Mixing is the stage after the recording. It is the art of enhancing the sound of every element separately and combining all recorded tracks to fit in a sound field. Mixing Equipment includes EQs, compression, panning, reverb, delays, and other effects used for this purpose.
Mastering is the stage after Mixing. It is the art of enhancing the quality of the entire mix, taking it a step further sonically to be ready for release, making sure it sounds well through multiple playback systems and compatible in different formats.
In case of an album or EP, all the tracks will have a consistent perceived loudness, Insertion of ISRC Codes, PQ codes, CD-Text, DDP, Apple Digital Master Formats, etc.Click to know more about Mastering
A crystal clear AD/DA converter with professional (preferably passive) 3-way midfield speakers are used in Mastering so as to reflect all frequencies to mimic a typical home listening situation. Nearfield monitors are best suitable for recording and mixing studio (for precise instrument localization and reverb/effects judgment).
Analog high-end processors are used in Mastering for best sonic results ( quality, noise, headroom) . The gears are often custom-made and carefully chosen due to each Mastering engineer's taste (in tube sound and other technical features).
Digital plug-ins are rarely used in Mastering (only a few limiting is needed) whereas Mixing nowadays relies on digital processing (especially in home studios) with the settings and all tracks are recalled by a single click within the session.
A special DAW is used for Mastering with an extended algorithmic calculation and other features ( ISRC and PQ codes, CD-Text, DDPi delivery, formats...). Other DAWs are more suitable for mixing/editing.
Early reflections in Mastering are kept at minimum for a precise judgment without any barriers between the monitors and the engineer (an inclined ceiling is a plus). We often see a large console or desk in mixing studios which is important and practical (but has its drawbacks...).
A dedicated room treatment with different specs (NC, ER, Reverb Time) is needed for Mastering. The mixing room is often surrounded by machines/fans/AC. Mixing is sometimes done in a live concert with PA system (or even bedrooms!)
Mastering Engineer is trained through years to listen in a macro perspective, concentrating on the whole stereo track as one group. Whereas the Mix Engineer is trained to listen more in a micro perspective, concentrating more on each track, the balance between each track and group, and the stereo image.
A fresh set of Healthy ears with a creative mind are essential in Mastering to snatch any minor issue since this is the last stage before releasing the Music!
So, since Mastering is about enhancing the quality of the entire mix, wouldn't it be more logical to enhance the quality of this mix during mixing?