Video
Field & Frames
A feature of video that I spent some time getting to grips with is Fields. This included getting it wrong in my first video, resulting in a few jerky sequences and transitions. Although I knew what fields are, as do most people who know anything at all about video, I hadn't realised all their implications when it comes to editing.
For the benefit of anyone else who has trouble or is confused with this, here is an explanation of fields as they affect video editing, as well as some tips on how to get best results.
What are fields?
Each TV frame is produced by scanning the screen twice, arranged so that the lines of the second scan fill in the gaps left by the first. Each of these scans is a field. So a 25 frame/sec TV picture is actually 50 fields/sec (30 & 60 respectively for NTSC - I'm English so I think PAL).
So to get one frame with maximum detail, you need to combine the information in both fields. Easy so far, but what happens when motion is introduced? Because the two fields are scanned sequentially in the camera, anything in the image that is moving is in a different place in the second field than it is in the first. This helps make for smooth motion in a TV picture, but is also the reason why fields can cause trouble when it comes to editing.
Video capture
Capture card drivers offer the option of capturing one field per frame (sometimes you can choose the first or second), or capturing both fields. In addition, when capturing both fields there is a question of what order the fields are packed into the AVI frame. Most drivers pack them in the order they are scanned, field-A first, but some pack them field-B first. Your capture card's documentation should tell you which field order it uses, but if you can't find it try order A and do a field-dependent trial edit. If the result plays badly, try order B
If you capture one field then the resulting AVI file is frame-based, in other words it has one image per frame. If you capture both fields the AVI is field-based - each frame consists of two distinct field images, separated by 1/50 (1/60) second.
For best quality you will want to capture both fields, so knowing how fields work is important.
Video editing
Most of an AVI file produced by the editor consists of frames that are simply copied from an input file to the output file (assuming you're not changing the CODEC). For this material it doesn't matter whether the files are frame or field based, the frame data is just copied verbatim - the output is the same as the input. But when you start adding effects, transitions, speed changes, etc. the difference becomes very important.
If the editor doesn't know that the input frames contain pairs of fields, it may do completely wrong things with the images. In addition, by creating the output in field mode, synthesised motion produced by transitions and moving-path options is considerably smoother.
AVI files contain a lot of information identifying the format of their contents, but the ability to mark a file as containing dual-field frames was not put into the AVI specification. As a result you have to know what your AVIs contain, and set the editor options correctly yourself.
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