Chapter 7: HD Audio
The two HD formats not only improves the picture quality, the sound quality has also been massively improved. Unlike the "matrixed" 7.1 sounds on DVDs, Blu-ray and HD DVD both support true 7.1 channel audio, and sometimes in either lossless or uncompressed format. Being lossless of uncompressed means standard audio compression techniques (needed to get the audio file size down to something acceptable) will no longer be needed, as this "lossy" type of compression reduces quality.
Before we get to the new HD formats, let's go through the many ways which you can enjoy these HD (or high bitrate) audio formats and the requirements of each method.
Raw Bitstream Transport: Also known as "pass-through" mode. This method sends the raw unprocessed digital data over HDMI 1.3 or newer connections, so the receiving device (your AV receiver/decoder) can do the decoding. HDMI 1.3 is required due to the high bitrates often used by HD audio formats. Note that eventually, there will be audio tracks that require audio mixing to be done on the player, and as such, bitstreaming may have limited usage.
Decoding to Linear PCM: The HD audio can be decoded by the player to Linear PCM 5.1/7.1 (uncompressed) audio and sent through the HDMI connection. HDMI 1.3, despite some reports, is not required since all versions of HDMI supports up to 8 channels of PCM audio at 192kHz, 24 bits per sample.
Transcoding to legacy format: The HD audio can also be transcoded/converted to an older format that your amp/receiver will support, like Dolby Digital AC3 or DTS. This type of audio will only require an optical/SPDIF connection, since the bitrate used is the same as standard DVDs (and so technically, this not "real" HD audio). Sometimes a legacy track is provided alongside the HD track to prevent the need for any processing to get to a legacy format.
Ok, so let's now look at the new HD formats in detail.
What are the new HD formats?
Linear PCM: LPCM has existed since the days of CDs, but now with HDMI's higher bandwidth, it can now carry 5.1 or 7.1 channels of audio, as opposed to the 2 channel audio found on CDs. LPCM is uncompressed audio, so it requires a lot of storage space. Linear PCM 5.1/7.1 requires HDMI due to the increased bandwidth required that optical/SPDIF cannot handle.
Dolby Digital Plus: Also known as E-AC3 or DD+, this is a lossy compression format designed to replace the Dolby Digital AC3 system. The advantages of DD+ over AC3 are several encoder based enhancements, as well as support for higher bitrates and up to 13.1 channels of audio at 96 kHz with 24-bit depth. Up to 6 Mbps streams are supported. DD+ is mandatory for HD DVD, meaning all HD DVD players must be able to decode DD+ to LPCM or downmix to a legacy format. DD+ is only optional for Blu-ray, meaning all discs with DD+ audio must also carry a separate legacy AC3 track.
DTS-HD High Resolution Audio: DTS-HD HR is the DTS's answer to DD+. It is also a lossy format that supports up to 7.1 channels of audio at 96 kHz with 24-bit depth. Up to 6 Mbps streams are supported as well. DTS-HD HR support is optional on both Blu-ray and HD DVD players.
Dolby TrueHD: TrueHD is a lossless audio format employing the MLP technique. It features 8 channel support at 96kHz/24-bit with a maximum bitrate of 18 Mbps - most movies so far have only used 5 or 6 Mbps of the maximum allowed bandwidth. TrueHD is an optional audio standard for Blu-ray, while it is mandatory for HD DVD. All HD DVD players must be able decode Dolby TrueHD to LPCM up to 2 channels as part of the mandatory specifications, although all current (as of December 2007) HD DVD players will decode up to 8 channels of LPCM through HDMI output.
DTS-HD Master Audio: DTS-HD MA is a lossless audio format, and DTS's answer to Dolby TrueHD and goes one step further. It is designed to replicate a movie studio's master audio track, which signifies the highest level of audio quality achievable. It supports virtually unlimited channels of audio, while also supporting 96kHz/24-bit audio. Bandwidth support is up to a massive 24 Mbps. It also features a lossy "core" DTS soundtrack for legacy compatibility. DTS-HD MA is optional for both Blu-ray and HD DVD.
So with the above information, what kind of audio equipment should you need? Since all the formats supports legacy output in one form or another, you existing AC3/DTS amp/receiver will give you multi-channel audio for all of these formats, but perhaps not at the higher bitrate, resolution and channels that these HD tracks may include. So if you are wary of spending more money on your sound system, then you don't need to upgrade to achieve a more than acceptable level of Blu-ray and HD DVD audio performance.
Please note that while the PS3 is recommended several times in this guide, the older "fat" variety cannot bitstream any of the high bitrate audio codecs. The new PS3 Slim models can. All can decode Dolby TrueHD to PCM, and the same for DTS-HD MA (when updated to firmware 2.30 or later).
Then there is the issue of audio mixing, which basically means that player decoding support is preferred to bitstreaming and receiver decoding. An example of audio mixing is the interactive elements such as the audio in menus - bitstreaming will mean no such audio is heard, while player decoding will mix the various sound elements together before sending it all out as a single PCM stream. With increased interactivity, audio mixing will become more and more important. In other words, a player doing the decoding is a more future proof way of handling audio than bitstreaming, so keep this in mind.
However, if you are in the market for a new sound system, the recommendation would be to get a system that supports at least PCM 8 channel audio input via HDMI. Note that not all HDMI receiver/amps will process the audio component of the HDMI signal - some only pass-through the HDMI video signal to your TV. If your amp/receiver can accept 8 channel PCM audio through the HDMI connection, then you will be able to enjoy a high level of audio from Blu-ray and HD DVD.
But if money is no object, then you will want a HDMI receiver that supports the decoding of all of the above HD audio formats, and of course a Blu-ray and HD DVD player that supports raw bitstream output for all of these formats to feed your receiver (not all do, unfortunately - for example the Toshiba HD-A30 does not support bitstream output while the Toshiba HD-A35 does, even though both has HDMI audio output). But prices for such receivers are dropping all the time, and you can pick a TrueHD and DTS-HD MA decoding receiver like the Onkyo TX-SR605S for less than $380 from Amazon. Is there really a huge difference between letting your player decode the TrueHD/Master Audio stream and feed it to your receiver via LPCM, versus feeding the receiver the bitstream and letting the receiver decode it? Both are handled differently and so there will be audible differences, but which is "better" or "worse" is a bit subjective. Then there is also the issue of audio mixing outlined above, and so bitstreaming is of limited use.
- There are three types of audio processing: bitstream/pass-thru, decoding and transcoding
- There are 5 new types of HD (high bitrate) audio: LPCM, DD+, Dolby TrueHD, DTS-HD HR, DTS-HD MA
- You do not need a new AV amp/receiver to get audio from HD movies, but you might want one to fully enjoy HD audio
- The PS3 used to have problem with bitstreaming and decoding of DTS-HD MA, but these have been resolved through firmware updates (enabled DTS-HD MA decoding) and the new PS3 Slim (enables bitstreaming)
- Audio mixing is when multiple streams are mixed into one track by the player - bitstreaming does not allow audio mixing, and so interactive elements may not have sound when bitstreamed!
- Getting an AV amp/receiver with HDMI audio support (LPCM 5.1/7.1) is recommended
- Getting an AV amp/receiver with TrueHD and DTS-HD MA audio decoding is better, but perhaps an overkill due to audio mixing issues