Tri-state delta modulation (TSDM) is introduced as an extension of two-state adaptive delta modulation. Its advantages for picture transmission are discussed as well as its implementaiton. Statistics from computer simulations of the TSDM on slow scan video signals show that the source messages (+1, 0, -1) can be entropy encoded for bandwidth reduction. In particular run-length coding of 0's gives a rate of less than 1 bit/pel in some cases.