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Mastering HD PC Audio, Part 1

Dolby Digital Plus, DTS-HD HR, Dolby True HD and DTS-HD MA

Dolby Digital Plus

Dolby Digital Plus (DD+) remains compressed, but handles higher bit rates and more efficient compression to deliver better sound quality. In addition to supporting 5.1 sound schemes, DD+ can also support 7.1, but you’ll only see this mentioned rarely in movie listings or details — most studios mix for 5.1 for the commercial recordings they sell to consumers. A separate Dolby Digital track will nearly always be included on Blu-ray Discs that also offer higher bitrate Dolby encodings, including both DD+ and Dolby TrueHD. Also, DD+ is optional for Blu-ray players, and thus is not a required support item: this means some players can handle DD+ while others can’t, though all can deliver the Dolby Digital core in any case.

dolby digital plus

Only a very few Blu-ray movies claim to include DD+ encodings, and PowerSquid includes no listings in this category. In fact, the only movies we could find that indicated support for DD+ were recorded for HD DVD, such as A View From Space With Heavenly Music, Unleashed, and so forth. But if a Blu-ray DD+ title were to appear, the entries would look something like this:

  • The Audio Codec is Dolby Digital Plus.
  • The Audio Channels (sound scheme) can appear as 5.1 (most typical) or 7.1 (unusual); the specification indicates it can support levels up to 13.1, but so far, 7.1 is as high as things go on commercial Blu-ray discs of any kind.
  • Audio Fidelity data: the DD+ specification indicates it can be either 16 or 24 bits deep, and can be recorded at 44.1 kHz, 48 kHz (typical for DVDs and Blu-ray discs), or 96 kHz.
  • Audio Bit Rate values can go as high as 6.144 Mbps for encoded data (this translates into 13.5 Mbps uncompressed), but might be either 1.5, 4.5, or 6.144 Mbps, depending on the bandwidth the studio decides to allocate to this type of audio track. Those HD DVDs that supported DD+ invariably used 1.5 Mbps bitrates.

See Table 1 for more information about sound schemes, SPDIF, and HDMI handling for DD+. According to Dolby Labs , DD+ achieves compression ratios of around 4:1 (3.75:1, according to some sources). DD+ is also a constant bitrate (CBR) encoding, which means it operates at the same bitrate at all times, regardless of the compressed audio material it is handling at any given moment. As with Dolby Digital and DTS, both DD+ and DTS-HD High Resolution (covered in the next section) use lossy but more efficient compression algorithms to encode data. This still results in some loss of fidelity and audio quality as compared to the studio master, primarily to achieve higher bandwidth reductions for audio information encoded in these formats.

DTS-HD High Resolution

The DTS alternative to DD+, DTS-HD High Resolution (often simply called DTS-HD HR) provides an enhancement to plain DTS similar to what DD+ offers over Dolby Digital, including higher bit rates and improved compression characteristics. Again, as with Dolby Digital and DD+, DTS-HD HR is encoded in the form of an extension to the core DTS data. Likewise, DTS-HD HR is optional for Blu-ray players as well, so many players extract only the 1.5 Mbps DTS core and ignore the extension data. As with DD+, an examination of high-def tracks available for commercial Blu-ray discs appears to indicate that the studios have forgone both of these formats in favor of the original core format, plus the uncompressed high-bandwidth versions — namely Dolby TrueHD and DTS-HD Master Audio. Apparently, the guiding notion seems to be to provide the highest possible fidelity with its inner compressed 5.1 core, so that equipment that can handle uncompressed audio can work with those bitstreams, and other equipment defaults to the compressed core 5.1 formats (Dolby Digital and DTS).

dts hd hr

Here again, we can find no Blu-ray discs that incorporate DTS-HD HR encodings. Though some foreign import HD DVD titles from Warner Brothers indicate support for this format, reviews of such discs indicate that Blu-ray versions that support Dolby TrueHD or DTS-HD Master Audio offer vastly superior sound when compared to their HD DVD counterparts — see, for example, this review of Ghost Rider (French Import). If there were to be a database entry for such an unlikely beast, following our earlier entries, here’s what it might look like:

