Just to be pedantic
Dolby Surround is two-channel analogue sound which has components of the signal recorded out of phase and which can be decoded into 5 audio channels by an amplifier which incorporated Dolby Surround (also called "pro Logic") decoding.
Dolby digital is a digital bitstream which can be decoded into 5 discrete audio channels using an amplifer which incorporates a Dolby Digital decoder.
Isn't quite right, dolby digital can be mono, 2 channel etc .....
Dolby® Digital
Delivers mono, stereo, or up to 5.1 discrete channels of surround sound: Left, Center, Right, Left Surround, Right Surround, and low-frequency effects (LFE).
Dolby Surround
The original Dolby multichannel analog film sound format that revolutionized movie soundtracks. This technology encodes four channels of audio (Left/Center/Right/Mono Surround) onto just two audio tracks for media such as TV broadcasts and feature films on VHS. Material encoded in Dolby Surround is fully compatible for playback on mono and stereo systems, and upon playback on systems containing Dolby Pro Logic® technology, the audio is reconstructed to its original four-channel surround sound.
Dolby Digital EX
Creates a rich surround-sound home theater experience by adding a sixth channel (Center Back) to Dolby Digital Surround EX-encoded content for enhanced localization of surround effects.
Dolby Pro Logic II
Transforms any high-quality two-channel source into five independent full-range channels: Left, Center, Right, Left Surround, and Right Surround. Brings new life to playback of CDs, MP3s, and other stereo content including surround encoded broadcast/VHS.
Dolby Pro Logic IIx
Delivers the most natural, seamless 6.1- or 7.1-channel listening experience. Processes any high-quality stereo, 5.1, or Dolby Digital Surround EXencoded content into 6.1 or 7.1 playback channels, depending on speaker configuration: Left, Center, Right, Left Surround, Left Back, Right Surround, Right Back, and subwoofer.
Dolby Digital Surround Pro Logic IIIII XXXXX
Format whose name and logo won't fit on the front of your newest amp
Just kidding!
Cheers
Kevin.