Introduction| Mono| Dolby Stereo| Dolby Surround|
Dolby Pro-Logic| Dolby Pro-Logic II| Dolby Digital|
Dolby Digital Surround-EX| DTS| DTS-ES| THX| Sony SDDS|

Dolby Surround Pro-Logic

Dolby Pro-Logic is not a sound format in the same way as Dolby Surround and Dolby Digital is. In fact it's a system of decoding used in home cinema hardware that decodes the extra front centre and rear channels from a Dolby Surround soundtrack. These extra channels are matrixed within a two-channel stereo soundtrack and are processed by the Dolby Pro-Logic circuit. Although Dolby Pro-Logic decoders are designed for use with Dolby Surround soundtracks, they can also create a virtual centre and surround channel using any two channel stereo source, but without the same channel separation and clarity. The rear channel, known as 'surround', is a single mono channel that is played back through two speakers at the back of the room. The surround channel is also limited in the bandwidth that it can handle, so only low volume atmospheric effects are sent through the rear speakers. The actual Dolby Pro-Logic decoder can be located in a range of hardware, including home cinema amplifiers and processors, surround sound televisions, and even in some high-end car stereos (look at the new Volvo). Dolby Laboratories have recently developed an updated analogue sound format called Dolby Surround Pro-Logic II, which is described on the following page.