High-Def FAQ: Blu-ray and HD DVD Audio Explained

Posted Fri Oct 12, 2007 at 01:37 PM PDT by

Editor's Note: As part of his twice-monthly column here at High-Def Digest, from time to time, Josh Zyber answers frequently asked questions related to High-Definition and both Blu-ray on HD DVD. This week: Josh provides a comprehensive rundown of all the audio formats currently available on next-gen disc.

Commentary by Joshua Zyber

If there's one request we get here at High-Def Digest more than any other, it's to help readers sort through all the confusion swirling around the new audio formats that come on HD DVD and Blu-ray discs. It seems that most early adopters can easily identify the benefit of a High Definition picture over Standard-Def DVD, but making sense of the difference between Dolby Digital Plus and DTS-HD, or PCM and TrueHD is a lot harder to get a good grasp on. It doesn't help that the companies who designed these sound formats (Dolby and DTS) haven't always been clear in their labeling or naming conventions.

A Quick Recap of the DVD Situation

On Standard-Def DVD, there are essentially only two competing sound formats to choose from: Dolby Digital or DTS. Both can accommodate movie soundtracks from monaural 1.0 to multi-channel 5.1, and in some cases add a matrixed center back channel as well (DTS also offers a discrete 6.1 option on selected titles). A small number of discs (mostly music concerts) may provide 2-channel PCM audio, but those are few and far between. As a rule of thumb, it's Dolby or DTS. The DVD spec requires all discs to contain either a Dolby Digital or PCM soundtrack as the base standard (pretty much everyone uses Dolby), and all DVD players are required to decode both. DTS is optional, and is generally considered (fairly or not) an added-value feature. Though it hasn't always panned out that way in actual practice, there is a perception in the DVD marketplace that DTS is the "better" sound option that will provide greater fidelity to the source. Sometimes that's true and sometimes not, but that's a discussion topic for another day. The reality of the situation is that both Dolby Digital and DTS are capable of delivering very good, sometimes even exceptional sound quality on DVD.

Both Dolby Digital and DTS are "lossy" compression codecs. Before making it to disc, each format selectively filters out data from the studio's digital audio master using perceptual encoding techniques. In theory, the data removed should consist mainly of either frequencies beyond the range of human hearing or frequencies that would normally be masked by other frequencies in the track anyway. If done properly, the end result should sound seamless to the listener. But if done poorly or over-compressed, the audio may lose fidelity.

Standard Dolby Digital can be encoded in a variety of bit rates, the most common being 192 kb/s (reserved for 1.0 or 2.0 soundtracks and generally poor fidelity), 384 kb/s (OK quality), and the maximum 448 kb/s (used on the majority of DVD 5.1 soundtracks). DTS has two bit rate encoding options: the commonly used 754 kb/s or a rarely offered high rate of 1.5 Mb/s. Within each format, the higher the bit rate means the less compression needed and the less data removed from the master. However, it also means that the audio track takes up more disc space, which can eat into the bit rate allocated to video quality. Also note that Dolby and DTS use entirely different compression techniques, and their bit rate numbers are not directly comparable to one another. While a 448 kb/s Dolby track is better than a 384 kb/s Dolby track, a 754 kb/s DTS track is not necessarily better than a 448 kb/s Dolby track just because the number is larger. Dolby uses more efficient compression techniques than DTS and can usually achieve results at 448 kb/s comparable to DTS at 754 kb/s.

Now on to the High-Def Formats

The advent of Blu-ray and HD DVD has brought a dramatic increase in picture quality from Standard Definition to High Definition. Along with that has come an expectation for an attendant boost in audio quality. When there's so much more disc space available on an HD DVD or a Blu-ray, why should we be limited to the heavily-compressed sound formats we got on DVD? High Definition video deserves High Definition audio to go with it.

Jumping into the fray once more are Dolby and DTS. Each company has developed a line-up of brand new sound formats to go with the new disc types, using advanced forms of audio compression to deliver high quality to the home listener, quality sometimes matching that of the studio master itself. On some discs we even have the option of raw PCM with no compression at all. But we haven't just been given one new codec choice per company. No, that would be too simple. Now we have a whole host of confusing new options. To help straighten out this tangled mess, let's break things out by High-Def disc format and take a look at what each supports.


