Filter sweep?

How could one go about modifying the Random Low Pass Filter to create a sweepable filter plugin?

It would be a Butterworth Filter but with a cutoff frequency that increases or decreases logarithmically from f_start to f_end.

Start frequency
End frequency
Filter slope
HighPass / LowPass

There are already a couple of Nyquist filters which may suffice …

High Pass Filter (LFO) , & Low Pass Filter (LFO)

[ LFO = Low Frequency Oscillator, which controls how the cut-off frequency of the filter varies with time ].

Nyquist has a pair of first order Butterworth filters (high pass and low pass) that support a variable frequency corner.
If you require a higher order filter, then the easiest way is to cascade these first order filters.

The code is in the form:

(hp <sound> <frequency>)

or

(lp <sound> <frequency>)

for high pass / low pass, respectively.
is the audio that will be processed.
may be either a number (for a fixed frequency), or a control signal (for a variable frequency).

To generate a frequency value that increases or decreases logarithmically, you can create a “piece-wise exponential envelope” with the command PWEV. For example, to rise from 100, to 2000 over the course of the selection, the PWEV command would be:

(pwev 100 1 2000)

So to create a first order low pass Butterworth filter that has a corner frequency starting at 100 and rising exponentially to 2000 Hz, and apply that filter to the selected audio, the code would be:

;version 4
(lp *track* (pwev 100 1 2000))

This code may be run in the Nyquist Prompt, or adapted to create a plug-in.

On second thoughts, Steve’s Filter fade plugin is a better match to the brief than David Sky’s LFO-controlled filters.