USF - Universal Subtitle Format

Subtitld's default format for subtitles

The Universal Subtitle Format (USF) is an ambitious project created in order to provide a clean, documented, powerful and easy to use subtitle format. The format is based on the XML format for many reasons:
  • Flexibility
  • Human
  • Readability
  • Portability
  • Unicode support
  • Hierarchycal system
  • Easier management
Features and Advantages
  • XML based for better flexibility and interoperability
  • Easy to learn syntax, similar to HTML
  • Time based to avoid problem with video frame rate conversion
  • 4 element types :
    • text
    • image
    • karaoke
    • shape
  • Powerful element positioning system :
    • 9 alignment types (TopLeft, ...,MiddleCenter, ..., BottomRight)
    • Relative to window or video
    • Horizontal and vertical margin in pixel or percent of video/window
  • Text attribute usable on a per letter basis :
    • color
    • italic, bold, underline
    • size
    • font
  • Definable style for quicker global modifications
  • Meta information to increase archiving/search facilities:
    • title
    • author
    • language (ISO639-2 plus a user definable string)
    • date
    • comment

Although Subtitld can read and write a variety of subtitle formats, it is necessary to have a default subtitle format to save and open safely. Saying "safely" we mean not loosing format and other features some subtitle formats do not support. Subtitld is set by default to save a USF file and the original subtitle format (this can be turned off).

The original specification and documentation from website ( are no longer online since 2006. The following is a copy of the now official documentation page for the format from Titlevision Subtitling Systems.

The creative company behind the the open source USF subtitle format, CoreCodec, that also invented the Matroska format, is now in private hands and no longer controls the development, so Titlevision, who has been using this format for years, has taken over the hosting for now.

Universal Subtitle Format (USF) Specification v1.1
Document revision:  V 1.1
Initial Release:    November 10, 2002
Updated:            November 28, 2010
Author(s):          Christophe PARIS (toffATcorecodecDOTcom)
                    Ludovic Vialle (blacksunATcorecodecDOTcom)
                    Uffe Hammer (hammerATanarkiDOTdk)

Table Of Contents
1 - Introduction
2 - Features and advantages
3 - The format syntax
    3.1 metadata
    3.2 styles
    3.3 effects
    3.4 subtitles
    3.5 other comments
4 - An example
5 - The future of USF
6 - History
7 - Resources
8 - Glossary

1 - Introduction

The goal of the Universal Subtitle Format (USF) is to create a new format
taking the best of currently available subtitles formats.
This format must be flexible enought to be used in every day subtitling,
as in more advanced subtitling projects. The key of the project is to
provide a set of tools and specifications to improve the quality of the
subtitles videos by preventing the process of encoding the subtitles with
the video which is providing a great loss of quality due to the contrast of
the subtitles. Also the format has been designed so the user can still
change the default style such as the font size, color, etc on the playback.

A big thank you to mf/Gabest/Pamel/Liisachan who are consultants on USF :)

2 - Features and advantages

- XML based for better flexibility and interoperability
- Easy to learn syntax, closed to HTML
- Time based to avoid problem with video frame rate conversion
- 4 element types :
  - text
  - image
  - karaoke
  - shape (rectangle, ...)
- Powerfull element positioning system :
  - 9 alignment type (TopLeft, ..., MiddleCenter, ..., BottomRight)
  - Relative to window or video
  - Horizontal & vertical margin in pixel or percent of video/window  
- Text attribute usable on a per letter basis :
  - color
  - italic, bold, underline
  - size
  - font
- Definable style for quicker global modifications
- Meta information to increase archiving/search facilities :
  - title
  - author
  - language (ISO639-2 plus a user definable string)
  - date
  - comment

3 - The format syntax

An XML file can be seen as a tree, with a root, branches and nodes.
<rootnode>            +-rootnode
  <node>                |
    <subnode/>          +-node
  <node>                  |
</rootnode>               +-subnode

The root of the USF tree is called "USFSubtitles".

