Understanding the Blu-ray Format
Dec 1, 2009 12:00 PM, By Glen O'Hara
AN AUDIO PRODUCER'S GUIDE
Secondary audio is designed primarily to accommodate director/actor commentary-type tracks, but it also offers some very cool options. DVD content creators had to make many tough decisions due to space and bandwidth limitations. Commentary tracks were usually encoded in stereo or Dolby Digital 2.0 instead of 5.1 to save space and were separate bitstreams. With Blu-ray, any number of commentary tracks can be encoded in Dolby Digital Plus or DTS Express in mono or stereo as secondary audio. (Due to limited bandwidth, PCM is not possible.) Not only is this secondary audio simply added to the primary audio mix (by user selection), it can be authored to do so on-the-fly, meaning through a pop-up menu while the feature is still playing. Blu-ray's specs require that the player be capable of mixing these sounds together, yet a secondary or tertiary track's sounds could be dropped, depending on player settings for co-ax or optical digital outputs. The big advantage here is commentary tracks no longer require another primary soundtrack as in DVD, allowing for more creative options with significantly less file size and bandwidth.
In addition to bringing this mono or stereo commentary track into the mix, what is called “AAF” metadata can be encoded into the secondary bitstream that allows fader moves of any and all channels in the primary mix. One can “ride faders” that reduce the primary audio level creatively around the commentary dialog and export that automation for inclusion in the secondary bitstream. When using this feature, the overall level of the primary audio can be adjusted +12 to -50 relative dB in 1dB increments, but any individual channel adjustment is limited to -28dB relative differential from the rest. Using metadata, the channels can be routed to any supported output in someone's home theater/player system setup (i.e., not necessarily center or L/R). I can't wait for someone to have the director talk over my shoulder in the surrounds. With Blu-ray, this is possible.
PCM With Reservations
Although Blu-ray supports PCM in any bit depth up to eight channels at 48kHz or 96kHz sample rates, or up to six channels at 192 kHz, there are two caveats about using PCM.
First, PCM requires at least double the file sizes as compared to using even the highest-quality, lossless features of both Dolby and DTS. Second, even if you have plenty of room on a 25- or 50GB disc, you lose many important features. The ability to set “dialnorm” (dialog normalization) and downmix options is only available when your content of any number of channels is “bundled” within a Dolby or DTS bitstream.
Not having the ability to set dialnorm significantly changes the game for both the consumer and the author. For instance, it may cause the end-users to alter their volume knob constantly due to the likelihood that different mixes from different sources can exhibit more drastic level differences than you might expect. At the production end, Blu-ray authors and their audio staff can simply adjust dialnorm settings as the audio content is encoded using any of the Dolby or DTS technologies. When using PCM, the authors would have to add what could be unfeasible workflow delay issues to kick the various audio mixes back to content-creation personnel to re-adjust audio levels for consistent playback from all the various content.
Also, when using PCM for multichannel tracks, one is at the mercy of the player manufacturer's default for downmixing. When either Dolby or DTS is used to encode multichannel mixes, metadata can be set by content creators through their compression (encoding) and authors as to exactly how they would like downmixes to be executed. This includes specifying how a 7.1 or 6.1 mix should be downmixed into 5.1, and how that resulting downmix should be presented when downmixed into stereo.
DVD vs. Blu-ray
The new Dolby and DTS extensions for Blu-ray are quite impressive. Both offer lossless encoding formats up to 24-bit/192kHz via DTS Master Audio and Dolby TrueHD. Both offer extra (beyond 5.1) channels with very high-quality lossy formats: DTS-HD and Dolby Digital Plus.
In the past, legacy Dolby Digital, AC3 bitstreams were limited to 448 Kb/sec on DVD. Legacy Dolby Digital always had a maximum transfer (or data rate) of 640 Kb/sec, but this was not supported on DVD specifications. No such limit exists for Blu-ray. Using their (legacy) AC3/Dolby Digital 5.1 at 640 Kb/sec makes a huge difference over the 384Kb or even the 448Kb rates available on DVD. For a Blu-ray that needs many 5.1 bitstreams, they still take up comparably small space vs. the Dolby Tru-HD. So if a content creator wanted multiple 5.1 mixes in various languages, both DTS and Dolby legacy formats could make a huge difference, allowing many 5.1 mixes to fit nicely on Blu-ray.
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