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Blu-ray Audio Explained
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 1509 kb/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 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

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