Theory of Operation

SRS Circle Surround ("CS") is an advanced surround format that allows full multichannel surround sound to be experienced in the home, car or theater. It can be used to create content for radio or television broadcast (live and produced), feature films, DVDs and CDs. CS can encode up to 6.1-channels of audio for transmission over standard stereo two-channel carriers and maintains highly accurate steering and channel separation during decoding. Circle Surround-encoded material is 100% compatible with other consumer surround decoders and can be played back in both mono and stereo systems without decoding. Circle Surround preserves the creative intent of the original mix, and brings the engineers and producers hard work to life.

It is the most flexible, compatible and accessible surround sound format available today.

Theory:
The Circle Surround encoding system accepts up to 6.1 channels of audio information for encoding to a two-channel Lt/Rt output for storage or transmission. This technology represents a substantial improvement over previous Lt/Rt-based encoding systems, which are not as well suited to handle modern, multichannel media with more than four channels of audio. The CS encoding method has remained consistent between CS and CS II decoding, providing full compatibility. A block diagram of the CS encoding process is shown in Figure 1.

Main, Center and LFE Encoding
Subsequent to the encoder bass management system, which will be discussed below, input summing amplifiers A1 and A4 combine the main left and right signals with the center input and the low passed LFE input.

Figure 1

Surround Encoding
Summing amplifiers A2 and A3 combine the surround back input with the surround left and right inputs. The outputs of these amplifiers are fed to constant phase networks F2 and F3 to encode the surrounds in quadrature with the main channels, which are processed through constant phase networks F1 and F4. The surround signals are then mixed antiphasically into the Lt/Rt output.

Encoder Bass Management
Bass from each of the main channels (L,C,R,Ls,Cs,Rs) would normally build up as a result of the encoding process producing more bass in the decoded LFE channel than intended by the content producer. To preclude this, the CS encoder incorporates high pass filters on the main channel inputs that are complimentary to the response characteristics of the LFE low pass filters. For more detailed information on the CS II decoder, please refer to the SRS Labs, inc. publication "Circle Surround Principles of Operation".