A Slew limiter is also known as Lag processor or Lag generator. It works by smoothing an incoming signal whether the input is audio or modulation signal. The end result is the same scenario in which the signal is smoothed out.
For example with extreme settings a square wave may end up looking like a triangle wave. This is why some people experiment with a lag generator to emulate a low pass filter.
The Slew limiter is mainly intended for control voltages. Even though a Slew limiter (Lag generator) can be used used as a form of a low-pass filter.
Examples of a Slew limiters are the Doepfer A170V slew limiter and the Lag generator found in u-he Bazille.
Common uses of a Slew Limiter
One of the most popular use of a Slew Limiter is to create portamento on a modular synthesizer. All you need to do is to patch the pitch control voltage from your keyboard into the input on the slew limiter. Then patch the output of the slew limiter to your oscillators input.
Low Pass Filter
You can use a Slew limiter as a low pass filter by patching audio output into the Slew limiter’s input. The harmonics of the audio will be attenuated based on the controls of the Slew limiter.
However depending on the settings a square wave into an LPF, when adjusted right, will come out looking like a sine. While a square wave into a Slew limiter typically comes out looking like a triangle wave.
As always you are free to experiment with a Lag generator in whatever way suits your needs. What’s essential is to grasp the fundamental concepts of how a Lag generator works. From there your imagination is the limit.
In the software world there not much synthesizers which have a Lag generator on board. But in the hardware modular synthesizers world there are various modules to choose from. Especially with the Eurorack modules.
Examples of software instruments which have a Lag generator are Native Instruments Reaktor, u-he Diva and u-he Bazille. Please note Reaktor is not necessarily a synthesizer but allows users to have access to Lag generators.