Understanding High-Definition Audio
Blu-ray discs can include movie soundtracks in any of the following formats (we list them first, then follow with more detailed descriptions for each item listed):
- PCM (aka Linear PCM or LPCM)
- Dolby Digital
- Dolby Digital Plus
- DTS-HD High Resolution
- Dolby TrueHD
- DTS-HD Master Audio
Before we tackle these formats, in the order of appearance in the preceding list, note that Dolby technologies originate at Dolby Laboratories, a well-known purveyor of professional, prosumer, and consumer audio noise reduction and multi-channel surround sound technologies. Likewise, DTS (also called Digital Theater Systems) comes from DTS, Inc., an equally well-known purveyor of digital sound capture and representation technologies that compete with those from Dolby Labs.
PCM (aka Linear PCM or LPCM)
PCM stands for pulse code modulation, and provides a digital representation of an analog signal where that signal is sampled regularly at uniform intervals, and represented in terms of binary digital values. In addition to its use for digital audio in computers and compact audio discs, PCM is also used in some digital telephone systems, and in some kinds of digital video formats. In PCM, sampled values are represented using varying bit depths; sound track audio is typically sampled using bit depths in a range from 12 to 24 bits, with 16 bit depth pretty typical of what studios use when encoding audio for writing to Blu-ray discs.
A PCM track can be an exact replica of the studio master, encoded to disc without compression, if the bit depth is the same as the master. If bit depth is reduced — as is often the case to save on the space devoted to storing audio information on disc — this may involve downsampling the data from 24 bit resolution to 16 bits instead. Technically speaking, downsampling is not the same as compression, though it does reduce the fidelity of the resulting audio information.
All Blu-ray players must support PCM Audio to comply with the Blu-ray specification, but not all Blu-ray discs include this format. Numerous movie databases provide detailed audio information about Blu-ray discs, most notably notably HighDefDigest and AVS Forum which offer moving ratings and reviews that include audio details. A typical listing indicates the Audio Codec used (LPCM 5.1 is only PCM format represented), the number of channels this encoding delivers on the disc (for LPCM, you’ll see values of 2.0 for stereo, and 5.1 with an occasional 6.1 or 7.1 sprinkled here and there). Occasionally you will also see Audio Fidelity (usually 48 kHz/24 bit or 48 kHz/16 bit) as well as audio bit rate (the highest value we find there is 13824 kbps for an unusual 96 kHz/24 bit entry, with 6912 and 4608 kbps most typical for 48 kHz/24 bit and 48 kHz/16 bit entries, respectively).
See Table 1 for information about sound schemes, SPDIF, and HDMI handling for PCM. The "good news" about PCM is that if your PC can deliver some form of this data via HDMI to your receiver, and that receiver handles PCM data streams, it can probably convert that stream into high-definition 5.1 or 7.1 audio, depending on how the bitstream is encoded and how many channels it contains. And any type of HDMI connection will do where PCM or LPCM is concerned, from HDMI 1.0 all the way up to HDMI 1.3a.