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What is a GOP?

GOP in an acronym which stands for "Group Of Pictures".  Literally, it just means a collection of some number of photographs.  As defined by the MPEG-2 spec it can be any number of frames but as defined by the DVD spec it is supposed to be a fixed number of related frames of video with audio attached.  One frame is made up of two fields of video which when interlaced make up one display image.   An NTSC GOP is supposed to be 18 frames and a PAL GOP is supposed to be 15 frames.   Fortunately for us, most DVD players are sufficiently flexible to handle GOPs with numbers of frames very much different from 18.  First of all, the TiVo itself does not follow the DVD spec, and even if it did, we TyTool users often want to edit the number of frames in a particular GOP.   'More on that later.

What is GAE?

GAE stands for GOP Accurate Edit point, or more generally GOP Accurate Edit mode.  This is a cut where an entire GOP is selected as an endpoint for editing.   At the highest level GOP Editing is the process of removing entire GOPs as single elements from the output stream. This is the simplest method of editing, and TyTool will simply pluck entire GOP elements from the stream, splicing the resultant fragment back together.  It's quick and dirty, but it works, it's easy, and it's fine if the user is not too particular about presentation quality.

What is FAE?

FAE stands for Field Accurate Editing or more generally Field Accurate Edit mode.  Each video frame of 525 lines is divided into two interlaced fields.  When editing in GAE mode, due to the very nature of the stream the cuts might not be precisely where the user wants them.  One either looses between 1 frame (2 fields) and 2-3 seconds of the show one is trying to keep, or somewhere between 1 frame and 2-3 seconds of commercial is included with the cut.  The amount either lost or kept is solely a function of the encoding and where the start of a GOP fell in the mix.

TyTool offers another option.  Using the open source tool mpeg2enc.exe, Tytool can decompress a single GOP, decode it into individual frames, and allow the user to select specific frames to be removed from the GOP.  This allows the user far more accurate control of edit points within the stream, right down to the field bounday.  Once the unwanted fields are removed, the GOP is re-assembled and re-encoded.  The down side is re-encoding loses a little quality in the stream (for the edited GOP only), and it requires a few additional steps for the user.  It also requires the user have the mpeg2enc.exe file available for TyTool to use.

Which do I choose, GAE or FAE?

The really good news is, the user does not have to select a particular mode of editing at the outset.  As long as the mpeg2enc.exe file is somewhere TyTool can find it (in the directory with TyToolxryy.exe is the best choice), the user can decide on an endpoint by endpoint basis whether to invoke FAE or just use GAE.  Not only can each cut be a different type of edit, the two endpoints of any single cut segment can be different types.  At the start of a cut, select [ with the mouse or press <Home> to create a GAE start point, or select [->F with the mouse or press <PgUp> to open the FAE window and create an FAE start point.  Similarly, the user clicks ] with the mouse or presses <End> to create a GAE end point, or selects F->] with the mouse or presses <PgDn> to open the FAE window and create an FAE end point.

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