Dolby TrueHD - What You Need To Know

Official Dolby TrueHD Logo
Official Dolby TrueHD Logo. Dolby Labs
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What Dolby TrueHD Is

Dolby TrueHD is one of several surround audio formats developed by Dolby Labs for use in home theater applications.

Specifically, Dolby TrueHD is designed for use in the audio portion of Blu-ray Disc and HD-DVD programming content. Although HD-DVD was discontinued in 2008, Dolby TrueHD has maintained its presence in the Blu-ray Disc format, but its direct competitor from DTS, referred to as DTS-HD Master Audio, is more commonly used on Blu-ray disc soundtracks than Dolby TrueHD.

Dolby TrueHD can support up to 8 channels of audio at 96Khz/24 bits (which is most commonly used), or up to 6 channels of audio at 192kHz/24 bits (96 or 192kHz represents the sampling rate, while 24 bits represents the audio bit depth). Blu-ray Discs that include Dolby TrueHD can either feature those options as a 5.1 or 7.1 channel soundtrack, at the movie studio's discretion.

Dolby TrueHD also accommodates data transfer speeds of up to 18mbps (to put this into perspective - for audio, that's fast!).

The Lossless Factor

Dolby TrueHD (as well as DTS-HD Master Audio), are referred to as Lossless Audio formats. What this means, in the case of Dolby TrueHD, is that unlike Dolby Digital, Dolby Digital EX or Dolby Digital Plus, and other digital audio formats, such as MP3, a type of compression is employed that results in no loss in audio quality between the original source, as recorded, and what you hear when you play the content back.

In other words, no information from the original recording is tossed away during the encoding process What you hear is what the content creator, or the engineer that mastered the soundtrack onto Blu-ray disc, wants you to hear (of course, the quality of your home theater audio system also plays a part).

Dolby TrueHD encoding even includes automatic Dialog Normalization to assist in balancing the center channel with the rest of your speaker setup (it doesn't doesn't always work well so you may still need to make a further center channel level adjustment if the dialog does not stand out well).

Accessing Dolby TrueHD

Dolby TrueHD signals can be transferred from a Blu-ray Disc player in two ways.

One way is to transfer Dolby TrueHD encoded bitstream, that is compressed, via HDMI (ver 1.3 or later) connection to a home theater receiver that has a built-in Dolby TrueHD decoder. Once the signal is decoded, it is passed from the receiver's amplifiers to the correct speakers.

The second way to transfer a Dolby TrueHD signal is by designating that a Blu-ray Disc player decode the signal internally (if the player provides such internal decoding) and then pass the decoded signal directly to a home theater receiver as a PCM signal via HDMI, or, via a set of 5.1/7.1 channel analog audio connections. In this case, the receiver does not need to perform any additional decoding or processing, and just passes the signal to the amplifiers and speakers.

However, to use the decoding to analog audio connection option, the Blu-ray Disc player has as to a set of 5.1/7.1 channel analog audio outputs, and the home theater receiver has to have a set of 5.1/7.1 channel analog audio inputs (which is now very rare).

It is also important to note that not all Blu-ray Disc players provide the same Dolby TrueHD internal decoding options - some may only provide internal two-channel decoding, rather than full 5.1 or 7.1 channel decoding capability.

In addition, unlike the Dolby Digital, Digital EX, and Digital Plus surround sound formats, Dolby TrueHD (either undecoded or decoded) cannot be transferred by Digital Optical or Digital Coaxial audio connections that are commonly used to access Dolby and DTS surround sound from DVDs and some streaming video content. The reason for this is that there is too much information, even in compressed form, for those connection options to accommodate.

More On Dolby TrueHD Implementation

Dolby TrueHD is implemented on Blu-ray disc in such a way that if your home theater receiver does not support the format, or if you are using a digital optical/coaxial connection instead of HDMI for audio, a default Dolby Digital 5.1 soundtrack automatically plays.

Also, on Blu-ray discs that have Dolby Atmos soundtracks, if you do not have a Dolby Atmos-compatible home theater receiver, either a Dolby TrueHD or Dolby Digital soundtrack can be accessed. If this is not automatically, those options can be offered as selections provided on the menu of the affected Blu-ray Disc. In fact, it is interesting to note that Dolby Atmos metadata is actually placed within a Dolby TrueHD signal so that backwards compatibility is is more easily accommodated.

For all the technical details involving the creation and implementation of Dolby TrueHD, check out two white papers from Dolby Labs Dolby TrueHD Lossless Audio Peformance and Dolby TrueHD Audio Coding For Future Entertainment Formats.