In multitrack recording, multiple sound sources are recorded from separate microphones to individualized audio channels. Downmixing is the process of combining some of those audio channels. This task, which is also known as fold-down, is performed by a mixing engineer.[citation needed]
Consumer electronics may also downmix automatically. For example, a DVD player or sound card may downmix a surround sound signal (four or more channels) to stereophonic sound (two channels) for playback through two-speakers.
Downmixing doesn’t just apply to audio signals. In radio communication, downmixing brings an IF signal down to baseband via demodulation with a complex carrier frequency.[citation needed]
Stereo downmixes / fold-downs
Left total /Right total (Lt/Rt)
Lt/Rt is a downmix suitable for decoding with a Dolby Pro Logic decoder to obtain 5.1 channels again. Lt/Rt is also suitable for stereophonic sound playback on a hi-fi or on headphones as it is.
Lt = L + -3dB*C + -3dB*(-Ls -Rs)
Rt = R + -3dB*C + -3dB*(Ls + Rs)
(where Ls and Rs are phase shifted 90°)[7]
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)
mixrecordstudio 