Signal to Noise - Dolby TrueHD & DTS-HD MA vs. Uncompressed PCM
How well do the new compression schemes from Dolby and DTS stack up against uncompressed audio? We buff up our golden ears to audition and compare the latest Blu-ray audio codecs, in the design labs that developed them.
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July 14, 2008 by EH Staff

Compared to DVD, the tremendous increase in storage capacity of the Blu-ray disc format, necessary to carry the increased high definition video data payload, also provides for expanded audio options, including the ability to carry so-called lossless audio formats such as Dolby TrueHD and DTS-HD Master Audio.

While many of the initial Blu-ray movie titles feature conventional Dolby and DTS digital soundtracks, a number of them also feature high resolution uncompressed pulse code modulation (PCM) soundtracks to appeal to owners of high end surround sound systems.

The upgrade of the High Definition Multimedia Interface to Version 1.3 includes cable upgrades along with transmitter and receiver modifications to allow substantially higher overall bitrate flows, and allows for lossless audio formats to be sent from source components to be decoded by newer A/V receivers and processors.

As with their movie theater equivalents, both Dolby and DTS home audio formats use what is called lossy compression, in order to fit into the relatively narrow amount of data space allotted on DVDs (and in the case of Dolby, with HDTV broadcasts as well). The need to compress digital audio stems from the way conventional PCM audio works – the bitrate remains the same at all volume levels and frequencies, even when there is little or no signal actually being coded.

With Blu-ray’s five-fold increase in data storage capacity (compared to DVD), both Dolby and DTS have developed new audio encoder/decoders (codecs) that are 100% bit-for-bit identical to the original PCM master, but with substantial bitrate reduction efficiency as well, freeing up more space on the disc for added content, extended/alternate versions and the like.

To get the latest scoop on these new codecs, Editor-in-Chief Geoffrey Morrison and I made arrangements to visit both companies’ respective headquarters, where we would be able to hear definitive A/B comparisons that would be otherwise impossible to properly set up in our own facilities.

Our first stop was at Dolby Laboratories’ headquarters in San Francisco. After a short tour of their impressive facilities, our hosts ushered us into what one of their engineers called their “codec killer room.” The specially designed room adheres to the ITU-R BS.1161-1 critical listening evaluation specification and companion BS.1284-1 Annex document that together specify in great detail the precise conditions, procedures and protocols necessary to achieve repeatable and truly useful results in the on-going development of these codecs. A suitably high resolution 5.1 system resides in the room, with five Revel Ultima Studio full range loudspeakers, along with a Paradigm subwoofer and a stack of Bryston power amplifiers rounding out the gear.

The control panel allowed for selection between a number of sources, including the original PCM multi-channel audio track, as well as TrueHD, Dolby Digital Plus, high bitrate 640 kilobits per second (kbps)Dolby Digital, and lower 448 kbps DVD-format Dolby Digital choices that have all been through the full encode/decode process.

The process of codec evaluation includes pre-screening potential listeners for their aural acuity as well as their consistency after multiple trials. Panelists are asked to listen to a reference clip, and then compare it against another clip that may be the same identical reference source, or a different clip that has been codec’d. They are then asked to score their perception of the audio quality on a five point scale. The lowest 1.0 grade is rated very annoying. The 2.0 grade is annoying, while the middle 3.0 grade is rated at slightly annoying. The 4.0 grade is rated perceptible, but not annoying, while the highest 5.0 grade is rated as imperceptible – the goal of the codec designers is to make the codec itself disappear, from an audio standpoint.

The computer chooses which clips are presented to the listener on a randomized basis to ensure true subjectivity, and the post-session scoring data is then entered into a database and statistically validated against the actual presentation order of the test clips. From that, the engineers can glean a useful score as to the performance of the codec compared to the reference uncompressed source clip, and the process ensures that individual biases are eliminated along the way. It is both time-consuming, and given the repetitive nature of listening to dozens or hundreds of clips in a given listening session, mind-numbingly boring (at least to me, anyway). This is why even keen-eared reviewers simply can’t perform an honest evaluation of codec sound quality in their own home theaters – it can only be done under these most rigidly controlled conditions, with specialized equipment and software that is designed expressly for the task.

Due to the masking of sounds that inevitably occurs during complex and bombastic passages, the best evaluation results are obtained using relatively simple program clips, limited in duration to around 10 seconds or so and on constant replay. For our limited test, our hosts chose a brief audio clip from the movie American Beauty, the so-called “Spectacular” dream sequence where Kevin Spacey’s character ruminates on his life while looking upward at the inviting Mina Suvari, barely dressed in rose petals and surrounded by additional petals that fall towards him. The track features simple, center-channel anchored dialog, along with gentle percussive bell-like notes (xylophone, perhaps?) along with even more gentle triangle bell embellishments—just the ticket for an A/B codec comparison.

Neither Geoff nor I could hear any differences between the original PCM track and the TrueHD version, which should be the case, as they’re bit-for-bit identical. The lossless coding process is analogous to “zipping” computer files—it’s simply a function of more efficient packing that loses nothing along the way. With movies, TrueHD typically provides a two- or three-to-one bitrate reduction compared to the original PCM source.

Next, we compared the original to the Dolby Digital Plus version (that codec is found on numerous BD titles, and like TrueHD, is fully backward compatible with regular Dolby Digital decoders). Even on this extremely high-end system, we couldn’t hear any difference between the uncompressed and the compressed. Then, we compared the higher bitrate (640 kbps) that is found on the Dolby Digital tracks on Blu-rays to the original. “Golden Ears” Morrison was able to hear the difference, but I, and most others in the room with us, did not. Each of us had our turn in the prime listening chair, and couldn’t know the origin of the clips or their order of presentation.

