainedbyinstantaneouscompanding(e.g.,acon-
versionofuniform14-bitPCMintoa11-bitnonuniformPCMpresentation)orbyforward-adaptive
PCM (block companding) as employed in various forms of near-instantaneously companded audio
multiplex (NICAM) coding [ITU-R, Rec. 660]. For example, the British Broadcasting Corporation
(BBC)hasusedtheNICAM728codingformatfor digitaltransmissionofsoundin severalEuropean
broadcast television networks; it uses 32-kHz sampling with 14-bit initial quantization followed by
a compression to a 10-bit format on the basis of 1-ms blocks resulting in a total stereo bit rate of
728 kb/s [12]. Suchadaptive PCM schemes can solve the problemof providing a sufficient dynamic
range for audio coding but they are not efficient compression schemes because they do not exploit
c
1999 by CRCPress LLC
