Slow scan procedure for high resolution graphics mode video scene compatible with conditional replenishment type of bandwidth reduction

Claim 1

In combination: a first scanner having a first frame time and a first number of lines per frame, for effecting line-sequential scan of a subject; means for sampling the video signals from said first scanner and for digitally encoding each said sample; frame memory means for receiving said encoded signal samples and for storing a complete frame of said samples as a reference frame; buffer means connectable between said frame memory means and a transmission channel, for storing and feeding video signal samples to said channel at a rate equal to the channel capacity; said transmission channel capacity being substantially less than the rate at which said first scanner generates picture Information of said subject; means for determining the magnitude of the difference between samples of said reference frame and the corresponding new samples of said video signal; means for monitoring the number of signal samples currently stored in said buffer; control logic means, reponsivie to the value of said difference and also to the number of samples presently in said buffer, for establishing a threshold for determining whether each said difference is significant enough to transmit in said channel, said threshold having multitudinous discrete levels that are increasingly higher as a function of said buffer fill; means for feeding said significantly different signal samples to said buffer and for stopping the buffer feed whenever said buffer is full, regardless of the extent of said difference between said corresponding samples; and a second scanner connected to said sampling means and comprising means for scanning a subject using a master frame with substantially more scan lines than the frame scanned by said first scanner, means for sequentially scanning successive subsets of lines within said master frame, each said subset comprising lines that with respect to the totality of lines of said master frame are separated from each other by a large and substantially constant number of intervening lines, the members of each said subset further being spatially adjacent in said master frame to the corresponding members of the preceding subset, and each said subset having substantially the same framing time as that of said first scanner.