Dolby Surround and Pro Logic use 4 and 5 speakers respectively. Surround Sound adds rear speakers to the main front pair, and Pro Logic adds an additional center channel. Although these systems use 4 or 5 speakers, the original input signal is still 2 channels: left and right. The 5 channels of sound are encoded into these 2 channels, stored on disc or tape, and then decoded by the home users Dolby decoder. This encoding is done to maintain backward compatibility with older 2 channel systems. A Dolby encoded source will still play correctly when using only 2 speakers without a decoder. The rear and center channels in a surround or pro logic system are not full range. The standard bandwidth for rear speakers is 70Hz to 8kHz, although some soundtracks will contain information as low as 50Hz. 
  Dolby Digital 5.1, also know as AC-3 because it is Dolby's 3rd audio code, uses 5 separate channels for encoding the 5 speakers, plus 1 separate channel for a subwoofer. The .1 subwoofer channel is for LFE (Low-Frequency-Effects), and is basically a deep bass channel. The 5 main channels will all have a full range signal. Most Dolby Digital processors will have a function that allows the bass from the 5 main channels to be redirected to the subwoofer if desired. This allows for having bass in a system even when using small surround speakers. 
  Dolby Digital 5.1 encoding is used primarily in DVDs. There are other mediums that use a 2 channel Dolby Digital format, such as CDs, TV satellite, digital cable, video games, and older laser discs. Although it is Dolby Digital, it is not the 5.1 encoding. The 2 digital signals are decoded into a 4 channel surround sound signal. This is used by companies that use a digital signal to save bandwidth or increase quality, but have not yet upgraded to the full Dolby Digital 5.1 system. Dolby Digital does not necessarily mean 5.1 encoding. 
  DTS (Digital Theatre Systems) is similar to Dolby Digital except that the signal quality is higher. To save space on a DVD, both Dolby Digital and DTS signals are compressed. The compression on Dolby Digital signals is higher than that of DTS, so a DTS signal retains more of the original signal and therefore is of higher quality. While Dolby Digital is standard on almost every DVD, many still do not use DTS. 
  Both Dolby Digital and DTS continue to add channels to their format. You can now get 6.1 (adds a rear center), 7.1 (uses 2 side channels), and 8.1 ( 

Last updated on May 5, 2011 by Alex Robles