Blu-ray discs can provide their movie soundtracks in any of the following formats:

Dolby Digital
What it is: The audio format familiar from DVD, Dolby Digital (sometimes known as AC-3) is one of the base standards of Blu-ray. It works basically the same way that it worked on DVD in configurations from 1.0 to 5.1, though it does offer a higher maximum bit rate of 640 kb/s (which is considered audibly indistinguishable from Dolby Digital Plus at the same rate).
Level of support: Full support for Dolby Digital is mandatory in all Blu-ray disc players.
Examples of discs that use it: Almost all Blu-ray discs from Sony, Warner Bros., Paramount, and Lionsgate, among others.
How to get it:

  • Toslink or Coaxial SPDIF - Using an SPDIF connection, the Dolby Digital bitstream is sent directly to your receiver for decoding, converting to analog, and amplifying to your speakers.
  • HDMI - Depending on the setting chosen in your Blu-ray disc player, the HDMI output can be used to transmit the Dolby Digital bitstream to be decoded in the receiver, or the player itself can perform the decoding to a PCM signal and transmit instead in that form. The receiver will still be needed for digital-to-analog conversion and amplification.
  • Multi-channel analog - With the analog connections, the player itself must decode the Dolby Digital bitstream and convert it from digital to analog. This will then be passed to the receiver for amplification. In this case, calibration adjustments such as speaker sizes and channel levels should be entered into the Blu-ray disc player's setup menus, not the A/V receiver's. The quality of the resulting sound will vary depending on whether the Digital-to-Analog (DAC) components in the player are as good as those in the receiver. If the receiver has superior DACs, a digital connection (either SPDIF or HDMI) will be preferred.


DTS
What it is: Sometimes referred to as DTS Encore (though DTS themselves don't seem to use that name anymore), this sound format is another familiar holdover from standard DVD. Blu-ray, however, more ably supports the codec at its higher 1.5 Mb/s bit rate.
Level of support: All Blu-ray disc players are required to support the transmission of a DTS bitstream over a digital connection and internal decoding up to at least 2 channels. Most players (other than early models such as the Samsung BD-P1000) will decode internally to 5.1.
Examples of discs that use it: 'Terminator 2', 'Lara Croft: Tomb Raider'.
How to get it:

  • Toslink or Coaxial SPDIF - The DTS bitstream will be sent to your receiver for decoding and processing.
  • HDMI - As with Dolby Digital, the HDMI connection can carry the raw DTS bitstream for decoding in the receiver, or the player may decode it to PCM first.
  • Multi-channel analog - The Blu-ray disc player will decode the DTS bitstream (only 2 channel decoding is required, but most players will do 5.1) and convert it to analog, after which it will be sent to the receiver for amplification. Once again, the final sound quality will depend on how well the audio components in the disc player compare to those in the receiver.


Dolby Digital Plus
What it is: An enhancement over standard Dolby Digital, DD+ offers higher bit rates and more efficient compression, resulting in improved sound quality. It can also support movie soundtracks up to 7.1 discrete channels (though honestly, the vast majority of Hollywood movies are only mixed for 5.1). On Blu-ray, DD+ is encoded as an extension to a "core" Dolby Digital AC-3 track.
Level of support: Unfortunately, DD+ is optional on the Blu-ray format, and not all disc players are required to support it. Many players will simply read the 640 kb/s core and disregard the extension. As a result, most movie studios prefer to use either basic Dolby Digital AC-3 or some of the other advanced formats.
Examples of discs that use it: 'A View from Space with Heavenly Music' claims a DD+ track, assuming that the packaging and menus aren't just mislabeled.
How to get it:

  • Toslink or Coaxial SPDIF - SPDIF cannot carry a full DD+ signal. If you use this connection method, the player will limit output to the Dolby Digital AC-3 core.
  • HDMI - If the player does not support DD+, it will simply extract the AC-3 core, in which case see the Dolby Digital listing above. Some players may decode the DD+ to PCM and transmit it over HDMI (any version). Other players will instead choose to transmit the DD+ bitstream to a receiver for decoding, but this requires HDMI 1.3 connections on both ends of the chain.
  • Multi-channel analog - Either the Blu-ray player will extract and decode the AC-3 core, or (on some models) will decode the full DD+ and convert it to analog.


