What is positive and negative modulation?

in order to transmit the video signal amplitude modulation is used. the composite video signal has distinct polarity of black and white levels in the picture.

1.positive modulation :- in this type of modulation, increase of brightness toward white causes increase in carrier amplitude peak white has 100% modulation while black and sync level have lower and minimum modulation respectively

2.negative modulation :-in this type of modulation,the sync tip level are at 100%.blanking level correspond to black is at 75%and increase in brightness toward white causes decrease in carrier amplitude

Negative modulation is preferred over positive modulation because impulse interference is difficult to deal with and random increase in signal is similar to existing sync pulse.

these interference result in generation of black spot and white blobs.