Help modulating a RLPF's frequency with a SinOsc

[dayunbao]

I'm trying to create something similar to the way an LFO can be assigned to a filter's cuttoff frequency on an analog synth. I'm 90% of the way there, I think, but the one thing I'm not quite sure how to do is make the LFO Rate cause the filter sweep to take longer for lower values, or take less time for higher values. I feel like this would be a good use of an XLine UGen, but there are a few things I'm still unsure of. I believe I could use an XLine to generate the frequency values for the SinOsc in the filter, and then use the LFO Rate in some way to control the duration? If I do that, I still need to map the output to some frequency range for the RLPF, but I'm not quite sure what's the best way to do all of this. Suggestions?

Here's a script that shows where I'm at right now:

import sys
from supriya import AddAction, Bus, Envelope, Server, synthdef
from supriya.ugens import EnvGen, In, LFSaw, Limiter, LinExp, Out, Pan2, ReplaceOut, RLPF, SinOsc, XLine

@synthdef()
def saw(frequency=440.0, amplitude=0.5, gate=1, out_bus=0) -> None:
    signal = LFSaw.ar(frequency=frequency)
    signal *= amplitude
    adsr = Envelope.adsr()
    env = EnvGen.kr(envelope=adsr, gate=gate, done_action=2)
    signal *= env
    signal = Pan2.ar(source=signal, position=0.0)
    Out.ar(bus=out_bus, source=signal)

@synthdef()
def filter(in_bus=2, lfo_rate=1, resonance=0.05, out_bus=0):
    signal = In.ar(bus=in_bus, channel_count=2)
    mod_signal = SinOsc.ar(frequency=lfo_rate)
    mod_freq = LinExp.ar(
        input_minimum=-1.0, 
        input_maximum=1.0, 
        output_minimum=32, 
        output_maximum=4200,
        source=mod_signal,
    )
    signal = RLPF.ar(frequency=mod_freq, reciprocal_of_q=resonance, source=signal)
    
    signal = Limiter.ar(duration=0.01, level=0.01, source=signal)
    ReplaceOut.ar(bus=out_bus, source=signal)

def main() -> None:
    server = Server().boot()
    server.add_synthdefs(filter, saw)
    server.sync()
    filter_bus: Bus = server.add_bus(calculation_rate='audio')
    server.add_synth(
        frequency=43.654,
        out_bus=filter_bus,
        synthdef=saw,
    )
    server.add_synth(
        add_action=AddAction.ADD_TO_TAIL,
        in_bus=filter_bus,
        lfo_rate=1,
        synthdef=filter,
    )

    while True:
        continue

if __name__ == '__main__':
    try:
        main()
    except KeyboardInterrupt:
        sys.exit(0)

[dayunbao]

Oh, wait. I think I figured it out.

Setting the phase of the SinOsc to 1.5 * pi starts it at the "trough," and using a value less than 1.0 for the LFO Rate is getting me the long, slow filter sweeps I want.

Any suggestions are still welcome, though.

import sys
from math import pi

from supriya import AddAction, Bus, Envelope, Server, synthdef
from supriya.ugens import (
    EnvGen, 
    In, 
    LFSaw, 
    Limiter, 
    LinExp, 
    Out, 
    Pan2, 
    ReplaceOut, 
    RLPF, 
    SinOsc,
)

@synthdef()
def saw(frequency=440.0, amplitude=0.5, gate=1, out_bus=0) -> None:
    signal = LFSaw.ar(frequency=frequency)
    signal *= amplitude
    adsr = Envelope.adsr()
    env = EnvGen.kr(envelope=adsr, gate=gate, done_action=2)
    signal *= env
    signal = Pan2.ar(source=signal, position=0.0)
    Out.ar(bus=out_bus, source=signal)

@synthdef()
def filter(in_bus=2, lfo_rate=1, resonance=0.05, out_bus=0):
    signal = In.ar(bus=in_bus, channel_count=2)
    mod_signal = SinOsc.ar(frequency=lfo_rate, phase=(1.5 * pi))
    mod_freq = LinExp.ar(
        input_minimum=-1.0, 
        input_maximum=1.0, 
        output_minimum=100, 
        output_maximum=4000,
        source=mod_signal,
    )
    signal = RLPF.ar(frequency=mod_freq, reciprocal_of_q=resonance, source=signal)
    signal = Limiter.ar(duration=0.01, level=0.03, source=signal)
    ReplaceOut.ar(bus=out_bus, source=signal)

def main() -> None:
    server = Server().boot()
    server.add_synthdefs(filter, saw)
    server.sync()
    filter_bus: Bus = server.add_bus(calculation_rate='audio')
    server.add_synth(
        frequency=43.654,
        out_bus=filter_bus,
        synthdef=saw,
    )
    server.add_synth(
        add_action=AddAction.ADD_TO_TAIL,
        in_bus=filter_bus,
        lfo_rate=0.09,
        synthdef=filter,
    )

    while True:
        continue

if __name__ == '__main__':
    try:
        main()
    except KeyboardInterrupt:
        sys.exit(0)