[Ardour-Users] Removing hum from track

[David Kastrup]

Ralf Mardorf <ralf.mardorf at alice-dsl.net> writes:

> On Wed, 26 Apr 2017 21:01:36 -0400, Sg.tla wrote:
>>I've been tracking audio for several years now but never got beyond
>>the basics of noise removal filters.
>
> PS: There's nothing wrong with not becoming an expert of noise removal,
> since it's better to become an expert in avoiding noise in the first
> place ;). I don't know what a modern "noise removal filter" is able to
> do, but in the old days, in the worst possible scenario, removing
> noise was done by combinations of gear. I would always start with
> the simplest approach, using a noise gate for silent parts (there are
> "smart" hardware noise gates available, unfortunately I don't know what
> Linux software provides in this domain) and as already mentioned by my
> previous mail, using parametric EQs to reduce hum in important
> recordings.

If it is a constant hum of power line frequency, you can use a _very_
narrow-band notch filter.  That has the advantage of mostly leaving the
music itself alone.  In the simplest case, this will be a 2-stage IIR
filter with an inner state having a resonance peak at the hum frequency.
Note that if it is very narrow-band, this also means that it is very
slow in converging to the hum's amplitude and phase: basically its
bandwidth corresponds with its convergence time.  For that reason, you
want to apply this sort of filter _before_ all editing (since otherwise
the filter will have to resynchronize at each cut or gain change) and
preferably _before_ any nonlinear processing stages.  Which may include
external stomp boxes if your source of hum is before that.

In other words: hum removal is an emergency measure for recordings that
cannot be redone properly.

It becomes nicer when the tracks in question do not feature instruments
or other relevant components with similarly low frequencies: in that
case you can employ less discriminating low-cut filters.  50Hz
corresponds to G1 (a bit more), 60Hz to B1 (quartertone less actually).
Western Bass singers start around D2 (which does not leave a whole lot
of breathing room for a low-cut filter to be effective against hum but
not singer), a bass guitar starts at E1 (41Hz) so you can't really
employ a low-cut here.  Bass drums also wash out if you take out the low
frequencies.

-- 
David Kastrup