Re: [LAU] Filter arrangement - was- Bitwig: what we can learn from it

On Sun, Mar 30, 2014 at 06:38:14PM -0700, Len Ovens wrote:

> My question is two (maybe three or four?) fold then. As I had always
> assumed that these filters would be in series (something about how they
> are laid out on the desks maybe?) ... How is this done in high end
> analogue desks?

They almost always have a series connection of individual sections.
Putting the sections in series means that they are independent of
each other, the gains (in dB) just add, and the order doesn't matter.

Parallel arrangements are used in e.g. 1/3 oct band equalisers. Putting
filter in parallel requires very careful design because strange things
may happen in the regions between the center frequencies due to phase
differences.

For the same reason, combining an equalised signal with the original
may produce unexpected results.

Things are different *within a single EQ section*. For example
implementing a parametric or shelf filter as the sum of the
input and a filtered version of the same has some advantages,
even if you get exactly the same response (including phase) as
when using another scheme. For one, if the gain is set to zero,
you get a bit-exact copy of the input as output. This is not
the case for e.g. a biquad set for a flat response - this will
act as two filters that just happen to cancel each other.
But even then such sections should be put in series. My 4-band
LADSPA (also available as an LV2) works that way.
 
> How is this done in the LV2 (etc.) plug-ins? (those with more than one
> filter in them) If some do it one way over the other, how can I tell which
> is which without going through code I don't understand?

The advertising blurb should at least tell you what the
thing is doing...
 
> Ardour, nonmixer, jackrack etc. are set up linear, one plugin into the
> next. So if I want to use a lowpass, highpass and bandpass filter, they
> will be one after the other. How will this affect the sound?

If you put filters in series with nothing non-linear in between
them then the order doesn't matter, provided 1) you use a sample
format with nearly unlimited dynamic range such as floats, and
2) none of the filters has gross defects. The second condition
is not satisfied with all LADSPA and LV2 plugins, there are some
that will happily add noise at -50 dB relative to the signal for
some combinations of inputs and settings.

Parallel arrangements have their place, e.g. for multiband
compressors used for mastering. Regarding these you should
be aware that such processing makes nonsense of whatever
careful EQ you have done before. Multiband compression
amounts an EQ that is changing all the time, and you usually
have no idea of what exactly it is doing. If the mixing and
mastering are done by the same person (as they usually are
in our context), everything done in mastering could have
been done better during the mix, e.g. by compressing tracks
instead of the mix.

Ciao,

-- 
FA
A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)
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