The test is to measure the phase at all luminance levels and compare the absolute differences, the amount, in degrees, is differential phase. The third mandated baseband test is Differential Gain. Yes it sounds almost the same, and yes, some have already figured out what this one means. In the baseband world, we quite often refer to both Differential Phase and Gain, because they are often interactive occurrences. Back to the Dallas Cowboys, same conditions, but now when the action goes from the sunny area to the shaded area, the hue stays green, but the intensity goes from bright green to almost no green at all.
Testing is performed with the exact same test signals used for differential phase.
The modulated stair step or ramp is used here also, and the difference in measurements is to measure the total chroma amplitude at all levels and compare the amplitude differences, the maximum difference, in percentage, is differential gain. There is a fourth measurement set, which is the subject of some discussion. This is testing the frequency response of the channel. I will not join in the question of using a sweep, or sin X over X, or RF spectrum analyzers, or the fact that FCC compliant modulators start to roll off at 4.1 MHz and have to be 20 dB down at 4.75 MHz. There are a couple of base band test signals which are there to measure frequency response.