DivX Free Codec , the most popular free audio / video codecs including DirectVobSub, AC3Filter, FFDSHOW ando so on.

DirectVobSub (vsfilter) [version 2.33]
DirectVobSub is a tool for displaying subtitles. Your subtitle file should have the same filename as your video file (for example Movie.avi and Movie.sub) and should be in the same folder as your video file. Then the subtitles will automatically be displayed in most players.

DirectVobSub is simply a DirectShow filter which can be used to display subtitle. It integrates automatically with any DirectShow player like the Windows Media Player, Media Player Classic, The Core Media Player and others.

AC3Filter
AC3Filter is a high quality free DirectShow filter designed for real time audio decoding and processing. It has a priority on wide functionality and convenient settings. Filter can decode following audio formats: AC3/DTS/MPEG Audio. It also supports multi-channel and digital (SPDIF) outputs.

Main Features:
· Decoding of AC3/DTS/MPEG1/2 Audio Layer I/II formats
· Support of DVD, AVI/AC3, AVI/DTS, WAV/AC3 and WAV/DTS
· Audio processing for any source
· Decomposition of any source to 6 channels
· Full information about audio track format
· Support of SPDIF passthrough mode
· Multi-channel audio output from all sources on SPDIF (on-the-fly AC3 encoding)
· Per-channel amplification for all input/output channels
· Per-channel delays (for compensation of distance difference to speakers)
· Automatic gain control
· Clipping
· Dynamic Range Compression, DRC
· Level indication for input and output channels
· Matrix mixer and ability of direct modification of mixing matrix
· Dolby Surround / Pro Logic / Pro Logic II downmix


FFDSHOW is a DirectShow decoding filter for decompressing DivX, XviD, H.264, FLV1, WMV, MPEG-1 and MPEG-2, MPEG-4 movies. It uses libavcodec from ffmpeg project for video decompression, postprocessing code from mplayer to enhance visual quality of low bitrate movies, and is based on original DirectShow filter from XviD, which is GPL'ed educational implementation of MPEG4 encoder.

First of all, the FFDSHOW consumes way less CPU power than original DivX codecs, even with maximum post-processing. At the same time the post-processing seems to give excellent quality results and allows more configuring than the original DivX 5 codec. It can even automaticly reduce post-processing if your CPU is getting overloaded. The noise filter adds a kind of a rasterising effect to the picture - might be useful in some cases. The alpha version also includes experimental playback for MPEG-1 and MPEG-2 files (such as VCD and SVCD movies), which works pretty damn well. Only problem is that MPEG-2 playback can't play the SVCD clips directly from the CD, but requires the video to be extracted to the HDD by using VCDGear first.

Features of FFDShow MPEG-4 Video Decoder:
· various compression methods:
· MPEG 4 compatible (Xvid, DivX 4, DivX 5) using libavcodec or XviD
· Divx 3 compatible, MSMPEG4v2, MSMPEG4v1
· WMV1/7, WMV2/8
· H263, H263+, H264
· HuffYUV in YV12 colorspace
· MJPEG
· MPEG 1
· Theora (not for regular use, format isn't stabilized yet)
· MPEG 2 by using mpeg2enc code from MJPEGtools project
· all Windows Media Video 9 supported encoders with output to asf/wmv file or AVI
· all common encoding modes: constant bitrate, fixed quantizer, fixed quality, two pass encoding (depends on compressor features)
· for libavcodec and XviD detailed selection of motion estimation method
· minimum and maximum I frames interval
· minimum and maximum quantizer selection, quantizer type and trellis quantization selection for libavcodec
· adaptive quantization (aka. masking) for libavcodec and XviD, single coefficient elimination
· selectable compression of starting and ending credits
· two curve compression algorithms for second pass of two pass encoding - thanks to XviD developers
· second pass simulation: although not very precise, still helpful for tweaking advanced curve compression parameters
· ability to use libavcodec internal two pass code
· image preprocessing with ffdshow image filters (latest ffdshow version must be installed)
· graph during encoding: if your encoding program doesn't provide one
· B frames support: from one to eight consecutive B frames
· support for MPEG 4 quarterpel and GMC
· selectable interlaced encoding
· decompression


Main components are:
· FFDshow MPEG-4 (Version rev.1050)
· AC3 Filter (Version 1.11 final)
· Matroska Splitter (Version 1.0.2.9 2006-05-12)
· AVI Splitter (Version 1.0.0.7 2006-03-20)
· RealMedia Splitter (Version 1.0.1.1 2006-05-03)
· MPEG-1/2 Decoder (Version 1.0.0.3 2006-05-12)
· MPEG Splitter (Version 1.0.0.3 2006-05-12)
· CoreAAC Decoder (Version 1.2.0.575 rev.3)
· CoreVorbis Decoder (version 1.1.0.79)
· Ogg Splitter (Version 1.0.0.0 2006-05-03)
· OggDS Filter 0.9.9.5 DLLs (Version aoTuVb 4.51)
· VSFilter (Version 2.37)