Home Entertainment`s guide to the array of surround sound formats.
Found on most TV broadcasts and VHS cassettes, Pro-Logic is an analog format that requires front L/R, front centre and rear L/R speakers. Sound to the two rear speakers is mono only.
Dolby Pro-Logic II
Like Pro-Logic, this analog system requires front L/R, front centre and rear L/R speakers, but sound to the two rear speakers is stereo. Pro-Logic IIx adds a Surround Back channel to the mix and requires an extra speaker. AV receivers with Pro-Logic II/IIx have the ability to create 5.1 and 6.1-like surround from a stereo sources, such as CDs, audio cassettes, VHS tapes and TV broadcasts.
Dolby Digital 5.1
The de facto standard for home theatre, Dolby Digital 5.1 has six fully discrete (independent) digital channels and requires five speakers (front L/R, centre, rear L/R) and a subwoofer (the .1 in the system). Dolby Digital is also available in mono and stereo flavours.
Digital Theater Systems is a digital 5.1 system similar to Dolby Digital, but because it employs less audio compression, claims higher sound quality. For most people the difference is negligible, and DTS discs are not as plentiful as Dolby 5.1 software.
Dolby Digital Surround EX
Just like Dolby Digital 5.1, but with an extra rear channel referred to as the Surround Back or Centre Rear. Positioned behind the listening position, this channel can be reproduced by one or two speakers. Because Surround EX employs six speakers, it is often described as a 6.1 system.
A 5.1 digital system with six independent channels and an additional matrix Surround Back channel that, like Surround EX, can be reproduced by one or two speakers. Because it employs six speakers it too is often described as a 6.1 system.
The Surround Back channel in a DTS-ES Discrete system is fully discrete, not matrixed, making it a true seven-channel 6.1 digital system. The extra channel can be reproduced by one or two speakers, and when the latter is the case, ES Discrete is often described as a 7.1 system.