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Volume 7, Number 4, 2008 — ARM in the Outback: Helping Farmers Protect Water from Wildlife
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 | | | Design Strategies & Methodologies | | MPEG Surround—A Bright New Future for Surround Sound |
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Author:
Matthias Rose, Head of Marketing Communications Audio and Multimedia, Fraunhofer IIS and Jan Nordmann, Director of Marketing and Business Development, Fraunhofer USA Digital Media Technologies
Synopsis:
Developed by Fraunhofer IIS, Dolby Laboratories, LSI Corporation and Philips, MPEG Surround is a feature-rich open ISO standard compression technique for multi-channel audio signals. It can operate on top of any core audio codec – for example, AAC, HE-AAC and MPEG-1 Layer II – and provides a comprehensive feature set, including: highest surround and stereo audio quality; bit-rates currently used for compression of stereo signals; full backward compatibility to legacy stereo equipment; wide scalability in terms of the bit-rate used to describe the surround image; optional compatibility with matrixed surround stereo signals; and virtualized binaural multi-channel sound reproduction capability for stereo headphones. Using transmitted side information in the receiver to produce a faithful reconstruction of the original surround source, MPEG Surround enables high-quality surround sound experiences at home, at work and on the move. It also has tremendous benefits for content producers and broadcasters, a number of whom have already adopted the technology. This article outlines the principal features, key benefits and possible applications of a technology that was approved as a part of the International MPEG-D Standard in early 2007.
As far as the average consumer is concerned, the music surround sound revolution began in the late 1990s with the introduction of the first SACD and DVD-A releases. In both cases, the developers claimed that their formats could open up a new dimension in home listening, allowing music lovers the opportunity to experience both classic albums and the latest releases in a new, compelling way.
However, the reality is that – thanks in large part to the requirement for new playback devices – neither format managed to connect with a sufficiently large percentage of the listening public. The initial high rate of 5.1 releases has decreased significantly in recent years, while the well-publicized commercial downturn affecting the major record companies has also dampened enthusiasm for the conventional distribution of surround sound content.
In many ways, it is a regrettable development. As anyone who has experienced an optimized 5.1 audio demonstration will know, the surround sound experience has the potential to re-engage the listener with music in a manner that can only be compared to a live performance. During an era in which music is often relegated to being merely a background accompaniment to conversation or household tasks, this is a quality that should not be overlooked.
While conventional distribution of 5.1 music content appears to be condemned to diminishing returns and possibly extinction, Fraunhofer is now offering a new, more accessible pathway to surround sound with its MPEG Surround technology. Co-developed by LSI Corporation, Dolby Laboratories and Philips, MPEG Surround is a feature-rich open standard compression technique for multi-channel audio signals. Operating on top of any core audio codec – including AAC, HE-AAC and MPEG-1 Layer II – the system provides a comprehensive feature set, including highest surround and stereo audio quality, bit-rates currently used for compression of stereo signals, full backward compatibility to stereo equipment, wide scalability in terms of the surround image description bit-rate, and virtualized binaural multi-channel sound reproduction capability for stereo headphones. And that is only to scratch the surface of this impressive new technology.
Finalized in September 2006 and then promoted by MPEG to part of International MPEG-D Standard in early 2007, MPEG Surround has already received significant levels of interest from consumers, content producers, broadcasters and consumer electronics (CE) manufacturers around the globe.
“We are still in the relatively early stages of communicating the benefits of MPEG Surround to the industry and consumers,” says Matthias Rose, Fraunhofer’s Head of Marketing Communications, Audio & Multimedia. “Despite this, it is absolutely clear that the technology has a long and productive future ahead of it. The world has been waiting for a more accessible, cost-efficient pathway to surround sound, and the MPEG Surround technology provides it.”
The Way Forward for Multi-channel Music
Working principle
(See Figure 1 on the next page)
MPEG Surround is based on the principle of Spatial Audio Coding (SAC). This entails the generation of a mono or stereo downmix from the multi-channel input signal in the encoder, with additional parametric side information (3-32 kbit/s or more) extracted to guide the up-mix process in the decoder. In so doing, MPEG Surround combines the advantages of the two existing methods for coding multi-channel content. Firstly, MPEG Surround provides very-high audio quality output just as discrete compression technologies, which process every channel separately. Secondly, it compresses surround sound at bit-rates typically used for the coding of stereo material, just like matrix surround systems which blindly embed multi-channel content in a stereophonic down-mix. In addition, MPEG Surround is not constrained by the disadvantages of discrete or matrix surround coding systems such as large file sizes or disappointing audio quality.
