243 lines
3.9 KiB
HTML
243 lines
3.9 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>
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What is a codec ?
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</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="VideoLAN Quickstart"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Introduction"
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HREF="intro.html"><LINK
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REL="PREVIOUS"
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TITLE="
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What is the VideoLAN project ?
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"
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HREF="x47.html"><LINK
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REL="NEXT"
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TITLE="
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How can I use VideoLAN ?
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"
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HREF="x156.html"></HEAD
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><BODY
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CLASS="sect1"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>VideoLAN Quickstart</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="x47.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 1. Introduction</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="x156.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="AEN131"
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></A
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>1.2. What is a codec ?</H1
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><P
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> To fully understand the VideoLAN solution, you must
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understand the difference between a <EM
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>codec</EM
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> and a
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<EM
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>container format</EM
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>
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</P
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><P
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></P
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><UL
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><LI
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><P
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> A <EM
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>codec</EM
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> is a compression
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algorithm, used to reduce the size of a stream. There are audio codecs
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and video codecs. MPEG-1, MPEG-2, MPEG-4, Vorbis, DivX, ... are
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codecs
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</P
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></LI
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><LI
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><P
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> A <EM
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>container format</EM
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> contains
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one or several streams already encoded by codecs. Very often, there
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is an audio stream and a video one. AVI, Ogg, MOV, ASF, ... are
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container formats. The streams contained can be encoded using different
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codecs. In a perfect world, you could put any codec in any container
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format. Unfortunately, there are some incompatibilities. You can
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find a matrix of possible codecs and container formats on the <A
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HREF="http://www.videolan.org/streaming/features.html"
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TARGET="_top"
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>features
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page</A
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>
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</P
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></LI
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></UL
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><P
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> To decode a stream, VLC first
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<EM
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>demuxes</EM
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> it. This means that it reads the container
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format and separates audio, video, and subtitles, if any. Then, each of
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these are passed <EM
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>decoders</EM
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> that do the mathematical
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processing to decompress the streams
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.</P
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><P
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> There is a particular thing about MPEG:
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</P
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><P
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></P
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><UL
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><LI
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><P
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> MPEG is a <EM
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>codec</EM
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>. There are
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several versions of it, called MPEG-1, MPEG-2, MPEG-4, ...
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</P
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></LI
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><LI
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><P
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>MPEG is also a container format, sometimes refered
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to as <EM
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>MPEG System</EM
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>. There are several types of MPEG:
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ES, PS, and TS</P
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><P
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>When you play an MPEG video from a DVD, for instance, the MPEG
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stream is actually composed of several streams (called Elementary
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Streams, ES): there is one stream for video, one for audio, another for
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subtitles, and so on. These different streams are mixed together into a
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single Program Stream (PS). So, the .VOB files you can find in a DVD are
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actually MPEG-PS files. But this PS format is not adapted for streaming
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video through a network or by satellite, for instance. So, another
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format called Transport Stream (TS) was designed for streaming MPEG
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videos through such channels.</P
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></LI
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></UL
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="x47.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="x156.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
|
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ALIGN="left"
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VALIGN="top"
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>What is the VideoLAN project ?</TD
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><TD
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WIDTH="34%"
|
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="intro.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>How can I use VideoLAN ?</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |