The active video portion of the signal has a chroma carrier on it also at 40 IRE in amplitude. Set the waveform monitor for a 1 H display rate. Select the chroma filter mode and adjust the vertical gain for 5 times normal gain. The burst reference should then just reach the 100 IRE marking, if not, then adjust the vertical gain to make it so. The rest of the line of video should also be just at the 100 IRE markings through the entire length of the line.
Any variation is a difference in chroma gain or differential gain.
Diff gain is measured directly from the IRE markings. Any variation in amplitude, either higher or lower, no matter where, is a difference in chroma gain. Diff gain is usually measured as the worst case variation on the line. The maximum deviation is then translated to percent. A Second method of measuring diff gain is done with a vectorscope.