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Downmixing

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Downmixing

Downmixing is a general term used for manipulating audio where a number of distinct audio channels are mixed to produce a lower number of channels. It is also known as fold-down.

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.

Example

Many DVD players and Personal computer sound cards can downmix multichannel audio (e.g. 5.1 surround sound) to stereophonic sound (2 channels) for output via two speakers.

Stereo downmixes/(or fold-downs)

Left total/Right total (Lt/Rt)

Lt/Rt is a downmix suitable for decoding with a Dolby Pro Logic upmixer 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°)[1]

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)

See also

  • Upmixing

References

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