Understanding the Blu-ray Format

Dec 1, 2009 12:00 PM, By Glen O'Hara



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DTS' legacy format, Coherent Acoustics (at its full 1,509 Kb/sec), was and still is available for DVD, as well as the 768Kb/sec version; i.e., both DVD and Blu-ray. So keep in mind that a Legacy DTS at 768 Kb/sec or a Legacy Dolby Digital AC3 stream at 640 Kb/sec — both supporting up to 6.1 channels — could come in very handy for lots of alternate-language multichannel bitstreams (tracks). On DVD, most creators only had room for encoded 2-channel downmixes for alternate languages. DVD maximum was eight audio bitstreams.

Dolby or DTS?

Although Dolby and DTS are different in their approaches, both companies offer transparent delivery of original master recordings for Blu-ray. On DVDs, many “golden-eared” audio/recording engineers creating 5.1 mixes were amazed at the Coherent Acoustics algorithm employed by DTS, even at their (half) speed of 768 Kb/sec. I would always recommend the full 1,509 Kb/sec for client's music or concert video 5.1 content, where total space was not a problem. Now, with the high-resolution and lossless options for Blu-ray, the sonic differences are perhaps even less than subtle, or nonexistent.

This doesn't mean there aren't some differences. DTS has an interesting advantage for Blu-ray, in that the DTS bitstream was structured to always have DTS' legacy 5.1 48 kHz (known as “Core”) embodied in its new formats, thus saving space. Dolby Digital's new TrueHD format uses the company's previous MLP (Meridian Lossless Packing) technology from the DVD-A specification; then for Blu-ray compatibility, a simultaneous AC3 (legacy) encode is required and is additive to the final file size.

While DTS is marketing this point as an advantage, the reality is that a compatible Dolby TrueHD bitstream typically adds only about 10 to 15-percent more data. This would result in about 400 extra MB on a file for a 2.5-hour feature that is already 3 to 6 GB. This would only be significant if someone was right at the 25GB (single) or 50GB (dual-layer) limit and needed a digital “shoehorn” option without sacrificing the “lossless” feature.

On the Dolby side, the advantage is seen in the Dolby Media Meter plug-in found in the company's offerings of encoding and editing tools. Using this awesome tool, you can analyze any existing mix for overall levels against industry-standard dialnorm, which is -27 dBFS. As I mentioned above, due to Blu-ray's much larger data capacity, content creators are including a lot of extra bonus material and dealing with different feature mixes, language overdubs, etc., all of which can have dramatically different overall levels. Dolby Media Meter can run faster than real-time analysis, whose result is a recommended dialnorm setting for that particular content. This application can run stand-alone or as an RTAS/AudioSuite plug-in in Pro Tools, among other platforms. It's also available for separate purchase from Dolby and can be invaluable for consistent levels over an entire Blu-ray disc's audio content.

Whatever format you use to author to Blu-ray, you can be sure that you'll have an abundance of great tools to access the deep feature set that Blu-ray discs offer. The bottom line is stellar picture and sound in numerous languages with lots of extra features and resources. What's not to like?

For more info on Blu-ray authoring, be sure to read Mix's review of the Dolby Media Producer software suite at mixonline.com/gear/reviews/audio_dolby_media_producer_2.

Glen O'Hara is an engineer and educator who has produced 5.1 music titles on DVD-A and DTS audio discs.

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