Understanding the Blu-ray Format

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



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DTS HD sounds best encoded at 1,509 Kb/sec.

DTS HD sounds best encoded at 1,509 Kb/sec.

Creators of audio content for consumer video and audio platforms have always had to deal with a list of do's and don'ts. As formats evolved, the list got longer, as did the learning curve. Early CD authors were presented with digital zero, 16-bit/44.1kHz and stereo PCM. Then came DVD and DVD-A, which brought 5.1/6.1 channels, DTS, Dolby Digital, 96kHz sample rate and MLP lossless. And so it goes with Blu-ray, the latest format to change the game for content creators. With the increasing importance and consumer acceptance of this new medium, let's explore some of the possibilities and the limitations that the latest high-definition disc has to offer.

Outstanding Possibilities

Blu-ray's audio and video specs are fantastic. The disc supports up to 32 different bitstreams for primary and 32 bitstreams for secondary audio, plus up to 16 MB for tertiary audio clips used in menu and button sounds. Bitstreams support any number of channels (mono, stereo, 5.1, 7.1 and more) using PCM, Dolby and DTS formats. There is also support for 96- and 192kHz audio sample rates. (There is no support for 44.1 or 88.2.)

An impressive example of Blu-ray's capabilities is on the film Harry Potter and the Order of the Phoenix, with 14 (yes, 14) 5.1 channel bitstreams, 13 different languages and 27 subtitle streams. In this title, Warner Home Video chose to have two English 5.1 channel mixes, in both raw PCM and Dolby Digital legacy, along with 12 other languages including Spanish and Castilian (Español and Castellano), European and Canadian French, German, Italian, Dutch, Japanese, Danish, Swedish, Catalan and Flemish. Now that's some authoring!

DTS Master Audio Suite uses straightforward entry screens.

DTS Master Audio Suite uses straightforward entry screens.

On the video side, a dual-layer, 50GB Blu-ray can store up to nine hours of HD feature content (1,920×1,080 resolution) and up to 23 hours of SD content (720×480). A Blu-ray disc's maximum sustainable data transfer rate is 48 Mb/sec, with the video part of that max limited to 40 Mb/sec. With variable bit rate encoding on both audio and video, audio could hit peak rates well above 8 Mb without affecting video quality at all.

However, before this stellar bit rate can be spun into the consumer's player, some questions should be answered. For instance: Should you give priority to the analog or digital outs? Should you send the original bitstream (only HDMI 1.3 or later) or a PCM stereo downmix to your HDMI output? Do you want any 96kHz sample-rate content down-sampled to 48 kHz for your digital out? These questions go on and on, and it's important that creators pay attention to these critical details — otherwise, consumers could end up with poor to downright annoying results.

For example, a stereo mix must be designated for LCRS (matrix or Dolby Pro Logic) decoding or straight through stereo intended for Left Front/Right Front playback. This is done via metadata flags known as Lt/Rt or Lo/Ro to indicate how audio playback should properly decode. Lt/Rt means “Left Total/Right Total” and is the indicator for surround sound (LCRS) decoding; Lo/Ro is the indicator for Left Only/Right Only decoding. Because industry authoring deals mostly with feature content, many hardware and software encoders, or the procedure within the authoring/compression departments, will default to Lt/Rt if not specified. In this case, our straight stereo mix's phantom center would cancel out of the Left and Right playback and instead be directed to the consumer's center channel and inadvertently decoded in LCRS surround mode. This actually occurred in a Steely Dan concert video DVD a few years ago. Now with Blu-ray's bevy of features, attention to this kind of detail is even more critical.

What's on First, Who's on Second

Blu-ray's primary audio bitstreams are designated to the main soundtracks. This is really where the “sky's the limit.” Currently, up to eight channels of any configuration (8.0, 7.1, 6.1, 5.1, 5.0, LCR, 2.0) down to mono are completely supported using PCM, Dolby or DTS formats. There are some channel limits with a 192kHz sample rate.

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