A self-sealing differential signal decoder (50) which accepts first and second input signals and provides an output representative of the difference therebetween which is scaled as if the sum of the input signals were equal to a predetermined value, avoids slow A.G.C. systems and sequence control systems, being operable at up to video speed by exploiting the properties of a differential long-tail pair amplifier (72, 74, 76) where the ratio between its output currents is proportional to the antilogarithm of the difference between its input voltages and the sum of the output currents is equal to the controlled value of a constant current source (76), the input to the long-tail pair being provided by a differential logarithmic amplifier (56, 58, 66, 68) and the decoder (50) being employed in a tribit servo signal reader.