<USFSubtitles version="1.0">     +-USFSubtitles   (1)
                                    @-version     (1)
  <metadata>...</metadata>          +-metadata    (1)
  <styles>...</styles>              +-styles      (0..1)
  <effects>...</effects>            +-effects     (0..1)
  <subtitles>...</subtitles>        +-subtitles   (1..N)

Between parenthesis the possible number of element. (1: required,
0..1: optional, 1..N: at least one, 0..N).
@ is the mark for an attribute.

The version attribute is used by the player to identify which feature
are used in the current file. The version will be incremented in the
 futur if new feature are needed.

  3.1 metadata

The metadata node is quite self explanatory.

<metadata>                                  +-metadata
  <title>My movie</title>                     +-title       (1)
  <author>                                    +-author      (1..N)
    <name>My name</name>                      | +-name      (1)
    <email></email>         | +-email     (0..1)
    <url></url>          | +-url       (0..1)
    <task>translator</url>                    | +-task      (0..1)    
  </author>                                   |
  <language code="eng">English</language>     +-language    (1)
                                              |  @-code     (1)
  <languageext code="DirectorComments">       +-languageext (0..1)
    Toff comments</languageext>               |  @-code
  <date>YYYY-MM-DD</date>                     +-date        (0..1)
  <comment>my comment</comment>               +-comment     (0..1)  

The language code attribute is normalized (ISO639-2), see for a full listing.
Othere examples :
<language code="fre">Français</language>
<language code="spa">Espagñol</language>
<language code="deu">Deutsch</language> or code can also be "ger".

The language extention must defined in this list : Normal, HearingImpaired,
DirectorComments, Forced, Children.

The date use the international standard date (ISO8601) [1].
  - YYYY : four-digit year
  - MM : two-digit month (01=January, etc.)
  - DD : two-digit day of month (01 through 31)
example : 2002-11-09

You can use basic tag (i, b, u, font, br) in the comment element.

  3.2 styles

Defining styles is optional, but it will make your subtitles more
structured and easier to maintain.

  <style name="MyStyleName">
    <fontstyle face="Arial"
               size="24" or size="+1" (+10%)
    <position alignment="TopLeft"

Styles are used to define settings applicable to a group of text subtitles.
A style has a required name attribute, it's the name you will use to
 reference it later. All other attribute are optional.

  <style name="MyStyleName"><fontstyle color="#FF0000" /></style>
  <text style="MyStyleName">your first subtitle text</text>
  <text style="MyStyleName">your second subtitle text</text>

If you change the style called "MyStyleName" you will change both lines.

In the player a default style is defined, it's used if you don't provide a
Moreover you can override this default style using the name "Default" :
<style name="Default><fontstyle size="36"/></style>

All the style heritate from the default style (the one of the player or the one
you have redefined).

Here is the hierarchy used :

  |Internal default style |
  |Redefined default style|
  +-----------+-----------+    +-----------+-----------+
  |       New style 1     |    |       New style 2     |  ...
  +-----------------------+    +-----------------------+

- fontstyle :

The family attribute specifies a prioritized list of font family names and/or
generic family names [1].
Here are some example :
family="Times, 'Times New Roman', serif"
family="Helvetica, Arial, sans-serif"
family="'Courier New', Courier, monospace"
family="'Comic Sans MS', cursive"
As you see it is recommended that one should specify a generic
font family after any named fonts as a fallback to a preferred family of font
types; one of serif, sans-serif, monospace, cursive or fantasy.

Colors are defined like in HTML, with 3 components, red, green and blue in
the range 0.255 coded in hexadecimal.
So pure red is coded  #FF0000, pure green #00FF00 and pure blue #0000FF.
The color can be extended by a 4th component, an alpha component specifing the
transparency of the color. An alpha component value of 255 specifies that the
color is transparent, and an alpha value of 0 specifies that the color is
opaque. Alpha component values from 1 through 254 specify the degree to
which the color is blended with the background when the color is rendered.
When you write #FF0000 in fact it's #00FF0000 a red opaque color. To make
a red transparent color you will write, for example #7FFF0000.

