downmixing

Downmixing is the process of reducing the number of audio channels in a multichannel recording or soundtrack to a smaller number of channels, most commonly from 5.1 or 7.1 surround sound to stereo. It is used to enable playback on devices with fewer speakers or on formats that do not support multichannel audio, such as many headphones, portable players, or standard television speakers.

Technique and practice: Downmixing typically uses a fixed downmix matrix that linearly combines the channel signals with predefined gains. For a stereo downmix, the left and right channels are preserved, while the center channel is usually distributed to both stereo channels and the low‑frequency effects channel is routed to the main channels at a reduced level. Other surround channels may be folded into the stereo pair as needed. Some codecs or playback systems also carry metadata that influences the preferred downmix or adapts it to the listening environment. Real-time downmixing may be performed by consumer electronics receivers or software players, while pre-downmixed copies are created during mastering for distribution.

Applications and considerations: Downmixing is common in film and television distribution, streaming, and gaming, enabling broad compatibility across devices. Potential downsides include loss of spatial cues, changes in perceived balance, and possible phase interactions or center-channel localization shifts. Good practice aims to preserve essential content and mono compatibility, while recognizing that some depth and directionality may be reduced when more channels are collapsed into two. Downmixing contrasts with upmixing, which attempts to recreate higher channel counts from stereo.