1. A differential detector for a receiver, said differential detector comprising: an analog to digital converter for sampling symbols of a signal to produce samples; a differentiator configured to differentiate the samples with a transfer function to produce a differentiated series of samples for each symbol; and a decision device configured to determine the value of each symbol by comparing values of the differentiated series of samples with boundary condition values; wherein the differential detector assigns a ternary value of either: i) +1 to a symbol if a value of the differentiated series of samples for that symbol exceeds an upper boundary condition value; or ii) −1 to a symbol if a value of the differentiated series of samples for that symbol exceeds a lower boundary condition value; or iii) 0 to a symbol if neither the upper nor lower boundary condition values are exceeded by the differentiated series of samples; wherein the boundary condition values are adaptive and are adapted for each symbol; wherein the differential detector either: a) adapts the upper boundary condition value of a symbol if the ternary value of its preceding symbol is +1; or b) adapts the lower boundary condition value of a symbol if the ternary value of its preceding symbol is −1; or c) maintains the upper and lower boundary condition values of a symbol if the ternary value of its preceding symbol is 0.