The alpha attribute can be used to scale the alpha component of all colors for
example color="#40FFFFFF" and alpha="50" would translate into

"color" is the foreground color of the text, or the highlighted color in
karaoke mode.
"backcolor" is the color used in karaoke when the text is not highlighted.

The font weight must be defined in this range :
'normal' is the same as '400'.
'bold' is the same as '700'.
lighter & bolder introduce a notion of inheritance.

- position :

The "alignment" attribute has 9 possible values :

  TopLeft      |    TopCenter   |      TopRight   
  MiddleLeft   |  MiddleCenter  |   MiddleRight
  BottomLeft   |  BottomCenter  |   BottomRight

The "relative-to" attribute has 2 possible values : "Window" or "Video".

Margin are defined in pixel or in percent of window or video depending
of what you have chosen for the "relative-to" attribute.

Rotation are defined in degree. You can use different rotation axis to
create some kind of 3D effect.

    Y^    Z
     |   /
     |  /
     | /
 ----+-----------> X

The common rotation used in 2D is the Z axis.
For example to write a text in diagonal from the bottom-left-corner to
the upper-right-corner you can use :

    <position alignment="BottomLeft"

  3.3 effects

Here is an idea, you define some key-frame and player interpolate the
required frame to make an animation.
Example :
    <effect name="50%-ZoomIn-ZoomOut">
            <keyframe position="0%"><fontstyle size="1"/></keyframe>
            <keyframe position="50%"><fontstyle size="24"/></keyframe>
            <keyframe position="100%"><fontstyle size="1"/></keyframe>
    <effect name="FadeInFadeOut">
            <keyframe position="$start"><fontstyle alpha="100"></keyframe>
            <keyframe position="$start+500"><fontstyle alpha="0"></keyframe>
            <keyframe position="$end-500"><fontstyle alpha="0"></keyframe>          
            <keyframe position="$end"><fontstyle alpha="100"></keyframe>
    <effect name="HScrolling">
            <keyframe position="0%"><position alignment="BottomRight" horizontal-margin="-$elemwidth"/></keyframe>
            <keyframe position="100%"><position alignment="BottomLeft"/></keyframe>

kf position : $start, $end, $duration

Attribute that could be "animated" :
<position alignment, horizontal-margin, vertical-margin, relative-to/>
<fontstyle size, color, alpha, outline-color, outline-level, shadow-color, shadow-level/>

  3.4 subtitles

It's the essential part of the file, the part where you put your subtitle

<subtitles>                                 +-subtitles     (1..N)
  <language code="eng">English</language>     +-language    (1)
                                              |  @-code     (1)
  <languageext code="DirectorComments">       +-languageext (0..1)
    Toff comments</languageext>               |  @-code
  <subtitle                                   +-subtitle    (1..N)
    start="hh:mm:ss.mmm"                        @-start     (1)
    stop="hh:mm:ss.mmm"                         @-stop      (0..1)
    duration="hh:mm:ss.mmm"                     @-duration  (0..1)
    type="SubtitleType">                        @-SubtitleType  (0..1)
    <text></text>                               +-text      (0..N)
    <image></image>                             +-image     (0..N)
    <karaoke></karaoke>                         +-karaoke   (0..N)
    <shape/>                                    +-shape     (0..N)
    <comment/>                                  +-comment   (0..N)

A subtitle has 2 required timestamps attribute : start and stop.
Timestamps use the following format :
hh : two-digit hour (00-23)
mm : two-digit minute (00-59)
ss : two-digit second (00-59)
mmm : three-digit millisecond (000-999)

Instead of using a stop timestamp you can use the duration attribute.
It's also allowed to use shorter timestamp : ss[.mmm]
100   -> 00:01:40.000
1.100 -> 00:00:01.100

SubtitleType can be either open or closed, but if not set then open is implied.

The content of a subtitle can be composed of different elements : text,
image or karaoke in any number.

All elements can be independantely placed with the same position attribute as
in style : "alignment", "horizontal-margin", "vertical-margin" and "relative-to".

