It all began with mono, a single channel of sound, and everyone thought that was simply wonderful at the time. How quaint! Now, most of us find mono a bore, but we’d have to concede that it’s easy to understand. A single speaker in the radio or phonograph reproduces one channel of audio information. Simple. Amazingly, that sufficed for more than 50 years, until the 1960s.
Now, it’s getting a bit confusing. If you’ve purchased an A/V surround sound receiver recently, it almost certainly has six separate channels (left, center, and right across the front, left and right surround channels to the sides, and a subwoofer channel for low-frequency effects). And if you paid a bit more, it may even contain two extra channels to process sounds for one or two back surround speakers.
Let’s sort them out.
The two competing surround soundtrack formats in which virtually all modern movies are mixed are Dolby Digital 5.1 and DTS 5.1. Each of these uses the aforementioned six channels, one of which is called the ".1" subwoofer channel because it’s limited to deep bass sounds below 100 Hz. It also represents one of the ten octaves of audible sound, hence its "5.1" label. In each system, a digital data stream of 1’s and 0’s carries all the information for all the channels over a single coaxial or optical digital cable to the innards of your receiver, which first identifies either a Dolby Digital or DTS signal, and then decodes the data, routes it to the appropriate channels, and converts it back to analog sound and music signals that can be amplified and reproduced by your five speakers and subwoofer. For the record, both Dolby Digital 5.1 and DTS 5.1 use perceptual encoding and decoding to save digital space and make all those channels manageable, as well as permitting a two-hour movie and six discrete channels of sound to be contained on a single DVD.
Recently, Dolby Labs added an extra channel to its Dolby Digital 5.1 channel scheme and called it Dolby Digital EX. Not to be outdone, DTS came up with its own version, DTS-ES. If your receiver has these modes, it will decode the small but growing number of movie soundtracks encoded with a back surround channel reproduced by one or two speakers behind the listener. If one, it’s called a "6.1"-channel system; if two, a "7.1"-channel setup. The idea of this extra channel(s) is to provide a more immersive listening experience and greater realism with more convincing surround effects. While the extra channel is not a truly discrete digital channel, it nonetheless will deliver a more seamless surround effect and better coverage, especially in larger rooms. If your receiver is equipped with Dolby Digital EX and DTS ES decoding, and you’ve added the extra one or two speakers at the back, when you play a DVD movie that’s been mixed for EX or ES, the receiver will automatically decode the extra information. You don’t have to do anything (this is good). And even if you play a standard 5.1-channel movie, your receiver will automatically extrapolate information for the extra one or two rear speakers. There is a further wrinkle with DTS, an ES mode called "ES Discrete". Some newer discs so-encoded deliver six entirely discrete channels plus a subwoofer channel. (With Dolby Digital EX, the extra channel is "matrixed"; DTS also has an ES-Matrix mode.)
If you’re old enough, you may recall that lots of movies back in the 1980s were mixed in "Dolby Surround," which used two analog channels to piggyback or "matrix" a center dialog channel and a single mono surround channel, the latter always played over two surround speakers. If you rent or own one of these old movies, you can use your receiver’s sexy new Dolby Pro Logic II Movie (or DTS Neo:6 Cinema) processing modes to simulate a digital 5.1-channel surround sound presentation from what is essentially a two-channel stereo source. And if you play a simple stereo CD or even a stereo vinyl LP, you can switch to a "Music" mode in DPL II or DTS that will recreate a six-channel surround mode. Choose the "Movie" mode of DPL II or the "Cinema" mode of DTS for old, Dolby Surround encoded movies or current television shows, or the "Music" modes for non-encoded stereo CDs or 2-channel sources. The latest A/V surround receivers have DPLIIx, which takes a two-channel stereo source and simulates 7.1-channel surround sound.
Finally, some manufacturers have developed their own proprietary multichannel processing. Harman/Kardon offers Logic 7 on most of its surround receivers, derived originally from Lexicon’s processors, which Harman owns. Logic 7 has its own Cinema and Music modes as well.
DPLII, DPLIIx, DTS Neo:6, and Logic 7 each has its own distinctive character. Depending on the source, you may prefer one over the other. I sometimes find Logic 7 to be very realistic with 2-channel Cds of classical orchestral, jazz, and opera. DTS Neo:6 tends to sound better with many pop or rock recordings, and DPLII or DPLIIx at times can be a knockout with both genres. DPLII or DPLIIx may also put too much information into the surround channels. But it’s fun to experiment and find which you prefer with which recording. Sometimes (I admit it’s rare), plain old 2-channel stereo sounds best!
Oops, I almost forgot. Most new A/V surround receivers offer a "5-channel stereo" or "7-channel stereo" mode, which mixes the left and right stereo channels from a 2-channel disc into ALL the amplifiers and speakers you’ve got running. As it turns out, it can be surprisingly enjoyable and it’s ideal for parties. No-one gets left out!