DTS-HD High Resolution
What it is: Similar to Dolby Digital Plus, DTS-HD High Resolution is an enhancement over standard DTS that offers higher bit rates and better compression. DTS-HD HR is also encoded as an extension to a "core" DTS track. (Note that DTS-HD HR is sometimes referred to as just "DTS-HD", which can be confusing and possibly misleading).
Level of support: Since this codec is also optional on Blu-ray, many players will only extract the 1.5 Mb/s DTS core and ignore the extension.
Examples of discs that use it: 'Basic Instinct', 'Total Recall'.
How to get it:

  • Toslink or Coaxial SPDIF - Because SPDIF cannot transmit a full DTS-HD HR signal, the player will extract the DTS core and send the bitstream for that instead.
  • HDMI - If the player does not support DTS-HD HR, it will extract the DTS core, replicating the DTS listing above. Some players may decode the DTS-HD HR to PCM and transmit it over any version of HDMI. Other players will instead transmit the DTS-HD HR bitstream to a receiver for decoding (this requires HDMI 1.3).
  • Multi-channel analog - Either the Blu-ray player will extract and decode the DTS core, or (on some models) will decode the full DTS-HD HR and convert it to analog.


PCM
What it is: A PCM track is an exact replication of the studio master, encoded on disc without compression. The benefit to this is that it maintains the purity of the source without any loss of fidelity that may come from compression. The downside is that an uncompressed audio track takes up a tremendous amount of disc space, which may (especially on single-layer BD25 discs) negatively affect the video quality of the movie. While the Blu-ray format is capable of utilizing PCM audio up to 24-bit resolution, studios may choose to encode at 16-bit resolution instead, depending on the bit depth of the original source or concerns about conserving bandwidth (downsampling a 24-bit master to 16 bits is technically not the same thing as compression).
Level of support: All Blu-ray disc players are required to support PCM audio.
Examples of discs that use it: Almost all discs from Sony and Disney, as well as selected titles from Lionsgate and other studios.
How to get it:

  • Toslink or Coaxial SPDIF - SPDIF does not have enough bandwidth to carry a full 5.1 PCM signal, so the audio track will be downgraded to 2 channels only. This is generally an undesirable result.
  • HDMI - A PCM track can be transmitted in full quality over any version of HDMI and delivered to the receiver for D-to-A conversion and amplification.
  • Multi-channel analog - In this case, the player converts the PCM to analog and sends it to the receiver for amplification. The quality of the DACs in the player will determine the final audio quality. If the disc player has inferior DACs to the receiver, an HDMI connection is preferred.


Dolby TrueHD
What it is: Dolby TrueHD is a "lossless" compression codec. Although it is compressed to take up less disc space than a PCM track, once decoded it is bit-for-bit identical to the studio master (at either 16-bit or 24-bit resolution, at the discretion of the studio). It may help to think of it like a ZIP file that holds a PCM track. Once you unZIP the file, you get a 100% identical copy of the original PCM, without compromising any sound quality.
Level of support: TrueHD is an optional format on Blu-ray. And since TrueHD is not built in a core+extension configuration, Blu-ray discs that contain a TrueHD track are also required to contain a standard Dolby Digital AC-3 track for compatibility with players that don't support TrueHD.
Examples of discs that use it: 'Ghost Rider', 'The Fifth Element' (Remastered).
How to get it:

  • Toslink or Coaxial SPDIF - SPDIF cannot carry a TrueHD signal. If using this connection type, the player will automatically revert to playing back the standard Dolby Digital AC-3 track instead.
  • HDMI - If the player does not support TrueHD, it will revert to the standard Dolby Digital track. Some players may decode the TrueHD to PCM and transmit it over any version of HDMI. Other players will instead transmit the TrueHD bitstream to a receiver for decoding (HDMI 1.3 required).
  • Multi-channel analog - Either the Blu-ray player will decode the standard Dolby Digital track, or (on some models) will decode the TrueHD and convert it to analog.


DTS-HD Master Audio
What it is: Another lossless audio codec similar to Dolby TrueHD. The difference between the two is that DTS-HD MA is built in a core+extension configuration (just like DTS-HD HR). Although a DTS-HD MA track takes up more disc space than a TrueHD track, it does not require a secondary standard track for backwards compatibility. Since both DTS-HD MA and TrueHD are lossless, they are both 100% identical in quality to the studio master, and hence identical in quality to each other.
Level of support: Like DTS-HD HR, Master Audio is optional on the Blu-ray format. If the player does not support DTS-HD MA, it can extract the standard DTS core.
Examples of discs that use it: Almost all titles from Fox Home Entertainment.
How to get it:

  • Toslink or Coaxial SPDIF - SPDIF cannot carry a DTS-HD MA signal. When using this connection type, the player will extract the standard DTS core instead and transmit that as a bitstream.
  • HDMI - If the player does not support DTS-HD MA, it will extract the DTS core. Some players may decode the DTS-HD MA to PCM and transmit it over any version of HDMI. Other players will instead transmit the DTS-HD MA bitstream to a receiver for decoding (HDMI 1.3 required).
  • Multi-channel analog - Either the Blu-ray player will decode the standard DTS core, or (on some models) will decode the DTS-HD MA and convert it to analog.