- text :

<text effect="MyEffectName" style="MyStyleName" speaker="Luke">
  <font face="Arial" size="16" color="#FF0000"
  >Red text in Arial 16</font>

Text element are defined by a subset of XHTML.
All available tag are used in the example above.
A special note for HTML users, XHTML is a bit more strict that HTML. In XHTML
you must precise that a tag is an empty tag, so line break use the tag <br/>
instead of <br>. Moreover the tag must be properly nested :
<b><i>This is </b>the bad example.</i>
<i><b>This is </b>the right example.</i>
XHTML is also case sensitive : <i>This right</i>, <I>This is wrong</I>

- image :

<image alpha="0-100" colorkey="#RRGGBB">Image_file_name</image>
The image must be in the same directory, or a sub-directory of the subtitle file.

- karaoke :

<karaoke style="MyStyleName"><k t="1000"/>song <k t="1000"/>text</karaoke>

The karaoke element is very similar to the text element. The main differrence
is that you can used the special tag <k t="duration_in_ms"/>.

So in the example below the text "song" has a duration of 400 ms, "cool " has
a duration of 300 ms...

<subtitle start="00:00:10.000" stop="00:00:11.000">
  <karaoke><k t="100"/>a <k t="200"/>very <k t="300"/>cool <k t="400"/>song</karaoke>

The sum of all the duration must be equal to the subtitle duration.
Here 100+200+300+400 = 1000 ms = 1s

- shape :


example :
<shape type="rectangle" width="160" height="120"/>

- comment:
Is a pure text element that cannot have any type of formatting.

  3.5 other comments

For expert :

If you can read an XML DTD, and for strict format syntax see the file USFV100.dtd

If you want to validate your file you can include in the XML prolog a link to the DTD.
<!DOCTYPE USFSubtitles SYSTEM "USFV100.dtd">

You can also add a reference to an XSL stylesheet, for brower display.
<?xml-stylesheet type="text/xsl" href="USFV100.xsl"?>

4 - An example


<?xml version="1.0" encoding="UTF-8"?>

<USFSubtitles version="1.0">
    <title>The Universal Subtitle Format sample</title>
    <language code="eng">English</language>
    <comment>This is a short example of USF.</comment>

    <!-- Here we redefine the default style -->
    <style name="Default" >
      <fontstyle face="Arial" size="24" color="#FFFFFF" back-color="#AAAAAA" />
      <position alignment="BottomCenter" vertical-margin="20%"
                relative-to="Window" />

    <!-- All others styles herite from the default style -->
    <style name="NarratorSpeaking">
      <fontstyle italic="yes" />

    <style name="MusicLyrics">
      <fontstyle back-color="#550000" color="#FFFF00" bold="yes" />

    <subtitle start="00:00:00.000" stop="00:00:05.000">         
      <text alignment="MiddleCenter">Welcome to
        <b>The Core Media Player</b></text>
      <image alignment="TopRight" vertical-margin="20" horizontal-margin="20"

    <subtitle start="00:00:06.000" stop="00:00:10.000">
      <text style="NarratorSpeaking" speaker="Toff">Hi! This is a <font size="16">
        small</font> sample, let's sing a song.</text>

    <subtitle start="00:00:06.000" stop="00:00:10.000">
      <karaoke style="MusicLyrics"><k t="700"/>La! La! La! <k t="1000"/>
        Karokeeeeeeeee <k t="100"/>is <k t="200"/>fun !</text>



5 - The future of USF

This is only the first revision of the Universal Subtitle Format and we will
enhance so it can become the most complete format. Please remember that it is
one of the key of XML: the flexibility.

