Sometimes, "Downmixing" is also used in radio communication to describe a process that brings an IF signal down to baseband via demodulation with a complex carrier frequency.
Stereo downmixes/(or fold-downs)
Left total/Right total (Lt/Rt)
Lt = L + -3dB*C + -3dB*(-Ls -Rs)
Rt = R + -3dB*C + -3dB*(Ls + Rs)
(where Ls and Rs are phase shifted 90°)
Left only/Right only (Lo/Ro)
Lo/Ro is a downmix suitable when mono compatibility is required. Lo/Ro destroys front/rear channel separation information and thus a Dolby Pro Logic upmixer will not be able to properly extract 5.1 channels again.
Lo = L + -3dB*C + att*Ls
Ro = R + -3dB*C + att*Rs
(where att = -3dB, -6dB, -9dB or 0)
- "Dolby Surround Pro Logic II Decoder Principles of Operation". Dolby Laboratories, Inc.