  • The Audio Codec is DTS-HD High Resolution.
  • The Audio Channels (sound scheme) can appear as 5.1 (most typical) or 7.1 (unusual).
  • Audio Fidelity data: the DTS-HD specification indicates it is 24 bits deep, and can be recorded at 48 kHz (typical for HD DVD) or 96 kHz (would probably be used for Blu-ray, but there aren’t any).
  • Audio Bit Rate values can go as high as 6.144 Mbps for encoded data, but might be either 1.5, 4.5, or 6.144 Mbps, depending on the bandwidth that the studio decides to allocate to this type of audio track. Those HD DVDs that supported DTS-HD invariably used 1.5 Mbps bitrates.

See Table 1 for more information about sound schemes, SPDIF, and HDMI handling for DTS-HD High Resolution. Interestingly, despite a total lack of DTS-HD encodings on Blu-ray discs, numerous players (including models from Panasonic, Onkyo, and Samsung) support DTS-HD, though I can find no mention of Sony’s support for this format, except through a firmware upgrade for the PS3. According to various sources, DTS-HD HR achieves compression ratios of around 3:1. DTS-HD HR is another constant bitrate (CBR) encoding, which means it operates at the same bitrate at all times, regardless of the compressed audio material it is handling at any given moment. Most serious listeners who compare DTS and Dolby Digital to DTS-HD HR and DD+ are quick to observe that despite their apparent similarities (constant bitrate encoding, lossy compression algorithms), both of the “advanced” versions deliver better sound quality than their standard DD and DTS counterparts. DTS-HD builds a set of enhanced extensions around a DTS encoding core, so even if extensions can’t be played back, the core DTS sound track remains available and accessible.

Dolby TrueHD

Dolby TrueHD is one of the first two lossless audio formats and codecs to become available only on high-definition optical media. For Blu-ray players and media, although Dolby TrueHD is an optional codec, it is widely supported (far more than DD+, which appears to be largely absent on Blu-ray discs of any kind).

Dolby TrueHD uses the Meridian Lossless Packing (MLP) algorithm as the basis for its audio compression, which routinely achieves 2:1 compression rations. A Dolby TrueHD bitstream can accommodate up to 14 discrete sound channels, but in practice will carry either 6 (5.1) or 8 (7.1) channels. The standard supports bit depth of up to 24 bits, and data sampling rates of up to 192 kHz — for an uncompressed maximum bit rate of 63 Mbps — but for Blu-ray the current maximum is 8 audio channels at 24 bits and 96 kHz (or as an alternative, 6 channels at 24 bits and 192 kHz) for a maximum encoded bit rate of 18 Mbps. A search of available Blu-ray titles indicates that the current maximum is about half that amount: 6 channels at 96 kHz and 24 bit depth (which translates into a bit rate of 13.5 Mbps uncompressed, and 9 Mbps compressed on a Dave Matthews and Tim Reynolds concert disc that earns a perfect audio score).

dolby true hd

Here’s what a typical audio entry looks like for a movie that includes a Dolby TrueHD track:

  • Audio Codec is Dolby TrueHD.
  • The Audio Channels (sound scheme) is nearly always 5.1, with a very few 6.1 and 7.1 entries.
  • Audio Fidelity is often absent, but usually takes a value of 48 kHz at 16 bits or 48 kHz at 24 bits; some concert discs are rated at 96 kHz at 24 bits.
  • Audio Bit Rate values are typically absent, but usually take a value of 4608 kbps (4.5 Mbps, which corresponds to 6 channels at 48 kHz at 16 bits). The highest value we’ve seen on a commercial concert Blu-ray disc is about 14.0 Mbps, which corresponds to 6 channels at 96 kHz at 24 bits. The maximum supported value for Blu-ray is 18 Mbps. That said, be sure to read what we report about “variable bitrate” (VBR) encodings at the end of this section.

