



Internet Engineering Task Force                         ssw. Whited, Ed.
Internet-Draft                                          20 February 2026
Intended status: Informational                                          
Expires: 24 August 2026


                             Ogg Stem Files
                       draft-swhited-ogg-stems-01

Abstract

   This document defines a multi-track profile of the Ogg container
   format for storing stems that is also backwards compatible with
   existing media players.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Bitstream Layout  . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Audio Streams . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Stem Metadata . . . . . . . . . . . . . . . . . . . . . .   3
     3.3.  DSP Metadata  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Mixing  . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Mastering . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Compressor Metadata . . . . . . . . . . . . . . . . . . .   5
     5.2.  Limiter Metadata  . . . . . . . . . . . . . . . . . . . .   6
   6.  Use with Ogg Skeleton . . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Stem are recordings of individual instruments, or clusters of
   instruments, used by DJs and music producers for live mixing of
   music.  Historically stem files have been stored as individual audio
   files, or using patent-encumbered or vendor specific proprietary
   container formats.  The Ogg file format developed by the Xiph.Org
   Foundation was formally specified in [RFC3533] and [RFC5334] and is
   ideally situated as a container for stems.  This specification
   documents a profile for the Ogg container format that allows it to
   store lossless or lossy stems as well as metadata about the stems for
   use in DJ applications or Digital Audio Workstations.

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:

   *  Backwards compatibility with existing media players




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   *  The ability to store at 5 audio tracks

   *  The ability to syncronize playback of multiple audio tracks

   *  The ability to store file-level metadata and per-stem metadata

3.  Bitstream Layout

3.1.  Audio Streams

   Each stem file may contain an arbitrary number of logical bitstreams
   containing audio and MUST include at least 3 streams (the original
   audio and at least two stems).  Each stream MUST be encoded using the
   same codec with the same parameters including bitrate, channel
   number, channel layout, and sample rate.

   The first logical bitstream containing audio data MUST be the final
   post-mix, mastered audio.  This helps preserve backwards
   compatibility in media players which do not support this format
   (which typically play the first audio stream found).  The remaining
   logical bitstreams will be individual stems and MUST have the same
   audio length as the first logical bitstream such that playing each
   stem stream from the beginning would result in the same audio
   (excluding mastering) as the final mix present in the first logical
   bitstream.

   For example, if the original logical bitstream is 3 minutes long and
   the stem file includes a percussion track but the percussion does not
   start until minute 2 the percussion stem would still be 3 minutes
   long but would contain a minute of silence at the start of the track.

3.2.  Stem Metadata

   The following tags MUST be stored in the Vorbis comment block
   encapsulated in the individual FLAC or Opus audio stream representing
   each stem.  Keys for these tags rae case insensitive.

      +============+===================================+============+
      | Tag        | Description                       | Example    |
      +============+===================================+============+
      | STEM:TITLE | Free text, used for the stem name | Percussion |
      +------------+-----------------------------------+------------+
      | STEM:COLOR | Color representing this track in  | #145374    |
      |            | RGB hex format                    |            |
      +------------+-----------------------------------+------------+

                                  Table 1




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3.3.  DSP Metadata

   For metadata that applies to all the stems it is not desirable to
   include it in the individual stream metadata blocks for several
   reasons:

   1.  In the absence of a standard many applications only store
       information on the first stream, but in the case of stems this is
       the one stream to which none of this metadata applies

   2.  Applications meant for writing general metadata may remove
       unknown values in the first streams metadata

   3.  Some stem metadata should be associated with all stem streams,
       but not the main mix stream and storing it on every stream is not
       ideal

   To work around these limitations stem files store metadata that
   applies to all stems (notably information about configuring a basic
   Digital Signal Processor or DSP) in a separate logical bitstream, the
   first packet of which is structured according to the following table:

       +=======+==================================================+
       | Data  | Description                                      |
       +=======+==================================================+
       | 8     | 0x53 0x74 0x65 0x6d 0x4d 0x65 0x74 0x61          |
       | bytes | ("StemMeta")                                     |
       +-------+--------------------------------------------------+
       | 2     | Version number of the metadata logical bitstream |
       | bytes | (notably this is not the version of the metadata |
       |       | stored in the mapping).  These bytes are 0x01    |
       |       | 0x00, meaning version 1.0 of the mapping.        |
       +-------+--------------------------------------------------+

                                 Table 2

   The remainder of the logical bitstream comprises a Vorbis comment
   metadata block containing human-readable information coded in UTF-8.
   The name "Vorbis comment" points to the fact that the Vorbis codec
   stores such metadata in almost the same way (see [Vorbis]).  A stem
   file MUST NOT contain more than one Vorbis comment metadata block The
   Vorbis comment metadata block is defined to be identical to the
   Vorbis comment metadata block defined in [RFC9639] section 8.6,
   "Vorbis Comment".

