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Meridian Lossless Packing:
as seen in MIX December 1998 by Philip De Lancie |
Product/MLP |
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It's been a long time coming, but the technical specifications for a new audio-oriented format based on DVD are now essentially complete. The DVD-Audio specification is being prepared by Working Group 4 of the DVD Forum, a group (composed largely, but not exclusively of consumer electronics hardware manufacturers) dedicated to ensuring the orderly development of a versatile and inter-operable family of DVD products. With DVD-Audio presentations at the 105th AES convention and a DVD Forum conference just two days later (also in the San Francisco area), WG4 formally revealed the basic outlines of the new format (much, but not all, of which was previously known). Still, WG4 chairman "Bike" Suzuki of JVC referred to the specification as being only "almost" ready. A final recomendation from WG4 was expected to go for approval to the DVD Forum's own steering committee at the end of October, at which time pressure was likely to be mounted on WG9, which deals with copy protection issues, to come up with a solution that will allow the DVD-Audio format to finally see the light of day. |
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The basis for WG4's work has been a set of requirements set forth by the International Steering Committee (ISC), representing the interests of the world's major record label trade associations (the RIAA, IFPI and JRIA). Most of the 15 requirements - things like support for high-resolution audio, multichannel (surround) playback and CD compatibility - have long been part of WG4's brief. But this summer the ISC recognized that even DVD is limited in terms of both bandwidth (the playback data rate) and total storage capacity, and added to its list the requirement that playing times, even for high-resolution multichannel sound, approach those of the 74-minute CD benchmark. To meet this requirement, WG4 mandated that all DVD-A players support a lossless data compression scheme championed by Meridian Audio of Cambridge, England, a 21-year-old hi-fi manufacturer best known for its audiophile CD players, receivers and other home stereo gear. Unlike "perceptual coding" schemes that reduce data rates by discarding "unneeded" audio data, Meridian Lossless Packing is more analogous to Stuffit and other software utilities that reduce the size of computer files without altering the information. According to Meridian and WG4, MLP yields a decoded output that is bit-for-bit identical with the original signal. |
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The actual degree of compression achieved by MLP depends on the program material itself, but the claimed reduction in bandwidth and storage requirements is significant (see Fig. 1). Without compression, for instance, 96kHz/24-bit audio requires 2.3 megabits per second per channel. That works out to 13.8 Mbps for 6-channel sound - which exceeds DVD-A's 9.6 Mbps bandwidth - and a total required capacity of 7.7 gigabytes, which exceeds the 4.7 GB capacity of DVD-5, the least expensive DVD to replicate (and thus most attractive to labels). Used on typical program material at that resolution and
channel configuration, MLP is expected to reduce bandwidth by 38 to 52%
(to 6.6-8.6 Mbps), allowing anywhere from 73 to 89 minutes on a DVD-5. To find out more about MLP and its application to DVD-A, I interviewed Meridian chairman Robert Stuart, a co-developer of MLP and longtime activist in the Acoustic Renaissance for Audio (ARA) group, which has pushed for super-fidelity audio on DVD since 1995. I also contacted Roger Dressler of Dolby Laboratories about his company's involvement in the licensing of MLP for DVD (see sidebar). |
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What is the history of MLP and Meridian's involvement
with its development? There was a long-standing relationship between myself, Meridian and two of the advisors to ARA - Michael Gerzon and Peter Craven. We had long had a passion for signal coding questions and had made several projects together. At that time, it was clear to all concerned that lossless compression was an obvious way to go. Over the following year, Peter, Michael and I evolved some strong ideas on good ways to approach this, and one or two key inventions appeared. As it turned out, at the time these first inventions were being patented, Michael Gerzon died. Around two years ago, I could see clearly that the
specification for DVD-Audio had moved forward - multichannel and
high-resolution were now accepted - but that there would still be a strong
requirement for lossless compression. So, Meridian decided to fund the
development of a lossless coding system. This development process has
taken two years of steady effort on the part of Peter Craven, Malcolm Law,
myself and others at Meridian Lossless Packing (MLP) is now integrated into MLP Ltd, a subsidiary company in the Meridian Audio Group. MLP Ltd will take the core technology and develop encoders, decoders and other products and ideas. |
