The Dolby MP Matrix was the professional system that encoded 4 channels of film sound into 2. This track used by the Dolby Stereo theater system on a 35mm optical stereo print and decoded back to the original 4.0 Surround. The same 4 channel encoded stereo track was largely left unchanged and made available to consumers as "Dolby Surround" on Home Video. However, the original Dolby Surround decoders in 1982 were a simple passive matrix 3 channel decoder : L/R and Mono Surround. The surround was limited to 7kHz. It also had Dolby Noise Reduction and an adjustable delay for improved channel separation and to prevent dialog leaking and arriving to listeners' ears first. The front center channel was equally split between the left and right channels for phantom center reproduction. This differed from the Cinema Dolby Stereo system which used active steering and other processing to decode a center channel for dialog and center focused on screen action. Later on in 1987, the Pro Logic decoding system was released to consumers. It featured virtually the same type of 4 channel decoding as the Dolby Stereo theater processor with active steering logic and much better channel separation (up to 30dB) as well as including a dedicated center channel output for the first time. Many standalone Pro Logic decoders also included a phantom center option for compatibility with earlier non-Pro Logic Dolby Surround equipped home theaters to split the center channel signal to the L/R speakers for legacy phantom center reproduction.