You are probably used to either receiving a stereo mixdown of the track you are to master, or you have exported your own project as a stereo track for mastering. Stem mastering is a more effective way of mastering. Instead of working with a stereo file you are working with multiple stereo files (also known as stems) which give you more means to process the track the way you wish to during the mastering process. I will be demonstrating how this is achieved in Ableton, but the process can be applied to any other DAW as well.
Step 1 '" Choose Your Stems
What first needs to be decided are what stems you are going to work with. I would recommend between 4-8 audio stems. Anything more then that might be a bit overkill for the mastering process and it might be required to return to the mix to adjust your mixdown. I usually work with the following audio stems:
- Sound Effects
The reason I keep the Kick and Snare separate from the other drums, is that sometimes after the mastering processors have been applied I find I need to change the volume and attack of the Kick or Snare. By having them separate I can process the Kick and Snare differently from the Drum Stem group. In Electronic genres the Kick and Snare are quite prominent and after the mastering processors are applied they can affect the character of the Kick and Snare. But every genre is different and you will need to assess what stems you wish to export out.
The audio stems.
Step 2 '" Treat it like a Stereo File
To start off, listen to your project as if it were only a stereo file. Start adding your mastering plugins to the master out, such as your Multi-Band Compressor, EQ, and Limiter, etc.
I have applied the following mastering effects chain to my Master channel: A low cut with the EQ Eight, next is some multi-band compression with Ableton's Multiband Dynamics plugin to. I have used the three frequency bands to apply different compression settings and volumes to the low, mid and high ranges. Next I have added a slight reverb to blend the elements together, and I have finished off with Ableton's Limiter to raise the overall gain of the track. You can also use Ableton's Spectrum plugin to visually analyze your frequency spectrums after the mastering processors.
Take a break, even a few days or so if you can. Them come back to your master with fresh eyes. Now listen to your stems and see what needs to be changed. The stems are only for subtle changes. I would recommend only slight volume, EQ and compressor changes. Nothing drastic like crazy modulation or huge reverb effects unless that's what you are after.
Step 3 '" Applying Processing to the Kick
Sometimes when you are mastering a stereo file you find that after applying the mastering processors your kick feels as if it has lost a bit of its presence and attack. With your Kick exported out separately now, you can adjust its volume separately in comparison to the other stems. You can also apply different compressor and EQ settings just to the kick so that it sits better with the other stems.
I wish to raise the volume of my Kick. I have added its own compression separate from the Drums stem. With this I can squash the Kick a bit more, and then raise its volume with the Gain parameter on the Compressor plugin. I have also added an EQ Eight to carve out some frequencies so the Kick sits better with the 'mastered' sound. I have low cut the frequencies below 430Hz, which opens up some space for the Bass to come through
Processing the kick separately.
Step 4 '" Vocal Stem Processing
Most mastering engineers recommend that when you provide them with a track you provide them with stereo files with different vocal volumes. One stereo file with the vocals levels as you want them; another with the vocals slighter lower (-1.5db); and one with the vocals slightly higher (+1.5db). The vocals can sometimes be affected in volume when mastering processors are applied across the stereo file. Vocals Stems alleviate this. Here you can adjust the vocal volume separate from the rest of the mix, and adjust the EQ and compression settings accordingly. This really gives you some flexibility when it comes to the mastering stage. Slight adjustments like this really do make the mastering process smoother. You don't always want to keep jumping back to your mix, as you may fiddle more with the mix than expected. Let's not get into the debate on 'When is a mix really done?'.
This is just the tip of the iceberg when it comes to Stem Mastering. There is really much more you can do with this process. Stem Mastering gives you more flexibility that cannot be found with a simply stereo mixdown file. Often during the mastering process elements get pushed back or brought forward too much. Stem mastering allows you to correct these issues without having to go jump back to the mix. It also allows for some creative decisions as well, which are not possible with the standard stereo file mastering process.
For more further mastering techniques check out these tutorial-videos: