Unlike traditional CD stereo playback, the world of surround sound is largely based on the need for encoding and decoding. Whether it’s a concert DVD-Video, HDTV broadcast or a video game, the 5.1 format currently reigns supreme. The term ‘5.1’ represents a Left, Center, Right, Left Surround, Right Surround and Low Frequency Effect (LFE) channel configuration. Virtually every home theater currently sold can handle this type of playback, with some even adding extra ‘rear’ speakers for more ambiance beyond 5.1.
To confuse matters a bit more (or to clear them up), there are two additional formats called Super Audio Compact Disc (SACD) and DVD-A (Audio), which both provide hi-resolution stereo and 5.1 audio. However, both need to be fed through six analog RCA outputs into that same home receiver that handles your movies, CDs, and TV broadcasts . . . ugh.
Since I happen to own an Onkyo HT-S780 7.1 home theater system with matching universal player (DVD-Vs, SACDs, and DVD-A discs), I asked their National Education Manager Bob Tamburri to elaborate on the matter at hand. “One of the important things about the surround receivers role is that in most cases, you can assign or pre-assign how it treats the various signals being fed into it,” he noted. “Typically, a surround mix from a DVD-V would be in the Dolby Digital or DTS formats. Both of these are encoded bitstream and would be handled by the player and the receiver through the coaxial or optical digital feed.”
“From there you have a couple of options,” he continued. “A Dolby Digital 5.1 mix will be handled as a Dolby Digital encoded signal and therefore you’ll get a discrete 5.1 playback from the receiver (Left, Center, Right, Left Surround, Right Surround, LFE). Similarly, if it sees a DTS signal, the receiver would treat it accordingly. DTS is the parallel process to Dolby Digital, and while it is still 5.1, the receiver uses a different codec to decode it. However, it is very similar in terms of what it outputs to the end consumer.”
And don’t always assume that when it says Dolby Digital, it means there’s a 5.1 mix involved. Often you will find a Dolby Digital 2.0 or 2.1 mix (with LFE) — especially in television broadcasts. If my Onkyo detects a Dolby Digital 2.0 mix, it will simply play it back as transmitted — in stereo.
This however, brings up our next issue. What are all those ‘Surround Modes’ on the receiver? “A Dolby 2.0 or 2.1 mix will playback on a two-channel system, assuming they have Dolby Decoding (in some cases there is a decoder in the player itself),” Tamburri continued. “But any two-channel signal that’s digital (or analog) can then be processed in the receiver with Pro Logic or Pro Logic II (if the receiver has it), which is a 5.1 matrix surround algorithm. It will take all the low frequency information, output it into the sub, interpolate a center channel from the left and right, and provide a matrixed surround. Often that surround information is matrix encoded into a Dolby Surround two-channel mix, and that’s what you get on a lot of VHS tapes that have the ‘Dolby Surround’ label on them. Pro Logic and PLII were designed to take that matrix surround and decode it for 5.1 systems. It’s surround through math.”
Tamburri continued to describe several other ‘Surround Modes’ available on a typical receiver. “PLII X (Pro Logic II X) is where the PLII (Pro Logic II) system decodes a 5.1 surround output from a two-channel source and takes it up to 6.1 or 7.1. So you can take a two-channel source and actually get a 7.1 surround output from that, if your home theater can handle 7.1. It will actually extrapolate an additional two channels for ‘Surround Back’, not just Left and Right Surround. Then there is DTS Neo6, which is that company’s version of Dolby PLII and PLII x. Neo6 will take a two-channel source and convert it to 5.1, or 6.1, and so forth. It uses an algorithm process — so again, it’s basically synthesizing a surround matrix.”
Now, stay with us here folks, nobody said this would be easy. We’ve gone this far into the receiver, so we have to keep going. “There is also Dolby Digital EX and DTS ES,” Tamburri says. “A Dolby Digital EX mix will generally include a single discrete surround back channel, providing 6.1 output. DTS happens to have two types of ES, matrix and discrete. Discrete will take DTS ES encoded tracks off the DVD and play them back discretely in 6.1, where as the ES matrix will take a 5.1 signal and give you a synthesized 6.1. On the higher end type home theater systems, you’re going to have THX processing, which also gives you 7.1. That’s essentially their version of Dolby Digital EX, but it gives you two channels in the back, not just one — although it’s the same information coming out of both. That’s applied more to movies, but they do have a spec for music.”
Now let’s look at a scenario where a 5.1 mix from your studio is placed onto a DVD-V, and encoded for both Dolby Digital and DTS. Once placed into the DVD player, you would have to choose either the Dolby Digital or DTS audio in a typical “audio preferences” menu. That encoded mix would then stream out the optical or coaxial output and be detected by the receiver — which would then decode and play the full discrete 5.1 audio. If you only output stereo, such as a Dolby Digital 2.0 mix or 16-bit CD, the receiver could either play it as such, or you can use one of the many ‘faux’ surround modes for a surround-like experience.
The next concept that’s important to understand about home theater audio is that of bass management. With a Dolby Digital or DTS mix, the receiver will filter out any bass information and direct it into the subwoofer. This crossover selection is located in the receiver, and typically varies from 60 up to 150Hz — with 100Hz being a common setting. Simply put, bass management allows home theater systems with small satellite speakers to reproduce bass properly. Some of them even allow you do direct bass to any speakers that can handle it, such as the Front Left and Right on my particular system.
But what about SACD and DVD-A discs, why are they hooked up analog into the receiver? “The universal player only outputs DSD [SACD’s audio format] or DVD-A out of the six analog outputs,” Tamburri noted. “They are not bass managed either from the player or at the receiver end. Essentially, it’s a channel for channel analog feed with no processing going on. Some higher end players do feature bass management for these formats, but many don’t.”
Another reason that the SACD and DVD-A output is generally only analog is that there is no standard “codec” for these formats. The processing for these must therefore be handled by the player and output as a discrete 5.1 signal. “Some newer players and receivers equipped with iLink (SACD and DVD-A) and HDMI (currently DVD-A only) can handle these formats in the digital domain — with no additional D/A or A/D processing,” Tamburri said. “Many, such as our Onkyo’s DV-SP1000, do feature bass management as well.”
What this all means to you as a producer creating a surround mix is that you need to consider what the consumer will hear in their home theater. If your project is encoded for Dolby Digital or DTS, it will generally be sent digitally into a receiver and be bass managed. If it’s a higher resolution DVD-A or SACD, chances are good it will be heard with no processing — so be careful of your low end. If it’s broadcast over a cable network in Dolby Digital, consumers with a surround home theater can hear it as such; otherwise they will just get stereo.
But wait, there’s more!
The next generations of hi-resolution disc formats are just around the corner. For now, enjoy the surround sound home theater experience, because that next mix pumping through the system might just be yours.