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Old 27th November 2007
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Playing file backwards is much more difficult because of the inter-frame compression used in most formats. To jump backwards an arbitrary number of frames the player must decode all video from the last keyframe. Keyframes in MPEG-4 are added in rather large intervals, normally every 5-10 seconds (variable). While jumping forward by a small step like 0.5 seconds the decoder needs only to process about 12 frames.

Also I suppose during normal forward decoding the system is pre-buffering a small amount of data.
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