High level of efficiency
At its core, this new technology provides a more efficient variation on traditional methods of coding two or more channels, with the result that it is possible to transmit multi-channel audio at significantly lower bit-rates. Thanks to the incorporation of a hybrid coding technique that uses additional residual waveform signals, content can be delivered at points across the scale – from ‘very low bit-rate’ to ‘near-transparent quality’. One initial route through the myriad of possibilities is offered by the selection of appropriate residual bandwidths and core codec bit-rates, thereby achieving a good balance between audio quality and bit-rate.
Superiority to matrix systems
In addition to its core down-mix/up-mix functionality, MPEG Surround can also integrate a matrix-surround-compatible down-mix signal. This provides backward compatibility to legacy receivers that are only able to decode the stereo core signal but are equipped with a conventional matrix surround decoder. An MPEG Surround decoder internally inverts the matrix process without signal degradation, prior to commencing its own up-mix process.
An Enhanced Matrix Mode is also available. Designed for transmission applications where additional MPEG Surround side information cannot be accommodated, Enhanced Matrix Mode enables the spatial parameters to be estimated from the inter-channel relations of the stereo down-mix signal. The overall result is a facility that offers superior spatial audio quality to conventional matrix-surround systems.
Backward compatibility
No matter what the specified bit-rate, content delivered via MPEG Surround remains fully backward compatible to stereo equipment. A playback device without a spatial audio decoder simply presents the transmitted mono- or stereophonic down-mix signal and discards the spatial parameter side information.
Playback
MPEG Surround can facilitate high-quality playback on a wide variety of equipment. At one end of the scale, MPEG Surround can be a vital component of a high-end 5.1 home theatre sound system. At the other, the technology’s incorporation of a binaural headphone mode allows listeners to experience a virtual multi-channel loudspeaker set-up when using headphones. This feature makes it particularly suitable for compact music players, mobile phones and other portable devices.
EBU acclaim
The high quality and effectiveness of MPEG Surround has been confirmed through a sequence of listening tests conducted during the last three years. Arguably the most significant is that carried out by the European Broadcasting Union (EBU) in September 2007, which compared the new technology to a number of existing discrete codecs. At a total bit rate of 96 kbit/s, MPEG Surround in combination with HE-AAC was shown to provide similar quality to AC-3 384 kbit/s. The performance of MPEG Surround combined with HE-AAC at a total bitrate of 64 kbit/s was also impressive.
Industry Adoption
MPEG Surround has been a recurrent topic of conversation in the audio, broadcast and consumer electronics industries since work commenced in 2004. Since its formal launch in 2006, this high level of interest has translated to an impressive degree of industry take-up. At time of writing in November 2008, the technology has been adopted by – amongst others – DAB+ (digital radio in Europe and Australia) and Digital Radio Mondiale. It has also become part of the DLNA digital home media standard, ATIS, an U.S. standard for IPTV, and been selected for the Korean T-DMB mobile TV system.
In addition, MPEG Surround is currently being considered for inclusion in the DVB-T (cable and satellite set-top boxes), DVB-H (mobile TV), 1seg-TV (mobile TV in Japan) and 3GPP (mobile phones) standards.
Ensuring further industry adoption is a critical part of the long-term strategy for MPEG Surround. Accordingly, Fraunhofer and its co-developers highlight the availability of the MPEG Surround patent pool license from VIA Licensing, Inc, thereby enabling manufacturers of encoding and decoding software/equipment to gain easy access to the essential IP under fair and reasonable terms of licensing.
Applications
As has already been established, MPEG Surround provides tremendous benefits for the consumer. However, its potential to revolutionize multi-channel content (music & video) delivery also has significant implications for content producers, broadcasters, consumer electronics (CE) companies and car manufacturers (see also Figure 2).
