56. The improvement recited in claim 52 wherein the predetermined event detecting means includes means for comparing the video envelopes of two frames.
56. The improvement recited in claim 52 wherein the predetermined event detecting means includes means for comparing the video envelopes of two frames. 58. The improvement recited in claim 56 wherein the video envelope monitoring means includes means for comparing the envelopes of at least one field of two non-consecutive frames.