   The Vorbis comment metadata block SHOULD NOT be used for arbitrary
   metadata that is unrelated to stems (ie. a track title or author).
   Vendor specific tags MAY be included in the metadata block.  Vendor



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   specific tags in the block MUST use a vendor specific namespace and
   MUST NOT prefix their tags with "STEM:".  Specific keys for the
   Vorbis comment metadata block are defined in the "Mastering" section.

4.  Mixing

   The stem tracks SHOULD NOT have any gain normalization applied.
   Instead they should retain the same levels as they would have in the
   final mix present in the first track so that if all stems were played
   at unity gain the levels would be equivalent to the final mix.

5.  Mastering

   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 previously defined Vorbis comment metadata block.
   After mixing the stems applications MAY choose to feed the mix
   through a Digital Signal Processor configured with the limiter and
   compressor settings read from the metadata.

5.1.  Compressor Metadata

      |  TK: I'm not really sure how this works for the NI stems,
      |  presumably they have a value range, but that probably depends
      |  on the specific compressor used and that's not likely something
      |  we can do in a standard format.  Instead we'd have to define
      |  exactly how the DSP works and say that you might need to
      |  normalize values for specific DSP's?  Unclear how best to
      |  handle this.





















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      +=============================+===================+===========+
      | Tag                         | Requirement Level | Values    |
      +=============================+===================+===========+
      | STEM:COMPRESSOR:ENABLED     | REQUIRED          | "TRUE" or |
      |                             |                   | "FALSE"   |
      +-----------------------------+-------------------+-----------+
      | STEM:COMPRESSOR:RATIO       | OPTIONAL          | TODO      |
      +-----------------------------+-------------------+-----------+
      | STEM:COMPRESSOR:OUTPUT_GAIN | OPTIONAL          | TODO      |
      +-----------------------------+-------------------+-----------+
      | STEM:COMPRESSOR:THRESHOLD   | OPTIONAL          | TODO      |
      +-----------------------------+-------------------+-----------+
      | STEM:COMPRESSOR:ATTACK      | OPTIONAL          | TODO      |
      +-----------------------------+-------------------+-----------+
      | STEM:COMPRESSOR:INPUT_GAIN  | OPTIONAL          | TODO      |
      +-----------------------------+-------------------+-----------+
      | STEM:COMPRESSOR:RELEASE     | OPTIONAL          | TODO      |
      +-----------------------------+-------------------+-----------+
      | STEM:COMPRESSOR:HP_CUTOFF   | OPTIONAL          | TODO      |
      +-----------------------------+-------------------+-----------+
      | STEM:COMPRESSOR:HP_DRY_WET  | OPTIONAL          | TODO      |
      +-----------------------------+-------------------+-----------+

                                  Table 3

5.2.  Limiter Metadata

    +========================+===================+===================+
    | Tag                    | Requirement Level | Values            |
    +========================+===================+===================+
    | STEM:LIMITER:ENABLED   | REQUIRED          | "TRUE" or "FALSE" |
    +------------------------+-------------------+-------------------+
    | STEM:LIMITER:RELEASE   | OPTIONAL          | TODO              |
    +------------------------+-------------------+-------------------+
    | STEM:LIMITER:THRESHOLD | OPTIONAL          | TODO              |
    +------------------------+-------------------+-------------------+
    | STEM:LIMITER:CEILING   | OPTIONAL          | TODO              |
    +------------------------+-------------------+-------------------+

                                 Table 4











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6.  Use with Ogg Skeleton

   Ogg [Skeleton] is a format designed to provide structuring
   information for multi-track Ogg files.  Its use is not defined for
   stem files, however, if a Skeleton logical bitstream is present each
   fisbone secondary header packet describing a logical bitstream
   containing a stem track SHOULD set the "role" header to the value
   "audio/stem".  Similarly, the fisbone secondary header packet
   describing the first logical bitstream containing the main audio
   SHOULD set the "Role" message header to "audio/main".

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

   This document should not affect the security of the Internet.

9.  References

9.1.  Normative

   [RFC3533]  Pfeiffer, S., "The Ogg Encapsulation Format Version 0",
              RFC 3533, DOI 10.17487/RFC3533, May 2003,
              <https://www.rfc-editor.org/info/rfc3533>.

   [RFC5334]  Goncalves, I., Pfeiffer, S., and C. Montgomery, "Ogg Media
              Types", RFC 5334, DOI 10.17487/RFC5334, September 2008,
              <https://www.rfc-editor.org/info/rfc5334>.

   [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>.

9.2.  Informative

   [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>.

   [Vorbis]   Xiph.Org Foundation, "Vorbis I specification", 4 July
              2020, <https://xiph.org/vorbis/doc/Vorbis_I_spec.html>.



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   [Skeleton] Xiph.Org Foundation, "Ogg Skeleton 4", 23 May 2016,
              <https://wiki.xiph.org/Ogg_Skeleton_4>.

Author's Address

   Sam Whited (editor)
   Email: sam@samwhited.com
   URI:   https://blog.samwhited.com











































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