If MPEG-2 is a standard, then why are there so many MPEG-2 audio "standards"? Here's the MPEG-2 Advanced Audio Coding (AAC) standard adopted for digital TV in Japan. Zoran gets the press, but then, it is audio.......
techweb.cmp.com
Posted: 9:00 p.m. EST, 3/5/98
Chip makers seek help for MPEG-2 Advanced Audio Coding
By Junko Yoshida
SANTA CLARA, Calif. -- With the Japanese
government's endorsement last month of the newly
standardized MPEG-2 Advanced Audio Coding
(AAC) as the audio coding algorithm for digital TV
in Japan, IC vendors are beating the bushes for
information on AAC and are seeking partners who
will give them access to AAC intellectual property.
To date, no one has launched a chip capable of
processing AAC, and there is no one-stop shop
for licensing the IP.
Now an ISO standard, MPEG-2 AAC is a
perceptual audio-coding algorithm that is not
backward-compatible with MPEG-1 or MPEG-2
audio. It provides high-quality sound at a rate of 64
kbits/second per channel for multichannel
operation.
AAC arrives at a time when a multiplicity of
audio-coding algorithms is fragmenting the digital
consumer market, bombarding system and chip
vendors with changing digital-audio requirements.
"A product with multiple [audio] standards is not
tomorrow's issue," said Paul Goldberg, vice
president of audio products and intellectual
properties at chip maker Zoran Corp. (Santa
Clara, Calif.). "It's very much today's issue."
AAC applications are not limited to the
satellite-based Japanese digital-TV service slated
for rollout in 2000; they extend to such new
services as delivery of downloadable CD-quality
music over the Internet, satellite or cable.
AT&T, for instance, has begun a trial service called
"a2b music" over the Internet in collaboration with
record companies. Some in the industry even
speculate that MPEG-2 AAC may become an
integral element for CD-recordable systems, or an
additional DVD audio feature when DVD becomes
recordable.
IC vendors are examining various digital-audio
algorithms. Besides linear pulse-code modulation
(PCM), MPEG-1 and 2 and Dolby Digital--the
mainstays of today's DVD players--at least a half a
dozen more have been proposed for DVD,
home-theater and Internet-audio applications,
some by such heavyweights as Bell Labs, Sony
and Philips.
For chip vendors seeking design wins in DVD
systems 18 months from now, the worst-case
scenario is a chip that's expected to support not
only Dolby Digital, Linear PCM and MPEG-1 and
2, but also AAC and any (or all) of the other audio
options. The challenge is to guess which
audio-coding algorithms are worth supporting, at
what cost and under what kind of silicon
architecture.
"Most people in the industry have underestimated
the complexity of meeting different digital-audio
requirements," said Paul Bundschuh, applications
and strategic marketing manager for digital-audio
operations at Motorola Inc. While a couple of
dozen solutions already exist to decode MPEG-2
video, few audio ICs are capable of decoding
multiple audio-coding algorithms.
The MPEG Committee's Audio Subgroup
reported just last month that MPEG-2 AAC,
tested under the stringent requirements of the
ITU-R test methodology, "demonstrated full
broadcast-quality audio at 128 kbits/s for stereo,
approximately half the bit rate of that needed by
the earlier MPEG-1 Layer II codec." The Layer II
audio codec is currently used in U.S. and
European digital-satellite TV services.
The purpose of the MPEG-2 AAC effort was to
achieve an audio coding solution that would be
"much more powerful than MPEG-1" and
"designed for efficient bandwidth-compression
applications," said Peter Kroon, supervisor of the
audio-communication group at Bell Labs.
Patent suitors
Chip companies are beginning in earnest to forge
partnerships with the essential MPEG-2 AAC
algorithm patent holders. Describing MPEG-2
AAC as "a priority for Zoran today," Goldberg
said Zoran has been "talking to Frauenhofer Institut
[Erlangen, Germany] for the last six months" to
work out necessary steps to port MPEG-2 AAC
to Zoran's DSPs. The German research institute is
one of several companies that claim to have
essential IPs for MPEG-2 AAC. Others include
Bell Labs, Sony Corp. and Dolby Laboratories
(San Francisco).
