Dolby Digital, DTS, THX: Surround Sound Standards Explained
When asked to explain the difference between 5.1 and 6.1 surround sound, most of us wouldn’t blink an eye. Ask that same crowd the difference between Dolby and DTX and you’re likely to get a blank stare.
I intend to fix that today.
Although there are only three main players in the surround sound industry, each has their own subsets of technology as it relates to surround. While there is rather significant overlap between each, the technologies themselves have enough differences that it’s worth exploring what makes each unique before settling on a home theater system that features just one of the formats.
Although Dolby has (seemingly) a million and one different formats, it’s really only important to understand two. Additional formats offer subtle variation, but no real change in primary technology. We’ll discuss some in a moment.
For now, we’re going to focus on Dolby Pro Logic and Dolby Digital.
Dolby Pro Logic
Dolby Pro Logic is the oldest of the audio decoding technologies. For you vintage audio equipment lovers, this is probably the technology you use with the most frequency.
It’s important to understand that although this option is offered on nearly every receiver sold, it’s not actually true surround. Technically it’s classified as “virtual surround,” which is to say, it’s an inferior technology for most of what you use your surround sound receiver for. Pro Logic was also a popular format for PC gaming until computer makers started creating 5.1, 6.1 and 7.1 sound cards that could support a true surround sound experience.
The original format was designed to utilize two channels and sometimes a subwoofer. Typically, the subwoofer was actually built in to the speakers it was designed for. Think 70s-style woodgrain cabinet-type speakers. Pro Logic decoded the audio from a record, cassette tape or VHS movie and played it back over the two channels with the only variation in sound being left and right.
Pro Logic II came along later, and added the ability to split this signal amongst 5 speakers and a separate subwoofer. The technology was fundamentally the same, but with the addition of multi-channel support. It still isn’t actual surround, as you aren’t getting different sounds from each speaker. The only variation is amid the sounds coming from the left and the right channels.
While this isn’t completely useless, it’s certainly not the format I’d choose if I were gaming or watching a movie (although there are typically settings in the audio menu for both). Where it is useful is in a strictly audio setting. If you’re listening to Spotify, for example the audio is in stereo anyway, so it really is a no harm, no foul situation when you’re simply adding more speakers to the mix.
Just to touch on it briefly, Dolby Pro Logic IIx and IIz are essentially the same as the original Dolby Pro Logic. Pro Logic IIx added the ability to process sound amongst 6 or 7 (6.1 and 7.1) channels depending on your speaker setup.
Pro Logic IIz added a new dimension, height. This allowed you to place additional speakers above the front left and right channels in order to create more realistic-sounding effects such as rustling tree branches, or rain.
Dolby Digital is the successor to Pro Logic, and came about when 5.1 surround started to become commonplace in households around the world. The technology uses a built-in decoder to decode the signal and separate individual sounds in to one of 5 (front left and right, rear left and right, center channel) independent speakers. Each speaker operates independently of the others (if the source is encoded in 5.1 surround) and thus allows for an all-encompassing, true surround sound experience.
It’s important to note that Dolby Digital is a lossy format. As such, the audio is encoded and compressed by the studio in order to fit on the disc, or to conserve bandwidth when broadcast digitally through cable or satellite. Netflix compresses the audio feeds as well in order to reduce server demand.
Two variations, Dolby Digital EX and Dolby Digital Plus are mostly the same as Dolby Digital, but with the addition of an independent channel (in the case of Digital EX) or two (for Digital Plus). This allows support for 6.1 and 7.1 surround systems.
One additional variant to note would be Dolby TrueHD. TrueHD, while still using compression, is lossless, and doesn’t actually lose any audio quality once it’s decoded and played back. TrueHD is only available on Blu-ray discs, as it’s far too large to fit on a DVD or CD. Support for this is rather varied and there are many receivers that offer no TrueHD support whatsoever.
DTS shares a lot of commonalities with Dolby Digital. Both feature lossy compression and encoding by the studio, both are decoded by the receiver, and both end up slightly diminishing the quality of audio from the original source. That said, DTS is transferred at a higher bitrate than Dolby Digital which leads to less sound degradation from the original studio master.
Now, that doesn’t mean that there’s a noticeable difference. For the average household, DTS and Dolby Digital will sound roughly the same. This is due to the quality of the speakers most of us own. If you don’t own relatively high-end speakers, the difference is negligible, and chances are that you won’t be able to tell the difference – even though many claim they can.
You can also get virtual 6.1 support if you have a sixth channel, and your receiver has DTS-ES support.
Also worth noting is the support for DTS. While it’s growing, it still isn’t supported on all consumer grade receivers like Dolby Digital.
DTS Neo:6 is DTS’s equivalent of Dolby Pro Logic II. The format isn’t true surround, but offers a virtual experience by splitting the signal between 5 or 6 speakers. Many claim that one format is better than the other when comparing DTS Neo:6 to Dolby Pro Logic II, but again, on most consumer-grade speakers, you aren’t going to notice any difference. They are, for all intents and purposes, the same unless you own premium equipment.
DTS-HD Master Audio
Are you starting to see the trend here? DTS-HD Master Audio is DTS’s version of Dolby TrueHD. Both remarkably similar in scope with the use of lossless compression, multi-channel audio support, and a big enough file size to make it only available on Blu-ray. The difference? Well, since both are lossless, it’s rather difficult to tell. Since Dolby and DTS aren’t exactly willing to hand over proprietary information, and all information points to them being the same, then for now we’ll just have to chalk it up to personal preference.
This is a bit of a wildcard, as unlike Dolby or DTS, it isn’t actually a format, rather a series of quality guidelines defined by THX. THX (according to their website) aims to “set the highest standards for home theater audio and video equipment.”
Now, while THX is kind of the end-all-be-all of audio quality, it’s typically only available on high-end receivers. Unlike the other two, THX also isn’t limited to audio. Video equipment can bear the THX certification as well.
The creator of THX, Tomlinson Holman, developed the standard after working with Lucasfilm Ltd. (a name that needs no introduction) to develop quality standards that accurately reproduced the soundtrack for Return of the Jedi – the third Star Wars film. THX has since branched off into its own company, a joint venture by Lucasfilm, and Creative Labs.
That just about wraps it up. So now, next time you’re left staring at your receiver menu with a plethora with a feeling of confusion about which format does what, hopefully you’ll refer back to the knowledge you gained today and make the correct choice.
Which one is the correct choice? Well, it’s whatever sounds best to you, of course.
For a modern listening experience, check out the best Dolby Atmos soundbars you can buy .
Image credits: Dolby entry by Mike Renlund via Flickr, THX Logo on Centre Speaker by William Hook via Flickr, 2.1 Home Theater via Shutterstock, 5.1 Home Theater via Shutterstock, 6.1 Home Theater via Shutterstock, 7.1 Home Theater via Shutterstock
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