Dolby Pro Logic was introduced shortly after for playback of the center channel as well. Pro Logic II added left and right surround channels (5.1), and Pro Logic IIx added support for 6.1 and 7.1 ...

This evolved into Dolby Pro Logic, which could intelligently extract 5.1 surround sound from a regular stereo soundtrack. 1992's "Batman Returns" was the first film released in Dolby Digital ...