[Ardour-Users] Removing hum from track

[Al Thompson]

On 04/27/2017 05:57 AM, David Kastrup wrote:
> 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.
>

Everyone is missing the simplest, least obtrusive method to eliminate
60Hz hum on multitrack.  Just flip the phase of some of the tracks.  As
long as the tracks aren't related to each other (i.e.- stereo pairs)
there should be no adverse affect, and you won't have to kill low end
response.