Dolby TrueHD – What You Need to Know

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

Dolby TrueHD is available 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.

Dolby TrueHD Specifications

Dolby TrueHD can support up to 8 channels of audio at 96 Khz/24 bits (which is most commonly used), or up to 6 channels of audio at 19 2kHz/24 bits. kHz represents the sampling rate, and bits represent the audio bit depth. Dolby TrueHD also supports data transfer speeds of up to 18mbps.

Blu-ray and Ultra HD Blu-ray Discs that include Dolby TrueHD can represent the 6 and 8 channel options as a 5.1 or 7.1 channel soundtrack, at the movie studio's discretion.

Channel distribution is front left/right, front center, surround left/right, and the subwoofer if 5.1 channels are used. The 7.1 channel version provides additional surround back left/right channels.

The Lossless Factor

Dolby TrueHD (as well as its competitor DTS-HD Master Audio), are referred to as Lossless Audio formats.

What this means is that—unlike Dolby Digital, Dolby Digital EX, or Dolby Digital Plus, and other digital audio formats like 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.

Said another way, 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. The quality of your home theater audio system also plays a part.

Accessing Dolby TrueHD

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

1. One way is to transfer a Dolby TrueHD encoded bitstream, which is compressed, via HDMI (ver 1.3 or later) connected 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.
2. The second way to transfer a Dolby TrueHD signal is by using a Blu-ray or Ultra HD Blu-ray Disc player to decode the signal internally. The decoded signal is then passed directly to a home theater receiver as a PCM signal via HDMI, or a set of 5.1/7.1 channel analog audio connections. When using the HDMI or 5.1/7.1 analog option, the receiver doesn't need to do any additional decoding or processing—it just passes the signal to the amplifiers and speakers so you can listen to the soundtrack as intended.

Unlike the Dolby Digital and Digital EX surround sound formats, Dolby TrueHD cannot be transferred by Digital Optical or Digital Coaxial audio connections, which 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 Dolby TrueHD.

Dolby TrueHD is implemented in such a way that, if your home theater receiver doesn't support it, or if you are using a digital optical/coaxial connection instead of HDMI for audio, a default Dolby Digital 5.1 soundtrack automatically plays for you.

Dolby TrueHD and Dolby Atmos

On Blu-ray or Ultra HD Blu-ray discs that have Dolby Atmos soundtracks, if you don't have a Dolby Atmos-compatible home theater receiver, either a Dolby TrueHD or Dolby Digital soundtrack can be accessed. If this is not done automatically, it can also be selected via the playback menu of the affected Blu-ray Disc.

For all the technical details involving the creation and implementation of Dolby TrueHD, check out two white papers from Dolby Labs:

• Audio Coding for Future Entertainment Formats