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Mastering HD PC Audio, Part 1

Dolby Digital and DTS

Dolby Digital

Dolby Digital is the venerable and familiar audio format, also known as AC-3, used on conventional DVDs; it’s also a base level audio standard for Blu-ray discs. It works just like it did on DVDs in sound schemes that range from 1.0 to 5.1, but offers a higher maximum bit rate of 640 kbps on Blu-ray, and sounds just like Dolby Digital Plus encoded at that same bit rate. (As you’ll see in the section on Dolby Digital Plus, however, its maximum bit rate is considerably higher, though seldom used.)

dolby digital logo

All Dolby logos courtesy of Dolby Laboratories.

All Blu-ray players must support Dolby Digital. When players can’t deliver higher bit rate Dolby encodings, they always default to Dolby Digital playback. Blu-ray requires that any time a high bitrate version of Dolby is encoded on a disc, it will also include a plain-vanilla Dolby Digital track as well. Thus if decoding of another higher bitrate form of Dolby audio doesn’t succeed, you will still be able to play back a Dolby Digital 5.1 audio track, with its maximum 640 Kbps bit rate.

Typical Dolby Digital audio tracks include the following information:

  • The Audio Codec is Dolby Digital.
  • The Audio Channels (sound scheme) can appear as 1.0 (rare, only for older movies), 2.0 (likewise, though not as old), or 5.1 (most typical).
  • Audio Fidelity data does not appear for this encoding, but the Dolby Digital specification indicates that it is always 16 bits, and can be 32 kHz, 44.1 kHz (the typical audio CD frequency), or 48 kHz (typical for DVDs and Blu-ray discs).
  • Audio Bit Rate values are usually 640 kbps (most 5.1 discs), but may be either 192 kbps (1.0) to 448 kbps (2.0 and some 5.1). 448 kbps is the maximum bit rate for conventional DVDs.

See Table 1 for information about sound schemes, SPDIF, and HDMI handling for Dolby Digital. Both Dolby Digital and DTS (covered in the next section) use what’s called “lossy compression” to create audio tracks. In the simplest of terms this means that audio information gets lost, and some sound fidely compromised (as compared to the original studio master recording) during the process of performing the data manipulations and reductions necessary to squeeze these types of audio tracks into their recorded forms. According to some sources, Dolby Digital can achieve compression ratios as high as 12:1, though ratios in a range 4:1 to 6:1 are more common. Dolby Digital 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.

DTS

DTS is another venerable and familiar sound format that originated for use on conventional DVDs. Like Dolby Digital, it is a compressed format that supports sound schemes that range from 1.0 to 5.1, where 5.1 is by far the most commonly used on Blu-ray discs. DTS supports a maximum bit rate of 1.5 Mbps, and Blu-ray players are generally more than equal to the task of handling this kind of data rate. All Blu-ray players must support transmission of a DTS bitstream across a digital link, and be able to decode at least two channels internally; that said, the vast majority of Blu-ray players can decode 5.1.

dts logo

All DTS logos courtesy of DTS, Inc.

The original 1997 DVD specification did not mention DTS, so older DVD players do not recognize DTS audio tracks. All modern DVD players can decode DTS internally, or pass it on to an external decoder through a digital link of some kind, but it is often reduced to a bit rate of 768 kbps. Higher bit rate DTS encodings are built around a DTS core with a maximum bit rate of 1.5 Mbps, so that if extensions fail to decode, base level DTS audio still remains available for processing or forwarding. If any DTS encoding is present on a Blu-ray disc, you can count on obtaining base-level DTS audio at a minimum level from a Blu-ray player.

A typical DTS audio track data will include the following information:

  • The Audio Codec is DTS.
  • The Audio Channels (sound scheme) can appear as 2.0 (rare, only for older movies), 3.0 (likewise, though not as old), or 5.1 (most typical).
  • Audio Fidelity data does not appear for this encoding, but the DTS specification indicates it can be either 16 or 24 bits deep, and can be recorded at 48 kHz (typical for DVDs and Blu-ray discs) or 96 kHz.
  • Audio Bit Rate values are usually 1.5 Mbps (for most 5.1 Blu-ray discs), but may be at 768 Kbps, a typical value for most conventional DVDs that include DTS encoded audio material.

