Upmixing algorithms extract spatial cues from stereo content and redistribute them to additional speakers. Center channel gets correlated (in-phase) content; surround channels get decorrelated (out-of-phase) ambience; height channels get spectrally-shaped decorrelated content. Dolby Pro Logic II, DTS Neo:6, and Dolby Surround Upmixer are common algorithms.
Upmixing quality varies: well-recorded stereo content upmixes convincingly; heavily compressed or phase-manipulated content can produce artifacts. Music upmixing is more subtle than film upmixing (where extracting dialogue to the center channel is the primary goal). Auro-3D and Dolby Atmos upmixers can also generate height information.