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Dolby Digital Sound on Blu-ray and HD DVD Explained
Last Updated on Friday, January 9th, 2009 11:29 AM
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The name Dolby Laboratories has a long history in the movies, and is probably one of the most recognizable brands in the industry when it comes to sound technology.
Not only is Dolby involved in the sound reproduction in movie theaters, it can be found on most DVDs. It is the most common sound format on DVD, and most all current A/V receivers and processors are capable of decoding the Dolby Digital format. However, some technical limitations of the sound format on DVD causes it to run in second place in the eyes (and ears) of many reviewers and enthusiasts to DTS Digital when both formats are available on DVDs.
The introduction of high definition disc formats allows many of those limitations to be removed. The additional storage capacity of these new formats provide for higher quality options to be available to the movie studios creating HD DVD and Blu-ray discs.
The Blu-ray format not allow provides for higher quality picture quality, it also makes possible the best sound quality ever seen on a home video format! The Dolby TrueHD and DTS-HD Master Audio formats are capable of delivering a movie soundtrack that is an exact match to the original as created by the movie studio.
This document covers information about the audio formats created/licensed by Dolby that can be on Blu-ray (and HD DVD) discs. If you are interested in more information about DTS-HD Master Audio and the other DTS audio formats that can be found on Blu-ray/HD DVD discs, please see DTS Digital Sound on Blu-ray and HD DVD Explained.
Dolby TrueHD is a lossless audio technology developed for high-definition disc-based media (Blu-ray and HD DVD). It is bit-for-bit identical to the studio master, supporting up to 8 full-range channels of sound at 24 bits / 96 kHz at a bit rate up to 18 Mbps.
Dolby TrueHD can be transmitted in bitstream format via HDMI 1.3 and above. The audio data also may be sent to the A/V receiver in Linear PCM form instead of raw Dolby TrueHD. A third option is that the audio is decoded by the player and output via 6-8 analog outputs. (See note below about audio mixing in HD DVD and Blu-ray players)
Dolby TrueHD support is optional on Blu-ray players, however. When a Dolby TrueHD soundtrack is included on a Blu-ray disc, a companion Dolby Digital track must also be included for compatibility reasons. You will need to consult the owners manual and release notes for your Blu-ray player to determine its level of support for Dolby TrueHD.
Dolby TrueHD is a mandatory audio format on HD DVD players, but only in 2-channel mode. Fortunately, all HD DVD players are capable of decoding multi-channel Dolby TrueHD, so support for that format is the de facto standard for HD DVD players.
Note: In the first and second generation HD DVD players, the Dolby TrueHD signal is converted to DTS Digital Surround at 1.5 Mbps for output over the standard digital audio output.
A historical tidbit is that Dolby TrueHD has its origins in the MLP Lossless technology, first used on DVD-Audio.
For more information, visit the Dolby web site: "Dolby TrueHD"
Dolby Digital Plus is the next generation of Dolby Digital for high definition programming and media. It is capable of delivering up to 7.1 channels of sound at data rates as high as 6 Mbps.
At this time, Blu-ray limits bit rate performance to 1.7 Mbps, and HD DVD limits it to 3 Mbps.
Dolby Digital Plus can be transmitted via HDMI 1.3 or above for single cable audio and video to A/V receivers, switchers, and display devices. For those not capable of decoding Dolby Digital Plus, the audio can be sent as the core Dolby Digital audio format over the standard digital audio output, or as PCM audio (a raw audio format) over HDMI to a compatible receiver.
Note: In the first and second generation HD DVD players, the Dolby Digital Plus signal is converted to DTS Digital Surround at 1.5 Mbps for output over the standard digital audio output.
Dolby Digital Plus is downward compatible with Dolby Digital, providing existing equipment with Dolby Digital quality sound and compatibility.
For more information, visit the Dolby web site: "Dolby Digital Plus: Audio that completes the high definition picture."
Dolby Digital EX is an extension of Dolby Digital which provides for an additional rear surround channel of sound to be encoded into the Dolby Digital soundtrack.
This additional channel is matrixed into the left and right surround channels, and is therefore not a discrete channel. Not all movies are encoded with a Dolby Digital EX soundtrack.
With the release of Dolby Digital Plus for Blu-ray and HD DVD, we cannot see much of a use for this method to be used when Dolby Digital Plus offers up to 7.1 channels of discrete sound. Thus, you probably won't see much of this sound format on high definition releases.
Dolby Digital is the multi-channel sound technology used on most all DVDs. It is capable of delivering 5.1 channels of sound, with five full-bandwidth channels with 3 Hz to 20 kHz frequency range for Front Left and Right, Center, and Surround, plus one "Low Frequency Effects" (LFE) subwoofer channel devoted to frequencies from 3 to 120 Hz.
On DVD, Dolby Digital is limited to a bit rate of 448 kbps. Blu-ray is capable of transporting Dolby Digital at a bit rate of 640 kbps. (By comparison, Dolby Digital in movie theaters is limited to 320 kbps) On HD DVD, it is limited to a slightly higher bit rate of 504 kpbs.
On Blu-ray, a Dolby Digital soundtrack is mandatory, while Dolby Digital Plus is optional. If a Blu-ray disc has a Dolby Digital Plus or Dolby TrueHD bitstream, there will also be a Dolby Digital track to ensure downward compatibility.
On HD DVD, Dolby Digital, Dolby Digital Plus, or Dolby TrueHD may be used as the sole soundtrack on the disc, as every player will have a decoder capable of processing any of those three bitstreams.
For more information, visit the Dolby web site: "Dolby Digital"
Compatibility and Connections
Blu-ray and HD DVD discs contain more audio than just what is contained strictly in the movie itself. There may be sounds when you bring up the overlay menus, and if you are viewing content that appears in a secondary video window (picture-in-picture), that audio needs to be mixed together with the primary audio from the movie soundtrack. Doing so requires that the player performs this mixing and then outputs a compatible audio stream to your A/V receiver or Preamp/Processor.
So, unlike with DVD, where the receiver needed to be able to decode Dolby Digital or DTS as sent straight from the disc to the receiver, the player now handles the initial decoding, mixing, and re-encoding. This process makes it possible for a Dolby TrueHD soundtrack to be decoded, mixed, and then sent via HDMI in PCM format (a raw audio data format) to an HDMI-equipped receiver that may not be able to decode Dolby TrueHD natively (such equipment is becoming more common).
Some players have an option to forgo this decoding/mixing/re-encoding process and the audio from the movie is sent to the receiver via HDMI in a bitstream format. When this happens, no menu audio or secondary audio of any kind is audible. The receiver is then required to decode the audio format that exists on the disc being played. For more information about this, please see the Help Document titled "Do I Need a New Receiver to Enjoy the Best Sound from HD DVD and Blu-ray?"
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