If you are looking to just acquire 1 line of video you will need to perform an acquisition at full sample rate for the appropriate line time (which will depend on the HD signal you are acquiring). Your external circuit will need to produce a trigger pulse when the appropriate line is detected. The acquired data could then be viewed with a graph indicator. Just bear in mind that with some HD signals that the Y signal bandwidth can be up to 30 MHz and the 5122 has 100 MHz sample rate so only 3 samples per cycle of that Burst block, this may not be enough for a very accurate measurement of the luma bandwidth so you might need to perform a number of acquisitions over a number of frames and average the measurement for a better measurement.
