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General Internet Engineering Task Force audio matroska stems djing This document defines a multi-track profile of the Matroska container format for distributing stems. It is intended to be used by DJ applications, Digital Audio Workstations, and multi-track recorders while remaining backwards compatible with existing media players.

Introduction Stems are recordings of individual instruments, or clusters of instruments, used by DJs and music producers when mixing music. Historically stems have been stored as individual audio files, or using patent-encumbered, proprietary container formats. In the context of Digital Audio Workstations (DAWs), trackers, and multi-track recorders, the term "stems" is used to mean the audio data for an individual track before or after printing the result of effects, EQ, compression, or other plugins that have been applied to the track. It is often desirable to save the stems alongside the final mix for archival purposes, or as an intermediary step after printing effects and automation before importing them into a different DAW to continue work on the final production. Many DJs also perform live remixing of music using stems, however, when performing live it is normally beneficial to keep track of fewer instruments at one time. Most DJ software limits the number of stems to 4, meaning that stems in the DJ context are often a mixdown of similar elements in the final track (ie. "stem" in the context of DJing is the output of a bus or a submix of the "stems" from the DAW context). For example, a DJ may have stems for bass that is actually a sub-mix of the bass guitar and a bass-like sythesizer, a drum track that is a mix of the 5 individual mics used to record the drum kit as well as a mix of the hand drums used later in the song, a vocals stem with a single vocal mic, and a Melodies stem with a mix of the various primary instruments that carry the melody and harmonies. A common feature of modern software used by DJs is "dynamic" or "live" stem separation where the DJ software attempts to algorithmically separate the audio signals in a track to allow the DJ to mute, solo, or apply effects to individual instruments. The results of such dynamic separation vary but are, generally speaking, noticeably different from the original stems used by the producer and frequently contain distortions and other artifacts that sound undesirable. For DJing, a better model is to have the producer release a collection of prepared stems along with the original track. This allows the final mix to sound closer to the producers original vision for the track, even while it is being remixed and re-interpreted by a DJ or another artist. This specification documents a profile for the Matroska container format that allows it to store the final mix for a track alongside the lossless or lossy stems used to mix the track, as well as separate stems meant for DJing, all in a single file. The target consumer of these stem files are DJ applications meant for live remixing and performance, as well as DAWs used by producers who want their music to be remixed or archived.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Versioning Files that meet the criteria of this document MUST have a global UTF-8 tag with the name "STEM_VERSION" with the value "1.0". The presence of this tag indicates that this is a stem file and the value indicates compatibility with this document. The value may or may not be changed in the future if new versions of this format are created.

Track Layout

Audio Streams Each stem file may contain an arbitrary number of audio tracks and stem files meant for use by DAWs MUST include at least two audio tracks (the mixed audio and at least one stem). Stem files meant for DJ use MUST include at least three audio tracks (the mixed audio and at least two stems). Stem files meant for DJ use SHOULD contain four or fewer stem tracks. For ease of decoding each track SHOULD be encoded using the same codec with the same parameters including bitrate, and sample rate. Stems are often recorded with a single channel and only the final mix is in stereo. For stem files that are meant to be re-mixed by a DAW this is fine, but DJs may want to maintain a similar balance and channel layout to the original track. Stems meant for DJ use MAY have a different channel count or layout than the main audio track, however it is RECOMMENDED that all stem tracks meant for DJ use maintain the same channel count and layout as the main track and have the same channel balance as their component parts in the final mix. For example, if the final mix is a stereo track that contains a fiddle that is 75% in the right channel and only 25% in the left channel, the stem track for the fiddle would also be in stereo with the stem mostly appearing from the right channel as in the final mix. The first track containing audio data MUST be the final post-mix audio in the default language (the mixdown track). All mixdown tracks regardless of language MUST have the Matroska "Default" flag set to "1" (, ). This helps preserve backwards compatibility in media players which do not support this format which typically play the first audio stream found or may select based on the default flag. In addition, the "Enabled" flag for any mixdown tracks MUST be set to "1" (). The remaining audio tracks will be individual stems and MUST have the same effective length as the mixdown track such that playing each stem track from the beginning would result in roughly the same audio (excluding the final stages of mastering) as the final mix present in the mixdown track. For example, if the original track is three minutes long and the stem file includes a percussion track but the percussion does not start until minute two, the percussion stem would still be three minutes long but would contain a minute of silence at the start of the track, or would have a block timestamp () that sets the effective start time to one minute. In addition, for stems meant for DJ use SHOULD have the same number of samples as the final mix to make seeking and syncing various combinations of stems easier on the DJ software. Each stem track MUST have the Matroska "Default" flag set to "0" and MUST have the "Enabled" flag set to "0". When creating the file the stem tracks SHOULD NOT have any intra-stem gain normalization applied to bring the stems up to the same perceived volume relative to each other. Instead, stem tracks meant for DJing should retain the same relative levels as they would have in the final mix present in the mixdown track so that if all stems were played at unity gain the overall level would be roughly equivalent to the level of the final mix. Steam tracks meant for archival and production use SHOULD be exported from the DAW or recorder as-is with no additional changes and MAY be exported before or after printing effects and the result of plugins. On playback, DJ software MAY choose to normalize the gain on any combination of stems currently being played to make it equivalent to the mixdown track or any other tracks being mixed in even if some stems are muted or have their individual gains adjusted. However, the exact mixing behavior of DJ applications is outside the scope of this specification.

