As in any small community, information, and sometimes disinformation, runs rapidly through the pro audio industry. One such recent minor controversy relates to DVD-Audio and, more specifically, to aspects of the data reduction algorithms used.
The Background
Even with the ample storage capacity of DVD-Audio discs, high sampling rate, and high word length, multichannel audio eats up a lot of storage space (leaving aside the optional components like video, still photo, and AC-3). There is also a data retrieval bottleneck built-in to the DVD standard; data leaving a disc must be within a 9.6 Mbps (Megabits per second) data rate. In its native PCM form, six channels of 96 kHz sampled, 24-bit audio would make a near 14 Mbps of data to be sucked off the disc on playback. Meridian Lossless Packing (MLP), created by UK-based Meridian Audio and licensed by Dolby Labs, is the method chosen by the creators of the DVD-Audio standard to minimize storage space used and to bypass the bottleneck issue. MLP works somewhat in the fashion of a Zip file process, with math crafted especially for the unique nature of digitized audio.
Input = Output
We have the ability to check our MLP streams by unpacking them into PCM output files after the conversion process, says Craig Anderson of Craigman Digital, a technical consultant to multiple record labels, production facilities, and hardware and software manufacturers, with authoring credit on near 90 DVD-Audio titles.
Though it is not standard practice, we have performed subtractive summing studies on occasion that prove that the original PCM file, when phase inverted in a workstation, will cancel (sum to zero) against the output PCM file. We remain quite confident that MLP is, as its name implies, lossless. Simply stated, Surround Professional found no end users that doubted the bit-for-bit transparency of an MLP encode/decode process.
Making it Fit
With any lossless data compression, the exact size of the encoded files depends upon the information in the original files. For digital audio, this can mean factors such as the amount of correlation between channels or the density of the music (heavy metal typically compresses significantly less than chamber music). As DVD-Audio has a very specific data rate target limit of 9.6 Mbps, it is conceivable that a project (particularly a high-resolution surround project) might not fit within that limit (Meridian provides an illuminating project calculation tool at www.meridian.co.uk/mlp/DVDA_cap_97_03.xls).
At the center of the current controversy are allegations that authoring facilities must routinely bandwidth limit source files to successfully fit them within the data rate limits. In practice, the engineers and facilities contacted by Surround Professional report that surround projects at 96 kHz, 24-bit routinely encode successfully. Anderson says that while the ability to perform filtering or bit-rate reduction is indicative of MLPs flexibility, filtering is an artistic choice that is left to the artist, the producer, and the mastering engineer. It has no place in the authoring facility.
Learning Curve
As with many new technologies, the early participants in DVD-Audio production faced a few hurdles in the form of immature tools and the industrys lack of experience with the new format. Signal chains had to be reexamined for extended bandwidth and dynamic range. New capabilities had to be explored and the knowledge developed to employ them properly as with MLPs downmix capabilities (MLP can store a template for downmixing source surround files to two channels). As Surround Professional investigated a chain of allegations regarding the efficacy of MLP, the bulk of the anecdotal comments alleging negative experiences led to a single such early adopter, 5.1 Entertainment.
Bob Michaels, president of the 5.1 Production Services division of the 5.1 Entertainment Group says that an exhaustive examination of their processing chain yielded a few surprises most notably, certain dithering and reverb algorithms in specific pieces of audio hardware that impressed an aliasing image, a mirror image of the source audio, into the ultrasonic range of the source material. With those spurious signal generators bypassed, Michaels reports that projects routinely MLP encode successfully in a single pass at 5.1 Entertainment. Michaels says that if an MLP process were to fail, he has no reason to suspect problems in the analog domain (analog tape bias leakage, high frequencies from synthesized square waves, and HF noise from tube gear all having been postulated as sources of problematic out of band signals). If we have problems MLPing, he says, we have to look at the chain of events happening in the digital world, were not just going to take the top off the mix and try and squeeze it through.
Though inevitably time consuming and occasionally requiring a second attempt at the math intensive process, the MLP encoder and its interface continue to mature. End users relate confidence in the process with few exceptions. The next generation of blue laser DVDs, with their extravagant storage capabilities, is cited by many as their hope for the future. But that possible future has yet to be realized. For high resolution, linear PCM, surround audio, DVD-Audio is here now, and Meridian Lossless Packing is what makes it work.
For more information, visit Meridian Audio at www.meridian.co.uk.
