Downmixing is a general term used for manipulating audio

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 where a number of distinct audio channels are mixed together 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

Frequency mixer

In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...

 with a complex carrier frequency.

Example

Many DVD player

DVD player

A DVD player is a device that plays discs produced under both the DVD-Video and DVD-Audio technical standards, two different and incompatible standards. These devices were invented in 1997 and continue to thrive...

s and Personal computer

Personal computer

A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

 sound card

Sound card

A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs. The term sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware...

s can downmix multichannel audio (e.g. 5.1 surround sound

Surround sound

Surround sound encompasses a range of techniques such as for enriching the sound reproduction quality of an audio source with audio channels reproduced via additional, discrete speakers. Surround sound is characterized by a listener location or sweet spot where the audio effects work best, and...

) to stereophonic sound

Stereophonic sound

The term Stereophonic, commonly called stereo, sound refers to any method of sound reproduction in which an attempt is made to create an illusion of directionality and audible perspective...

 (2 channels) for output via two speakers.

Left total/Right total (Lt/Rt)

Lt/Rt is a downmix suitable for decoding with a Dolby Pro Logic

Dolby Pro Logic

Dolby Pro Logic is a surround sound processing technology developed by Dolby Laboratories, designed to decode soundtracks encoded with Dolby Surround. Dolby Stereo was originally developed by Dolby in 1976 for analog cinema sound systems...

 upmixer to obtain 5.1 channels again.
Lt/Rt is also suitable for stereophonic sound

Stereophonic sound

The term Stereophonic, commonly called stereo, sound refers to any method of sound reproduction in which an attempt is made to create an illusion of directionality and audible perspective...

 playback on a hi-fi or on headphones

Headphones

Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...

 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°)

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

Dolby Pro Logic

Dolby Pro Logic is a surround sound processing technology developed by Dolby Laboratories, designed to decode soundtracks encoded with Dolby Surround. Dolby Stereo was originally developed by Dolby in 1976 for analog cinema sound systems...

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)