The Difference Between Stereo And Stem Mastering
This is a guest post fom Barry Gardner who operates SafeandSound mastering studio, explaining what stem mastering is and how it differs from standard stereo mastering.
Stereo Audio Mastering
The procedure of mastering is traditionally one of working with 2 track tapes or 2 track stereo interleaved files. The goal being ensuring a collection of recordings work well together and in their own right, tonally and dynamically and with a view to enhancement. Mastering engineers have developed techniques for working with stereo 2 track mixes that can enhance and correct audio, however there is fundamentally a limitation with working with a stereo file.
To explain some of the limitations of stereo 2 track mastering it is best to provide an example. Occasionally you might have a mix down that has a somewhat dark and warm sounding instrumental backing and an upfront bright, airy sibilant vocal. This can be problematic because a mastering engineer will want to brighten the backing track and yet reduce the high frequencies in the voice (by using equalization or de-essers). This is very difficult to do even with extremely precise tools and detailed monitors. This is a contradictory requirement which sometimes ends up meaning a compromise for the track must be found or it needs a re-mix. This is fine sometimes if there is available studio time at the producers and mastering engineers end. However if there is deadline looming then a compromise must be struck.
Stem Mastering (Or Seperation Mastering)
Stem mastering which is also known as separation mastering is an extension of the 2 track stereo mastering process. Stem mastering uses groups of instruments which are in common with each other. So typical stems from a track might be : horn section, percussion, drums, bass, vocals, backing vocals, synths1, synths 2. Stems provide the mastering engineer with an opportunity to make sightly more detailed and targeted adjustments to the music.
Is stem mastering just more mixing ?
Some mastering engineers see stem mastering as simply just more mixing. I do not personally agree with this. To me mixing is a very painstaking procedure which uses lots of automation (fader, send effects etc.) , vocal tuning, detailed fader riding and individual dynamic and tonal control of instrumentation. It is also the stage where the overall character of a piece of music is formed. Good mixing is not a trivial pursuit it takes hours and hours of refinement to ensure an excellent mix down is achieved. Often a vocal will have automation on every word to ensure it is 100pct levelled correctly and in balance with the music.Not everyone is a world class mix engineer and mixes are of course going to be completed to differing degrees of competency.
Stem mastering is not the same as mixing, there will generally be much less need for automation of events and instead the mastering engineer can focus on the job much as he or she would working with a 2 track mix. This approach affords the added flexibility of being able to respond to and adjust relative balances, correct tonal issues and alter dynamics (including de-essing if required) in context of groups of instruments as well as the mix itself. It is also possible to enhance the music by creating a slightly deeper sound field by the use of both analogue and digital tools. These tweaks can sum up to be greater than their individual parts and produce a more satisfactory end result. In addition the standard stereo 2 track processes can also be applied across the mix as required.
One very important issue before choosing a stem mastering service is to ensure you have made the best possible mix down. This mix should be “signed off” as being as good as you can get it. Then stems should be exported from a digital audio workstation at exactly the same length so they will play back in time when the mastering engineer receives them. Before sending stems they should combine (sum) to sound the same as your mix down. As a general rule stems should be created as un-limited, un-normalized stereo interleaved 24 bit .wav or .aiff files.
Does stem mastering always produce better results?
Not always, and before I suggest stem mastering myself I always listen to see if a track can be improved or not by stem mastering. It is very quickly apparent whether a track will benefit from stem mastering upon listening to the 2 track stereo mix. I will be listening to overall fidelity, depth, balance, quality, tonal response, dynamics vs. my own internal references for the genre of music at hand. If the end result is likely to be improved I may suggest it. A great mix with all the factors which make a good mix is highly unlikely to benefit from stem mastering. Stem mastering is usually quite a low percentage of a mastering studios throughput. I estimate stem mastering to be around 3% of my work the other 97% being stereo 2 track mastering work.
When approaching a stem mastering job the mastering engineer must make effective communications with a client in order to understand their needs and goals. It would be quite easy for the mastering engineer to end up in role of producer if he or she did not communicate clearly and listen to the goals of the artist, producer, band or record label. Some people benefit from significant input and some do not so these jobs are judged on a case by case basis.
Note from 2infamouz:
Barry Gardner is a reputable and experienced mastering engineer, and we highly recommend you visit his website if you need an album or other audio project mastered. You can visit his site at SafeandSound Audio Mastering.