Blu-ray Audio Codecs Revisited
There are seven basic types of digital audio tracks that can appear on BDs:
- Linear PCM
- Dolby Digital 5.1
- Dolby Digital Plus
- Dolby TrueHD
- DTS 5.1
- DTS-HD High Resolution Audio
- DTS-HD Master Audio
- Three front channels: front left, front center, and front right
- Four surrounds: left side, left back, right back, and right side
- A low-frequency effects channel, the ".1," carrying just deep bass for a subwoofer
Linear PCM is recorded with no digital compression whatever. It offers uncompromised sound quality, at the expense of taking up a lot of space on the disc.
Linear PCM is specified as having so many bits per sample (the resolution), with so many samples per second (the sampling frequency). For example. the Blu-ray release of the movie Unbreakable has an English soundtrack with PCM 5.1 (48kHz/24-bit) surround sound. There are 48,000 samples per second of each of the six audio channels. Each sample contains 24 bits.
48kHz/16-bit sampling is also common. 96kHz at 16, 20, or 24 bits per sample is another sampling frequency that is permitted. It is also possible to sample each channel at a rate of 192kHz, using 16, 20, or 24 bits per sample, as long as the number of channels doesn't exceed six — i.e., for 5.1-channel audio.
All Blu-ray players are required to be able to pass linear PCM tracks through as is for use by an external receiver or a TV set. It can do this via an HDMI connection or via a digital optical output. All Blu-ray players must also be able to decode and downmix linear PCM tracks into two-channel stereo for output on analog audio connections.
Linear PCM is one of the three mandatory types of audio track which every BD must contain at least one of. The other two "mandatory" codecs are Dolby Digital 5.1 and DTS 5.1 — see below.
Dolby Digital 5.1 and DTS 5.1 are competing codecs for the lossy compression and decompression of up to 5.1 channels of audio. There are no left side and right side surround channels with either of these codecs. The compression is lossy, in that the decompressed audio stream is not absolutely identical to the original stream ... though the differences ideally can't be detected by the human ear.
DD 5.1 and DTS 5.1 have long been used on regular DVDs, with the former being present on virtually all DVD titles in the U.S. and the latter being used in addition to Dolby Digital (and/or linear PCM) on some DVD titles. For both, the maximum allowed bitrates (how many bits per second can appear in the encoded bitstream and need to be processed by the decoder) is low by today's standards. DD 5.1 has a maximum bitrate of just 640 kbps on BD. DTS 5.1 tops out at 1.524 Mbps.
If a Blu-ray disc lacks a linear PCM track, it must provide a DD 5.1 track and/or a DTS 5.1 track, since Blu-ray players are required to support only these three types of audio.
Dolby Digital Plus and DTS-HD High Resolution Audio are two competing formats for extending the audio fidelity of their respective predecessors, Dolby Digital 5.1 and DTS 5.1. Both allow up to 7.1 channels of audio on Blu-ray discs, including two side surround channels in addition to the customary rear surround channels, and both permit far higher bitrates going into the decoder, making for sound with much higher resolution. DD+ can use bitrates up to 4.736 Mbps on BD; DTS-HD HR, up to 6.0 Mbps.
However, both DD+ and DTS-HD HR remain lossy, so the output audio stream is not perfectly identical to the original input stream that is fed into the encoder when the disc is authored. Neither DD+ nor DTS-HD HR is required to be included as audio tracks on Blu-ray discs, nor are Blu-ray players required to be able to use these tracks if they do appear on a disc. DD+ and DTS-HD HR are "optional" codecs on Blu-ray.
Dolby TrueHD and DTS-HD Master Audio are competing codecs that extend Dolby Digital 5.1 and DTS 5.1, respectively, into the realm of lossless 7.1-channel audio compression on Blu-ray discs. Unlike DD+ and DTS-HD HR, these two, when decompressed, yield audio streams identical with the original streams fed into their compressor, bit for bit.
On BD, bitrates for TrueHD encoded bitstreams can be up to 18.64 Mbps. Master Audio encoded bitstreams can use up to a whopping 24.5 Mbps.
Blu-ray players are not required to support either Dolby TrueHD or DTS-HD Master Audio tracks, should they happen to appear on a BD. These two lossless codecs are strictly optional for players to be able to decode and for discs to include.
