Dolby TrueHD,
Developed by Dolby Laboratories, is an advanced lossless audio codec based on Meridian Lossless Packing. Support for the codec was mandatory for HD DVD and is optional for Blu-ray Disc hardware. TrueHD supports 24-bit, 96 kHz audio channels at up to 18 Mbit/s over 14 channels (HD DVD and Blu-ray Disc standards currently limit the maximum number of audio channels to eight). It also supports extensive metadata, including dialog normalization and Dynamic Range Control.
Channel configurationsAlthough most commonly associated with the 5.1 channel configuration, Dolby Digital allows a number of different channel selections. The full list of available options are:
Mono (Center only)
2-channel stereo (Left + Right), optionally carrying matrixed Dolby Surround
3-channel stereo (Left, Center, Right)
2-channel stereo with mono surround (Left, Right, Surround)
3-channel stereo with mono surround (Left, Center, Right, Surround)
4-channel quadraphonic (Left, Right, Left Surround, Right Surround)
5-channel surround (Left, Center, Right, Left Surround, Right Surround)
All of these configurations can optionally include the extra Low Frequency Effect (LFE) channel. The last two with stereo surrounds can optionally use Dolby Digital EX matrix encoding to add an extra Rear Surround channel.
Many Dolby Digital decoders are equipped with downmixing functionality to distribute encoded channels to available speakers. This includes such functions as playing surround information through the front speakers if surround speakers are unavailable, and distributing the center channel to left and right if no center speaker is available. When outputting to separate equipment over a 2-channel connection, a Dolby Digital decoder can optionally encode the output using Dolby Surround to preserve surround information.
The '.1' in 5.1, 7.1 etc. refers to the LFE channel, which is also a discrete channel.