See Table 1 for information about sound schemes, SPDIF, and HDMI handling for DTS. According to some sources, DTS can achieve compression ratios as high as 10:1, though ratios around 4:1 are more common. DTS is another constant bitrate (CBR) encoding, which means it operates at the same bitrate at all times, regardless of the structure of the audio material it is handling at any given moment. This arrangement tends to favor data reduction at the cost of audio quality, just as the use of a lossy compression algorithm does likewise.

  • dogman-x
    How many people are dissatisfied with DVD sound quality?

    Sure, DVD standard definition picture quality leaves something to be desired on a 1080p monitor, but AC-3 through an S/PDIF cable into a nice home theater receiver and decent speakers seems perfectly good already.

    I doubt a higher bit rate coder would make a perceptible difference to most ordinary listeners. The limit of human hearing is just over 20KHz (analog). For digital sampling, you multiply that by around 2.2. For example, phone transmissions are engineered for 3KHz analog and transported at 8KHz digital. So I think these 96KHz and 192KHz sampling rates are a waste. The standard 48KHz is more than adequate for the limits of human hearing.

    And then there's the 24-bit sampling depth. There is no audio recording equipment on earth that can record that much dynamic range. And even if it could, nobody would want it. When the source audio track goes to near silent, many listeners will be annoyed. Most well engineered movie sound tracks always have something going (music, birds chirping, etc.) to keep the listener interested. So the dynamic range of most movies could probably be captured with around 12 bits. 16 bits is already more than adequate.

    Then there's all the hoopla over compression algorithms. Many audio purists will dismiss any lossy compression algorithms without even giving it a proper listening test. The concept of audio compression goes like this: If you hear an alarm clock ticking, then the alarm bell rings, you don't need to encode the ticking when the alarm rings since no one will hear it. Double-blind listening tests have shown that many lossy encoded sound tracks can't be picked out even by audio mastering engineers (golden-ears people who do this for a living). Interestingly, it's often people with significant hearing loss that tend to pick out the lossy compression algorithms. If you can't hear the high frequencies of the alarm bell, then you'll notice the ticking has gone away.

    I suspect all these new Dolby / DTS standards are mostly aimed at getting people to buy new home theater sound equipment. Remember that Dolby Labs & DTS both make a small royalty for each new receiver unit sold. Given that existing audio equipment can always use the core Dolby Digital track (without the higher bit rate extensions) over S/PDIF, I would welcome a Tom's listening review of these new sound schemes to see if there is any real difference.
    Reply
  • etittel
    Dear Dogman:
    If you check out the quality sites that rate DVD audio, you'll see comments from people with very "good ears" that give Dolby TrueHD and DTS-HD Master Audio considerably higher ratings than they give to core Dolby Digital or DTS sound tracks. My own experience on my "less-than-state-of-the-art" but good quality audio gear (5.1 is as high as I go right now, because that's as many speaker as I've cared to purchase, position, and EQ for my home theater rig) confirms these confirms these other claims. I can hear more detail and richer sound when decoding DolbyTrue HD than I can when decoding normal Dolby Digital.
    The sites that do a good job of rating Blu-ray DVD audio include Cinema Squid, www.highdefdigest.com (I cited a couple of their reviews in this story), www.blu-ray.com, www.soundandvisionmag.com, and www.hometheaterforum.com. A little digging can turn up loads of opinions around this subject matter, most of which leans in the direction that decoding the lossless codecs indeed requires new (and more expensive) gear, but is worth to those to whom such things matter.
    HTH,
    --Ed--
    Reply
  • wiyosaya
    Hi Ed,

    I really hope that you continue to do home theater articles for Tom's. This is, by far, the best article that I have ever seen Tom's publish on home theater. It sounds like a.) you really know your stuff, or b.) you took the time to thoroughly research the topic or c.) both. The in-depth info on the various HD audio formats is fantastic.

    I hope that the part 2 article is just as in-depth, and I am hoping that you will at least partly cover the Asus Xonar AV1.

    My personal preference is to have a stand-alone Blu-ray recorder, but in the US, those do not exist so far. I have heard rumors that Toshiba will release one later this year, however, if they do, it remains to be seen if it will record from an HDMI input. Perhaps the combination of the Xonar AV1 in a HTPC would provide an acceptable alternative.

    Thanks again for the superb article.

