When we think of surround sound today, we think of 5.1 digital transmission, with the most popular transmission codecs being supplied by either Dolby or DTS. However, there are still many cases when transmission of discrete, multichannel surround is impractical due to bandwidth limitations or other constraints such as infrastructure restrictions. It may also be desirable to use a surround format that does not require a proprietary decoder and its associated costs for cases when multichannel surround decoding is not required.
In these situations, technologies exist today that enable surround sound transmission over two-channel paths. Those technologies are backward compatible with stereo in such a way that content that is not surround-decoded plays back as stereo. When multichannel surround decoding is desired, the appropriate decoder can take the input stereo information and render it as an effective surround presentation.
These systems typically fall into two categories: composite and parametric. The composite systems essentially encode the information required to reconstruct the surround field in the audio itself. This audio can then be transmitted or stored on or over any two-channel media, including analog. 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. Examples of composite systems are the SRS Circle Surround and the Dolby ProLogic II. An example of a parametric system would be MPEG surround.
Digging deeper
From a functional standpoint, composite, or “matrix,” systems embed surround decoding cues in the audio through the use of amplitude and phase information. Matrix systems evolved from the earliest days of surround. The original system was Dolby Stereo, which was applied to motion picture sound tracks. When the home video era began with Beta and VHS, and when each eventually incorporated high-quality FM-based stereo audio systems, people discovered that the surround cues remained embedded in the audio tracks of the home releases. This, in essence, was the dawn of home theater.
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. Dolby ProLogic was developed to overcome this limitation by “steering” the signal based on surround cues encoded in the audio. Dolby ProLogic provided an improved experience, and it had the advantage of being able to play back unencoded content as basic stereo. However, it was still limited to a mono surround channel, which was often reproduced over two rear speakers even though they carried identical information. In addition, a 7kHz high-frequency roll-off was applied to the surround channel to enhance the perception of isolation from the front channels. Even with these issues, Dolby ProLogic was the standard method of storing and transmitting surround sound for many years.
In the mid-1990s, a composite surround encode/decode system was developed by a company called Rocktron. It was designed to address the limitations of Dolby ProLogic cited above. The Rocktron Circle Surround system provided full-bandwidth stereo surround by using a more advanced multiband, variable-time-constant steering system. 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.
In response to Circle Surround's success, Dolby created ProLogic II, which addressed the limitations of the original ProLogic. Both systems, however, could decode each other's encoding and still present a credible surround sound experience. It is this characteristic, along with the ability to encode surround to two channels and the lack of necessity for a decoder when surround is not required, that makes composite surround systems of this type attractive, even today, for certain cases.
Parametric systems such as MPEG surround can create a “near-discrete” surround presentation under the right conditions, but they require changes to the two-channel infrastructure to carry the sidechain data. This extends to both the transmission codecs as well as the internal distribution and playout structure of the content provider.
Conclusion
When transmission is limited to two channels, an excellent surround sound experience can still be made available to consumers through highly effective and mature technologies currently available in the market.
Alan Kraemer is the CTO of SRS Labs.
