Matroska Stem Files
draft-swhited-mka-stems-08
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| Document | Type | Active Internet-Draft (individual) | |
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| Author | Sam Whited | ||
| Last updated | 2026-04-04 | ||
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draft-swhited-mka-stems-08
Internet Engineering Task Force ssw. Whited, Ed.
Internet-Draft Independent
Intended status: Informational 4 April 2026
Expires: 6 October 2026
Matroska Stem Files
draft-swhited-mka-stems-08
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 and Digital Audio Workstations 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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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 6 October 2026.
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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Track Layout . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Audio Streams . . . . . . . . . . . . . . . . . . . . . . 3
4. Digital Signal Processor . . . . . . . . . . . . . . . . . . 5
4.1. Compressor Metadata . . . . . . . . . . . . . . . . . . . 5
4.2. Limiter Metadata . . . . . . . . . . . . . . . . . . . . 6
5. Format Support . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . 8
9. Informative References . . . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Stems are recordings of individual instruments, or clusters of
instruments, used by DJs and music producers for live mixing of
music. Historically stems have been stored as individual audio
files, or using patent-encumbered or vendor specific, proprietary
container formats.
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. A better model is to have the producer release the
original stems and information about the mastering alongside 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 interpreted by the DJ or another remixer.
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 in a
single file. In addition it specifies metadata for storing mastering
information so that remixes using the stems can remain as close as
possible to the producer of the tracks original intent. The target
consumer of these stem files are DJ applications meant for live
remixing and performance, as well as Digital Audio Workstations
(DAWs) used by producers who want their music to be remixed.
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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. Requirements
STEM files have a few basic requirements including:
* Backwards compatibility with existing media players,
* The ability to store multiple audio track,
* The ability to store file-level metadata and track-level metadata,
and
* Backwards compatibility when additional tracks have unknown
formats that cannot be decoded.
3. Track Layout
3.1. Audio Streams
Each stem file may contain an arbitrary number of tracks containing
audio and MUST include at least three audio tracks (the mixed audio
and at least two stems). For stem files meant for live DJ use, it is
RECOMMENDED that four or fewer stem tracks be used (as opposed to
stem files meant for music production or non-live remixing where a
DAW may utilize a significantly larger number of 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. Stems MAY have a different channel count or layout
than the main audio track, however it is RECOMMENDED that all stem
tracks 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.
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The first track containing audio data MUST be the final post-mix
audio in the default language. All tracks containing the final post-
mix audio 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 main 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 first track such that playing each stem
track from the beginning would result in the same audio (excluding
mastering) as the final mix present in the first 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.
Each stem track MUST have the Matroska "Default" flag set to "0" and
MUST have the "Enabled" flag set to "0".
The stem tracks SHOULD NOT have any gain normalization applied to
bring the stems up to the same perceived volume. Instead they should
retain the same levels as they would have in the final mix present in
the default track so that if all stems were played at unity gain the
overall level would be equivalent to the level of the final mix.
Each stem track (ie. all tracks that are not the first 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 (generally no more
than one word), but no specific format or length requirement is
defined. The track Name element MAY also be duplicated or overriden
as a tag, in which case the order of precedence from Section 24.1 of
[RFC9559] SHOULD be respected.
For each stem track a tag ([RFC9559], Section 5.1.8) SHOULD 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).
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4. Digital Signal Processor
Because mastering happens post-mix and the stems are pre-mix audio
the stem tracks SHOULD NOT have any mastering steps applied.
Instead, metadata for configuring a compressor and limiter SHOULD be
included in the file's global metadata as simple tags (see
Section 5.1.8.1.2 of [RFC9559]). After mixing, playback applications
MAY choose to feed the mix through a Digital Signal Processor (DSP)
configured with the limiter and compressor settings read from the
metadata.
Each binary setting for the compressor or limiter is stored as a
floating-point number in the 32-bit and 64-bit binary interchange
format, as defined in [IEEE_754_2019] with the additional restriction
that they are limited to a minimum value of 0.0 and a maximum value
of 1.0. Because different DSPs may use different ranges or scales
for each value the playback software SHOULD interpret the 0-1 values
as a linear scale and map them to the range and scale required by the
DSP when configuring the DSP for playback. This may result in a loss
of fidelity on some DSPs, but this is deemed an acceptable trade off
for stem playback which would not normally be able to have a
mastering step at all.
During production of a stem track, vendor specific metadata MAY be
embedded in the Matroska file for more accurately configuring a
specific DSP, but if such metadata is included the scaled values
SHOULD also be present for those without access to the specific DSP
used for the track and such metadata MUST select tag names in such a
way that they do not conflict with the tag names defined for the
generic compressor or limiter.