At the time of this writing, only one Blu-ray disc player (the Samsung BD-P1400) supports the transmission of a DTS-HD Master Audio bitstream over HDMI 1.3. No current Blu-ray disc players will yet decode the DTS-HD MA track to PCM internally. All other players are limited to extraction of the standard DTS core. This situation is expected to change in the near future as more player models are released, and manufacturers issue firmware updates to existing players.


HD DVD discs can provide their movie soundtracks in any of the following formats:

Dolby Digital
What it is: All HD DVD players are required to support standard Dolby Digital AC-3 up to 448 kb/s (the same maximum bit rate as used on DVD). However, they're also required to support the advanced Dolby Digital Plus (see next listing), and for movie soundtracks that's what almost all studios have gone with instead.
Level of support: Mandatory.
Examples of discs that use it: Dolby Digital is commonly used on the bonus features on most HD DVDs. To my knowledge, no discs have used this format for the feature soundtrack.
How to get it:

  • Toslink or Coaxial SPDIF - The Dolby Digital bitstream would be transmitted to a receiver for decoding and processing.
  • HDMI - Depending on the HD DVD player settings, the Dolby Digital can be transmitted as a bitstream or decoded to PCM first.
  • Multi-channel analog - In this method, the player decodes the Dolby Digital internally and converts it to analog.


DTS
What it is: As with Blu-ray, the legacy DTS format familiar from DVD is available as an option on HD DVD, encoded at the higher 1.5 Mb/s bit rate.
Level of support: All HD DVD players are required to support DTS.
Examples of discs that use it: 'The Chronicles of Riddick', 'Sleepy Hollow'.
How to get it:

  • Toslink or Coaxial SPDIF - The bitstream is transmitted to a receiver.
  • HDMI - Either the raw bitstream is sent to the receiver, or the player decodes to PCM first.
  • Multi-channel analog - The player decodes the DTS internally and converts it to analog.


PCM
What it is: While all HD DVD players support uncompressed multi-channel PCM, and the internal decoding of all Dolby and DTS formats is converted to PCM, movie discs with PCM appear to be limited to 2-channel audio. Whether this is a requirement of the HD DVD format itself or a choice of the disc authors is not clear. In any case, due to the enormous amount of disc space required, PCM is not a very popular option on HD DVD.
Level of support: Mandatory.
Examples of discs that use it: 'Chronos', 'Moonlight Jellyfish' (a Japanese film included as a pack-in item with the Toshiba HD-XA1 player released in that country).
How to get it:

  • Toslink or Coaxial SPDIF - SPDIF can carry 2-channel PCM just fine and send it to the receiver for D-to-A conversion and amplification, but cannot carry more than 2 channels.
  • HDMI - HDMI likewise will transmit the PCM to a receiver for D-to-A and amplification.
  • Multi-channel analog - The PCM will be converted to analog and then sent to the receiver to be amplified.


Dolby Digital Plus
What it is: DD+ is the base standard audio format for HD DVD. Unlike its application on Blu-ray, DD+ on HD DVD does not utilize a core+extension configuration. The format can be encoded at bit rates of 640 kb/s (considered equivalent to Blu-ray's use of standard Dolby Digital at that same rate) or 1.5 Mb/s. However, note that although the latter version of DD+ shares a similar bit rate as standard DTS, this does not mean that these two are equivalent to one another. DD+ uses better encoding and more efficient compression to provide improved quality at the same rate. At least one professional Hollywood sound mixer has described Dolby Digital Plus at 1.5 Mb/s as audibly transparent to the studio master.
Level of support: All players are required to support Dolby Digital Plus.
Examples of discs that use it: Almost all domestic HD DVD releases. Discs from Warner Bros. default to the lower 640 kb/s rate, while those from Universal and Paramount tend to favor the higher 1.5 Mb/s (with some exceptions).
How to get it:

  • Toslink or Coaxial SPDIF - SPDIF cannot transmit DD+ in full quality. When using this connection method, the player will decode the DD+ and then transcode it to either standard Dolby Digital AC-3 or sometimes even DTS (depending on player model).
  • HDMI - Almost all HD DVD players decode the DD+ track internally to PCM for transmission over HDMI. Some may transmit the DD+ bitstream to a receiver instead (HDMI 1.3 required).
  • Multi-channel analog - In this case, the player decodes the DD+ track and converts it to analog. The quality of the DACs in the player will determine the resulting sound quality.


