What is difference in Alternate mode triggering and Chop mode triggering?

These are the 2 triggering modes available in a CRO.

In alternate triggering mode, the first horizontal sweep is reserved for the first channel to output the waveform, the second sweep for the second channel and this keeps alternating. This is suitable when sweep time is small or else we may observe flickering of signals in the display [This is because, during a sweep, one of the channel is inactive and thus its output clears off in a certain time( remember phosphorescent coating)].

In chopped mode, during a sweep, parts of the output wave are drawn in both the channels in an alternating manner, the frequency of which is kept high to provide in-sensitiveness to human eye. This works well for higher sweep times.