Most musicians and bands are aware of the benefits of professional audio mastering, the final stage where a mastering engineer with considerable experience will enhance, quality control and optimize a piece of music before it’s release. Mastering is usually performed using a 24 bit stereo interleaved file, the result of the producers/engineers final mix down. The mastering engineer can equalize, compress and adjust other parameters of the mix to aid translation across all the varying types of audio system the music will be played on.
Stem mastering is a type of mastering that differs in that it uses ‘groups’ of instruments such as bass, guitars, drums, keyboards, vocals. There are a number of advantages to stem mastering. The ‘collections’ of instruments allow the mastering engineer to create a master with more targeted corrections and enhancement to the equalization, dynamics, depth and balance of the instruments in mix down.
Stem mastering does not extend itself to full mix down, mix down itself uses copious fader automation, pitch correction, delay based effects processing such as delay, echo, reverb, flanging and phasing. If the mastering engineer has chosen his tools in a prudent manner the depth and dimension that stem mastering can add to the mastering process is very valuable. The advantages can be clear when it comes to overall end result. It is an option to be considered if there are issues of balance that the producer, engineer or musician cannot nail without some additional assistance. It can also benefit ‘in the box’ computer productions that may need an injection of character, thickness and warmth.
When you prepare for stem mastering you need to ensure that all the groups (or stems) are of exactly the same length. In your digital audio workstation you set locators at the beginning and end of the tracks (ideally with a 2 second pre and post roll at each end of the bounce/export to avoid cutting off reverb tails etc.) The engineer will require 24 bit files and they should be stereo interleaved files. This way when the mastering engineer receives the files they can be imported and lined up on multiple tracks so they will all play perfectly in synchronization.
Ideally when you have exported your stems you would import them into your own sequencer and check that they sound the same as your mix down. If so they are ready to send to the mastering engineer. Some people may have been mixing into a limiter which causes a complication because stems need to be unlimited when they are exported (preserving the punch and dynamics of the tracks). The originally summed mix down will have been affected globally by the limiter on the stereo master output. If you were to export the stems through a limiter vie the stereo output (as is often the case) each stem will be limited in a different way because the stems are exported in isolation.
In this case I recommend exporting the stems without the limiter and also provide a stereo limited mix down so it is possible to gauge how much limiting was applied to the overall mix, this gives some insight to what was being heard during mix down and can help identify any specific problems you may have been having. Mixing into a limiter is not at all ideal but it provides another layer of information during mastering if a stereo limited mix is provided along with the stems at the time of stem mastering.
Mastering enginers have various professional engineering backgrounds and it is comforting to know that your engineer has had plenty of mixing experience when considering whether to try stem mastering for your music projects.
Barry Gardner operates SafeandSound Mastering and has extensive professional recording, mixing and mastering experience (now focusing on mastering as sole income) and offers stem mastering services to add extra depth, clarity and dimension during the mastering process.