For TV work a 5 step waveform is used which has the chrominance subcarrier (4.43 MHz) sitting on it. So as the device has each step going into it, it should continue to amplify both the steps and the sub-carrier equally, but typically as the amplitude of the steps increase the amplitude of the chrominance falls. You view the output via a band pass filter and the chrominance can be seen as 5 blocks of carrier, each of a decreasing amplitude. The first block of carrier is taken as 100% and the lowest one is the measured differentially. first step = 100%, 5 step = 94% diff gain = 6%. Exactly the same with the phase but a special instrument must be used such as the Tektronix vectorscope. You are measuring how the phase of the sub-carrier changes with the level of the steps.
Frank