Content producers and distributors
With illegal download services continuing to exert their influence, the conventional recording industry is under pressure like never before. Traditional revenue streams are drying up and budgets for recording and reissuing music are the subject of drastic re-evaluation. Leaner, cheaper and simpler are the defining characteristics of the new business model that is now beginning to emerge. In addition, the importance of streaming and download services for music and video content is increasing rapidly.
MPEG Surround is a perfect fit for these changing times in that it allows record companies to issue 5.1 content without incurring the high packaging and production costs associated with conventional disc-based releases. Many labels spent considerable sums in the early part of this decade on 5.1 remixing work, only to find that the available formats did not allow them to capitalize on their investment. In addition, numerous albums were prepared for 5.1 release, only for their proposed reissue to be postponed or delayed indefinitely. MPEG Surround offers a resolution to both scenarios.
It should also be noted that, with several large record companies now re-focusing their energies on alternative distribution of existing or ‘archive’ content – rather than the creation of new music – MPEG Surround’s relevance is only likely to increase since it can be implemented for delivery of music via online, mobile and other ‘next generation’ services. The provision of surround sound content for computer games is another avenue that many record companies are exploring, and again one for which MPEG Surround has relevance.
Online music & video distribution and mobile TV services are two of the priority market segments highlighted by Fraunhofer and its collaborators for further development. Fraunhofer is currently working to supply encoding tools to record labels and content aggregators, as well as the decoder SDKs to facilitate hard- and software player support. Fraunhofer, Dolby and Philips also have established a partnership with mediaHYPERIUM – a celebrated Los Angeles media production and publishing house that has won several prestigious awards for SACD and DVD releases – with a view to providing safe, high-quality encoding facilities for studio-master content.
Broadcasters
The provision of multi-channel content for non-broadcast/offline home listening is one consideration; readying it for broadcast is quite another. MPEG Surround, however, offers a cost-effective method for 5.1 services, and has already been implemented successfully by high-profile broadcasters in the US and Germany.
Based in Boston, CBS radio station WZLX became the first commercial radio station to broadcast full-time in MPEG Surround in spring 2007. Responding to research indicating that many consumers now want to be able to experience surround sound in their cars, the station introduced an MPEG Surround-based service via a digital multi-channel studio infrastructure provided by Telos Systems’ Axia division.
Meanwhile, Bavaria-based broadcaster ROCK ANTENNE recently became the first European station to realize a 24-hour DAB Surround service. The station deployed MPEG Surround in conjunction with the SX Pro upmixing technology (also developed by Fraunhofer) to achieve this historic landmark.
In addition, German public broadcasters are beginning to introduce DAB Surround. Amongst others, Bayern-4 Classic already provides true 5.1 content for broadcasts via DVB-S which could easily be reused for DAB Surround.
CE companies
The cost-efficiency and flexibility of MPEG Surround has the potential to provide a major boost for consumer electronics manufacturers looking to develop new surround sound-compatible devices. Surround capable docking stations, AVRs and HTiB systems are just some of the devices to which MPEG Surround could bring substantial extra functionality.
Car manufacturers
Until recently, the implementation of surround sound systems in automobiles has been the subject of eager discussion but relatively little action. With disc-based surround sound formats now gradually receding from the picture, there is now a demand for a more cost-efficient pathway to multi-channel playback. Fraunhofer and its partners are convinced that, with MPEG Surround, they have found the solution.
“Fraunhofer has successfully collaborated with car manufacturers such as BMW and Audi in introducing consumers, broadcasters and content providers to the MPEG Surround format,” says Jan Nordmann, Director of Marketing & Business Development, Audio & Multimedia Division, Fraunhofer USA. “To date, surround playback in cars has mostly been associated with DVD or matrix stereo upmixing technologies, but MPEG Surround gives automobile manufacturers the opportunity to bring 5.1 content into the vehicle via digital radio, mobile TV, internet streaming and connectivity to docked nomadic devices such as iPods.”
