Dolby Surround Pro Logic is the full name that refers to the matrix surround format and decoding system in one. When a Dolby Surround soundtrack is created in post production MP Matrix, four channels of sound are matrix-encoded into an ordinary stereo (two-channel) soundtrack. The center channel is encoded by placing it equally in the left and right channels minus 3dB; and the surround channel is encoded using phase shift techniques for out of phase information (L-R). The surround channel was often used for ambient background sounds in the original recording, music scores and effects.