I suppose someone at TiVo, Inc. knows why the extension .ty and the name tystream were chosen, but no one else seems to. Nonetheless, .ty files are the way the TiVo stores its video programming. If the content is transferred directly to the PC with no processing, this is the format in which it is saved. Fortunately, the .ty format is really just MPEG Layer II video with variable audio streams attached, and some differencews in arrangements and headers. This made it possible to create programs like TyTool which can readily convert the tystreams into .mpg or .VOB files to be played on the PC or burned on a DVD.
MPEG stands for Moving Picture Experts Group. They are a group of engineers and developers in charge of the development of standards for coded representation of digital audio and video. DVDs were developed based on the MPEG-2 standard, and every PC codec out there recognizes and can play .mpg files. Most DVD players do not, however, directly recognize .mpg files.
Video Object Files
Video Object Files are MPEG-2 files with a very specific format designed to be read by DVD players. All VOB files are MPEG-2 files, but not all MPEG or even all MPEG-2 files are VOBs. It is also generally not sufficient just to copy a .VOB file over to a blank DVD in order to view the program on a DVD player. Most DVDs are created with two folders labeled AUDIO_TS and VIDEO_TS in which are contained not only the VOB files, but also .IFO and .BUP files. For compatibility the .VOB files are usually broken into 1GB logical chunks, although on the DVD they need to be physically contiguous. If the audio is embedded in the VOB files, then technically the AUDIO_TS directory is not required, but some DVD players refuse to play the material unless AUDIO_TS is present, even if it is empty.
DVD Information Files
DVD information files contain information about the video streams contained in the VOB files, including chapter information, angles, selectable audio streams, etc. The VIDEO_TS.IFO file must always be present in the VIDEO_TS folder, but other .IFO files may also be present in the final DVD.
Backup Information Files
Backup information files are not absolutely required, as they are nothing but exact copies of the respective DVD information files bearing the same filename, but they are a good idea to have. DVD information files are comparatively quite small, but they are absolutely essential to the operation of the DVD, and they must be locally error free. While a small error in a video file may be only modestly problematical, the DVD will not function without accurate .ifo files. If the video information files are corrupt, the DVD simply won't play. If there is a .bup file, however, when the DVD encounters an error in the .ifo file, it can try to find the information in the .bup file. If successful, the DVD player can still play what would be an otherwise useless DVD.
Windows Bitmap Files
The .bmp file format was developed by Microsoft as a means to display wallpaper and other photographic data. It is not a particularly compact or efficient means of storing photographic data, but it is well supported. TyTool uses .bmp files as a background for the DVD menu when creating the IFO fileset if the user so chooses. One may obtain a bitmap under the Options menu of the GOP editor, or supply one from an external source. TyTool only uses .bmp files for this purpose. If the user has a picture saved with another file type (.jpg, .gif, .tga, .png, etc.), then it will need to be converted using photo editing software before it can be used as a menu background.
Elemental Video Stream
Splitting is the process of taking a multiplexed source stream and outputting 2 separate Elemental Stream (ES) files: one for audio and one for video. When the option to split the files is selected in TyTool, it will produce an audio stream file and a separate video stream file. The .m2v file is an MPEG-2 file containing only video information.
Elemental Audio Stream
Extension: .m2a or .mpa (depending on the settings in the Options - Naming pull-down menu)
Splitting is the process of taking a multiplexed source stream and outputting 2 separate Elemental Stream (ES) files: one for audio and one for video. When the option to split the files is selected in TyTool, it will produce an audio stream file and a separate video stream file. The .m2a (or .mpa) file is an MPEG-2 file containing only audio information.
Key Index Files
Key files are produced by TyTool in order to be able to make edits (cuts and chapter marks) of the video content. The .key file conatains timing, audio / video synchronization, and GOP boundary information about the source .ty file of the same name. Without an index file, it would not be possible to know where in the .ty file to remove sections and splice the fragments back together, or where to add manual chapter markers. TyTool uses 2 files to implement these functions: the .ket file and the .cut file. The .key file does not contain any user information. It only contains the information about the source file, irrespective of any edits made in the GOP editor.
Cut files contain the location of the chapter markers and cut locations created by the user in the GOP editor. It does not contain any atual details about the source .ty file, but when combined with the .key file of the same name, it allows TyTool to perform cuts and add chapter markers when muxing the .ty file to a VOB, an MPEG or a split stream.
Chapter Index Files
Chapter Index Files are created by TyTool when it muxes a VOB from a .ty, .key, and .cut file. When muxing, whatever sections specified by cuts during editing with the GOP editor are removed, and the updated locations of the manually selected chapter markers are specified in the .chp file. This information is used only by TyTool when creating am IFO file set and only if the "chap" setting for the specific VOB is checked in the IFO creation tool.