DTS-HD High Resolution
What it is: DTS-HD HR works on HD DVD just as it does on Blu-ray, in a core+extension configuration.
Level of support: DTS-HD HR is optional on HD DVD. Players that don't support it can extract the standard DTS core.
Examples of discs that use it: 'Sky Captain and the World of Tomorrow' claims a "DTS-HD" track, which is presumably DTS-HD High Resolution, unless the packaging and menus are mislabeled.
How to get it:

  • Toslink or Coaxial SPDIF - SPDIF cannot transmit DTS-HD HR in full quality. The player will extract the standard DTS core for transmission as a bitstream.
  • HDMI - If the player does not support DTS-HD HR, it will extract the DTS core. Some players may decode the DTS-HD HR to PCM and transmit it over any version of HDMI. Other players may instead transmit the DTS-HD HR bitstream to a receiver for decoding (HDMI 1.3 required).
  • Multi-channel analog - Either the HD DVD player will extract and decode the DTS core, or (on some models) will decode the full DTS-HD HR and convert it to analog.


Dolby TrueHD
What it is: Once decoded, the lossless Dolby TrueHD format is bit-for-bit identical to the studio master (at either 16-bits or 24-bits, at the discretion of the studio).
Level of support: Support for TrueHD up to at least 2 channels is mandatory on all HD DVD players, but the majority will support it all the way to 5.1. Because there are rare cases of disc players that limit TrueHD to 2 channels (such as the LG model BH100), discs with TrueHD tracks must also contain a Dolby Digital Plus track for 5.1 compatibility.
Examples of discs that use it: '300', 'Superman Returns'.
How to get it:

  • Toslink or Coaxial SPDIF - SPDIF cannot transmit TrueHD in full quality. When using this connection method, the player will decode the TrueHD and then transcode it to either standard Dolby Digital AC-3 or possibly DTS (depending on player model).
  • HDMI - Almost all HD DVD players decode the TrueHD track internally to PCM for transmission over HDMI. Some may transmit the TrueHD bitstream to a receiver instead (HDMI 1.3 required).
  • Multi-channel analog - The player will decode the TrueHD track and convert it to analog.


DTS-HD Master Audio
What it is: The other losslessly compressed format, DTS-HD Master Audio is also bit-for-bit identical to the studio master once decoded (and hence identical to Dolby TrueHD, assuming an equal bit depth is used). DTS-HD MA works in a core+extension configuration.
Level of support: Since the DTS-HD MA format is optional on HD DVD, players that don't support it will extract the standard DTS core.
Examples of discs that use it: Almost all HD DVDs released by Studio Canal in Europe.
How to get it:

  • Toslink or Coaxial SPDIF - SPDIF cannot transmit DTS-HD MA in full quality. The player will extract the standard DTS core for transmission as a bitstream.
  • HDMI - If the player does not support DTS-HD MA, it will extract the DTS core. Some players may decode the DTS-HD MA to PCM and transmit it over any version of HDMI. Other players may instead transmit the DTS-HD MA bitstream to a receiver for decoding (HDMI 1.3 required).
  • Multi-channel analog - Either the HD DVD player will extract and decode the DTS core, or (on some models) will decode the full DTS-HD MA and convert it to analog.


At the time of this writing, no HD DVD players will decode either a DTS-HD High Resolution or a DTS-HD Master Audio audio track. The players are limited to extracting the DTS cores, or (on selected player models) may be able to transmit the codec bitstream to a receiver using HDMI 1.3. Support for these DTS formats will hopefully expand with future player models or firmware updates to existing models.

An Exhausting List

I believe that covers all the bases for audio options a viewer is likely to run into on either HD DVD or Blu-ray. The purpose of this list is to explain the basic workings of each sound format offered on High-Def discs. I will save the arguments over whether lossless formats are really as good as uncompressed formats, debates on the importance or relevance of bit rate numbers, concerns about bit depth, and the controversy about Dolby's "Dialog Normalization" function for another day.

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