The bitrate is only a target. If you use single-pass encoding, the codec will attempt to match the bitrate, but if it encounters a segment that uses more bits, it'll hope that it can make it up later. But if the keyframe interval is too low or resolution is too high, it may never be able to, and it'll run over. With 2 pass encoding, the codec has a better understanding of where to distribute the bits over time, but it still may not be able to meet the target if the keyframe interval is too low. For example, setting a keyframe for every frame will likely cause the codec to go way over its target bitrate. There are often lower limits on the size of keyframes (based on the resolution), to maintain a minimum of quality. Imagine if you jpeg compressed a 640x480 image with jpeg quality 1. It would be so blocky that it would be unacceptable.
You will often find that a codec is both tuned for a particular bitrate (and frame size), and that the underlying codec algorithms have a minimum bits per pixel below which they won't go. For example, if you're encoding HD resolutions, you'll need to be able to handle much greater motion (in terms of pixels moved per frame) than if you're encoding the same source scaled down to 160x120. --Brian _______________________________________________ Flashcoders@chattyfig.figleaf.com To change your subscription options or search the archive: http://chattyfig.figleaf.com/mailman/listinfo/flashcoders Brought to you by Fig Leaf Software Premier Authorized Adobe Consulting and Training http://www.figleaf.com http://training.figleaf.com
