As described in my earlier article "How Surround Sound Works," there are two primary ways to deliver a surround sound signal. The older analog matrix technology works by mixing (matrixing) multiple channels into two main channels; the matrixed information then has to be decoded into the original channel configuration. The newer digital discrete technology keeps all the channels separate from start to finish, producing a more clearly isolated surround effect.
Over the years there have been several surround sound formats using these various technologies. It can all get a bit confusing, especially when you look at the back of a DVD case and see a number of different logos denoting different surround sound formats. Fortunately, most newer audio/video receivers contain multiple surround sound decoders, and handle all the format-switching automatically. But it still helps to know what you're dealing withso read on for a basic guide to surround sound formats.
Surround sound was first heard in the home via a matrixed technologythe original Dolby Surround format. Matrix technology has become more effective over the years, but still isn't as realistic as discrete surround. That said, we've seen three different matrix technologies in the home, all analog in nature, and all from Dolby Laboratories.
The original surround sound format for home video was Dolby Surround. This is a simple three-channel format, with left, right, and surround channels. The surround channel is matrixed into the left and right channels; the Dolby Surround decoder separates the surround channel from the mix and runs it into one or two surround speakers. When you use two surround speakers, they both play the same surround channel.
Dolby Surround was first used in some prerecorded videotapes, and in some augmented stereo television broadcasts. It's not used much, if at all, today.
Dolby Pro Logic
Dolby Pro Logic is a more sophisticated version of Dolby Surround. Pro Logic adds a center channel to the mix; it's a four-channel system with left, center, right, and surround channels. The center and surround channels are matrixed into the left and right channel information, and then separated out with a decoder. As with Dolby Surround, the single Dolby Pro Logic surround channel is often fed to two surround speakers, which both play the same track.
Dolby Pro Logic is the surround technology used in many television, cable, and satellite broadcasts, as well as in most prerecorded videotapes. It's still in use today.
Dolby Pro Logic II
Dolby Pro Logic II is a version of Dolby Pro Logic with a different purpose. This surround format is designed to simulate a surround effect from a two-channel source. You use Dolby Pro Logic II to play stereo soundtracks and CDs on a surround sound system. (The newer Dolby Pro Logic IIx format applies the same technology to 7.1-channel systems.)