[Ardour-Users] more on parallel eq'ing
nettings at stackingdwarves.net
Fri Dec 14 12:19:01 PST 2012
On 12/12/2012 09:07 PM, Jörn Nettingsmeier wrote:
> i doubt that "parallel eqing" as a technique makes sense. if this way of
> working inspires you (i can see how the two faders provide a very direct
> means of shaping the sound) _and_ you find musical use for the heavy
> coloration that results from combining a zero phase signal path with a
> non-linear phase one (the eq), then fine, but this is a rather
> idiosyncratic way of working, and the results won't be entirely
> predictable. i would venture than most engineers wouldn't be happy with
> such an arrangement.
in order to see how bad the results actually are, i did some
measurements of a "parallel" eq scheme.
while i maintain that it doesn't make much sense to do it this way,
there is less harm done than i thought with standard parametric filters.
for steeper filters however, the combination is really deadly.
for testing, i set up a single pink noise source (JAPA), fed to two
ardour busses: bus1 had no plugins, bus2 contained three different
filters, only one of which was active at any one time: a 4th order
highpass at 85 Hz, a 4th order lowpass at 12 kHz, and a four-band
mitra-regalia filter (written by fons, with a gui by nedko arnaudov)
with a pretty drastic curve meant to illustrate a practical "worst case" eq.
The four-band parametric is quite well-behaved in the parallel
the blue graph in JAPA is just the EQ, while the red graph shows the
combination of the unfiltered signal with the EQ'd one. as expected, the
boosts and cuts are gentler in the latter, while the interference
artefacts are pretty minor (they are audible, but not really visible in
the graph). the window on the right shows ardour3's "plugin analysis",
where you can see the phase response of the filter (red) - it's really
gentle. green is the amplitude response, ignore the funkiness in the
bass, it's an artefact. likewise, the little "teeth" in the green curve
are artefacts of using pink noise for measuring.
now, my droogs, the old higher order filters (with knives in them) kick
it real horrorshow:
yummy. the phase response of such a filter is something an engineer
should be aware of. its fall-off is 24db per octave, and while this is
not a typical mixer strip eq, you might find it behind a rumble filter
switch. analog synth fans will have a couple of those on their machines.
you can imagine what a 48db/oct filter would look like when recombined
with the straight signal. totally unusable for any systematic approach
to mixing, but certainly interesting from a synthesis point of view.
a fourth-order lowpass is pretty much the same:
these are chebyshev filters, and you could argue that with their
passband ripple, they're not exactly high-end anyways. but a
fourth-order butterworth (as often found in loudspeaker crossovers)
doesn't look any better:
(i've had to cascade four 1st-order bw filters here for lack of a better
bottom line: i don't see anything interesting in parallel EQing, and
many pitfalls. but if parallel EQing inspires you, go ahead. just make
sure that if you saddle that horse, you know how to ride it.
now i guess some of these artefacts sound a lot nicer and fuzzier in
all-analog systems, which have their ways of "rounding off the edges",
so i could imagine some weathered old hands from the analog days
obtaining great results with such techniques.
but a textbook-correct recombination in the digital domain can get
really nasty, so it shouldn't be blindly emulated in a DAW.
my 10 ¢
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