accessing a random access memory as a primary storage medium on a realtime basis for recording/playing back video data from/to a video device, the random access memory having a large capacity to store {HEAD ~ a plurality} of {TAIL ~ frames} of video data representing a plurality of seconds of video data; and 
accessing a random access memory as a primary storage medium on a realtime basis for recording / playing back video data from / to a video device , the random access memory having a large capacity to store { HEAD ~ a plurality } of { TAIL ~ frames } of video data representing a plurality of seconds of video data ; and
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