AC3 file, a Dolby Digital audio file, can be found as the standard audio track on Digital Versatile Discs (DVD) and High Definition Television (HDTV). This coder has been designed to take maximum advantage of human auditory masking in that it divides the audio spectrum of each channel into narrow frequency bands of different sizes optimized with respect to the frequency selectivity of human hearing. This makes it possible to sharply filter coding noise so that it is forced to stay very close in frequency to the frequency components of the audio signal being coded. By reducing or eliminating coding noise wherever there are no audio signals to mask it, the sound quality of the original signal can be subjectively preserved.
Dolby Digital is the trademarked marketing name for Dolby Laboratories' 'lossy' AC-3 codec. The common version contains 5.1 channels (five primary speakers and an LFE channel), but the format supports Mono and Stereo usages as well.
Dolby Digital EX is very similar in practice to Dolby's earlier Pro-Logic format, which utilized Matrix technology to add a center and single rear surround channel to stereo soundtracks. EX adds an EXtension to the standard 5.1 channel Dolby Digital codec in the form of matrixed rear channels, creating 6.1 or 7.1 channel output. However, the format is not considered a true 6.1 or 7.1 channel codec because it lacks the capability to support a discrete 6th channel like the competing DTS-ES codec.
Dolby Digital Surround EX: Whereas Dolby's Pro-Logic IIx format creates 6.1 and 7.1 channel output from stereo 2 channel (2.0). Dolby formats, the Digital Surround EX codec adds a sixth and sometimes seventh channel to standard (non-EX) 5.1 channel Dolby Digital soundtracks.
Dolby Digital Plus is an enhanced coding system based on the AC-3 codec. It offers increased bitrates (up to 3 Mbit/s), support for more audio channels (up to 13.1), improved coding techniques to reduce compression artifacts, and backward compatibility with existing AC-3 hardware.