[FFmpeg-user] Audio normalization using "volume" and "compand"filters

[Robert Krüger]

On Wed, Nov 27, 2013 at 10:08 PM, Thierry Lelégard
<thierry.lelegard at free.fr> wrote:
> Le 27/11/2013 21:49, Paul B Mahol a écrit :
>
>> On 11/27/13, Thierry Lelegard <thierry at lelegard.fr> wrote:
>>
>>> Second problem: The usage of the filter "compand" is extremely obscure.
>>> Its
>>> documentation (http://ffmpeg.org/ffmpeg-filters.html#compand) can hardly
>>> be
>>> understood if you do not already know the meaning of each parameter. See
>>> below some tests I made without deeply understanding what they mean.
>>>
>>> [....]
>
>>>
>>> Could someone please explain how to use "compand" and its parameters?
>>>
>>> More precisely, how can we compress an input audio with the
>>> characteristics
>>> in_rms and in_peak into a given target out_rms and out_peak?
>>
>> compand filter is port of sox effect filter of same name.
>> I really doubt that its documentation is obscure.
>
> This is of course a subjective matter. To make it more objective, the
> following
> is a complete copy/paste of the "compand" documentation from
> http://ffmpeg.org/ffmpeg-filters.html#compand
>
> Honestly, how can you get an answer to the above question ("how can we
>
> compress an input audio with the characteristics in_rms and in_peak into
> a given target out_rms and out_peak?"), using this when you are not an
> expert in audio engineering?
>
> The description of the function transfer using the "points" is not clear.
> Especially when the experimentation gives results which seem inconsistent.
> I do not mean that the filter does not work well. I mean that the way it
> works using the parameters is far from clear. This is why I am looking for
> help in this list.
>
> Thanks,
> -Thierry
>
> 6.27 compand
>
> Compress or expand audio dynamic range.
>
> A description of the accepted options follows.
>
> ‘attacks’
> ‘decays’
>
> Set list of times in seconds for each channel over which the instantaneous
> level of the input signal is averaged to determine its volume. ‘attacks’
> refers to increase of volume and ‘decays’ refers to decrease of volume.
> For most situations, the attack time (response to the audio getting louder)
> should be shorter than the decay time because the human ear is more
> sensitive
> to sudden loud audio than sudden soft audio. Typical value for attack is 0.3
> seconds and for decay 0.8 seconds.
>
> ‘points’
>
> Set list of points for transfer function, specified in dB relative to
> maximum
> possible signal amplitude. Each key points list need to be defined using the
> following syntax: x0/y0 x1/y1 x2/y2 ....
>
> The input values must be in strictly increasing order but the transfer
> function does not have to be monotonically rising. The point 0/0 is assumed
> but may be overridden (by 0/out-dBn). Typical values for the transfer
> function are -70/-70 -60/-20.
>
> ‘soft-knee’
>
> Set amount for which the points at where adjacent line segments on the
> transfer function meet will be rounded. Defaults is 0.01.
>
> ‘gain’
>
> Set additional gain in dB to be applied at all points on the transfer
> function and allows easy adjustment of the overall gain. Default is 0.
>
> ‘volume’
>
> Set initial volume in dB to be assumed for each channel when filtering
> starts. This permits the user to supply a nominal level initially, so that,
> for example, a very large gain is not applied to initial signal levels
> before the companding has begun to operate. A typical value for audio
> which is initially quiet is -90 dB. Default is 0.
>
> ‘delay’
>
> Set delay in seconds. Default is 0. The input audio is analysed immediately,
> but audio is delayed before being fed to the volume adjuster. Specifying
> a delay approximately equal to the attack/decay times allows the filter
> to effectively operate in predictive rather than reactive mode.
>
> 6.27.1 Examples
>
> Make music with both quiet and loud passages suitable for listening in a
> noisy environment:
>
> compand=.3 .3:1 1:-90/-60 -60/-40 -40/-30 -20/-20:6:0:-90:0.2
>
> Noise-gate for when the noise is at a lower level than the signal:
>
> compand=.1 .1:.2 .2:-900/-900 -50.1/-900 -50/-50:.01:0:-90:.1
>
> Here is another noise-gate, this time for when the noise is at a higher
> level than the signal (making it, in some ways, similar to squelch):
>
> compand=.1 .1:.1 .1:-45.1/-45.1 -45/-900 0/-900:.01:45:-90:.1
>

Maybe it helps if you google for Sox compand examples or questions or
even check the Sox manual. It's been part of Sox for a long time and
only rather recently has been ported to ffmpeg. So the knowledge base
in the Sox community is likely to be more helpful in this regard.