Re: [sc-users] Problem in Demand/Dwhite?

[jamshark70@xxxxxxxxx]

On Fri, Dec 6, 2019 at 7:01 AM <walters@xxxxxxxxxxxxxxxxxx> wrote:
> The Demand/Dwhite combo seems to output 0 throughout the first buffer even if triggered at audio rate on the first sample with 0 out of range.

Not exactly. The first output sample is the demand-ed value (there are
no zeros in the output stream).

b = Buffer.alloc(s, 512, 1);

a = { RecordBuf.ar(Demand.ar(Impulse.ar(1), 0, Dwhite(4, 8, inf)), b,
loop: 0, doneAction: 2); Silent.ar(1) }.play;

b.getn(0, b.numFrames, { |data| d = data });
d.plot;

d.first == 0
-> false

d.first == d.last
-> true

The real problem is kind of a classic one, which I've investigated at
length in the past (and complained about at length), but we probably
aren't going to fix it.

https://github.com/supercollider/supercollider/issues/2343

Impulse, with initial phase 0, produces 1 as its first "real" output
sample. But every UGen's constructor (Ctor in resources) needs to
produce a pre-sample for initialization. Normally we assume that this
will be the same as the first "real" output: y(-1) = y(0). But for
some UGens, including Impulse (which is a really big problem), y(-1)
!= y(0) -- for Impulse, the pre-sample is 0.

So then 'reciprocal' produces a pre-sample = inf, and the RLPF Ctor
pre-initializes its coefficients based on rq = inf, and after the
first block, those faulty coefficients get replaced by proper ones.

I have argued that we should make it an absolute, inviolable rule that
every UGen's Ctor's initial output sample should be its real output.
This would complicate some UGens' code, but it would make this class
of initialization problem disappear forever (instead of cropping up a
couple times a year, in new contexts).

Unfortunately https://github.com/supercollider/supercollider/issues/2343#issuecomment-296443946
is the current status.

> Fixing this issue will take a lot of work and a lot of API changes. Furthermore, there are still plenty of absent and inconsistent initialization issues in sc3-plugins and other third-party plugins.
>
> We can introduce a workaround UGen that allows manipulation of the initialization sample only. Just a UGen like "PreTrig" that outputs an initial sample of 1 and then outputs silence would work. For example, you could set the initial sample to an arbitrary value with (1 - PreTrig.ar) * sig + (PreTrig.ar * initValue).

I don't like this, but that's where we are.

Probably the easiest workaround would be "Demand.ar(...).clip(4, 8)".

hjh

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