List of the future enhancements and usability:
- Level choices: level 1: pure text output, level 2 : text with b i u tag
level 3 : style, level 4 : animation
- Directshow Filter Integration
- Portable rendering solution via expat/freetype
- USF will be supported in the most advanced file format as a subtitle track: the
Matroska file format. For more information on Matroska, please visit
- USF in Ogg Media (OGM) files
- Encryption (reverse-engineering protection) of the XML file, so you can protect
your work. We actually plan to use a (binary ?) file format using vectors. This may
also be used for hardware support and will suppress the need of the fonts.
- UCI subtitle codec, so the USF can be played on every player supporting the powerfull
UCI API even if the player don't have any kind of support for subtitles.
- Enhancements of the specifications, such as animations, rotations, scalings, and
- The Universal Subtitle Format Editor development supported in The Core Media Player.
Which features incredibly easy-to-use facilities with real-time previews.
- Much more...

6 - History

2010-11-28 v1.1 (Uffe Hammer)
  - Added type (SubtitleType) attribute to a subtitle element.
  - Added comment element to subtitle element.

2003-06-18 v0.16 (Christophe PARIS)
  - Corrected typo EaringImpaired -> HearingImpaired
  - Added speaker attribute to the "text" element
  - Added "BasicTagAndText" support in the "comment" element
  - Multiple instances of the "subtitles" elements is supported with
    "language" and "languageext" elements to put several language in one
  - Several "author" element is allowed
  - Added "task" attribute to the "author" element

2003-02-22 v0.15 (Christophe PARIS)
  - Added font family attribute
  - Changed bold attribute in weight attribute with more levels.
  - Changed the alpha component definition, now "0" is opaque and "FF" is
  - Added relation between alpha attribute and alpha color component
  - Added wrap attribute to fontstyle

2003-01-19 v0.14 (Christophe PARIS)
  - Used ISO639-2 in place of ISO639-1 language code
  - Color definition extended from RGB to ARGB
  - Added rotation-x, rotation-y and rotation-z attribute to the position
  - Added duration attribute, short timestamp

2002-12-04 v0.13 (Christophe PARIS)
  - Renamed attribute font name in font face
  - Renamed attribute backcolor in back-color
  - Added outline-color, outline-level, shadow-color, shadow-level
    attribute to fontstyle
  - font size can now be relative (+1 mean +10%)
  - Added language extention element (languageext) in metadata

2002-11-23 v0.12 (Ludovic Vialle)
  - Introduction
  - Future of USF

2002-11-09 v0.10 (Christophe PARIS)
  - DTD specs. converted to this document.

2002-10-30 v0.03 (Christophe PARIS)
  - Added karaoke support
    - Added backcolor attribute in fontstyle and font element
    - Added karaoke element
    - Added k element
  - Added alpha attribut in image and fontstyle element
  - Added image/colorkey
  - Corrected typo, alignement in alignment

2002-08-17 v0.02 (Christophe PARIS)
  - Added element metadata/language
  - Added attribute position/relative-to

2002-08-16 v0.01 (Christophe PARIS)
  - First draft                           

7 - Resources

ISO-639-2 : Language Code

ISO8601 : Date and Time format

Generic font families

Extensible Markup Language (XML) 1.0 (Second Edition) :

XML in 10 points :

8 - Glossary

XML - extensible markup language is an open standard (W3C) that provides a
data format and a data modeling language for defining data.

DTD - document type definition is the modeling mechanism for XML. It provides
the rules for how XML data is defined and logically related.

Well-formed XML - an XML document in proper XML format, but with no structural
conformance to a DTD (flat XML).

Valid XML - an XML document in proper XML format with a structural conformance
to a DTD (structured XML).

XSLT - extensible stylesheet language for transforming XML documents into other
XML documents.

9 - TODO

- Add a way to scale the subtitles according to the original resolution used
- shapes
- effects
- embedded image & font (ex: embedded:\\image1 + an embedded section with binary
  encoded data ...)
- short color as in css
- remove language from metadata
- The font element should have italic, weight, underline attributes. Having
  these in the fontstyle element only is insufficient. (see
  -> merge font & fontstyles
- remove font/face, family is enough
- merge text & karaoke

- add all attributes of fontstyle to font (maybe merge the two, or add a %fontattributes% set)
- Add StrikeOut (maybe shorthand <s> too)
- Non-uniform font scaling -> (ASS \fscx, \fscy)
- Box display around subtitles: around text and around whole block; borders, padding...