See Table 1 for more information about sound schemes, SPDIF, and HDMI. Dolby TrueHD uses a lossless compression algorithm to compact the data it encodes. In the simplest of terms, this means all transforms applied during compression are completely reversible, so that the uncompressed audio that results from decoding Dolby TrueHD is bit-for-bit identical to the studio master. Of course this requires more bandwidth to accommodate and explains why the bitrate numbers are so much higher for both Dolby TrueHD and DTS-HD Master Audio (explore in the next section) which more or less shares the same characteristics. That said, both Dolby TrueHD and DTS-HD Master Audio use variable bitrates, which means they can increase or decrease the amount of bandwidth they require to match the data in the corresponding audio information they store (in different terms, quiet or simple sound tracks require lower bitrates while noisy or complex sound tracks require higher bitrates). When we talk about bitrates for these encodings, we are addressing their maximum handling capacities, which will seldom be fully exercised during encoding or playback. Remember also that Blu-ray recordings with Dolby TrueHD sound tracks are required to include Dolby Digital versions of the same sound track separately as well: this is to more or less guarantee access to the lower fidelity, lower bandwidth, but more generally reproducible Dolby Digital data in case something hampers or prevents Dolby TrueHD decoding or playback.

DTS-HD Master Audio

DTS-HD Master Audio, sometimes abbreviated as DTS-HD MA, is the other of the two lossless audio formats and codecs to become available only on high-definition optical media. As with Dolby TrueHD, DTS-HD Master Audio is an optional codec for Blu-ray players, but also like Dolby TrueHD and DD+, DTS-HD Master Audio is much more prevalent and widely supported than is DTS-HD High Resolution. In fact, a comparison of the number of Dolby TrueHD entries on the Blu-ray.com and at avsforum.com sites as compared to those for DTS-HD Master Audio, shows that the number of entries for the latter (135/45) sometimes outnumbers the former (109/107). Perhaps this is because DTS-HD supports variable bits rates on a Blu-ray disc of up to 24.5 Mbps, with a maximum of 192 kHz sampling and 24 bit depth in two-channel stereo mode, and up to 8 channels at 24 bits at a sampling frequency of 96 kHz.

dts hd ma

According to the DTS-HD specification itself, the format can accommodate an arbitrary number of channels. Yet, if you look at the information for Blu-ray discs with DTS-HD MA tracks currently available, you’ll see that the vast majority features 6 (5.1) channels at 48 kHz with a bit depth of 24. You will find a fair number of 7.1 recordings, and some 6.1 (and even 5.0) Blu-ray discs, where the maximum sampling rate appears to be 96 kHz for multichannel media. The maximum audio bit rate we found for an available Blu-ray disc was 13.5 Mbps (for example I Am Legend, which earned perfect audio scores from Blu-ray.com, DVDTown.com, and Hi-Def Digest).

Here’s what a typical audio entry looks like for a movie that incorporates a DTS-HT Master Audio soundtrack:

  • Audio Codec is DTS-HD Master.
  • The Audio Channels (sound scheme) is nearly always 5.1, with a few 2.0, 4.0, 5.0, and 6.1 entries, and a higher proportion of 7.1 entries (about 10%) than for Dolby TrueHD (about 7%).
  • Audio Bit Rate values are absent, but with typical values of 48 kHz at 24 bits for 6 channels calculate to 6.75 Mbps. The value doubles for 96 kHz, and increases proportionally for 8 channel soundtracks.

See Table 1 for more information about sound schemes, SPDIF, and HDMI. Remember again that the DTS core remains available from a DTS-HD MA bitstream, so that DTS playback is possible even when issues may present themselves with decoding the high-definition audio extensions around that core.

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