OGG Stem Files

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 and 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.

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.

Requirements STEM files have a few basic requirements:

Backwards compatibility with existing media players
The ability to store at least 5 stereo audio tracks
The ability to syncronize multiple audio tracks
The ability to store global metadata and per-stem metadata

Bitstream Layout

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 MUST be the final post-mix, mastered audio. This helps preserve backwards compatibility in media players which do not support a bitstream. The remaining logical bitstreams will be the stems and MUST have the same audio length as 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.

Skeleton Stem files MUST contain a bitstream. For each fisbone secondary header packet describing a stem logical bitstream (ie. not the fisbone packet describing the first stream containing the post-mix audio) the following message headers are defined:

Message Header	Requirement Level	Description
Role	REQUIRED	MUST always be "audio/stem"
Title	REQUIRED	Free text, used for the stem name (eg. "Percussion")
Stem-color	OPTIONAL	Color representing this track in RGB hex format, eg. "#145374"

The fisbone secondary header packet describing the first logical bitstream containing the main audio MUST set the "Role" message header to "audio/main".

Mixing The stem track 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.

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 stem file. 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.

Compressor Metadata Metadata used for configuring the compressor should be stored alongside the stem files global metadata (ie. in the primary VorbisComment).

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

Limiter Metadata Metadata used for configuring the limiter should be stored alongside the stem files global metadata (ie. in the primary VorbisComment).

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

IANA Considerations This memo includes no request to IANA.

Security Considerations This document should not affect the security of the Internet.