Skip to main content

Mastering HD PC Audio, Part 1

Understanding High-Definition Audio

Audio Formats

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
  • DTS
  • 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.

  • 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