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How did MLP come to be adopted by the WG4 for the DVD-Audio specification? MLP was selected in a committee process that took place this year because of its superior compression and peak data-rate performance. These were central factors for best supporting the DVD-Audio format and providing a safety margin for encoding and playing time. Interestingly, these were the very features on which we had initially concentrated and laid out in the ARA Proposal. |
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How and by whom has the bit-for-bit accuracy of decoded MLP material been independently confirmed? Bit-for-bit accuracy is determined by encoding and decoding audio files (as .WAV, AIFF or DVD format) and doing a bit comparison. We also do this by capturing the output of a real-time decoder and making the same bit comparison. In the selection process for DVD-Audio, JVC performed independent tests on a number of different audio items. The MLP encoder also inserts check data into the MLP bitstream. A patented check method is used, and this allows the MLP decoder to indicate that the complete process from encode to decode has been lossless. We imagine a "lossless" light. The MLP decoders in the Meridian Surround Processors implement this feature. |
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What is the input format to MLP encoding? Does it work from audio that is already in linear PCM form, or are the data reduction techniques applicable as well to audio in other digital formats? Is there the possibility of direct conversion from analog, or is there always an analog-to-PCM conversion before MLP enters the picture? The MLP core always accepts and delivers PCM. It is not applicable to low-resolution bitstreams, or text and picture compression. What is the range of word lengths and sampling rates to which MLP can be applied? Although the limits are not strict, we envisaged MLP for sample rates from 32kHz up to 192kHz and for word-lengths between 14 and 24 bits. |
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Describe the MLP process: the stages the material goes through, how MLP distinguishes redundant from unique data, what patterns it looks for, and what assumptions are made about the content of the incoming signal. The MLP process uses a number of proprietary technologies. It uses three methods to reduce the data rate. Lossless processing and lossless matrixing are used to reduce the correlation between channels. Lossless waveform prediction is used to reduce the inter-sample correlation, with a very large palette of special filters. And Hufman coding is used to reduce the data rate by efficiently encoding the most likely occurring successive values in the serial stream. This combination of methods has been designed to ensure not only a very good compression ratio, but also to ensure the all-important issue of holding down peak data rate; lossless coding produces a variable rate of output data, and it is necessary to keep the peaks low enough for use on carriers. MLP also uses a proprietary data buffering scheme to smooth the data rate. MLP makes no assumptions about the incoming data, and it is not looking for patterns. It operates on waveforms. However, the lossless processing is applied to lossless IIR prediction methods, and this is particularly helpful on real-world audio signals from real microphones which tend to have an output that falls with frequency. |
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Since MLP's ability to reduce bandwidth requirements varies depending on the program material, how is absolutely lossless performance at a given data rate assured in the fixed-bandwidth mode? The fixed data rate mode may be achieved by a two-pass process, in which case the encoder and packetiser determine the bandwidth that is needed. Alternatively, the operator may "dial-in" a fixed data rate. If the MLP encoder cannot achieve the compression, the encoder will issue an error message and the operator needs to take action. The likelihood of this arising depends on the rate requested. There are a very small number of signals that can be made up to prevent MLP from achieving adequate compression, but on real audio material, we have never been able to catch it out in the task of fitting any audio within the bandwidth of the DVD-Audio specification. That said, the issue of the amount of compression is important. If there isn't enough compression to control peak data rate or to get that extra 2 minutes of playing time, MLP's method allows the content provider to make fine adjustments to the incoming signal to save on data rate. For example, input precision can be moved in 1-bit steps. In addition, the system is very effective at detecting band-limiting. So playing time can be increased by redithering a 24-bit signal to 23 or 22 bits - with or without noise-shaping - or by filtering one channel to LFE [low frequency effects], or reducing the audio bandwidth at 96kHz sampling from 48k to 40k before feeding it to the MLP encoder. This gives content providers almost limitless scope to tweak playing time and channel options. |
