First things first. Stems, also known as sub-mixes, subgroups or busses involve a group of audio sources that are mixed together. A stem can exist in mono, in stereo or in multiple tracks for surround sound. When talking specifically about sound mixing for film, stems are usually used to facilitate the final mix. In order to achieve the desired final mix, engineers join together to dialogue, music and sound effects as independent items. This process is known as D-M-E. The whole process requires a certain order. First of all, engineers use the dialogue stem when creating and editing the trailer of a film. Then, they add music and effects to the sequence.
Stem mastering is a technique derived from stem mixing where engineers arrange the individual audio tracks and join them together. By doing this, users are able to have independent control and signal processing of each stem and they are able to handle each element by itself. Stem mastering is a result of sub-mixes known as stems. When all the stems are played at the same time, a complete mix emerges. When stems are offered, the user has total control over the mix. Stem mastering is frequently used for complex productions or for productions than have too many restrictions. Stem mastering is usually expensive because it requires more time and special knowledge. Stem mastering has become popular since it allows the user to make alterations to each of the elements involved in the mix before applying the final compression.
Stem mixing is a technique which involves mixing audio material obtained from different groups of audio tracks and separating them before merging them into the final mix. A stem and a separation are quite similar. For some, stem manipulation and stem mastering are frequently considered practically the same. For others, stem are considered sub-mixes used for separation mastering. Apparently, the distinction lies within how many separate channels of input are available for mixing and/or at which phase they are towards reducing them to a final stereo mix. In stem mixing, users group audio tracks that have similar properties, into stems or tracks to allow an individual signal processing for each stem.
A little bit of history. The stem technique originated around the same time as mixing boards. From the very beginning, stems were capable of signing individual inputs to subgroup faders and to work with each stem mix independently. The stem technique is widely used to control, process and manipulate entire groups of instruments, as well as to streamline and simplify the mixing process. In order to achieve a variety of effects, engineers can route the stem-mix through its own signal processing chain. Within the music production universe, specialists use DAWs to digitally process and manipulate individual groups of audio tracks through plugin chains.