A Dolby Pro Logic home theater system includes four channels and five speakers of surround sound. Dolby Pro Logic is a matrix decoded multichannel surround sound system, meaning that it turns two-channel (stereo) sound into four channels of sound. The more advanced Dolby Pro Logic II produces five channels of sound.
The output channels in Dolby Pro Logic are electronically derived from input channels. None of the four outputs are identical to the two inputs used to transport the encoded signals, but all the signals being output from the decoder are indeed present in the two-channel source — nothing new, such as reverb, is being added.
The two surround speakers in a Dolby Pro Logic system play essentially the same monophonic sound. You often see a subwoofer channel on Pro Logic receivers, as well; note that this is not a separate channel but is derived from the low-frequency information from the front channels.
One problem with the dual monophonic surround-sound channels is that, in some ways, they defeat the purpose of having surround sound. You can't send information specifically to one speaker, as is done with a left or right front speaker. And because they have the same signal, they tend to create a localized sound field between the speakers — and that goes against surround sound's goal of creating a large, diffuse background sound field. Proper placement of the surround speakers on the side walls aiming across the listening area helps achieve the optimal results.
The biggest new feature that Dolby Pro Logic II offers is a fuller sound experience for two-channel stereo or Dolby Surround-encoded sources that were being played over 5.1-channel systems. Pro Logic II does extra processing and extends the Pro Logic work substantially. Importantly, Pro Logic II decodes the surround-sound speakers in full-bandwidth stereo and therefore provides a more complete rear sound field.
Just about all surround-sound decoders sold since 2002 or so support Dolby Pro Logic II. Most now support an even newer and more advanced version of Pro Logic — Pro Logic IIx. This system provides a full 6.1- or 7.1-channel output — meaning you can take any incoming stereo (2.0) or 5.1-channel signal and turn it into a 6.1- or 7.1-channel output that uses all the speakers in your home theater system.