Mid/side (M/S) coding is the most common form of joint stereo: instead of storing left and right, the encoder stores a mid channel (the sum, L+R) and a side channel (the difference, L-R). For most material the two real channels are highly similar, so the side channel is small and compresses very efficiently, freeing bits that improve overall quality at no cost to the stereo image — the original L and R are recovered exactly by simple addition and subtraction. M/S is reversible and lossless in itself; the savings come from how little data the side channel needs. The same sum/difference idea appears in microphone technique and in mastering tools that process the centre and sides separately.
The book · Volume 4 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From PCM and Codecs to Loudness, Accessibility, and Real-Time Voice Processing
Mid/side coding is how most stereo codecs save bits on music, and it is also why phase problems in a mix can turn into odd artifacts after encoding. This volume explains mid/side coding in depth with joint stereo, channel layouts and the AAC and Opus encoders that switch it on, so you can predict how stereo content survives compression. Written by Nikolay Sapunov, CEO at Fora Soft.
Stereo mixes sounding phasey or hollow after encoding with mid/side coding?
Send us affected clips and encoder settings and we will pinpoint the mid/side behavior behind the artifacts and suggest settings that fix it. 250+ video products shipped since 2005.
