[FFmpeg-user] asubboost and asupercut

[Paul B Mahol]

On Sat, Nov 28, 2020 at 5:18 PM Michael Koch <astroelectronic at t-online.de>
wrote:

> Am 28.11.2020 um 14:48 schrieb Paul B Mahol:
> > On Sat, Nov 28, 2020 at 2:46 PM Paul B Mahol <onemda at gmail.com> wrote:
> >
> >>
> >> On Sat, Nov 28, 2020 at 2:24 PM Michael Koch <
> astroelectronic at t-online.de>
> >> wrote:
> >>
> >>> Am 28.11.2020 um 13:44 schrieb Paul B Mahol:
> >>>> On Sat, Nov 28, 2020 at 1:35 PM Michael Koch <
> >>> astroelectronic at t-online.de>
> >>>> wrote:
> >>>>
> >>>>> Am 28.11.2020 um 12:57 schrieb Paul B Mahol:
> >>>>>> On Sat, Nov 28, 2020 at 12:41 PM Michael Koch <
> >>>>> astroelectronic at t-online.de>
> >>>>>> wrote:
> >>>>>>
> >>>>>>> Am 27.11.2020 um 20:50 schrieb Paul B Mahol:
> >>>>>>>> On Fri, Nov 27, 2020 at 8:24 PM Michael Koch <
> >>>>>>> astroelectronic at t-online.de>
> >>>>>>>> wrote:
> >>>>>>>>
> >>>>>>>>> Am 27.11.2020 um 19:25 schrieb Paul B Mahol:
> >>>>>>>>>> On Fri, Nov 27, 2020 at 7:09 PM Michael Koch <
> >>>>>>>>> astroelectronic at t-online.de>
> >>>>>>>>>> wrote:
> >>>>>>>>>>
> >>>>>>>>>>> Hello,
> >>>>>>>>>>>
> >>>>>>>>>>> I have a few questions about the asubboost and asupercut
> filters.
> >>>>>>>>>>>
> >>>>>>>>>>> -- In asubboost it's not yet clear how the block diagram of the
> >>>>> filter
> >>>>>>>>>>> looks like. Especially the "decay" and "feedback" options are
> >>>>> unclear.
> >>>>>>>>>>> What's the input of the the delay line? Before or after the low
> >>> pass
> >>>>>>>>>>> filter? Where does the feedback go to? Before or after the
> >>> lowpass
> >>>>>>>>>>> filter? I have attached a sketch of a possible block diagram,
> but
> >>>>> it's
> >>>>>>>>>>> only a wild guess.
> >>>>>>>>>>>
> >>>>>>>>>> This filter just adds delayed sub frequencies set by cut off
> >>>>> frequency
> >>>>>>>>> back
> >>>>>>>>>> to output. Decay sets decay of old sub echo in buffer and
> feedback
> >>>>> sets
> >>>>>>>>> how
> >>>>>>>>>> much
> >>>>>>>>>> new sub frequencies are added to the delay buffer.
> >>>>>>> I did try to reverse engineer the asubboost filter from its output
> >>>>>>> signal. Is the attached sketch correct?
> >>>>>>> It seems the "feedback" parameter is unnecessary because it does
> >>> exactly
> >>>>>>> the same thing as "wet".
> >>>>>>>
> >>>>>> No, your reasoning is invalid.
> >>>>>>
> >>>>>> I do not have time to draw graphs or do consulting for free.
> >>>>> When you swap the values of "wet" and "feedback", the output does
> >>> always
> >>>>> remain the same.
> >>>>> If you think that I'm wrong, please show an example to prove the
> >>> opposite.
> >>>> Make sure that you take into account decay parameter, delay buffer is
> >>> still
> >>>> used.
> >>> When you swap the values of "wet" and "feedback", the output does
> always
> >>> remain the same, regardless which values you use for "dry", "decay" and
> >>> "delay".
> >>> As can be shown with this example:
> >>>
> >>> set "A=0.4"
> >>> set "B=0.7"
> >>>
> >>> ffmpeg -f lavfi -i aevalsrc='0.5*gt(t,0.1)':d=1 -lavfi
> >>>
> asplit[a][b];[b]asubboost=dry=0.3:wet=%A%:decay=0.4:feedback=%B%:delay=50[c],[a][c]join,showwaves=draw=full:s=800x300:r=1
> >>>
> >>> -frames 1 -y out1.png
> >>>
> >>> ffmpeg -f lavfi -i aevalsrc='0.5*gt(t,0.1)':d=1 -lavfi
> >>>
> asplit[a][b];[b]asubboost=dry=0.3:wet=%B%:decay=0.4:feedback=%A%:delay=50[c],[a][c]join,showwaves=draw=full:s=800x300:r=1
> >>>
> >>> -frames 1 -y out2.png
> >>>
> >>> ffmpeg -i out1.png -i out2.png -lavfi vstack -y out.png
> >>>
> >>>
> >>> Red is input step signal, green is output step response of the
> asubboost
> >>> filter. In the lower half of the output image the "wet" and "feedback"
> >>> values are swapped.
> >>>
> >> What we said previously about delay buffers and that above command?
> >> It was literally less than 24h.
>
> The previous example was "showfreqs" which has a frequency domain
> output. Not suitable for analyzing filters which contain delays.
> Now I'm using "showwaves" which has time domain output. That's a
> different thing. Of course delay lines can be analyzed in time domain.
>
>
> >> That command can not show you the action of delayed input as it is not
> >> designed for it.
>
> A step signal contains all frequencies and is the best possible source
> for analyzing unknown black boxes that may contain delays. You can
> replace the input by any other source, but you will never find any
> difference in the two outputs. It's a fact, "wet" and "feedback" are
> interchangeable.
>

Nope, you are very mistaken. Try with real audio.


>
> Michael
>
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