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Giz Explains: Dolby, DTS and Home Theatre Audio Codec Confusion

Posted by Matt Buchanan at 4:00 AM on July 24, 2008

You actually know what some of the crazy doodles on the side of an HDTV means when it comes to video--720p, 1080i, 1080p. Congrats, you're ahead of most people, like my mother. But do you understand the alphabet soup of audio, the confounding constellation of logos on your Blu-ray player's box? While there are basically two rival home-theatre audio encoders--Dolby and DTS--they each have several different quality levels and options for different scenarios. Yeah, it's a lot to keep up with, and it annoys us too. So we asked Dolby and DTS to put down their guns for a sec and help us sort it out.


 

We're assuming you know some of the basics--like that 5.1 audio is five channels of audio positioned at centre, front right, front left, back right and back left, and then one subwoofer channel. And that a higher bit rate means more audio data is coming through, which, generally, means it's higher quality and gonna sound better, since you're losing less of the original studio sound.

The building block of digital audio is "pulse code modulation"—an old technology used for CDs and everything since. It can be rendered in several resolutions, from 16-bit stuff on CD to 24-bit on newer DVD and higher-res formats. It can also have varying frequency ranges, typically from 44.1KHz to 96KHz. Without going into more detail, you just need to know that PCM is bulky, and it is this PCM data that both DTS and Dolby work to encode into more manageable files. When audio tracks are decoded in a disc player, they are either sent out analogue via multichannel RCA outputs, or they become PCM tracks that any digital receiver can easily interpret.

We're taking you through the major branded audio formats that you'll run into if you're dealing with a home theatre, or hell, a Blu-ray player.

First up: Dolby. There are basically three tiers of audio: Dolby TrueHD at the top, then Dolby Digital Plus, then good old Dolby Digital.

Dolby TrueHD is a lossless compression format that is bit-for-bit identical to the studio masters. It can handle a bit rate of up to 18 megabits per second, and support as many as 14 channels of audio, though you're more likely to see it at 7.1. It's actually optional in the Blu-ray spec, but it's supported by the PS3 and most other new Blu-ray players. Some players decode the TrueHD internally, then stream out uncompressed PCM audio through HDMI, while others can send the TrueHD file itself out over HDMI in bitstream for the receiver to decode.

Dolby Digital Plus is the next step down. It still delivers 7.1 audio, but at a max bit rate of 3Mbps. It's a more efficient codec than the original Dolby Digital, and is a mandatory minimum in the Blu-ray 1.1 spec. Dolby Digital Plus can be used for Bonus View picture-in-picture audio tracks on a Blu-ray disc, with the main audio track encoded as TrueHD.

Dolby Digital is the lowest rung, at 5.1 audio channels, running at 448Kbps on DVD (though a richer 640Kbps on Blu-ray, used, again for special features or supplement language tracks).

DTS's offerings follow a similar tiered setup.

DTS-HD Master Audio is at the top. It's a lossless format that is also bit-for-bit identical to the studio master. It supports a bitrate up to 24Mbps (though the average Blu-ray flick's audio is only about 2-3Mbps, with 4-5Mbps spikes) and up to eight channels (like 7.1). (It too, is supported by the PS3.)

DTS High Resolution Audio is below that. It also supports eight channels at a constant bit rate of up to 6Mbps. It's for situations where a studio doesn't want to eat up disc space with a full lossless track (like bonus features or tracks), though DTS told us 95 percent of studios who use DTS use the full HD Master Audio.

DTS Digital Surround is down at the DVD end, with support for 5.1 channels and bandwidth up to 1.5Mbps, though post-2000 DVDs typically keep the track at 768Kbps to save disc space.

You may have heard a few things about Dolby ProLogic II or IIx, or maybe DTS Neo:6. These aren't digital codecs, so much as they are "matrix" programs that take stereo tracks and route it to to the different speakers in a surround system. A vestige from pre-digital days, people used to master stereo tracks deliberately for ProLogic—try watching The Simpsons opening credits through your receiver with ProLogic turned on.

Dolby and DTS also have virtual surround technologies that do the opposite of matrixing: They take 5.1 tracks and perform hocus pocus on them so that they sound surround-y, but play through stereo speakers or headphones. It's more subjective, and has a whole different science to it, so maybe we'll save it for another time.

That, in a nutshell, is what all of those Dolby and DTS logos on the back your Blu-ray player, A/V receiver or movie box means. If you want to know how "golden-eared" audiophiles feel about the highest qualities, and how well they fare against uncompressed PCM, check out this informative piece from Home Entertainment Magazine. As a rule, DTS HD Master Audio or Dolby TrueHD will kick arse, but unless you have a $50,000 sound system, you may not be able to tell the difference between the middle and top tiers anyway.

Something we missed, or you still wanna know? Send any questions about Dolby, DTS, Dubbly, Dobby or anything else to tips@gizmodo.com, with "Giz Explains" in the subject line.

Tags: audio | blu-ray | dolby | dts | feature | giz explains | home entertainment | home theatre | playstation 3 | regulars | top

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