![]() In response to Circle Surround's success, Dolby created ProLogic II, which addressed the limitations of the original ProLogic. ![]() Circle Surround was later acquired and further refined by SRS Labs and served as a mainstay for sports and music broadcasting prior to the digital era. The Rocktron Circle Surround system provided full-bandwidth stereo surround by using a more advanced multiband, variable-time-constant steering system. It was designed to address the limitations of Dolby ProLogic cited above. In the mid-1990s, a composite surround encode/decode system was developed by a company called Rocktron. Even with these issues, Dolby ProLogic was the standard method of storing and transmitting surround sound for many years. In addition, a 7kHz high-frequency roll-off was applied to the surround channel to enhance the perception of isolation from the front channels. However, it was still limited to a mono surround channel, which was often reproduced over two rear speakers even though they carried identical information. Dolby ProLogic provided an improved experience, and it had the advantage of being able to play back unencoded content as basic stereo. Dolby ProLogic was developed to overcome this limitation by “steering” the signal based on surround cues encoded in the audio. The original iteration of Dolby Stereo had limitations that prevented it from working well, as it only provided about 3dB of separation between adjacent channels. An example of a parametric system would be MPEG surround. Examples of composite systems are the SRS Circle Surround and the Dolby ProLogic II. Parametric systems analyze the spatial characteristics of a multichannel input and encode the resultant information into a low-bit-rate digital sidechain that can be used at the receiving end to spatially reconstruct something close to the original surround presentation. This audio can then be transmitted or stored on or over any two-channel media, including analog. ![]() The composite systems essentially encode the information required to reconstruct the surround field in the audio itself. These systems typically fall into two categories: composite and parametric. When multichannel surround decoding is desired, the appropriate decoder can take the input stereo information and render it as an effective surround presentation. Those technologies are backward compatible with stereo in such a way that content that is not surround-decoded plays back as stereo. In these situations, technologies exist today that enable surround sound transmission over two-channel paths. ![]()
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