Implementation with ARM and OMAP Processor Platforms
As might be interpreted from the preceding sections on in-car and portable music applications, MPEG Surround also has tremendous implications for companies implementing the ARM processor architecture. A significant percentage of current portable devices – PDAs, mobile phones, media players and more – incorporate an ARM CPU. The advent of MPEG Surround, therefore, promises to add a further dimension to the functionality that chip manufacturers can offer to their clients. Specifically for implementations on limited-resource mobile platforms, MPEG Surround features a low-power decoder mode, which employs a real-valued representation only instead of a complex-valued algorithm. This reduces the amount of necessary operations for the decoding process.
Examples of possible applications include MPEG Surround in binaural headphone mode for PMPs and mobile phones, where MPEG Surround’s low processing requirements would be a significant asset. For in-car audio, meanwhile, it would be possible to play MPEG Surround from an iPod or other portable music player plugged into an MPEG Surround-equipped automotive entertainment system.
Fraunhofer has been working hard to help encourage the introduction of convenient connecting devices into the market, and is preparing to present an ARM-based MPEG Surround decoder for portable media players at the upcoming round of international trade shows. This will enable surround playback from a legacy iPod connected to an AVR via an ARM-powered docking station.
MPEG Surround on ARM-powered Processors
As indicated above, the ARM processor architecture dominates the music player and mobile phone markets where AAC has been applied, and is Fraunhofer’s natural choice for its first embedded port of MPEG Surround multi-channel and MPEG Surround Binaural decoding. With the processing power available in today’s ARM cores, there is little requirement for separate DSP-based processors, even for encoding. For example AAC encoding and decoding is possible directly on the ARM processor through fixed-point arithmetic implementations of AAC. This simplifies product design since the control, file system and user interface code can all run on the same processor as the audio codec.
Fraunhofer is continuing to port its floating-point reference code – implemented in the MPEG standardization process – to fixed-point production software that will run efficiently in embedded devices. As shown in Figure 3, it initially converts the floating point code to a generic fixed-point prototype by replacing floating-point operations with their fixed-point equivalents, possibly by using double precision fixed-point or via the implementation of an internal software block floating-point representation where necessary. Fraunhofer’s reference code also configures the data storage for efficient use of an embedded processor’s more limited memory, and will also remove the runtime overhead of calling transcendental functions and other math libraries, either through its own optimized real-time ones or by pre-computing look-up tables.
Once this is done, and some other techniques to compact and speed up the code have been implemented, there is a test for stability and conformance. For encoders, this often means the conduction of double-blind listening tests, similar to those employed for Fraunhofer’s MPEG standards work. When this code passes these tests, it becomes a fixed-point template code that is then modified only in a bit-exact way to port to each processor, taking advantage of specific memory models or hardware instructions to improve the execution speed. In this way, it is readily possible to keep all of the ports current if an improvement is made simply by changing the fixed-point template and conditionally including the optimization details for each processor when re-compilation takes place. For the ARM processor family, this is usually very straightforward as the architecture does not require much in the way of special optimization in Fraunhofer’s code.
At this point, MPEG Surround decoders are implemented and running in real-time on a number of ARM cores, including ARM9E, ARM11 and ARM Cortex A8. Demon-stration platforms include OMAP- or ARM11-powered development boards, as well as portable devices such as PDAs, mobile phones or the Apple iPhone®.
Further Fraunhofer pro-ducts on ARM processors
In addition to MPEG Surround, Fraunhofer offers many other audio and video codecs for the ARM processor architecture, as shown in Figure 4 on the next page.
Realizing the 5.1 Dream
As should be clear from this article, MPEG Surround is a technology with numerous potential applications and beneficiaries. Most of all, however, it promises to prize open an exciting new future for surround sound.
“No longer the preserve of the technophile, surround sound is something that an increasing number of listeners and viewers expect to have access to on a daily basis,” concludes Matthias Rose. “Deploying this technology for radio and other forms of broadcast has been more problematic, but finally, MPEG Surround offers a practical way forward. Digital radio, IPTV, mobile service providers and more can all benefit from a technology whose true potential is only starting to be realized.”
At the 2009 CES Show in Las Vegas, visitors can experience MPEG Surround in Fraunhofer’s show car and an iPod docking station prototype demonstration at north hall booth 3219 of the Las Vegas Convention Center. In addition, further significant music industry support for the new format will be announced during the spring industry events.
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Author: Matthias Rose and Jan Nordmann, Fraunhofer |
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