Since Dolby TrueHD is an optional codec, a companion Dolby Digital bitstream (2.0 192Kbps or 5.1 448Kbps/640Kbps) must accompany the Dolby TrueHD bitstream on Blu-ray discs. The maximum encoded bitrate is 18 Mbit/s. Dolby TrueHD audiotracks may carry up to 8 discrete audio channels (7.1 surround) and 20 objects of 24-bit audio at 96 kHz or up to 6 channels (5.1 surround) at 192 kHz. In the Blu-ray Disc specification, Dolby TrueHD is an optional codec.
Part 2: 3 Best Tools to Rip Blu-ray to MKV with Dolby TrueHD Audio Track Kept.Part 1: Brief Introduction to Dolby TrueHD and Dolby TrueHD Audio in Blu-ray Disc.