Audio for DVD Seminar

It's no secret that Dolby Laboratories doesn't aim its audio compression technologies at the high-end consumer audio market. After all, Dolby excels at finding ways to get maximum performance out of limited-bandwidth environments such as the audio cassette, or the space alloted for 5.1-channel soundtracks on DVDs.

At the recent Audio for DVD Seminar held at company headquarters in San Francisco, the folks from Dolby often referred to audiophiles as "them"---as in "us and them." Nonetheless, Dolby seems sympathic to the concerns of tweaks troubled by any compression schemes being used to squeeze music into small places. The seminar, held June 25 and 26 for industry professionals---recording and mastering engineers, equipment manufacturers, chip and software designers, and members of the press---emphasized the need for consistent and high-quality studio environments for those involved in creating audio for DVD.

The first day of the session offered a broad overview of DVD, home theater, and multichannel sound, as well as a history of Dolby's involvement with multichannel audio. Day two was an in-depth technical exploration of DVD multichannel audio encoding and decoding techniques. Those who attended both days received certificates of completion.

Beginning his presentation, Dolby Technology Strategy Director Roger Dressler mentioned DVD-Audio and the Sony/Philips Super Audio CD (SACD), and acknowledged that the two approaches are "non-compatible, at present." He said that Dolby is collaborating with the DVD Forum Working Group 4, who should have "Version 1.0" of a DVD-Audio spec ready in September.

Dolby Digital Technology Licensing Manager Ramzi Haidamus then announced the company's recent agreement with Meridian (also announced at HI-FI '98 earlier this month), in which Dolby will license Meridian's MLP lossless-compression technology (see previous story for details) to manufacturers worldwide. "Even saying the word 'compression' causes an allergic reaction among some purists," he said. "MLP, which gives you back exactly what you put in, should satisfy everyone."

Ramzi also pointed out that working with Meridian on MLP is only the second time in history that Dolby has gone outside of the company for technology, the first time being for HXPro from Bang & Olufsen.

While still on the topic of DVD-Audio, he stated that Dolby doesn't feel people need more than 20-bit resolution in a digital audio signal, since they've concluded that folks can't hear any difference when going to higher rates. "If the audio community thinks it needs 24 bits, well, good luck."

The next hour or so was spent reviewing Dolby's perceptual-coding technologies for reducing data in digital-audio storage systems. The idea is to throw out the information that is masked by the louder or more noticeable portions of the signal, thus saving up to 91% of the space needed to encode a multichannel audio recording. The current consumer implementation of this technique is Dolby Digital (DD), which is being promoted as both the 5.1 surround format for DVD-Video and HDTV, and also as a multichannel format for surround-sound music on DVD discs. Dolby likes to point out that, unlike the proposed DVD-Audio spec, DD 5.1 audio for music releases is compatible with existing DVD players. Audiophiles like to point out that, to them, it sounds worse than CD.

Another drawback, however, to encoding and decoding a digital audio stream is that decoders exhibit latency---a slight delay of 5 to 25 milliseconds to receive and decode the audio data. The result: a decoded signal doesn't work well in an interactive environment in which the user is triggering sounds in real time, such as in a computer-based game.

A series of slides was used to demonstrate how a recording/mixing studio might want to set up its 5.1 surround system. The preferred surround speaker type, according to Dolby, is a direct radiator (or monopole) placed on either side of and slightly behind the engineer. Dolby feels that this is the best method for "properly determining directional information." Next up were dipole surrounds, although Dolby notes that this makes it "more difficult to determine directional cues." Tripole surround speakers were suggested as a compromise between mono and dipoles because tripoles are less dependent on room layout and can provide decent directional information. Equal amplifier power is recommended for all channels.

Also noted for engineers in the audience were the time delays that digital processing can introduce into a digitally based recording studio. Each digital processor needs a little time to add its effect (equalization, reverb, panning, etc.), which can seriously alter arrival times of a signal from the various channels, getting them out of sync. The trick here is to check your mixes not only in 5.1 or stereo modes, but also in mono before encoding to DD. An example was given of a Madonna single that used a digital process that rendered it so unlistenable on mono radios (okay, it was probably unlistenable on even a good stereo system) that the BBC banned it from broadcast.

We were treated to several listening tests, both in Dolby's state-of-the-art, theater-sized presentation studio, and in smaller rooms designed to more closely resemble a consumer listening environment. Recordings were used to demonstrate how surround channels can place a listener inside a venue, and "moves" the listener forward or back in the hall. While this was interesting as an effect, one member of the audience stated that the original stereo presentation was still his favorite. We have to agree for now, but these are the early days of the format---it'll be interesting to see how others exploit it.

One more interesting fact from the seminar: Dolby estimates that there are 34 million surround decoders in the market (10% of households), but that only half of them are hooked up!

See related story on the Stereophile Guide to Home Theater website.

Share | |
Site Map / Direct Links