Mid-Side Mastering Basics: Wider Stereo Image and Clearer Bass Frequencies

the mid-side technique can be used during mixing or mastering to help widen the stereo imaging of the track as well as separate the mix allowing you to fine tune the left and right channels separately from that of the center channel. This gives you the flexibility to tighten up the bass and vocals which are mixed down the center while giving the mix more stereo depth and shine. 

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