    Best Regards,
    Matthew
    Reply
  • etittel
    Gee thanks Matthew. As far as your a-b-c list goes, it's mostly (b) with a growing amount of (a), hopefully heading toward (c)! I've had the good fortune to work with more accomplished home theater gurus including Mike Chin of SilentPCReview.com, and also Matt Wright (formerly of HTPCnews.com, now with missingremote.com), and to establish good relationships with home theater/audio experts at AMD, NVidia, and Intel. With a support network like that, I'm glad to dig into these topics for the readership, as much out of a desire to get out of home theater PCs what I think they should deliver, as out of plain orneriness (when it doesn't work that way) and curiousity (when it does).

    And you're right: the only Blu-ray recorders I'm aware of that are currently available include $3,000+ models from Panasonic and Sony available only in Japan at present. It would be quite a coup for the HD DVD champion to be first to market in the US (or anywhere) with an affordable Blu-ray burner.

    And yes, I do plan to touch on the Asus Xonar as well as some other upcoming products (most notably from Auzentech, a Korean-based outfit that plans to release direct HDMI 1.3a audio support later this year, and upcoming stuff from Nvidia that promises to do likewise). If anything, I hope you'll find Part 2 even *more* interesting than Part 1...

    --Ed--
    Reply
  • jawshoeaw
    People claiming music of one standard sounding superior to another may not be doing a double blind test. I've read serious posts about how XM radio sounds as good as a CD - anecdotal evidence is an oxymoron. If studio engineers could not tell the difference in a blind test - well, I trust science not opinion. Unless it's my opinion of course
    Reply
  • vskatusa
    This is an excellent article on the subject!

    I have researched the subject and stopped my build of the media pc when the truth became clear....

    The audio part is not ready for prime time yet!
    Reply
  • I am of the middle-aged plus males group where hearing loss becomes rather common, due to nerve damage. Somehow I can still discriminate between media and hardware that are very good versus not good, although there is much of the dynamic range that is no longer heard. Simply, there is much of what is new each year that is just wasted $$$ for those of us with hearing losses. I like new equipment like the next person, but I'm not sure what is worth my money and what isn't.

    One avenue that is open to us for saving some money, currently, is by building our own speakers. There are many places via Internet and elsewhere for tutorials and less than retail priced components.
    Reply
  • Awesome article - can't wait for the follow-ups. Quick response to one of the comments above -

    wiyosayaI have heard rumors that Toshiba will release one later this year, however, if they do, it remains to be seen if it will record from an HDMI input.
    Unfortunately there's no way any device that is HDCP compliant will be recording via HDMI as the HDCP license explicity defines what an HDCP "Presentation" device (any device capable of receiving, decrypting and displaying HDCP protected content) is allowed to do.

    The HDCP license says:

    ---------
    A Presentation Device shall not make any copies of Decrypted HDCP Content for any purpose, except for such temporary buffers
    ---------
    And
    ---------
    Decrypted HDCP Content may be temporarily buffered in a Presentation Device to enable and perform the Presentation Function, image processing function (e.g., picture-in-picture display, image overlay, image enhancement and brightness adjustment) or ?freeze frame? of a single frame of Audiovisual Content, provided that such buffer shall not persist for more time than is necessary to perform such function.
    ---------
    Reply
  • etittel
    Dear Jawshoeaw:

    You're absolutely correct about the double-blind comparison. That said, if you check the various Web sites I cited in my previous post, they do provide separate ratings and rankings for Dolby Digital versus Dolby TrueHD, and DTS versus DTS-HD Master Audio *for the same movies*. Two separate auditions, two separate rankings from people who know their stuff is good enough for me, though that doesn't mean it needs to be good enough for you. That said, I have personally compared the two sets of tracks on my own equipment and (though I didn't do a double blind comparison because I have only one Blu-ray player to work with) I believe I can hear an audible difference betweeen Dolby Digital and Dolby TrueHD, as do other members of my family, and friends who've come over to watch movies. Again that's only FWIW, which may be more to me than to you.

    --Ed--
    Reply
  • etittel
    Dear AVenVY:

    Great interpretation of the standards. I didn't mean to suggest that this device would allow copying of decrypted HDCP content, though it should finally provide a superior way to burn HD camcorder content onto DVD. The content providers (aka movie studios) would go ballistic if such devices permitted knockoffs of *gasp!* copyrighted material.
    --Ed--
    Reply