Stem Metadata

Use For each stem track a binary tag () MUST be set with its target set to the stem track and a tag name of "STEM_USE". The value of this tag is an unsigned 64-bit integer and MUST be one of the following values: Defined Stem Uses

Value	Description
0x00	Stems meant for live playback by DJs that may contain a mix of various instruments and sounds.
0x01	Stems output from multi-track recorders or DAWs containing individual tracks that have been output before any effects, EQs, or other plugins.
0x02	Stems output from multi-track recorders or DAWs containing individual tracks that have been output after printing any effects, EQs, or other plugins.

Color For each stem track a tag MAY also be set with its target set to the stem track and a tag name of "STEM_COLOR". The tag value must be a string in RGB hex format set to a color representing the stem (ie. "#145374").

Name Each stem track MUST set the value of the track "Name" element () to a short, human-meaningful, track name for the stem that describes its contents, for example "Percussion" or "Vocals". These names are intended for display in playback applications and therefore should remain concise, but no specific format is defined. The track Name element MAY also be duplicated or overridden as a tag, in which case the order of precedence from SHOULD be respected. The maximum length of the name string (whether in the "Name" field or a tag) MUST NOT be longer than 20 bytes.

Type The free-text "Name" field provides a label that is backwards compatible with existing software that supports Matroska, but does not necessarily provide enough information for DJ software implementing this format. Some software uses pre-defined stems (ie. always has "Vocal", "Melody", "Bass", and "Drum" stems), but the name is free-text and may not exactly match the name of the specific stem in that software. Or the software may want to display icons that represent each stem, or display translations in a language that is not already embedded in the files "Name" tags. For example, in most tracks with words there might be a stem with its name set to "Vocals" but another track may call it "Singer" or a traditional called folk dance track might name it "Caller". Software for stem playback might want to show a Microphone icon next to each of these tracks. To help map stems to individual icons or controller buttons, a binary tag with the name "STEM_TYPE" and a target of the stem track MAY be added to the file. The value of this tag is an unsigned 64-bit integer from the Matroska Stem Types Registry. The following initial values are defined: Defined Stem Types

ID	Recommended Name	Description
0x00	Other	Stems that contain a mix of other sounds that are not covered by the other stems in the file.
0x01	Vocals	Stems that contain sung vocals.
0x02	Spoken Word	Stems that contain spoken vocals including traditional dance calls, or spoken words over a tune.
0x03	Melody	Stem that contains the melody (regardless of instrument).
0x04	Bass	Stems that include bass elements (regardless of instrument).
0x05	Percussion	Stems containing percussive elements.
0x06	Synth	Stems containing synthesizers.
0x07	Piano	Stems containing piano or keyboard tracks.

Mastering Because the final stages of mastering happens post-mix and the stems are pre-mix audio the stem tracks SHOULD NOT have the final post-mix mastering steps applied (including any final compression or limiter steps). This means that a DJ playing the track using the stem tracks instead of the mixdown track will result in different audio from the final mix. This is deemed an acceptable trade off since the final sound of the DJs version of a track is likely to be significantly different from what the original track producer had created either way. Even without the final mastering steps this method still gives the producer more control over the final sound than if the DJ were to use an auto stem separation algorithm. Other mastering steps MAY be applied to the stems as well as to the final mix at the mastering engineers discretion.

Format Support The Matroska container format can store many types of audio, not all of which are suitable for DJing or music production. To ensure compatibility between playback and encoding applications the following formats SHOULD be supported depending on the use case of the software as shown in the following table. Formats with the use case "Live remixing" are intended largely for playback applications meant for live performance (ie. DJ software). Formats with the use case "Production" are intended to be distributed for remixing in a non-live setting (ie. with a DAW or music tracker). Files with the use case "Recording" are meant to be written by multi-track recorders and later read by DAWs and MAY only support one of the recommended formats (ie. support for various forms of PCM audio is not required for a field recorder which likely only has hardware for integer or float PCM data, not both). Audio codec support

Codec	Use Case	Codec ID
FLAC	Live remixing, Production	A_FLAC
Opus	Live remixing	A_OPUS
PCM, 32-bit float	Production, Recording	A_PCM/FLOAT/IEEE
PCM, integer little endian	Production, Recording	A_PCM/INT/LIT

IANA Considerations

Matroska Tag Names Registry This document modifies the "Matroska Tag Names" registry defined in to add the following values: Additions to the "Matroska Tag Names" Registry

Tag Name	Tag Type	Reference
STEM_VERSION	UTF-8	This document,
STEM_COLOR	UTF-8	This document,
STEM_TYPE	binary	This document,

Stem Types Registry IANA will create a new registry called the "Stem Types Registry" which will store a list of specific stem types which may be mapped to localizable strings, icons, etc. by stem file playback software. This registry is not meant to be an exhaustive list of possible stem types. Instead, it is meant to contain commonly used general stem names to provide playback software a hint at how a specific stem might be displayed. To register a new Stem Type in the registry a unique unsigned 64-bit integer ID, a recommended name, and an optional reference to a document describing the stem type are required. The initial values of the Stem Types Registry are as follows: Initial Contents of the "Stem Types" Registry

ID	Recommended Name	Reference
0x00	Other	This document,
0x01	Vocals	This document,
0x02	Spoken Word	This document,
0x03	Melody	This document,
0x04	Bass	This document,
0x05	Percussion	This document,
0x06	Synth	This document,
0x07	Piano	This document,

Security Considerations This document inherits security considerations from both and . It does not have additional security considerations.

Normative References Informative References

Acknowledgements Thanks to the members of #matroska on the libera.chat IRC network, and to mosu and JanC in particular, for patiently explaining the basics of the format to me and for all their feedback. Thanks also to the members of the Ardour forums for their feedback on DAWs and mastering. Thanks to the developers of Mixxx and the members of the Mixxx forums for pointing out the potential pain points for DJ software. Finally, thanks to the members of the IETF CELLAR working group, especially Steve Lhomme, for their feedback.