A circuit which limits the rate at which a signal can increase or decrease in value. This circuit, or a software simulation of it, is most often used to produce portamento by processing a control voltage which is subsequently used to control the frequency of a VCO. In the analog synthesizer world, two types of lag processor exist: the more common type is basically a low pass filter with a below-audio cutoff frequency. It exhibits an exponential transfer curve; that is, if the gap between the input and output voltage is large, the ouptput will rise/fall more rapidly, and if the gap is small, the output rises or falls slowly. A second, less common type is known as "slew rate limiting"; it simply imposes a hard upper limit on the slope of the output.