Matroska Stem Files
draft-swhited-mka-stems-12

Internet Engineering Task Force                         ssw. Whited, Ed.
Internet-Draft                                               Independent
Intended status: Informational                            14 August 2026
Expires: 15 February 2027

                          Matroska Stem Files
                       draft-swhited-mka-stems-12

Abstract

   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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 15 February 2027.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Versioning  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Track Layout  . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Audio Streams . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Stem Metadata . . . . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Use . . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.2.2.  Color . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.2.3.  Name  . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.2.4.  Type  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Mastering . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Format Support  . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Matroska Tag Names Registry . . . . . . . . . . . . . . .   9
     6.2.  Stem Types Registry . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  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

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   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 [RFC9559] 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.

1.1.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  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.

3.  Track Layout

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3.1.  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" ([RFC9559], Section 18.1, 5.1.4.1.5).  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" ([RFC9559],
   Section 5.1.4.1.4).

   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 ([RFC9559],
   Section 10) that sets the effective start time to one minute.

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   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.

3.2.  Stem Metadata

3.2.1.  Use

   For each stem track a binary tag ([RFC9559], Section 5.1.8) 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:

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      +=======+====================================================+
      | 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. |
      +-------+----------------------------------------------------+

                        Table 1: Defined Stem Uses

3.2.2.  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").

3.2.3.  Name

   Each stem track MUST set the value of the track "Name" element
   ([RFC9559], Section 5.1.4.1.18) 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 Section 24.1 of [RFC9559] 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.

3.2.4.  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

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   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:

        +======+=============+====================================+
        | ID   | Recommended | Description                        |
        |      | Name        |                                    |
        +======+=============+====================================+
        | 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.                            |
        +------+-------------+------------------------------------+

                        Table 2: Defined Stem Types

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4.  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.

5.  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).

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    +==============+================+=================================+
    | Codec        | Use Case       | Codec ID                        |
    +==============+================+=================================+
    | FLAC         | Live remixing, | A_FLAC [RFC9639], Section 10.2  |
    | [RFC9639]    | Production     |                                 |
    +--------------+----------------+---------------------------------+
    | Opus         | Live remixing  | A_OPUS [I-D.ietf-cellar-codec], |
    | [RFC6716]    |                | Section 3.4.32                  |
    +--------------+----------------+---------------------------------+
    | PCM, 32-bit  | Production,    | A_PCM/FLOAT/IEEE                |
    | float        | Recording      | [I-D.ietf-cellar-codec],        |
    |              |                | Section 3.4.33                  |
    +--------------+----------------+---------------------------------+
    | PCM, integer | Production,    | A_PCM/INT/LIT                   |
    | little       | Recording      | [I-D.ietf-cellar-codec],        |
    | endian       |                | Section 3.4.35                  |
    +--------------+----------------+---------------------------------+

                        Table 3: Audio codec support

6.  IANA Considerations

6.1.  Matroska Tag Names Registry

   This document modifies the "Matroska Tag Names" registry defined in
   Section 6.1 of [I-D.ietf-cellar-tags] to add the following values:

         +==============+==========+============================+
         | Tag Name     | Tag Type | Reference                  |
         +==============+==========+============================+
         | STEM_VERSION | UTF-8    | This document, Section 2   |
         +--------------+----------+----------------------------+
         | STEM_COLOR   | UTF-8    | This document, Section 3.2 |
         +--------------+----------+----------------------------+
         | STEM_TYPE    | binary   | This document, Section 3.2 |
         +--------------+----------+----------------------------+

         Table 4: Additions to the "Matroska Tag Names" Registry

6.2.  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.

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   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:

         +======+==================+============================+
         | ID   | Recommended Name | Reference                  |
         +======+==================+============================+
         | 0x00 | Other            | This document, Section 3.2 |
         +------+------------------+----------------------------+
         | 0x01 | Vocals           | This document, Section 3.2 |
         +------+------------------+----------------------------+
         | 0x02 | Spoken Word      | This document, Section 3.2 |
         +------+------------------+----------------------------+
         | 0x03 | Melody           | This document, Section 3.2 |
         +------+------------------+----------------------------+
         | 0x04 | Bass             | This document, Section 3.2 |
         +------+------------------+----------------------------+
         | 0x05 | Percussion       | This document, Section 3.2 |
         +------+------------------+----------------------------+
         | 0x06 | Synth            | This document, Section 3.2 |
         +------+------------------+----------------------------+
         | 0x07 | Piano            | This document, Section 3.2 |
         +------+------------------+----------------------------+

          Table 5: Initial Contents of the "Stem Types" Registry

7.  Security Considerations

   This document inherits security considerations from both [RFC8794]
   and [RFC9559].  It does not have additional security considerations.

8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9559]  Lhomme, S., Bunkus, M., and D. Rice, "Matroska Media
              Container Format Specification", RFC 9559,
              DOI 10.17487/RFC9559, October 2024,
              <https://www.rfc-editor.org/info/rfc9559>.

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9.  Informative References

   [RFC6716]  Valin, JM., Vos, K., and T. Terriberry, "Definition of the
              Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
              September 2012, <https://www.rfc-editor.org/info/rfc6716>.

   [RFC8794]  Lhomme, S., Rice, D., and M. Bunkus, "Extensible Binary
              Meta Language", RFC 8794, DOI 10.17487/RFC8794, July 2020,
              <https://www.rfc-editor.org/info/rfc8794>.

   [RFC9639]  van Beurden, M.Q.C. and A. Weaver, "Free Lossless Audio
              Codec (FLAC)", RFC 9639, DOI 10.17487/RFC9639, December
              2024, <https://www.rfc-editor.org/info/rfc9639>.

   [I-D.ietf-cellar-codec]
              Lhomme, S., Bunkus, M., and D. Rice, "Matroska Media
              Container Codec Specifications", Work in Progress,
              Internet-Draft, draft-ietf-cellar-codec-20, 14 August
              2026, <https://datatracker.ietf.org/doc/html/draft-ietf-
              cellar-codec-20>.

   [I-D.ietf-cellar-tags]
              Lhomme, S., Bunkus, M., and D. Rice, "Matroska Media
              Container Tag Specifications", Work in Progress, Internet-
              Draft, draft-ietf-cellar-tags-25, 3 May 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cellar-
              tags-25>.

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.

Author's Address

   Sam Whited (editor)
   Independent
   Email: sam@samwhited.com

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   URI:   https://blog.samwhited.com

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