Mixing vs. Mastering
I would say the majority of people just starting to learn about audio engineering, or any musician just beginning to record their music in a professional studio, can’t tell you the difference between a “mix” and a “master.” Both are important parts of the process, but they are entirely different.
Most professional recordings are recorded through an array of microphones into large consoles that allow the separation of vocals and instruments into separate channels. This is called multi-channel recording. It’s important to keep the instruments and vocals separated so that the engineer can tweak the sound in each channel separately. For instance, the engineer may want the acoustic guitar to be panned slightly to the left speaker, the piano to the right, and the lead vocal equally balanced in the middle; and he may want to add a short delay to the guitar, a different special effect to the piano, and some reverb to the vocal; and he may elect to have the guitar and piano volumes about the same and the vocal to be a tad louder than the sum of the instruments, etc., and this process for all other channels that were recorded. None of these actions would be possible if these instruments or vocals occupied the same channel. The act of deciding what sonic tweaks will be implemented to each channel of an entire multi-channel recording is called mixing. The end result of mixing is a pleasant and interesting sounding 2-channel stereo recording. A typical multi-channel mix may take several days if not weeks to finalize, with listening sessions in between.
Once a mix is finalized – that is, everyone is happy with it -- the next step is to have the mix mastered. Mastering is usually (but not always) performed at a studio that specializes in this. The mastering engineer applies global compression and equalization to the final mix to help smooth out peaks and bring the overall track volume up to industry standards (“radio ready”). Mastering does not change the mix, it enhances the overall sonics, and is the final step before the songs are sent off for duplication or submitted for online distribution. Mastering a song usually takes less than one hour.