So there are four new codecs that can be used on Blu-ray discs as audio tracks, in addition to old standbys Dolby Digital and DTS (and in addition to linear PCM, which is not strictly speaking a "codec"). The four include Dolby Digital Plus and DTS-HD High Resolution Audio, both with lossy compression. They also include Dolby TrueHD and DTS-HD Master Audio, with lossless compression. For none of these four new codecs is a given Blu-ray player model required to be able to use them at all (!). The only compression-decompression schemes a player must support are Dolby Digital 5.1 and DTS 5.1.
In addition, always keep in mind, Blu-ray discs often include an uncompressed linear PCM track, which the player is required to support.
There are several ways in which a player can support any or all of the four optional advanced compression schemes (in addition to Dolby Digital, which the player has to support fully, and DTS, which the player also has to support fully). The four advanced codecs are:
- Dolby Digital Plus
- Dolby TrueHD
- DTS-HD High Resolution
- DTS-HD Master Audio
- decode it fully
- decode just the "core" of it
- pass through as a bitstream the entire audio track
- pass through as a bitstream just the "core" portion of the audio track
This, accordingly, is the "core" bitstream for Dolby TrueHD. The remainder of the data in the input Dolby TrueHD bitstream is referred to as the "extension."
A Dolby Digital Plus track likewise has a "core" bitstream that is, in effect, equivalent to DD 5.1, along with a copious amount of "extension" data giving additional channels and/or higher resolution.
In like manner, DTS-HD High Resolution and Master Audio tracks have "core" bitstreams, equivalent to DTS 5.1 (not to Dolby Digital 5.1, which comes from DTS' rival company, Dolby Labs). These are, again, in both cases augmented by "extension" data.
(What about a DTS 5.1 track? It's already in effect just a "core" bitstream, with no "extension" data present. The player will decode all of it. If a player can pass it through as a bitstream at all, it passes through all of it.)
When a Blu-ray player decodes into linear PCM a digitally compressed audio track that includes both "core" and "extension" data, it may have the ability to decode both sources of data, the "core" data and the "extension" data, giving you all the audio information in the track.
Or, it may simply decode the "core" information and ignore the "extension," giving you the equivalent of just Dolby Digital 5.1 or DTS 5.1, depending on the "family" (Dolby or DTS) of the track on the disc.
Similarly, when a Blu-ray player is set up by the user to pass digitally compressed audio tracks through to an external receiver as an output bitstream — under the assumption that the receiver which receives it as input will be able to decode it — it may have the ability to pass through the entire audio track, including both the "core" and the "extension" information.
Or it may be limited to passing through only the "core" bitstream and ignoring the "extension" information. In the latter case, the receiver will act as if it is receiving just a Dolby Digital 5.1 bitstream or a DTS 5.1 bitstream, depending on the family of the track on the disc.
Unfortunately, it is not easy to find out how a particular Blu-ray player actually handles all these various types of audio streams.
If you are in the market for a player, you can easily search in vain for information about how the various models you are considering actually handle audio. Professional reviews, manufacturers' websites, and enthusiast forums often fail to specify, or specify incorrectly, how players react to the various kinds of audio.
If you actually buy a player, you might think its manual or user guide would clear up the matter once and for all, but, no: the literature is apt to add to rather than eliminate the confusion.
For example, the online manual for the Sony PlayStation 3, which is not just a game machine but also plays Blu-ray discs, has this to say:
BD / DVD Audio Output Format (HDMI)
Set the audio output format to use when playing a BD or DVD containing audio recorded in Dolby Digital or DTS format. This setting is used when an audio output device is connected to the system via an HDMI cable.
Bitstream Set to output audio with the original digital signal prioritized. Linear PCM Set to output audio by converting the digital signal to Linear PCM format.
If [Bitstream] is selected, some portions of the audio content may not be output.
That, sadly, is basically the only guidance you get. In truth, thanks to firmware updates that have happened since the manual was written, the PS3 can internally decode Dolby TrueHD fully, including the "extension" bitstreams. And it can, as of the latest firmware release (version 2.30), decode DTS-HD Master Audio as well.
The PS3 also passes through all four of the advanced codecs as bitstreams, but only in their "core" form, minus their "extensions." Additionally, it allows bitstreams of linear PCM tracks, Dolby Digital 5.1 tracks, and DTS 5.1 tracks — Blu-ray's three "mandatory" audio formats — to be passed through as is.
But you'd never know those things from Sony's PS3 online manual.
And it is often equally hard to find out what other Blu-ray players manufactured by Sony or ones made by the various other makers can do with the various kinds of audio tracks on BDs. Sigh.