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What are the various multichannel options supported by MLP for the DVD-Audio application, and what kind of mixed-resolution playback options are available? MLP's bitstream supports up to 63 channels, with sampling rates ranging from 32kHz up to 192kHz - including the set based on 44.1kHz - and up to 24-bit resolution. Mixed rates are supported in that some channels can have sampling at twice the rate of others. Mixed resolution is supported automatically - you can feed 24-bit to some channels and 16 to others, and the compression will be automatic. The MLP stream has an information channel that allows decoders to know if it is dealing with speaker feeds - and if so which speakers - or if the signals are hierarchical. It also has provenance data describing if speaker feeds are from hierarchical or binaural sources, as well as copyright and ownership data. This allows the stream to be very complete and enables sophisticated receiving equipment to reconstruct more accurate surround. |
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How will MLP handle downmixing from surround to stereo on DVD-Audio, and what are the MLP options allowing producers to influence the downmixed result? DVD-Audio has four ways of delivering a 2-channel version of a multichannel mix. Which one will be used will depend on the playing-time requirements, the data rate of both mixes, and the desires, prejudices, budget and time-scale of the content provider. A separate 2-channel mix can be placed on a second layer, on a second side, or in a separate track or separate audio stream in a track on the same layer as the multichannel mix. Alternatively, the player can make a downmix at playback time of a multichannel mix stored on the disc. The downmix is not fixed; DVD-Audio has a huge number of options for the tables of mix coefficients used to create the 2-channel mix, and the mix parameters can change. The downmix option can be very helpful if the multichannel content is very long, or if it is high-precision, which uses a lot of the disc budget, or if the provider cares less about the 2-channel or cannot afford to make two different mixes. MLP supports all of the options offered by the DVD-Audio specification, but it offers a number of added benefits. It allows longer playing time and/or higher resolution in both mixes. It can, in some cases, remove the need to use a second layer. Also, with MLP the downmix is made by the MLP encoder rather than the player. The MLP stream then carries both the 2-channel and multichannel versions in the same stream and delivers both losslessly. The 2-channel version is carried virtually for free in the payload. And because the MLP encoder is in a piece of pro gear and its downmix can be monitored, the mixer can make the mix using the encoder and listen to it before signing off on it. |
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Many producers would prefer to create their own stereo mix rather than rely on an algorithm to downmix from a 5.1 mix. Some will, some won't. Some will care more about the 2-channel mix and may put on a restricted-precision multichannel mix. Assuming a limited-bandwidth LFE channel, side-by-side delivery of separate 5.1 and stereo mixes would require storage space equivalent to 7.1 channels. Using MLP on all 7.1 channels, how many minutes of 24-bit/96kHz audio would fit on a DVD-5 DVD-Audio disc? DVD-Audio only allows 6 channels in a stream. So, rather than having the multichannel and 2-channel mixes in the same stream, you have two separate regions, one containing multichannel and the other containing two channels. This is necessary anyway in order to constrain peak data rate. Using MLP, these two sets of content at 96kHz/24-bit would typically play for more than 70 minutes each. The play time increases to 76 or 83 minutes for 23-bit and 22-bit resolution. It also increases if the bandwidth of the audio is less than 48 kHz. The reduction in word-size can be made with any suitable box and can use an arbitrary set of dithering/noise-shaping methods, including no shaping. Aside from the capacity issue, are there other obstacles to putting both a 5.1 MLP and a stereo MLP mix on one disc, and letting the listener choose depending on personal preference and the configuration of the playback system? There are none. The limit will be what the content provider wants to do. A multichannel player will allow playback of the multichannel or the 2-channel versions. Of course, a 2-channel player must play the 2-channel version. |
