What is stem mastering?
Stem mastering is a variant of "classical" mastering, which is done from a track (most of the time stereo): the export of the mix. As its name suggests, stem mastering is based on stems, which are stereo tracks that group together several of your tracks. It will allow the audio engineer to have more flexibility in his work, to be able to make small corrections, to optimize and improve the sound quality of each stems if necessary. The result will inevitably be better! The interest is to group your tracks by "theme", such as vocal stem, percussive stem, bass stem, etc. We will go into more detail on the choice of stems a little lower....
Most of the time, the treatments applied to the mastering will - slightly or not - change the balance between the instruments (because of the compression applied), and modify the color of some elements (because of the equalization provided). These modifications are usual and desired, it is the principle of mastering, but often, the audio engineer must compromise between correcting defects and the repercussions that this will have on the other elements of the song. Thanks to the separation of the elements offered by the stems, many corrections will not lead to any or very few modifications on the rest of the instruments!
Do I need it?
If your mix is close to perfection in all its components (dynamic, frequency, space, etc.), then "classical" mastering is sufficient. On the other hand, if, as in 99% of cases, your mix suffers from some deficiencies, there is a good chance that during mastering, the audio engineer will have to make some compromises between correcting defects and obtaining the most perfect sound. In these cases, mastering by stems will often lead to a better result.
Some very frequent examples of problems encountered during a classical mastering by the audio engineer:
- - the vocals are too quiet, but using a multi-band compressor or equalizer also increases guitars and synths
- - the bass drum doesn't have enough weight (not enough low frequency), but the bass already has too much
- - the drums lacks spatialization, but everything else has enough of it
- - the voice has too many "sss", but adding a de-esser (even in mid-side) removes too much clarity from cymbals and charleys
- - guitars and synths have unnecessary deep low frequencies that interfere with the bass
- - ...
All these technical problems do not have a real solution in a "classical" mastering, compromises must be found and the sound potential of the song will be limited. With stem mastering, each problem can be fixed without affecting (or negligibly affecting) the other elements. The rendering of stems mastering will therefore undoubtedly be better.
Beware, however, not to confuse mixing and stem mastering ! In mixing, the working latitude is total, and many processes, automation, etc. are used. With stem mastering, stems are present to allow small corrections to be made to the mix, so traditional mastering processes are even more effective. A bad mix will not be compensated by stems mastering!
How to prepare your stems?
First of all, you must determine what your stems will contain, and how many to use. Concerning their quantity, generally 4 to 6 stems are more than enough, to propose more would end up being mixing ;). You still have to determine the components of your stems. In most cases, it will almost always be necessary to start with 4 basic stems:
- - "percussive" stem, containing the drums and all percussive elements
- - "bass" stem, containing the bass(s)
- - "vocals" stem, containing the vocals
- - "others" stem, which contains everything else!
With these 4 stems, you already give the audio engineer a lot of work latitude. It can happen that an instrument which is very present, and very much in the spotlight (such as a particular synthesizer, or a trumpet which answers the voice, etc.) can benefit from having its own stem. It's really up to you to decide, you know the most important artistic elements of your song.
Finally, you have to create the stems! To do this, disable all the effects you put on the "master" bus, then "group" the selected tracks for each stem (for example all vocals and their effects) into a "group" track on your console (physical or virtual). Once your 4 to 6 groups have been created, you will have to export each group separately in stereo, having previously used the "solo" button on each group. Each stem must start at the same time in order to be synchronized with the others.
To check your work, import your stems into an empty project on different stereo tracks, set to the same starting position. Start the playback and you should get the same rendering as a classic mix export: the sum of the stems recreates the entire song.
Stem mastering is very effective, for a price generally a little higher than classical mastering, and is particularly well adapted to the projects made in home studios, which often encounter mixing problems related to the poor acoustics of the room or the lack of experience of the mixer.