[PD] more |vcf~| things

[julien.breval at tremplin-utc.net]

hello

(still talking about the |vcf~| filter)

as there doesn't seem to be any simple nor complex mathematical relationnship
between Q, cutoff frequency and level factor (|*~|) for having a constant output
level, I am now trying to find something empirical ...

I made a table that gives the level factor you need to keep the same level when
you change Q
the source is a |noise~| and the amplitude is measured by |env~|, and 'f' refers
to the required level factor, which was found empirically (and quickly)
I attached the test patch

some values for vcf~_cutoff=50 Hz:
Q=0 -- f=1
Q=1 -- f=15
Q=2 -- f=20
Q=3 -- f=24.5
Q=4 -- f=28
Q=10 -- f=42
Q=20 -- f=60
Q=40 -- f=86

some values for vcf~_cutoff=12800 Hz:
Q=0 -- f=1
Q=1 -- f=0.75 (see the remark2 below)
Q=2 -- f=0.8 (see the remark2 below)
Q=4 -- f=1.35
Q=20 -- f=3.4
Q=40 -- f=5


remark1: for a constant Q, multiplying the cutov by 2 adds +3 to the output
value of |env~| (resp. dividing, -3)

remark2: for Q=1 or Q=2 at 12800 Hz, the filter should normally not increase the
volume of the signal like this ... maybe it's due to some software
implementation contraints (?)

also, do you think the method I used for measuring is correct ?


any suggestion, idea, web link about this, etc will be appreciated a lot ...
I will try to use a logarithmic-based compensation factor now

cheers,
-j
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