Stem mastering vs stereo mastering: what is the difference?
Updated · onyr1x
Stereo mastering works from one finished stereo mix, while stem mastering works from a few grouped stems, such as drums, bass, music and vocals, that add up to that mix. Stem mastering lets the engineer adjust the balance between those groups. Stereo mastering keeps the approved mix intact and is the standard route.
What stereo mastering is
In stereo mastering, the mastering engineer receives a single stereo file: the final, approved mix. The work happens on that file as a whole. Typical tasks include EQ, compression, limiting, stereo width, setting the final loudness and preparing release files. On an EP or album, it also makes the songs sit well together.
Because everything is on one file, any change affects the whole mix. The engineer can brighten a mix or tighten the low end, but cannot turn up the vocal alone without also affecting everything that shares its frequency range. If a real balance problem turns up, the usual answer is to ask for a revised mix.
What stem mastering is
In stem mastering, the engineer receives a small number of stems instead of one stereo file. Each stem is a group of related parts, and together they add up to the full mix. A common split looks like this:
- Drums and percussion
- Bass
- Music: synths, keys, guitars and pads
- Lead vocal
- Backing vocals and ad-libs
- Effects, such as risers, reverb returns and transitions
Stem mastering is not a remix
Stem mastering is still mastering. The engineer works with a few broad groups, not the dozens of individual tracks a mixing engineer would have, so they can adjust the level and tone of a group but cannot rebuild a single part inside it. The groups are processed lightly and summed, and the result is mastered in the usual way.
The tradeoffs
Stem mastering gives more control. The engineer can bring the vocal up a little, reduce harshness in the cymbals without dulling the voice, or rebalance the kick and bass, all without a new mix. That helps when a mix is close but not quite there.
That control has costs. It takes more of the engineer's time and involves more decisions, and it blurs the line between mixing and mastering, so the balance you approved can change. Mix bus processing is the biggest technical catch. A bus compressor or clipper on the mix reacts to everything at once. When stems are exported with it switched off, they no longer sound exactly like the approved mix. When it is left on each stem, it reacts differently to each group than it did to the full mix. Either way the engineer has to rebuild that glue, and on styles where heavy bus processing is part of the sound, as it often is in hyperpop, that can change the character of the track.
Stereo mastering keeps the mix exactly as it was signed off, keeps the roles of mixer and mastering engineer clear and involves fewer files and fewer chances of export mistakes. It is how most records are mastered.
When to choose each one
Stem mastering tends to make sense when:
- You mixed the song yourself and are unsure about the balance between the main elements.
- One element, often the vocal or the low end, is clearly off and a remix is not practical.
- The mixing engineer is not available for revisions.
When stereo mastering is the better choice
Stereo mastering is usually the better route when:
- A mixing engineer mixed the song and you approved the result.
- Mix bus compression, saturation or clipping is an important part of the sound.
- You want the master to stay as close as possible to the mix you signed off.
A middle ground: revise the mix
If only one thing is wrong, the simplest fix is often a revised stereo mix rather than stem mastering. Some artists also ask for alternate mixes with the vocal slightly up or down, which gives a choice at mastering without stems.
How to prepare stems for mastering
If you choose stem mastering, the stems have to be exported with care, because the engineer will treat them as your mix.
- Use the same format for every stem: uncompressed WAV or AIFF, 24-bit or higher, at your session's sample rate.
- Export every stem from the same start point to the same end point, so they line up exactly.
- Turn off processing on the master bus for the stems, and tell the engineer what was there.
- Print each group's effects into its stem, or give shared reverbs and delays a stem of their own.
- Make sure nothing clips and that normalisation is off in the export settings.
- Send the stereo mix as well, with its master bus processing on, as the reference for how the song should sound.
Check that the stems add up to the mix
Before sending, import all the stems into an empty session at the same start point with faders at their default level, then play them against your stereo mix exported without master bus processing. They should sound the same. For a stricter check, flip the polarity of the stereo mix. If the stems match, the two cancel out and you hear little or nothing. Anything you do hear is missing or different in the stems.
How onyr1x can help
onyr1x is a music producer based in Almada, Portugal, across the river from Lisbon, working in cinematic hyperpop, art-pop and K-pop-leaning production. onyr1x offers mixing and mastering alongside custom production and finishing songs, remotely or in studio sessions in Almada.
If you are deciding between a stereo master and a stem master, send the song and a short note on what you want to change, and onyr1x will reply personally. Each project is quoted on request.