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What is the current state of MLP encoding gear? Are encoding devices commercially available? From whom? Right now, MLP encoders exist as development tools, but they should be available for content generation by year's end. We consider that there will be a significant market for MLP encoders in recording, mastering and distribution systems, so real-time and near-real-time stand-alone hardware encoders are under development, as are PC-based encoders. And Meridian is in discussion with all the major makers of DVD authoring systems. What is the current availability of MLP decoding equipment? What major semiconductor suppliers have agreed to market MLP-capable playback chips? Meridian has already added MLP decoding to its surround decoders. The next commercial phase is expected to be MLP decoders in DVD-Audio players. We are in discussion with ten semiconductor makers, all of whom we expect to make the decoder available in IC form. |
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What applications do you foresee for MLP in addition to the DVD-Audio format? What steps have been taken or planned by Meridian or others to ensure widespread adoption of MLP decoding capability in settings other than DVD-Audio players? We see applications in recording, archiving, distribution and filmmaking, but our current efforts are concentrated on DVD-Audio. Following this, we will focus on outboard decoders and professional applications. |
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DOLBY TO HANDLE MLP LICENSING When it comes to the licensing of technology for consumer electronics products, few companies have the depth of experience of Dolby Laboratories Licensing Corporation. Perhaps it's not surprising that Meridian Audio turned to Dolby - already deeply involved in DVD formats due to the mandated inclusion of Dolby Digital (AC3) surround playback in DVD-Video players - to handle licensing for MLP. "Dolby is the exclusive licensee of Meridian for MLP and is acting as the licensing entity for MLP technology," says Dolby's Roger Dressler, director of technology strategy. "Dolby will handle all the licensing and technical support aspects in the same manner as for other Dolby technologies, such as Dolby Digital. This includes IC implementations, consumer products and professional encoders." Dressler describes MLP as a "good fit with Dolby's technology and strategy. MLP is a form of signal processing used to reduce data rate and storage capacity, which is very much in line with our core businesses at Dolby, and it represents a very high standard in its field of lossless coding. Furthermore, MLP incorporates a 'systems approach,' which is very much in line with our philosophy for Dolby Digital. It's not just a compression system, but a comprehensive system for efficiently delivering audio to a range of user requirements, and it includes particular attention to factors such as compatible stereo reproduction, sidechain data for assisting the decoder with future, additional processing applications, and additional channels." |
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It is actually a bit unusual for Dolby to handle licensing for technology developed out-of-house. "We don't seek to handle outside technologies," Dressler says, "and have turned away many such proposals. But we were approached by Meridian to consider handling the licensing of MLP should it be adopted in DVD-A. Recognizing that the fit was so good - and that most of the prospective licensees for MLP were already Dolby licensees - made the decision quite straightforward on a business basis." Beyond the strict business reasons, a touch of nostalgia may have played a role in deliberations at Dolby, whose beginnings in studio gear have been somewhat overshadowed by later ventures into sound for film and home theater. "MLP gives Dolby the opportunity," Dressler says, "to provide technology for a unique market segment that harkens back to our roots: the music industry. With the transition to digital meaning less use of A-type and SR in studios, we feel that MLP once again gives Dolby the opportunity to offer music producers and artists greater freedom of creativity: more channels, better quality or both, and to assure it can be delivered through to the consumer." Deep as Dolby was in the technology and politics of DVD, the company was well-positioned to support Meridian in its efforts to win MLP's incorporation into the DVD-A specification. "Much of our role was in helping Meridian with their technical presentations through our liaison office in Tokyo. Several such occasions occurred well before the WG4 decided to formally consider adopting a lossless system. Dolby also has been in touch with the U.S. music industry to explain the benefits of lossless technology, which we believe helped move the concept forward. This grew into Dolby helping Meridian with some of the meetings in the U.S. and Japan during the evaluation of the lossless systems for the format. Many of the questions about how MLP would be supported and licensed for all the manufacturing companies were quickly answered once Dolby and Meridian were able to announce their relationship. Since Dolby is a known quantity in this regard, that was of some comfort. But it did not affect the selection of MLP, which was done on a purely technical basis." - Philip De Lancie
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Philip De Lancie is Mix's media & mastering editor |
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