The shocker came when we compared the lower 448 kbps Dolby Digital DVD bitrate to the original. There was an audible difference, but it was only ever-so-slightly noticeable (and this is with a high end audio system in an acoustically controlled environment that is so far beyond what typical home theater systems are capable of resolving). There was just the slightest decrease in presence with the DD version, not exactly a softening of the sound, but just a tad less ambience and a similarly small tightening of the front soundstage’s depth. Quite a remarkable result, I thought, and I was highly impressed with how much fidelity can be packed into such a relatively small amount of bitspace. If I was doing actual scoring, I would have awarded a 4.8 grade to the results I heard – the audible difference was that subtle.

DTS

The following week found us at DTS’ facilities in Agoura Hills, just northwest of Los Angeles. There we had a tour of their deluxe screening room, and soon found ourselves in one of their demonstration sound studios, where a 7.1 system featuring seven KRK Expose E8T monitor speakers was teamed with two Bag End PS18E subwoofers. In lieu of a stack of power amplifiers, this system was instead easily powered by a Denon AVR-2808CI audio/video receiver, and we were treated to a number of high resolution audio and HD video clips from the latest DTS Blu-ray demonstration disc.

After an informative presentation which explained the benefits of their latest codec technologies, we dove right into the A/B comparisons between the original PCM versions and the various DTS codec’d versions. The short clip chosen for us came from a DTS Blue Man Group recording, again using a spare, sparse selection for an easier and more revealing A/B comparison. Again, we found no differencebetween the uncompressed original track and the DTS-HD Master Audio version.

In addition to the DTS HD-Master Audio lossless codec, DTS also offers up a nearly lossless high bitrate format called DTS-HD High Resolution Audio, with up to four times the bitrate of their core DTS format, which we were able to audition via their Blu-ray demonstration disc. We then conducted A/B comparisons between the high resolution Blue Man Group PCM original soundtrack and the core DTS codec which has a Blu-ray and DVD bitrate of either 768 kbps or 1.5 Mb/s, in a somewhat similar but not totally blind fashion that we went through the week before.

It was déjà vu all over again. We switched back and forth between the original PCM master and the core DTS version, and here we found only the slightest, barely noticeable difference. From a frequency response standpoint, both versions were identical, with clearly delineated high frequency details, but the compressed version differed slightly only in barely noticeable presence —that sense of being “there”, with the original PCM track having just slightly greater overall richness. Whatever acoustic elements were removed in the code/decode process were clearly superfluous, at least for the most part, as the audible differences were so minor as to be almost unnoticeable—again, another testament to the capabilities of this highly refined codec.

A/V receiver and surround processor makers are quickly adding these advanced decoders in their new model offerings, and broadening the price range to a wider audience, with A/V receiver models so equipped priced at under $600 available later this summer. Owners of Sony’s Playstation 3 gaming console got the DTS-HD Master Audio decoder as a freebie this past spring, as a feature added during a PS3 system software upgrade, and more HDMI version 1.3 Blu-ray players that can pass these high resolution bitstreams to downstream high resolution surround decoders are entering the market as well.

Conclusion
From both listening sessions, I came away with a newfound respect for the abilities of these audio codecs to deliver excellent sound quality at dramatically reduced bitrates. Ideally, I’d like to see future Blu-ray releases moving away from bit-hog multi-channel PCM tracks and instead use one of these high resolution codecs, as a typical Blu-ray movie’s 5.1 channel PCM soundtrack consumes a whopping 6.9 Mb/s all the time. That’s a large chunk of a disc’s available bitspace, some of which could be better used for maximized video coding precision, for example.

These new high-resolution codecs are backwards compatible with existing decoders, but only in their most basic form. For the better sound you’ll need either a player that decodes (and sends that audio out via PCM over HDMI or analog 6-channel out) or a receiver/processor that decodes the format, and a player that will output the bitstream of these codecs.

Dolby Digital Plus and DTS-HD High Resolution offer tremendous sound quality and are extremely efficient for the quality they provide. While still compressed audio, they’re closer to the original master than most people will be able to hear.

For those who will settle for nothing but the best, the bit-for-bit accuracy of Dolby TrueHD and DTS HD-Master Audio provide them the confidence that what they’re listening to at home is identical as the original studio master soundtrack.

So Subtle
What impressed, or perhaps surprised, me most about these tests was how good the base codecs actually are. The difference between the original audio and the basic Dolby Digital and DTS is a lot subtler than you’d expect, given the extreme amount of compression (around 10:1, a similar ratio to that of 128 kbps MP3).

That said, I could definitely pick out the difference between the lesser (or perhaps it’s more accurate to say “better”) compressed versions and the higher compressed versions. The difference is mostly in the presence, or ambience. The lossless, Dolby Digital Plus, and DTS-HD High Resolution compressed tracks were just a little more open and airy. I hate to say it, but they just sounded more realistic and transparent. The 448 kbps Dolby Digital and standard DTS tracks were less so, a little more closed off. Between the 640 kbps Dolby Digital and the uncompressed, the difference was even less noticeable. Enough so that most people, even those trained to listen for it, probably won’t be able to hear the difference.

The core DTS call is a little harder, as there wasn’t the same blind system in place to A/B as precisely as at Dolby. Results were similar, though.So by all means go for the new codecs, as they definitely sound better than what was on DVD. Uncompressed PCM, on the other hand, is just a waste of space (though compatible with everything).

If you’ve been listening at home and are sure you can hear a difference on your favorite discs, be wary. There is absolutely no way to tell that compressed and uncompressed tracks on any disc have anything to do with each other. They could come from different masters, they could be mixed differently, or any number of other variables that makes an in-home test, unfortunately, impossible. That said, trust your ears, and go with the one that sounds best to you.

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