This guest post comes from Barry Gardner, mastering engineer at SafeandSound online mastering.
In an ideal world once music has been mixed it would simply be committed to cd or uploaded ready for distribution. In some rare occasions this does happen, though it is fairly uncommon. In any event people who are very accomplished mix engineers still appreciate the fresh ears of the mastering engineer and final quality control before the music is released to the general public. However, there does seem to be a skewed understanding of what mastering is and what it can do. It certainly is not mixing which is arguably the most important stage of audio production.
At this stage it is very important that the overall balance has been performed at the best of the mixers ability. Vocal balance and all the intricate dynamic interplay between musicians should have been well controlled to present a clear and focused sound that articulates the musical message. Mastering music relies very much on a good mix down. It is at the mastering stage that the stereo 2 track mix is worked upon to try and squeeze every last bit of quality from the mix. This could involve adding depth, air, sheen, warmth, perceived volume, clarity and punch and making tweaks to the stereo image is required. These processes when added correctly can very much improve and enhance the final results. However mastering is not normally capable of adjusting mix balances more than 0.5dB -1dB without detrimental effects. This would be performed using either mid and sides compression, eq or gain/attenuation so it is vital that the main balances are spot on so the mastering engineer can make the subtle, accumulative tweaks which add up to a larger subjective improvement to the audio. Where possible employing a professional mix engineer would ensure that the mix down was as good as it could be, the experience of a professional audio engineer should not be discounted.
These days many mastering engineers completely understand the lowered budgets that are encountered in the world of music production. As such many will offer a second set of fresh ears over a mix and provide a little advice or suggest tweaks in the mix down to ensure better mastering results. This benefits the band, musicians or producer and of course the mastering engineer through better end results. In most instances the engineer will provide this service on the basis that the job is proceeding. (normally taking payment up front) Then the mastering engineer will have a listen to the mix down before proceeding with mastering processes. Mixes come in all shapes and sizes and it will be normal to suggest any very obvious problems be adjusted in the mix. A judgment will have to be made on the quality of the mix as to how deep a mix appraisal should be given. There is no point in opening a can of worms that a novice mixer cannot resolve through lack of technical know how. In any event when a mastering engineer provides a mix appraisal the musician/mixer should save a copy of the original mix down in case something goes awry when changing a mix balance. If you need to brush up your mix skills there are numerous great books available to help you with mixing in a small studio environment.