Of course, IC vendors need to soberly assess
exactly how big the MPEG-2 AAC market is
going to be and when its growth might reach critical
mass. U.S. vendors, however, can't afford to
spend too much time fence-sitting. Japanese
system vendors have been aggressively developing
proprietary consumer digital ICs through their
in-house semiconductor arms.
Although the Japanese DTV system remains the
current focus of MPEG-2 AAC development,
Motorola's Bundschuh regards AAC as a new
force that may open the door for broader
worldwide applications when MPEG-4 comes
online. AAC is defined as a part of the emerging
MPEG-4 multimedia representation standard.
Whether MPEG-2 AAC becomes an important
element for the recordable DVD standard remains
an open question. Considering the heavy Japanese
interest in AAC, especially by Sony, some see a
possible link.
To complicate matters further, a Bell Labs concept
called Perceptual Audio Coding (PAC), which has
been proposed for U.S. digital radio and
multicasting on the Internet, could compete with
AAC for some U.S. applications. Despite a history
of close involvement with AAC development, Bell
Labs today is promoting its proprietary PAC,
claiming that PAC's low-bit-rate, high-quality audio
coding laid the basis for AAC. According to
Kroon, PAC's coding has substantially evolved,
both computationally and in terms of quality, since
1996, when AAC was frozen as a standard.
PAC supports bit rates as low as 8 kbits/s and is
commercially available through Elemedia, Lucent
Technologies' internal venture. Unlike MPEG-2
AAC, PAC comes with such application-specific
features as digital audio broadcasting, resistance
against communication errors and distortion, and
robust error recovery. PAC at 128 kbits/s requires
processing power equivalent to 10 to 15 Mips for
decode; encoding requires 40 to 50 Mips. PAC at
16 kbits/s needs only 3 to 4 Mips for decoding and
20 Mips for encoding, according to Deepen Sinha,
research staff member at the audio communication
group of Bell Labs.
Surrounding issues
If AAC is still a ways off, a more pressing matter
for most chip vendors today is how to respond to
Japanese consumer-electronics manufacturers' new
demand that they integrate a DTS Digital Surround
stream output feature into a DVD chip set.
Developed by Digital Theater Systems Inc.
(Westlake Village, Calif.), DTS Digital Surround is
an encode/ decode system that delivers 5.1
channels of master-quality, 20-bit audio. It is
derived from the surround-sound technologies the
company developed for motion pictures and movie
theaters.
The DVD standard does not mandate DTS. But as
Hollywood studios have released more DVD
movie titles featuring DTS Digital Surround,
"consumers' awareness has been going up," said
Darren Neuman, director of DVD engineering for
LSI Logic Corp. (Milpitas, Calif.). Although DVD
players may not have a built-in DTS-decoding
capability, manufacturers expect to give them at
least an ability to output a DTS bit stream, with
decoding in a separate audio/video receiver.
With developers uncertain about which
digital-audio coding schemes to run with, many are
embracing programmable approaches.
"You need a powerful enough architecture to
handle a wide range of audio algorithms," said
Zoran's Goldberg. When Goldberg worked for
Dolby Labs several years ago, "my assumption
was that nothing would stay the same as far as
audio is concerned." Zoran's ZR38600, its latest
programmable audio processor, is the only audio
processor capable of 3-D sound/virtual surround
processing simul-taneously with Dolby Digital
decoding, he claimed.
The goal for Zoran is to continue to develop a
DSP-based audio solution for multistandard
products that detect a code in the header,
download an appropriate decoder software and
decode the stream accordingly, he said.
Bundschuh at Motorola agreed on the basic
concept for "a software-configurable hardware
architecture." Motorola's recently announced DSP
56362 is the only chip now certified to decode
Dolby Digital, DTS and multichannel MPEG-2
audio algorithms. |