4.1. Compressor Metadata
+========================+========+===================+
| Tag Name | Type | Values |
+========================+========+===================+
| COMPRESSOR_ENABLED | UTF-8 | "TRUE" or "FALSE" |
+------------------------+--------+-------------------+
| COMPRESSOR_RATIO | binary | 0.0-1.0 |
+------------------------+--------+-------------------+
| COMPRESSOR_OUTPUT_GAIN | binary | 0.0-1.0 |
+------------------------+--------+-------------------+
| COMPRESSOR_THRESHOLD | binary | 0.0-1.0 |
+------------------------+--------+-------------------+
| COMPRESSOR_ATTACK | binary | 0.0-1.0 |
+------------------------+--------+-------------------+
| COMPRESSOR_INPUT_GAIN | binary | 0.0-1.0 |
+------------------------+--------+-------------------+
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| COMPRESSOR_RELEASE | binary | 0.0-1.0 |
+------------------------+--------+-------------------+
| COMPRESSOR_HP_CUTOFF | binary | 0.0-1.0 |
+------------------------+--------+-------------------+
| COMPRESSOR_HP_DRY_WET | binary | 0.0-1.0 |
+------------------------+--------+-------------------+
Table 1: Compressor metadata tags
4.2. Limiter Metadata
+===================+========+===================+
| Tag Name | Type | Values |
+===================+========+===================+
| LIMITER_ENABLED | UTF-8 | "TRUE" or "FALSE" |
+-------------------+--------+-------------------+
| LIMITER_RELEASE | binary | 0.0-1.0 |
+-------------------+--------+-------------------+
| LIMITER_THRESHOLD | binary | 0.0-1.0 |
+-------------------+--------+-------------------+
| LIMITER_CEILING | binary | 0.0-1.0 |
+-------------------+--------+-------------------+
Table 2: Limiter metadata tags
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 "Music production" are intended to be distributed for remixing
in a non-live setting (ie. with a DAW or music tracker).
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+================+==================+==========================+
| Codec | Use Case | Codec ID |
+================+==================+==========================+
| FLAC [RFC9639] | Live remixing, | A_FLAC [RFC9639], |
| | Music production | Section 10.2 |
+----------------+------------------+--------------------------+
| Opus [RFC6716] | Live remixing | A_OPUS |
| | | [I-D.ietf-cellar-codec], |
| | | Section 3.4.32 |
+----------------+------------------+--------------------------+
| Raw PCM (IEEE | Music production | A_PCM/FLOAT/IEEE |
| float, little | | [I-D.ietf-cellar-codec], |
| endian) | | Section 3.4.33 |
+----------------+------------------+--------------------------+
| Raw PCM | Music production | A_PCM/INT/BIG |
| (integer, big | | [I-D.ietf-cellar-codec], |
| endian) | | Section 3.4.34 |
+----------------+------------------+--------------------------+
| Raw PCM | Music production | A_PCM/INT/LIT |
| (integer, | | [I-D.ietf-cellar-codec], |
| little endian) | | Section 3.4.35 |
+----------------+------------------+--------------------------+
Table 3: Audio codec support
6. IANA Considerations
This memo modifies the "Matroska Tag Names" registry to add the
following values:
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+========================+==========+============================+
| Tag Name | Tag Type | Reference |
+========================+==========+============================+
| STEM_COLOR | UTF-8 | This document, Section 3.1 |
+------------------------+----------+----------------------------+
| COMPRESSOR_ENABLED | UTF-8 | This document, Section 4.1 |
+------------------------+----------+----------------------------+
| COMPRESSOR_RATIO | binary | This document, Section 4.1 |
+------------------------+----------+----------------------------+
| COMPRESSOR_OUTPUT_GAIN | binary | This document, Section 4.1 |
+------------------------+----------+----------------------------+
| COMPRESSOR_THRESHOLD | binary | This document, Section 4.1 |
+------------------------+----------+----------------------------+
| COMPRESSOR_ATTACK | binary | This document, Section 4.1 |
+------------------------+----------+----------------------------+
| COMPRESSOR_INPUT_GAIN | binary | This document, Section 4.1 |
+------------------------+----------+----------------------------+
| COMPRESSOR_RELEASE | binary | This document, Section 4.1 |
+------------------------+----------+----------------------------+
| COMPRESSOR_HP_CUTOFF | binary | This document, Section 4.1 |
+------------------------+----------+----------------------------+
| COMPRESSOR_HP_DRY_WET | binary | This document, Section 4.1 |
+------------------------+----------+----------------------------+
| LIMITER_ENABLED | UTF-8 | This document, Section 4.2 |
+------------------------+----------+----------------------------+
| LIMITER_RELEASE | binary | This document, Section 4.2 |
+------------------------+----------+----------------------------+
| LIMITER_THRESHOLD | binary | This document, Section 4.2 |
+------------------------+----------+----------------------------+
| LIMITER_CEILING | binary | This document, Section 4.2 |
+------------------------+----------+----------------------------+
Table 4: Additions to the "Matroska Tag Names" 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>.
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[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>.
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>.
[IEEE_754_2019]
IEEE, "IEEE Standard for Floating-Point Arithmetic",
IEEE IEEE 754-2019, DOI 10.1109/IEEESTD.2019.8766229, 18
July 2019, <https://ieeexplore.ieee.org/document/8766229>.
[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-17, 15 February
2026, <https://datatracker.ietf.org/doc/html/draft-ietf-
cellar-codec-17>.
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.
Finally, thanks to the members of the IETF CELLAR working group,
especially Steve Lhomme, for their feedback.
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Author's Address
Sam Whited (editor)
Independent
Email: sam@samwhited.com
URI: https://blog.samwhited.com
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