Working with many independent producers, I have noticed that sometimes there is a bit of confusion about the differences between Stereo and Stem Mastering.
In this article, I'm going to explore the signal flow of these ways of processing audio stereo mastering vs stem mastering.
For this article, I've taken some screenshots of Logic Pro X only for the demonstration purpose. The same signal flow can be reproduced with any DAW.
STEREO vs STEM MASTERING...WHAT IS THE DIFFERENCE?
SHORT ANSWER:
From the mastering engineer's point of view, the real differences between stereo and stem mastering are signal flow and flexibility.
Let's start from the beginning
Mastering is the final step of the music production process. At this stage the mastering engineer will look after the final polishing of the song, correcting some minor problems if needed, applying an overall enhancement to the song, and getting it ready for distribution.
We talk about stereo mastering when a client provides the L+R mixdown (one stereo file).
We talk about stem mastering when instead the client provides multiple stems.
But...what are stems?
As I've explained in the article STEM MASTERING: HOW TO PREPARE THE STEMS stems are multiple stereo tracks containing consolidated groups of instruments of the same mix. In this article, I've also explained when you should go for stem mastering and how a mix can benefit from it (it might be worth to have a read).
Stereo Mastering Signal Flow
From the stereo mixdown, the mastering engineer will apply his/her mastering chain. This can be digital, analog, or a mix of both. Usually, this process includes equalization, compression, and limiting, but can also include other kinds of processes like saturation, multiband compression, mid-side processing, etc.
Stem Mastering Signal Flow
Providing different stems will allow the mastering engineer to work more in detail. In fact, before going to the main mastering chain, s/he'll be able (if needed) to work on each stem to improve at a deeper level some aspect of the mix like the intelligibility, depth, frequency distribution, and dynamic.
At this level is also easier to fix problems potential problems like muddiness or resonances for instance. The mastering engineer, in fact, can work only on the problematic groups of instruments leaving the rest unchanged. The overall texture, glue, and enhancement are then applied to the main mastering chain.
Conclusion
As you can imagine, stem mastering is more expensive but also give much more flexibility and the ability to work on a deeper level. That said if you have a good mix there is no reason to go for the stem mastering and a common stereo mastering process will do the job excellently. On the other hand, if you are not 100% sure of your mix and you feel it can be improved at the mastering stage it is worth talking with your mastering engineer and discuss with him the possibility of stem mastering.