TV input source identifier.

摘要

The circuit determines whether a video signal is from a video tape player or a broadcast signal source by measuring jitter of the vertical synchronization pulses. In one embodiment, every other time interval between the vertical synchronization pulses is measured and saved as a count, and each count is compared to the preceding count to determine whether sufficient jitter is present to indicate that the video signal is from a local tape player. In another embodiment all intervals between the vertical synchronization pulses are measured and a predetermined number thereof are saved as counts. The even and odd counts are then analyzed separately, and the extreme count (the most deviant count) of each group is eliminated. With the extremes eliminated, the overall deviation of the counts from an average is determined for each group (even and odd) and such deviations are compared to predetermined data (a threshold) to decide whether jitter is sufficiently large to indicate that the source of the video signal is a local tape player. In still another embodiment timing data is generated corresponding to the period of at least one selected field in each of a plurality of selected frames. From this timing date, a maximum number is stored that corresponds to the greatest period and a minimum number is stored that corresponds to the least period of the selected fields. The minimum number is subtracted from the maximum number and the difference is compared to a threshold to determine the type of video signal. In an alternate embodiment, numbers corresponding to fields from successive frames are compared by a subtraction and the difference is compared to a threshold in order to make a decision as to the type of video signal. In both of the last two embodiments, the noise level of the video signal is constantly monitored. When the noise is above a threshold, the local signal is generated regardless of the information derived from the timing data.