Changing SynthDef amplitude in realtime using OSC trigger/message and UGen - osc

Question

Sorry if this is very obvious, I'm very new to SuperCollider. I've followed a few suggestions from other threads but this application appears unique as I'm using OSC data from Max 8. I've run out of time so I'd hugely appreciate any suggestions.
I'm trying to change the amplitude of my Synth using AmpCompA. I can change the frequency in realtime using OSC messages from Max 8, however, I can't apply AmpCompA in realtime using the same trigger/message. Is this possible another way?
Here is the code:
// setup synth
(
SynthDef.new("sine", {arg out = 0, freq = 200, amp = 1.0;
var sin;
sin = SinOsc.ar(freq);
Out.ar(out, sin * amp);
}).send(s);
)
x = Synth.new("sine", [\freq, 200, \amp, 1.0]);
//test parameter setting
x.set(\freq, 400);
x.set(\amp, 0.1);
x.free;
//read from OSC message
(
f = { |msg|
if(msg[0] != '/status.reply') {
b = 1.0 * AmpCompA.kr(msg[1]);
x.set(\freq, msg[1]); //this does work when I send an OSC message from Max 8
x.set(\amp, b); //this doesn't work? Can't set a control to UGen error
}
};
thisProcess.addOSCRecvFunc(f);
)
s.sendMsg("/n_free", x);
Max 8 Screenshot
Error:
ERROR: can't set a control to a UGen
CALL STACK:
Exception:reportError
arg this =
Nil:handleError
arg this = nil
arg error =
Thread:handleError
arg this =
arg error =
Object:throw
arg this =
UGen:asControlInput
arg this =
Object:asOSCArgEmbeddedArray
arg this =
arg array = [*1]
< FunctionDef in Method SequenceableCollection:asOSCArgArray >
arg e =
ArrayedCollection:do
arg this = [*2]
arg function =
var i = 1
SequenceableCollection:asOSCArgArray
arg this = [*2]
var array = [*1]
Node:set
arg this =
arg args = [*2]
< FunctionDef in Method Collection:collectInPlace >
arg item =
arg i = 1
ArrayedCollection:do
arg this = [*2]
arg function =
var i = 1
Collection:collectInPlace
arg this = [*2]
arg function =
FunctionList:value
arg this =
arg args = [*4]
var res = nil
Main:recvOSCmessage
arg this =
arg time = 626.8060463
arg replyAddr =
arg recvPort = 57120
arg msg = [*2]
^^ The preceding error dump is for ERROR: can't set a control to a UGen

Answer 1

You can use the x.set to send numbers to a synthdef, but you can't use them to send functions or uGens. AmpCompA is a UGen - and therefore can only be used inside a synthdef.
You can modify your synthdef to use it:
(
SynthDef.new("sine", {arg out = 0, freq = 200, amp = 1.0;
var sin;
sin = SinOsc.ar(freq);
Out.ar(out, sin * amp * AmpCompA.kr(freq));
}).send(s);
)
Your OSC message function only would ever send the frequency:
x.set(\freq, msg[1]);
The SynthDef uses the frequency to compute the psychoacoustic amplitude change.